Cover: Satellite-based relief image, Red River Valley

 Courtesy of Jonathan Caine
Questa Baseline and Pre-Mining GroundWater Quality Investigation. 25. Summary 

of Results and Baseline and Pre-Mining 

Ground-Water Geochemistry, Red River 

Valley, Taos County, New Mexico, 

2001–2005

By D. Kirk Nordstrom

Prepared in cooperation with the New Mexico Environment Department

Professional Paper 1728

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Nordstrom, D.K, 2008, Questa baseline and pre-mining ground-water quality investigation. 25. Summary of results 

and baseline and pre-mining ground-water geochemistry, Red River Valley, Taos County, New Mexico, 2001–2005: 

U.S. Geological Survey Professional Paper 1728, 111 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................3

Questa Baseline and Pre-Mining Ground-Water Quality Investigation ......................................4

Meaning of Natural Background and Baseline......................................................................4

New Mexico Ground-Water Quality Standards......................................................................4

The Questa Molycorp Molybdenum Mine...............................................................................5

Acknowledgments................................................................................................................................5

Study Site ........................................................................................................................................................7

Physical Features..................................................................................................................................7

Geology...................................................................................................................................................7

Climate and Vegetation........................................................................................................................9

Hydrology ...............................................................................................................................................9

Surface Water..............................................................................................................................9

Ground Water.............................................................................................................................10

Database for Speciation and Mineral Saturation Calculations ...........................................................10

Summary Conclusions from Previous Reports .......................................................................................10

Historical Review of Water Quality..................................................................................................11

Ground Water.............................................................................................................................11

Red River Surface Water..........................................................................................................11

Seeps, Springs, and Tributaries...............................................................................................14

Geologic Investigations .....................................................................................................................14

Airborne Visible-Infrared Imaging Spectrometry.................................................................14

Geologic Characterization of Weathering Processes .........................................................14

Mineralogical Characterization...............................................................................................16

Leaching Studies .......................................................................................................................16

Bedrock Structure and Ground-Water Flow.........................................................................17

Lake-Sediment Chemistry ........................................................................................................17

Seismic Profiles..........................................................................................................................17

Geomorphology..........................................................................................................................18

Hydrologic Investigations..................................................................................................................19

Quality Control and Quality Assurance (QA/QC) of Water Analyses.................................19

Synoptic and Tracer Studies of Red River.............................................................................19

Reactive-Transport Modeling in the Red River.....................................................................21

Diel, Storm Event, and Long-Term Trends in Red River.......................................................22

Ground Water.............................................................................................................................24

Water Balances ........................................................................................................................24

Straight Creek....................................................................................................................24

Red River Valley ................................................................................................................25

Integrating Water-Flow and Sulfate Mass-Load Balances for the 

Hottentot–La Bobita Reach ..........................................................................................................25

Interpretation of Ground-Water Geochemistry ......................................................................................30

Median Concentrations from Monitoring Data..............................................................................30
iv

Chemistry of Water Developed from Scar Weathering................................................................30

Water-Chemistry Classification for the Red River Valley Ground Water...................................30

Geochemical Mass Balance on Straight Creek Drainage...........................................................40

Geochemical Controls on Solute Concentrations ..................................................................................42

Trends in Specific Conductance ......................................................................................................42

Geochemical Controls on Dissolved Sulfate Concentrations .....................................................45

Geochemical Controls on Dissolved Iron Concentrations...........................................................47

Ferric Iron Concentrations .......................................................................................................48

Ferrous Iron Concentrations....................................................................................................48

Geochemical Controls on Dissolved Manganese Concentrations.............................................48

Geochemical Controls on Dissolved Aluminum and Silica Concentrations .............................54

Aluminum.....................................................................................................................................54

Silica.............................................................................................................................................57

Geochemical Controls on Dissolved Fluoride Concentrations....................................................57

Geochemical Controls on Dissolved Calcium Concentrations....................................................62

Geochemical Controls on Dissolved Magnesium Concentrations.............................................65

Geochemical Controls on Dissolved Strontium Concentrations.................................................67

Geochemical Controls on Dissolved Barium Concentrations .....................................................70

Geochemical Controls on Dissolved Zinc Concentrations ..........................................................70

Geochemical Controls on Dissolved Cadmium Concentrations .................................................70

Geochemical Controls on Dissolved Copper Concentrations .....................................................73

Geochemical Controls on Dissolved Nickel and Cobalt Concentrations ..................................73

Geochemical Controls on Dissolved Chromium Concentrations................................................76

Geochemical Controls on Dissolved Lithium, Sodium, and Potassium 

Concentrations ......................................................................................................................76

Geochemical Controls on Dissolved Beryllium Concentrations .................................................79

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site.............................................79

Pre-Mining Ground Water at Molycorp’s Questa Mine .............................................................. 79

Capulin Canyon....................................................................................................................................82

Goat Hill Gulch.....................................................................................................................................87

Sulphur Gulch......................................................................................................................................90

Sugar Shack Catchments..................................................................................................................94

Bedrock Ground Water......................................................................................................................95

Summary........................................................................................................................................................96

References Cited..........................................................................................................................................98

Appendix 1 ..................................................................................................................................................105

 Table 1–1. Thermodynamic data used by WATEQ4F for modeling aqueous 

speciation and mineral solubility......................................................................................106

 Table 1–2. List of analytic equations for linear best fits of correlated data in

 the form of y = mx + b ........................................................................................................110

Appendix 2. Mathematical derivation of curves for mixing lines shown in 

figures 8 and 9...............................................................................................................................111
v

Plate

(View this plate by following the link at http://pubs.usgs.gov/pp/1728)

1. Surficial geologic map showing late Quaternary sediments, wasterock piles, wells, catchments, and tributary streams, in the middle 

Red River Valley, New Mexico. Modified from Vincent (in press)

Figures

1. Maps showing the Red River Valley and the study reach between the

town of Red River and the U.S. Geological Survey gage near Questa, 

New Mexico ..................................................................................................................................6 

2. Satellite image of the Red River showing scar areas, Molycorp mine site, 

and the town of Red River with photographs of scar areas superimposed on the satellite image...................................................................................................8

3. Diagram showing study components and their relationships for inferring 

pre-mining ground-water chemistry .......................................................................................13

4. Schematic geologic cross section of alteration zones with overlying 

dashed lines depicting the relative position of the erosional surface

for five catchments along the Red River.................................................................................15

5. Example of a cross section for seismic line 3 showing bedrock, water, 

and ground surface and unsaturated zone from seismic refraction

 tomography..................................................................................................................................18

6. Longitudinal cross section of the Red River along the reach of the mine 

site projected onto a two-dimensional plane with bedrock surface, 

water tables, projections of the debris-fan surface, and Red River

streambed surface .....................................................................................................................19

7–10. Graphs showing:

 7. Longitudinal profiles of discharge, pH, alkalinity, dissolved sulfate,

manganese, zinc, total recoverable aluminum, and dissolved 

aluminum with distance from the synoptic/tracer study of 2001.................................20

 8. Mixing line constructed between Red River upstream from groundwater emergence (7,200 meters) and well water from SC–1A, using 

calcium/sulfate molar ratios relative to sulfate concentrations .................................22

 9. Mixing line constructed between Red River upstream from groundwater emergence (7,200 meters) and well water from SC–1A using 

magnesium/sulfate mass ratios relative to sulfate concentrations............................23

 10. Changes in discharge, pH, and sulfate and manganese concentrations at the U.S. Geological Survey Questa gage during the storm

of September 17–18, 2002...................................................................................................24

11–14. Diagrams showing:

 11. Water-flow balance for the Red River alluvial aquifer shown 

schematically for synoptic/tracer studies of 2001 and 2002 for the 

reach from Hottentot debris fan to La Bobita .................................................................26

 12. Sulfate mass-load balance for the Red River alluvium shown 

schematically for the synoptic/tracer studies of 2001 and 2002 for 

the reach from Hottentot debris fan to La Bobita ..........................................................27
vi

 13. Sulfate mass loads for Model II in which the sulfate mass loads of 

the catchments are back-calculated from the ground-water 

emergence mass loads ......................................................................................................28

 14. Sulfate mass loads for Model III in which the sulfate mass load of 

the alluvial ground-water inflow is back-calculated from the groundwater emergence mass loads and the catchment inflows are kept 

constant ................................................................................................................................29

15. Map showing location of observation wells in Straight Creek catchment 

and Advanced Waste-Water Treatment facilities.................................................................31

16. Graphs of (A) frequency distribution plot of pH for scar-drainage water 

in the Red River Valley and (B) frequency distribution plot of sulfate 

concentrations for scar-drainage water in the Red River Valley (excluding

Goat Hill water) ...........................................................................................................................40

17. Plot of pH against median sulfate concentration for all ground waters 

and Straight Creek surface water............................................................................................41

18. Graph showing decrease in pH with the incremental oxidation of pyrite 

to (a) an acid ferrous sulfate solution with no dissolved iron oxidation, 

(b) oxidation of the dissolved iron sulfate solution but no precipitation of 

hydrous ferric oxides, and (c) oxidation with precipitation of ferrihydrite .......................42

19. Diagram showing evolutionary flow paths based on changes in groundwater chemistry with downstream gradient..........................................................................43

20. Plot of specific conductance against sulfate concentrations for (A) Straight 

Creek surface and alluvial ground waters and (B) median values for all 

ground waters and Straight Creek surface water ................................................................44

21–40. Graphs showing:

 21. Correlation between specific conductance and sulfate concentration 

for Red River surface-water samples from this study...................................................45

 22. Relation between sulfate concentrations of Straight Creek well waters 

and elevation of the bottom of the screened interval for each well...........................46

 23. (A) Iron concentrations plotted against sulfate concentrations for 

all ground waters and Straight Creek surface water. (B) Ferrous 

iron: total dissolved iron molar ratio plotted against sulfate concentrations for all ground waters and Straight Creek surface water...............................47

 24. (A) Ferric iron concentrations plotted against pH for all ground waters 

and Straight Creek surface water. Ferrihydrite saturation index plotted 

against (B) pH and (C) iron for all ground waters and Straight Creek 

surface water fulfilling the Eh measurement requirements described 

in Naus and others (2005) and in Nordstrom and others (2005)...................................49

 25. Saturation indices for potassium jarosite plotted relative to pH.................................50

 26. (A) Ferrous iron plotted against alkalinity. Siderite saturation index 

plotted against (B) pH, (C) calcium concentrations, and (D) dissolved 

inorganic carbon concentrations for all ground waters and Straight 

Creek surface water ...........................................................................................................51

 27. Manganese concentrations plotted against sulfate concentrations 

for (A) Straight Creek surface and alluvial ground waters and 

(B) median manganese and sulfate concentrations for all ground 

waters and Straight Creek surface water.......................................................................52

 28. (A) Dissolved manganese concentrations plotted against alkalinity. 

Rhodochrosite saturation index plotted against (B) dissolved inorganic 

carbon concentrations and against (C) pH for all ground waters and 

Straight Creek surface water............................................................................................53
vii

 29. Dissolved aluminum concentrations plotted against sulfate concentrations for (A) Straight Creek surface and alluvial ground waters, 

(B) median aluminum and sulfate concentrations for all ground 

waters and Straight Creek surface water, and (C) dissolved aluminum

concentrations plotted against pH...................................................................................55

 30. (A) The log of the activity of free Al3+ plotted against pH for all ground 

waters and Straight Creek surface water, (B) amorphous aluminum 

hydroxide saturation index plotted against pH for all ground waters 

and Straight Creek surface water. Alunite saturation index normalized to the total stoichiometric coefficient plotted against (C) pH, 

and (D) aluminum concentration ......................................................................................56

 31. (A) Silica concentrations plotted against pH for all ground waters. 

(B) Silica concentrations plotted against aluminum concentrations 

for all ground waters. For samples having a pH value less than 4, 

silica concentrations plotted against sulfate concentrations for

(C) Straight Creek alluvial ground water; and (D) median silica and 

sulfate concentrations for all ground waters and Straight Creek 

surface water.......................................................................................................................58

 32. (A) Silica saturation index plotted against pH, and (B) kaolinite 

saturation index plotted against pH .................................................................................59

 33. Fluoride concentrations plotted against sulfate concentrations for 

(A) Straight Creek alluvial ground waters and (B) median fluoride 

and sulfate concentrations for all ground waters and Straight Creek 

surface water. Fluoride concentrations plotted against calcium 

concentrations for (C) Straight Creek and alluvial ground water and 

(D) median concentrations for all ground waters and Straight Creek 

surface water.......................................................................................................................60

 34. (A) Fluoride concentrations plotted against calcium concentrations. 

Fluorite saturation index plotted against (B) pH, (C) aluminum 

concentrations, and (D) aluminum concentrations less than 

10 milligrams per liter for all ground waters and Straight Creek 

surface water.......................................................................................................................61

 35. Fluoride concentrations plotted in relation to aluminum concentrations for all ground waters and Straight Creek surface water....................................62

 36. Calcium concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) for 

median ground-water concentrations and Straight Creek surface 

water. (C) Ground waters and Straight Creek surface water, all data. 

(D) Calcium:sulfate molar ratio plotted against pH for all ground 

waters and Straight Creek surface water.......................................................................63

 37. Gypsum saturation index plotted against (A) calcium concentrations, 

(B) sulfate concentrations, (C) the calcium:sulfate molar ratio for 

Straight Creek ground water and surface water. (D) Gypsum saturation

index plotted against calcite saturation index ...............................................................64

 38. Calcite saturation index plotted against pH for all ground waters 

and Straight Creek surface water ...................................................................................65

 39. Magnesium concentrations plotted against calcium concentrations for

(A) Straight Creek surface and alluvial ground waters and (B) median 

concentrations for all ground waters and Straight Creek surface water. 

Magnesium concentrations plotted against sulfate concentrations for 

(C) Straight Creek surface and alluvial ground waters and (D) all ground 

waters and Straight Creek surface water.......................................................................66
viii

 40. Dolomite saturation index plotted against calcium concentrations...........................67

41. (A) Plot of strontium concentrations against sulfate concentrations for all 

ground waters and Straight Creek surface water. (B) Plot of strontium concentrations against calcium concentrations for all ground waters and 

Straight Creek surface water. (C) Strontianite saturation index relative to 

calcium concentrations for ground water and Straight Creek surface water. 

(D) Celestine saturation index relative to calcium concentrations for all 

ground waters and Straight Creek surface water ................................................................68

42–50. Graphs showing:

 42. (A) Barium concentrations plotted against sulfate concentrations for 

all ground waters and Straight Creek surface water. Barite saturation 

index plotted against (B) barium concentration and (C) pH for all 

ground waters and Straight Creek surface water. (D) Dissolved 

(0.45-micrometer filtration) barium concentrations plotted against

total recoverable barium concentrations for all ground waters and 

Straight Creek surface water............................................................................................69

 43. Zinc concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) median 

values for all ground and surface waters. Zinc concentrations plotted 

against manganese concentrations for (C) Straight Creek surface and 

alluvial ground waters and (D) median values for all ground and 

surface waters.....................................................................................................................71

 44. Cadmium concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) all ground 

and surface waters. Cadmium concentrations plotted against zinc 

concentrations for (C) Straight Creek surface and alluvial ground 

waters and (D) all ground waters and Straight Creek surface water ........................72

 45. Copper concentrations plotted against sulfate concentrations for 

A) Straight Creek surface and alluvial ground waters and (B) all 

ground and surface waters. Copper concentrations plotted against 

zinc concentrations for (C) Straight Creek surface and alluvial ground 

waters and (D) all ground waters and Straight Creek surface water ........................74

 46. Nickel concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) all ground 

and surface waters. Cobalt concentrations plotted against nickel 

concentrations for (C) Straight Creek surface and alluvial ground 

waters and (D) all ground waters and Straight Creek surface water ........................75

 47. Chromium concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) all ground 

and surface waters. Chromium concentrations plotted against nickel

concentrations for (C) Straight Creek surface and alluvial ground 

waters and for (D) all ground waters and Straight Creek surface water ..................77

 48. Lithium concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) all ground 

and surface waters. Potassium concentrations plotted against sodium 

concentrations for (C) Straight Creek surface and alluvial ground waters 

and for (D) all ground waters and Straight Creek surface water................................78

 49. Beryllium concentrations plotted against sulfate concentrations for 

(A) Straight Creek surface and alluvial ground waters and (B) all ground 

and surface waters ............................................................................................................ 80
ix

 50. Beryllium concentrations plotted against fluoride concentrations for 

(A) Straight Creek alluvial ground waters and (B) all ground and surface 

waters. Beryllium concentrations plotted against lithium concentrations 

for (C) Straight Creek alluvial ground waters and for (D) all ground waters 

and Straight Creek surface water ....................................................................................81

51. Topographic map of mine site with watershed boundaries, scar areas, debris 

fans, and Red River alluvium ....................................................................................................82

52. Plot of pH against sulfate concentration for (A) seeps and ground waters 

in Capulin Canyon with less than 3,000 milligrams per liter sulfate concentration and (B) same waters including those surface-water impoundment 

waters with greater than 3,000 milligrams per liter sulfate concentration.......................83

53. (A) Plot of fluoride concentrations against calcium concentrations for Capulin 

Canyon waters. (B) Plot of fluoride concentrations against sulfate concentrations. (C) Same plot as figure 53A but showing the entire range of concentrations. (D) Same plot as figure 53B but showing the entire range of 

concentrations ...........................................................................................................................84

54. (A) Plot of zinc against manganese concentrations for Capulin Canyon 

waters. (B) Plot of zinc against sulfate concentrations for Capulin Canyon 

waters. (C) Same plot as figure 54A but showing the entire range of concentrations. (D) Same plot as figure 54B but showing the entire range of 

concentrations ...........................................................................................................................85

55. (A) Plot of nickel against sulfate concentrations for Capulin Canyon waters. 

(B) Plot of cobalt against nickel concentrations for Capulin Canyon waters. 

(C) Same plot as figure 55A but showing the entire range of concentrations. 

(D) Same plot as figure 55B but showing the entire range of concentrations .................86

56. Topographic map of the three subcatchments that constitute Sulphur Gulch–

upper Sulfur Gulch, Blind Gulch, and Spring Gulch..............................................................91

57. Geologic map of the three subcatchments that constitute Sulphur Gulch–

upper Sulfur Gulch, Blind Gulch, and Spring Gulch..............................................................92

Tables 

1. New Mexico ground-water quality standards.........................................................................5

2. Compilation of reports published within this project............................................................12

3. Catchments that contribute ground water and sulfate loads to the Red 

River alluvium near natural scar drainages (Hottentot to La Bobita) for 

August 2001..................................................................................................................................27

4. Catchment yields, sulfate concentrations, and sulfate mass loads using 

Model II for 2001 .........................................................................................................................28

5. Median values for pH, temperature, specific conductance, redox 

potential, dissolved oxygen (DO), dissolved solute concentrations, 

dissolved organic carbon (DOC), cation- and anion-equivalent sum, 

and speciated charge imbalance for Straight Creek drainage water 

and ground water analyzed in this study................................................................................32

6. Weathering mass balances of minerals that can account for the median 

chemistry of Straight Creek scar-drainage water.................................................................42

7. Range of values for water-quality constituents of concern for Capulin 

Canyon alluvial ground waters unaffected by scar drainage.............................................88
x

8. Range of values for water-quality constituents of concern for Capulin 

Canyon alluvial ground waters affected by scar drainage..................................................88

9. Range of values for water-quality constituents of concern for Capulin 

Canyon bedrock ground waters...............................................................................................88

10. Preliminary values for water-quality constituents of concern for alluvial 

ground waters in Goat Hill.........................................................................................................89

11. Estimated preliminary values for pre-mining ground-water quality constituents for Goat Hill Gulch debris fan compared to the range of values

from water in wells MMW–42A and MMW–44A..................................................................89

12. Estimated final values for water-quality constituents of concern for 

alluvial ground waters in Goat Hill...........................................................................................90

13. Compositions of water used for the reactive mixing calculations .....................................93

14. pH, aluminum, and iron concentrations calculated by assuming different aluminous 

minerals precipitate upon mixing in the Sulphur Gulch catchment...................................94

15. Water-quality constituents of concern for lower Sulphur Gulch drainage 

and debris fan..............................................................................................................................94

16. Water-quality constituents of concern for two Sugar Shack catchments 

(8 and 10 on plate 1)....................................................................................................................95

17. Water-quality constituents of concern for middle Sugar Shack catchment (area 9 on plate 1).............................................................................................................95

18. Water-quality constituents of concern for the range of pre-mining 

bedrock ground waters exclusive of those in Capulin Canyon...........................................96
xi

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft) 

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

Area

square kilometer (km2

) 247.1 acre

square kilometer (km2

) 0.3861 square mile (mi2)

square meter (m2

) 10.76 square foot (ft2) 

square centimeter (cm2

) 0.1550 square inch (ft2) 

Volume

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 264.2 gallon (gal) 

liter (L) 0.2642 gallon (gal)

liter (L) 1.057 quart (qt)

liter (L) 33.82 ounce, fluid (fl. oz)

cubic centimeter (cm3

) 0.06102 cubic inch (in3) 

Flow rate

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

cubic meter per second (m3

/s) 22.83 million gallons per day (Mgal/d) 

liter per second (L/s) 15.85 gallon per minute (gal/min) 

liter per second (L/s) 0.03532 cubic foot per second (ft3

/s)

Mass

ton, short (2,000 lb) 0.9072 metric ton

kilogram (kg) 2.205 pound avoirdupois (lb)

gram (g) 0.03527 ounce, avoirdupois (oz)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32.

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Horizontal coordinate information is referenced to North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius

(μS/cm at 25°C).
xii

Abbreviations used in this report

°C degree Celsius

AVIRIS Airborne Visible-Infrared Imaging Spectrometry

AWWT Advanced Waste-Water Treatment 

cm centimeters

DIC dissolved inorganic carbon

DOC dissolved organic carbon

Eh redox potential relative to the Standard Hydrogen Electrode

ft3

/s cubic feet per second

HFO hydrous ferric oxides

IAP ion-activity product

kg/day kilograms per day

km kilograms

km2 kilograms squared

Ksp solubility-product constant

m meters

MAD mean absolute deviation

MCL maximum contaminant level

meq/L milliequivalents per liter

mg/L milligrams per liter

mm millimeter

mmol/L millimoles per liter

m/m meters per meter

NMAC New Mexico Administration Code

NMED New Mexico Environment Department 

NMSA New Mexico Statutory Authority

QA/QC quality assurance/quality control

QSP quartz-sericite-pyrite

SI saturation index

TDS total dissolved solids

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

μg/L micrograms per liter

μm micrometer

μS/cm microsiemens per centimeter

WQCC Water Quality Control Commission
Questa Baseline and Pre-Mining Ground-Water Quality 

Investigation. 25. Summary of Results and Baseline and 

Pre-Mining Ground-Water Geochemistry, Red River Valley, 

Taos County, New Mexico, 2001–2005

By D. Kirk Nordstrom

Abstract

Active and inactive mine sites are challenging to remediate because of their complexity and scale. Regulations meant 

to achieve environmental restoration at mine sites are equally 

challenging to apply for the same reasons. The goal of environmental restoration should be to restore contaminated mine 

sites, as closely as possible, to pre-mining conditions. Metalliferous mine sites in the Western United States are commonly 

located in hydrothermally altered and mineralized terrain 

in which pre-mining concentrations of metals were already 

anomalously high. Typically, those pre-mining concentrations 

were not measured, but sometimes they can be reconstructed 

using scientific inference.

Molycorp’s Questa molybdenum mine in the Red River 

Valley, northern New Mexico, is located near the margin of 

the Questa caldera in a highly mineralized region. The State of 

New Mexico requires that ground-water quality standards be 

met on closure unless it can be shown that potential contaminant concentrations were higher than the standards before 

mining. No ground water at the mine site had been chemically analyzed before mining. The aim of this investigation, in 

cooperation with the New Mexico Environment Department 

(NMED), is to infer the pre-mining ground-water quality by 

an examination of the geologic, hydrologic, and geochemical controls on ground-water quality in a nearby, or proximal, 

analog site in the Straight Creek drainage basin. Twenty-seven 

reports contain details of investigations on the geological, 

hydrological, and geochemical characteristics of the Red 

River Valley that are summarized in this report. These studies 

include mapping of surface mineralogy by Airborne VisibleInfrared Imaging Spectrometry (AVIRIS); compilations of historical surface- and ground- water quality data; synoptic/tracer 

studies with mass loading and temporal water-quality trends of 

the Red River; reaction-transport modeling of the Red River; 

environmental geology of the Red River Valley; lake-sediment 

chemistry; geomorphology and its effect on ground-water 

flow; geophysical studies on depth to ground-water table 

and depth to bedrock; bedrock fractures and their potential 

influence on ground-water flow; leaching studies of scars and 

waste-rock piles; mineralogy and mineral chemistry and their 

effect on ground-water quality; debris-flow hazards; hydrology and water balance for the Red River Valley; ground-water 

geochemistry of selected wells undisturbed by mining in the 

Red River Valley; and quality assurance and quality control 

of water analyses. Studies aimed specifically at the Straight 

Creek natural-analog site include electrical surveys; highresolution seismic survey; age-dating with tritium/helium; 

water budget; ground-water hydrology and geochemistry; 

and comparison of mineralogy and lithology to that of the 

mine site.

The highly mineralized and hydrothermally altered 

volcanic rocks of the Red River Valley contain several percent 

pyrite in the quartz-sericite-pyrite (QSP) alteration zone, 

which weather naturally to acid-sulfate surface and ground 

waters that discharge to the Red River. Weathering of wasterock piles containing pyrite also contributes acid water that 

eventually discharges into the Red River. These acid discharges are neutralized by circumneutral-pH, carbonatebuffered surface and ground waters of the Red River. The 

buffering capacity of the Red River, however, decreases 

from the town of Red River to the U.S. Geological Survey 

(USGS) gaging station near Questa. During short, but intense, 

storm events, the buffering capacity is exceeded and the river 

becomes acid from the rapid flushing of acidic materials from 

natural scar areas.

The lithology, mineralogy, elevation, and hydrology of 

the Straight Creek proximal analog site were found to closely 

approximate those of the mine site with the exception of the 

mine site’s Sulphur Gulch catchment. Sulphur Gulch contains 

three subcatchments—upper Sulphur Gulch, Blind Gulch, and 

Spring Gulch. Blind and Spring Gulches are largely propylitic zones with negligible pyrite mineralization, and although 

this lithology is not found at Straight Creek, it is found to the 

west of the Hansen Creek area where the geology extends to 

the Spring Gulch catchment. In lower Sulphur Gulch the erosion surface has cut deeper into the hydrothermal-alteration 

sequence than that found at Straight Creek, exposing the 
carbonate-fluorite alteration zone. Nevertheless, concentration 

limits from mineral solubilities and from the highest measured concentrations in ground water undisturbed by mining 

in the Red River Valley provide a basis for constraining the 

most likely concentrations for pre-mining conditions at the 

mine site.

The Straight Creek natural-analog site consists of acid 

surface drainage derived from the weathering of QSP altered 

rocks at the headwaters. The surface drainage of pH 2.5–3 

disappears into a debris fan and becomes acid alluvial ground 

water of pH 3–4. Most dissolved constituents behave conservatively during downgradient ground-water transport and are 

diluted by side-canyon seepage waters and by Red River alluvial ground water at the toe of the debris fan. The main change 

in chemistry is a transition from strongly oxidizing conditions 

in the surface drainage (dissolved iron is in the Fe[III] oxidation state) to moderately reducing conditions (dissolved iron 

is in the Fe[II] oxidation state, without sulfate reduction) in 

the alluvial ground water. As a result of reduction, substantial amounts of copper are removed from solution and some 

chromium is removed. A substantial increase in silica concentration is observed between Straight Creek surface drainage 

and the alluvial ground water. Aluminum and silica appear to 

coprecipitate once the alluvial ground waters have reached a 

pH of 4. Bedrock ground waters are circumneutral pH (6–7.8) 

but also high in iron, manganese, and sulfate. Ferrous iron 

concentrations in bedrock are limited by siderite saturation, 

manganese by rhodochrosite saturation, calcium by calcite and 

gypsum saturation, barium by barite saturation, and fluoride 

by fluorite saturation.

A water-flow and sulfate-load balance was developed 

for the reach of the Red River from the Hottentot debris fan 

to the La Bobita campground. In spite of uncertainties in the 

data, these estimates indicate that ground waters in the Red 

River alluvium leaving the La Bobita area are 8 to 9 cubic 

feet per second and the sulfate flux is approximately 3,000 to 

5,000 kilograms per day. Further fluxes of sulfate-laden waters 

enter the Red River alluvium between La Bobita and the Columbine Park area, but more field work and calculations would 

be needed to define the ground-water flow and sulfate fluxes 

in this area.

The origin and geochemical behavior of individual solutes in Straight Creek surface drainage, acid alluvial ground 

water, and neutral-pH bedrock ground water are compared 

to solute concentrations and their geochemical behavior in 

other catchments undisturbed by mining. The same trends 

apply generally with a few exceptions that can be explained 

by local mineralized zones that may be suboxic without 

substantial pyrite oxidation; that is, pyrite (if present) has not 

been exposed at the ground surface to intense weathering. The 

results show that acid ground waters derived from scar areas 

follow regular solute trends demonstrated by linear correlations of elements with sulfate based on the Straight Creek 

analog trend. The only mineral solubility controls that apply to 

oxidized acid ground water are hydrous ferric oxide saturation for ferric iron, gypsum for calcium, barite for barium, 

and aluminum and silica solubility control by an as-yetunidentified phase when pH values reach 4. Circumneutral-pH 

anoxic ground water achieves mineral saturation for siderite, 

rhodochrosite, calcite, fluorite, Al(OH)3

, gypsum, and barite. The results also show that a wide range of ground-water 

chemistry is possible over rather short distances. This wide 

range of solute concentration greatly complicates the objective 

of obtaining the pre-mining ground-water quality.

The geochemical behavior of solutes that were interpreted for Straight Creek, then applied to other undisturbed 

catchments, were then applied to catchments on the mine site 

for pre-mining conditions. The wide range of water chemistry 

that depends on changes in lithology and alteration, the presence or absence of scar, whether alluvial or bedrock ground 

water along with the uncertainty that accompanies the data 

led to the conclusion that a range of concentration for each 

solute of regulatory concern had to be applied, and that different solute concentrations had to be applied to bedrock than to 

alluvial ground waters, and that different concentrations had to 

be used for different parts of the same catchment if the lithology changed markedly. Different ranges of solute concentrations had to be estimated for each different watershed on the 

mine site because the geology changed markedly from one 

catchment to another. In spite of these complications, there is 

little doubt that under natural, pre-mining conditions, several 

elements of regulatory concern must have exceeded New 

Mexico ground-water quality standards by as much as tenfold 

in several locations both at the mine site and along the Red 

River Valley between the towns of Red River and Questa. The 

most common exceedances were for iron, manganese, sulfate, 

and fluoride. Manganese exceeded New Mexico ground-water 

standards by as much as 250 times. This exceedance is caused 

by the dissolution of rhodochrosite, MnCO3

, and manganiferous calcite, (Ca,Mn)CO3

, that are common accessory minerals 

from hydrothermal alteration.

Introduction

Ore deposits are defined as mineral deposits that contain 

economic concentrations of minerals. In the Western United 

States, metalliferous ore deposits are found commonly in 

mountainous regions where rapid weathering has exposed 

them, making them visible to prospectors and more easily 

mapped and evaluated by economic geologists. Mining of 

metal-sulfide deposits exposes more minerals to oxidation 

and solubilization, which leads to deleterious contamination 

of receiving streams if unabated (Blowes and others, 2004). 

Water quality commonly is degraded by acid mine waters containing high concentrations of metals such as iron, aluminum, 

copper, zinc, cadmium, arsenic, and mercury. Precipitation and 

transport of iron and aluminum colloids as well as naturally 

occurring clay minerals can cause high turbidity during stormrunoff events at mine sites. Numerous fishkills and elimination 

of benthic invertebrates have been documented for many of 

2 Investigation 25—Summary Report
these mine sites. Today, these sites are subject to regulations 

that require remediation and restoration of mined lands and 

affected waters.

Important questions that often arise are: How clean do 

these waters have to be? If natural waters unaffected by mining exceeded water-quality goals or criteria because of highly 

mineralized rock, shouldn’t different standards apply? What 

criteria can provide a basis for water-quality goals at any particular mine site? The answers to these questions have a direct 

bearing on the costs of remediation. It seems reasonable to 

assume that improvements to water quality need not proceed 

beyond those conditions that existed before mining took place. 

But what was the quality of water prior to mining? Before the 

1950s, ground-water quality (regardless of whether before, 

during, or after mining) was rarely, if ever, sampled and 

analyzed at mine sites. The same situation was still common 

until the last decade or two. Most mine sites have no direct or 

reliable information regarding ground-water quality, although 

many surface-water samples have been analyzed for acid and 

metal contaminants. Documented ground-water chemistry 

(not just the water chemistry of mine water discharged to the 

surface) affected by mining operations include midwestern 

lead-zinc mines of the United States (Playton and others, 

1980; Parkhurst, 1987; Toran, 1987), coal mines in Illinois 

and Appalachia (Booth, 2002; Brady and others, 1998), mines 

in Europe, South America, Russia, and South Africa (Goleva, 

1977; Vartanyan, 1988; Fernández-Rubio, 1999; Younger and 

Robins, 2002; Younger and others, 2002).

Before the 1970s, routine analytical techniques were not 

available to determine low concentrations of many trace elements in water samples. Concentrations and detection limits 

at the level of today’s water-quality standards could not have 

been determined reliably, if at all, by routine analytical techniques at that time. Therefore, any attempt to determine premining ground-water quality at older mine sites must rely on 

indirect methods and scientific inference. Inference of groundwater quality constituents in a complex mineralized terrain of 

poorly known hydrology will always carry larger uncertainties 

than any direct methods of measuring ground-water quality, 

and the degree of uncertainty depends on the approach used.

Mineralized areas undisturbed by mining are known to 

contain acid-sulfate water with toxic concentrations of metals 

(Runnells and others, 1992; Posey and others, 2000; Mast and 

others, 2000; Yager and others, 2000). These conditions occur 

because the same geochemical and microbiological processes 

that produce acid mine water also produce natural acidic 

water. Important differences between acid mine drainage and 

natural acidic drainage are that (a) mining greatly increases 

the surface area of pyrite and other sulfides exposed to air 

and water, thereby increasing substantially the rate of acidity 

produced, (b) mining increases the total mass amount of pyrite 

and other sulfides exposed to air and water, thereby increasing 

substantially the amount of metal-rich acid waters produced, 

and (c) the increased quantities of acidic production from 

mining increases the mobility of heavy metals to waterways. 

Hence, the risks to aquatic, terrestrial, and human health 

increase substantially from mining and mineral-processing

activities compared to pre-mining conditions in the same 

areas, unless remedial measures offering environmental protection are incorporated into the mining plans.

The key to understanding pre-mining water-quality 

conditions depends directly on understanding the processes 

that produce acid rock drainage and high concentrations 

of soluble constituents for a given hydrogeological setting. 

Our understanding of rock weathering, pyrite oxidation, the 

inorganic and microbial processes that lead to the development of acidity, and processes affecting the mobility of metals 

in surface and ground waters has improved considerably over 

the last 2 decades (Alpers and Blowes, 1994; Plumlee and 

Logsdon, 1999; Nordstrom, 2000, 2004; Jambor and others, 2003; Nordstrom and others, 2007). A prudent choice of 

empirical relationships interpreted by geologic and hydrologic 

context and geochemical models can provide the necessary 

constraints to estimate pre-mining water-quality conditions 

for Molycorp’s Questa molybdenum mine. To address this 

need, the U.S. Geological Survey (USGS), in cooperation with 

New Mexico Environment Department (NMED), conducted a 

multidiscipline investigation to infer pre-mining ground-water 

chemistry at the mine site.

Purpose and Scope

This report is a summary of a 5-year investigation 

(2001–2005) on geological and hydrological conditions that 

affect ground water and surface water in the mineralized area 

of the Red River Valley from the town of Red River to the 

USGS gaging station (08265000, Red River near Questa) 

at the Questa District Ranger Station of the USDA Forest 

Service. The size and complexity of the field site necessitated 

a multidisciplinary approach gathering field data and utilizing 

expertise in surface-water hydrology, ground-water hydrology, 

geology (environmental, economic, and structural geology), 

mineralogy, geochemistry, geomorphology, and geophysics. 

Information gained from all of these fields of study is integrated in this report for the purpose of inferring the pre-mining 

ground-water quality at the Questa mine. This report does 

not attempt to determine how much of Molycorp’s current or 

past mining activities have contributed contaminants to the 

Red River or other aspects of the Red River Valley ecosystem. 

Those assessments are proceeding independently under an 

agreement between the U.S. Environmental Protection Agency 

(USEPA) and Molycorp. Results from the USGS study may be 

helpful in the ongoing site characterization and risk assessments, but they are not part of the USGS purpose or scope of 

work.

Among several useful approaches that could be used to 

estimate pre-mining water quality (Runnells and others, 1992; 

Alpers and Nordstrom, 2000), the most direct one is the proximal analog. Other approaches are a distal or remote analog site 

(examples are found in Runnells and others, 1992; unnecessary if a proximal analog site is available); sediment sampling 

Introduction 3
(provides no information on water chemistry); stable isotopes 

of water and dissolved sulfate (can be used in conjunction with 

analog site studies; the composition of oxygen isotopes in dissolved sulfate can be complicated to interpret); geochemical 

modeling with or without kinetics (numerous assumptions are 

usually necessary; strongly dependent on practical experience of the modeler); statistical analysis (sites having more 

than two sources may not be interpretable; interpretations 

still dependent on geochemical experience); mass balance on 

oxygen flux (requires considerable site-specific knowledge); 

and mass balance based on age of weathered sulfides (can give 

long-term metal and sulfate fluxes, but geologic reconstruction 

over long periods is needed). Several of these approaches were 

used in the present investigation.

The most useful proximal analog should be similar to the 

mine site in lithology, geologic structure, topography, hydrology, climate, and ground-water characteristics. This study is 

likely the first comprehensive study to use a proximal analog 

site to estimate pre-mining ground-water quality at a mine 

site by a third party (a party independent of those who have a 

vested interest in the outcome). The site chosen for the natural 

analog is the Straight Creek catchment, located approximately 

3 miles to the east of the eastern boundary of the mine site.

Questa Baseline and Pre-Mining Ground-Water 

Quality Investigation

Meaning of Natural Background and Baseline

There is a tendency to think of “natural background” as 

unpolluted or pristine preindustrial conditions. Unfortunately, 

such concepts are not helpful because water-quality data for 

such conditions are nonexistent; natural variations around the 

world can be large and do not permit a simple, single analytical result for a given element or compound; and there is 

often a value judgment or anticipated “use” judgment associated with natural substances (rocks, soils, air, and water) that 

precludes a simple, unambiguous result. Bates and Jackson 

(1980) state that a natural area is “An area of land or water that 

has retained its wilderness character, although not necessarily completely natural and undisturbed... .” John Hem (1985) 

stated that “Natural waters need not be pristine—unaffected 

by the works of man. Indeed, probably few are completely 

free from such influences.” The USGS, therefore, has avoided 

use of the word in the course of this study. Instead, the term 

“baseline” is preferred to refer to current conditions of water 

quality and “pre-mining” to describe those conditions that 

likely existed during the few hundred to few thousand years 

before mining took place. The time period must be constrained 

to the recent Holocene (the last 4,000 years). Early Holocene 

or earlier would have had markedly different climatic conditions with different weathering rates. Furthermore, the ground 

surface would have been higher with respect to the depth of 

mineralized zones (that is, pyrite-rich rock currently exposed 

would have been deeper and less exposed to weathering in 

several areas).

Baseline conditions refer to current conditions either 

affected by mining or not affected by mining. Those areas not 

affected by mining could have been affected by anthropogenic 

activities such as exploration drilling, clearings for roads and 

power lines, and other structures, but not by the large-scale 

excavation of rock or the dumping of waste rock or tailings 

typical of metals extraction and concentration. Baseline conditions unaffected by the responsible person’s facility refer to the 

same background conditions stated in Section 20.6.2.7.E of 

the New Mexico Administration Code (NMAC), though other 

anthropogenic activities or natural sources might have affected 

the analytical results.

“Background” is primarily used in this study to refer to 

the conditions defined by the State of New Mexico and their 

regulatory framework. The regulatory enforcement agency 

for the State is the New Mexico Environment Department 

(NMED).

The Water Quality Control Commission (WQCC) Regulations were adopted in 1977 pursuant to the New Mexico 

Water Quality Act, New Mexico Statutory Authority (NMSA) 

Sections 74–6–1 through 74–6–17. The purpose of these 

regulations is to protect ground- and surface-water quality. The 

ground-water discharge permit requirements of the WQCC 

Regulations are the State’s primary tool for protecting groundwater quality. These regulations limit certain discharges to 

ground water at current mining operations and closure of 

those discharge sites. In addition to the permit program, the 

regulations specify requirements for certain reclamation 

activities, including ground-water cleanup standards. Section 20.6.2.4103.F NMAC also allows an operator to petition 

the WQCC for approval of alternative abatement standards, 

which are a type of variance from the numerical WQCC 

ground-water quality standards. It is within this regulation that 

the term “background” is defined. Background is defined by 

Section 20.6.2.7.1101.E NMAC and means “...the amount 

of ground contaminants naturally occurring from undisturbed 

geologic sources or water contaminants which the responsible 

person establishes are occurring from a source other than the 

responsible person’s facility... .”

New Mexico Ground-Water Quality Standards

According to WQCC regulations on September 15, 2002, 

and pursuant to discussions with NMED (§20.6.2.3103; www.

nmenv.state.nm.us./NMED_Regs/gwb/20_6_2_NMAC.pdf), 

the list of constituents in table 1 were considered potential 

contaminants of concern for the purposes of characterizing 

pre-mining ground-water quality in the study area. However, 

this list does not preclude the possibility of other dissolved 

inorganic constituents of concern in the study area, especially mercury, molybdenum, uranium, and vanadium. These 

additional constituents were considered and analytically 

determined in the present study, but no instances of anomalous concentrations were observed. There is no New Mexico 

4 Investigation 25—Summary Report
ground-water quality standard for beryllium, but it is considered a potential element of concern for this site for which the 

drinking-water maximum contaminant level (MCL) is used 

(U.S. Environmental Protection Agency, http://www.epa.gov/

safewater/mcl.html).

The Questa Molycorp Molybdenum Mine

Molycorp, Inc., operates a molybdenum mine east of the 

town of Questa, northern New Mexico, in the Red River Valley 

(fig. 1 and front cover image). The Red River approximates 

the southern boundary of the mine site and flows to the west 

where it joins the Rio Grande. Molycorp is subject to New 

Mexico’s mine closure regulations and operates in a highly 

mineralized area likely to have solute concentrations greater 

than New Mexico ground-water quality standards. There are 

no known ground-water chemistry data prior to 1982 when the 

utility Advanced Waste-Water Treatment (AWWT) near the 

town of Red River drilled an observation well in the Straight 

Creek alluvium and analyzed their first sample collected in 

November of that year. Although wells were installed in the 

mid-1960s in conjunction with the milling operations, no 

water-chemistry data were reported until 1992. Consequently, 

there are no pre-mining ground-water analyses; they must be 

inferred based on best available scientific methods. That inference is the main objective of this investigation.

Prior to 1916, prospectors discovered outcrops of bright 

yellow, weathered rock that looked like sulfur (hence the name, 

Sulphur Gulch) and a greasy mineral that looked like graphite. 

The graphitic mineral was identified as molybdenite (MoS2

) in 

1916, and the yellow mineral was ferrimolybdite, the oxidized 

product derived from the weathering of molybdenite (Schilling, 

1956). By 1920 the Molybdenum Corporation of America had 

acquired the property to mine the deposit, and the company 

subsequently shortened its name to Molycorp, Inc. Mill and 

flotation plants were set up by 1923 for production, and smallscale underground mining of high-grade veins (averaging 4 

percent with a maximum of 35 percent molybdenum) continued until 1958. During the 1950s exploration had identified a 

large, low-grade deposit (about 0.3 percent) below the highgrade deposit, and the decision was made to extract this ore 

by open-pit methods (Carpenter, 1968). Extraction of ore from 

the open pit began in 1965. Mill tailings were transported 9 

miles by pipeline to a tailings facility in the Rio Grande Valley 

just west of Questa. Peak molybdenum production occurred 

in 1976 with 11.5 million pounds per year. In 1977 Molycorp, 

Inc., became a wholly owned subsidiary of Union Oil Company of California. In 2005 Molycorp became a wholly owned 

subsidiary of Chevron Corporation.

Open-pit mining ceased in 1983 and underground mining 

restarted. Waste rock was no longer dumped onto piles at the 

mine site, but the amount of tailings continued to increase. 

Low market values for molybdenum have periodically caused 

the mine to shut down (1986–89 and 1992–95). Active mining 

continues at Molycorp’s Questa Mine in response to market 

demand. Further history and related information are available from Molycorp, Inc. (www.molycorp.com) and from the 

USEPA (www.epa.gov/superfund/sites/npl/narl1599.htm).

Acknowledgments

I am indebted to Blaine McCleskey, who artfully provided graphs and tables for this report; to Jim Ball, who also 

provided graphs, tables, and PHREEQC and WATEQ4F 

calculations; to Geoff Plumlee, Kirk Vincent, Jonathan Caine, 

Jim Ball, and Doug McAda, who graciously provided figures 

from other reports; and to the reviewers who carefully read and 

commented on the report: Roger Lee, Charles Alpers, Geoff 

Plumlee, Phil Verplanck, Christoph Wels, Bruce Walker, Anne 

Wagner, Mike Reed, and Kevin Myers. My appreciation is 

extended to NMED for clarifying the regulatory statutes. I am 

grateful to Nathan Myers and Doug McAda for assistance with 

the sulfate flux calculations, figures, and additional hydrologic 

data. I am deeply grateful to the 30+ USGS personnel who 

made this investigation possible. This investigation could not 

have been accomplished without the support and cooperation 

of Molycorp, Inc., NMED, and the USDA Forest Service, and 

the helpful comments of Amigos Bravos. The encouragement 

of Mark Purcell of the USEPA is also appreciated. This project 

was truly collaborative and was only successful insofar as all 

parties gave generously and openly of their time and expertise 

to see its completion.

Table 1. New Mexico ground-water quality standards.

[mg/L, milligrams per liter; MCL, maximum contaminant level]

Constituent or property Concentration (mg/L)

1

Aluminum (Al) 5.0 

2

Beryllium (Be) .004

3

Cadmium (Cd) .01

3

Chromium (Cr) .05

1

Cobalt (Co) .05

4

Copper (Cu) 1.0

3

Fluoride (F) 1.6

4

Iron (Fe) 1.0

3

Lead (Pb) .05

4

Manganese (Mn) .2

1

Nickel (Ni) .2

4

Sulfate (SO4) 600.0

4

Total dissolved solids (TDS) 1,000.0

4

Zinc (Zn) 10.0

4

pH Between 6 and 9

1

 Standards for irrigation use.

2

 National Drinking Water MCL (U.S. Environmental Protection 

Agency).

3

 Human-health standard.

4

 Other standards for domestic water supply.

Introduction 5
Figure 1. The Red River Valley and the study reach between the town of Red River and the U.S. Geological Survey gage near Questa, New Mexico.

6Investigation 25—Summary Report Mine site Open pit Town of Red River

Study area

Study area Red River

drainage basin

NEW

MEXICO

Index map

EXPLANATION

Alteration scar area

USGS streamflow-gaging

station 08265000

Questa Ranger 

Station

Waldo Spring

June Bug

Mill

Upstream diel site

Taos

Sangre de Cristo

Mountains

USGS streamflow-gaging

station 08264500

Taos Range

unnamed

drainage

MILE

KILOMETER
Study Site

Physical Features

The study area is located in Taos County in the Taos 

Range of the Sangre de Cristo Mountains of north-central New 

Mexico (fig. 1; a detailed map with well locations at a scale 

of 1:50,000 is provided on plate 1, from Vincent, in press). 

The Red River drainage basin is a tributary to the Rio Grande 

within the Carson National Forest. The area has a rugged terrain with steep slopes and V-shaped valleys. The study area 

is the Red River Valley between the Questa ranger station 

at 2,280 m (7,480 ft) elevation at the west end and the town 

of Red River at 2,646 m (8,680 ft) elevation at the east end 

(fig. 1). The canyon walls of the Red River basin rise rapidly 

from about 2,400 m to over 3,000 m at the ridgecrest on both 

the north and south sides of the river. Some peaks reach nearly 

3,500 m. The 600-m difference in elevation between the Red 

River and the adjacent ridgecrest can occur over a horizontal 

distance of less than 2,000 m.

The Questa Molycorp molybdenum mine, referred to as 

the mine site, is located on the north side of State Highway 

No. 38 and the Red River 13 km east of the ranger station. The 

mine site is approximately 16 km2

 (6 mi2) and encompasses 

three main tributary valleys to the Red River—Capulin Canyon, Goat Hill Gulch, and Sulphur Gulch, from west to east, 

respectively (pl. 1 and fig. 1).

Mining activities produced extensive underground workings and an open pit of approximately 0.65 km2

 (162 acres) 

near or in Sulphur Gulch. Waste-rock piles cover steep slopes 

on the north side of the Red River between Capulin Canyon and Spring Gulch (a tributary valley of Sulphur Gulch). 

Hydrothermally altered bedrock is present in Capulin, Goat 

Hill, Sulphur, Hansen, Straight, June Bug, Hottentot, and 

Bitter Creek drainages (fig. 1). The latter five drainages are 

examples of unmined drainages with the exception of minor 

prospects in Bitter Creek. Weathering of extensively altered 

rock has resulted in steep, highly erosive, sparsely vegetated 

“scars” that are clearly visible from the ground and in aerial 

photographs (fig. 2).

Geology

The geochemical interaction of water with soil, colluvium, alluvium, and fractured bedrock produces the composition of surface and ground waters that are the focus of 

this study. Hence, the geology is a fundamental component 

to understanding the ground-water chemistry. This section 

briefly summarizes the general geologic framework from 

Schilling (1956), Rehrig (1969), Lipman (1981), and Meyer 

and Leonardson (1990, 1997) in addition to observations made 

by the USGS scientists working at the site. More detailed 

geologic results related to ground-water geochemistry appear 

in later sections of this report.

The Taos Range is composed of Precambrian metamorphic assemblages and granitic intrusive rocks overlain by 

Tertiary volcanics. The volcanic rocks are primarily intermediate to felsic composition (andesites to rhyolites), and 

they have been intruded by late Oligocene and early Miocene 

quartz monzonites and granites that provided the source of 

the hydrothermal fluids and molybdenite mineralization. The 

hydrothermally altered volcanics often contain pyrite mineralization (generally 1–3 percent). The Red River Valley 

is located along the southern edge of the Questa volcanic 

caldera and contains complex structural features and extensive 

hydrothermal alteration. The mineral deposits in the Red River 

Valley are considered Climax-type deposits that are associated 

with silica- and fluorine-rich rhyolite porphyry and granitic 

intrusives. The three principal alteration zones include a highly 

altered quartz-sericite-pyrite (QSP) zone, less-altered argillic 

(dominantly kaolinite) zones, and mildly altered propylitic 

zones (containing calcite mineralization). QSP alteration, as 

the name implies, produces a mixture of quartz, pyrite (as 

much as 10 percent), and fine-grained mica (sericite) or illite. 

Chlorite, epidote, albite, and calcite typically are found in the 

propylitic assemblages. Ore deposits contain quartz, molybdenite, pyrite, fluorite, calcite, manganiferous calcite, dolomite, and rhodochrosite. Lesser amounts of galena, sphalerite, 

chalcopyrite, magnetite, and hematite also are present. The 

hydrothermal alteration related to mineralization overprints an 

older, regional propylitic alteration. In these areas, rocks can 

contain a mixture of quartz, pyrite, and illite clays replacing 

feldspars, chlorite, carbonates, and epidote. Abundant minerals 

in waste rock produced by mining activities include chlorite, 

gypsum, illite, illite-smectite, jarosite, kaolinite, and muscovite (Gale and Thompson, 2001).

Andesite volcanic and volcaniclastic rocks are present in 

most scar-area bedrock outcrops and are the dominant bedrock 

units in the Straight Creek, South and Southeast Straight 

Creek, South Goat Hill, Sulphur Gulch, and Southwest Hansen 

scars. Amalia Tuff, a mildly alkaline, rhyolitic tuff, is the 

dominant rock type in the Goat Hill and Hansen scars, and 

quartz latite porphyry is the main rock type in the June Bug 

and Southeast Hottentot scars. Rhyolite porphyry is the main 

rock type in the Hottentot scar, and quartz latite and rhyolite 

porphyries form the hillslopes of several scars. Rhyolite porphyry and tuff do not seem to have been substantially affected 

by propylitization. Advanced argillic alteration was identified 

in the Hansen and Hottentot scars and in areas southwest of 

the Molycorp open pit. Propylitized andesite bedrock is present in the La Bobita drainage, an area that does not contain 

alteration scars.

Samples collected from a weathering profile in the 

Straight Creek scar were studied in detail to characterize the 

mineralogic variations. Unweathered bedrock exposed in the 

creek bottom is propylitized andesite with a QSP overprint. 

Depending on location within the weathering profile, altered 

rocks contain variable amounts of quartz, illite, chlorite, and 

plagioclase feldspar, with smaller amounts of pyrite, gypsum, 

rutile, jarosite, and goethite (Livo and Clark, 2002; Ludington 

Study Site 7
Figure 2. Satellite image of the Red River showing scar areas, Molycorp mine site, and the town of Red River with photographs of scar areas supe

image (from Ludington and others, 2004).

4 KILOMETERS

2 MILES

0 10 1

2 3
and others, 2005). Calcite, goethite, and sericite are widely 

distributed in the Red River Valley rocks and soils as revealed 

by Airborne Visible/InfraRed Imaging Spectrometry (AVIRIS; 

Livo and Clark, 2002). Calcite is an important mineral in the 

Red River Valley because its dissolution effectively neutralizes 

the acid inflows so that pH values in the Red River tend to be 

slightly alkaline (pH 7–8). Gypsum is common throughout 

the Red River Valley and forms as a secondary product of 

acid-sulfate weathering when pyrite oxidizes and reacts with 

calcite. Because gypsum is readily soluble in water, calcium 

and sulfate are the dominant ions for most surface and ground 

waters in the Red River Valley.

The major minerals in rock samples collected during mineral exploration and mining are biotite, calcite, chalcopyrite, 

fluorite, galena, molybdenite, pyrite, quartz, rhodochrosite, 

and sphalerite. Mining activities produced roughly 328 million 

tons of rock overburden deposited in Capulin Canyon, along 

the north slope of the Red River, and in Goat Hill, Sulphur, 

and Spring Gulches (Robertson Geoconsultants, Inc., 2000b). 

The abundant minerals in waste-rock samples include chlorite, 

gypsum, illite, illite-smectite, jarosite, kaolinite, and muscovite (Gale and Thompson, 2001). Recently, magnesite has 

been found in carbonate-altered rock at the mine site (Molycorp, Inc., oral commun., 2006).

Climate and Vegetation

The Red River Valley is located within a semiarid desert 

that receives precipitation throughout the year and sustains 

moderate biodiversity. Between 1915 and 2002, the annual 

average temperature was 4o

C, and the precipitation and 

snowfall were 52 and 370 cm, respectively, with average daily 

temperatures fluctuating by 18o

C (Western Regional Climate 

Center, 2003).

Climate and vegetation vary greatly within short distances because of differences in topography, weather, and 

soil composition. The altitude in the study area ranges from 

2,280 m at the ranger station to 2,704 m at the Zwergle gage 

(USGS station number 08264500, upstream from the town of 

Red River) and 3,000 m at the ridge crest (fig. 1). Orographic 

effects of mountainous topography lead to precipitation on the 

windward slopes and localized storms within tributary valleys. 

Major precipitation events include summer thunderstorms and 

winter/spring snowstorms. Thunderstorms and monsoon rains 

are responsible for mass wasting in hydrothermally altered 

areas, producing debris flows that potentially affect vegetation, 

alluvial aquifers, and the Red River. Winter snowpack contributes to ground-water recharge through snowmelt infiltration 

and runoff.

Prevalent vegetation in the Red River Valley is representative of the following altitude zones: piñon-juniper woodland 

(1,800–2,300 m), mixed conifer woodland (2,300–2,740 m), 

and spruce-fir woodland (2,740–3,660 m; Knight, 1990). 

Willows, cottonwoods, shrubs, perennial grasses, and flowering vegetation are common near the banks of the Red River. 

Extending from the river are widely spaced piñon pines and 

junipers. This mountainous valley has an abundance of ponderosa pines and limber pines except at higher altitudes where 

Douglas fir and white fir grow. This typical mountain community, although diverse, is dominated by ponderosa pines (Larry 

Gough, U.S. Geological Survey, oral commun., 2003).

Hydrology

Surface Water

The Red River originates at an altitude of approximately 

3,658 m (12,000 ft) near Wheeler Peak, the highest peak in 

New Mexico (4,011 m or 13,161 ft), and flows roughly 55 km 

(35 mi) to its confluence with the Rio Grande at an altitude 

of 2,012 m (6,600 ft). The USGS maintains a streamflowgaging station at the USDA Forest Service ranger station at an 

elevation of 2,271 m (7,452 ft) about 1 mi downstream from 

the western boundary of the mine site. Total basin drainage 

area is 492 km2

 (190 mi2); the drainage area upstream from 

the Questa ranger station gaging station is 293 km2

 (113 mi2). 

Peak streamflow usually occurs from late May to mid-June, 

with snowmelt-related flows typically beginning in late March 

and increasing through mid-April. Summer thunderstorms are 

prevalent in July and August. Between 1930 and 2001, the 

mean annual discharge of the Red River at the Questa ranger 

station gage has ranged from 12.8 to 103 cubic feet per second 

(ft3

/s), while the average daily discharge ranged from 2.5 to 

750 ft3

/s with an average of 46.8 ft3/s (U.S. Geological Survey, 

2004).

The main drainages in the vicinity of the mine site are 

Capulin Canyon, Goat Hill Gulch, and Sulphur Gulch on the 

north side of the Red River (fig. 1). Upstream from the mine 

site, Little Hansen, Hansen, Straight, Hottentot, and Bitter Creeks drain scar areas, whereas Mallette Creek drains a 

nonscar area on the north side of the Red River. Bear Canyon, Columbine, Pioneer, and Placer Creeks drain largely 

unmineralized land on the south side of the river. Bear Canyon 

and upper Pioneer Creeks contain some mineralization. Downstream from the mine site, the Red River joins with Cabresto 

Creek, entering from the north side of the Red River before it 

discharges to the Rio Grande.

Springs and shallow alluvial ground-water discharge 

to the Red River, rendering it a gaining stream over much of 

its length (Smolka and Tague, 1989). Between the town of 

Red River and the gaging station near Questa, there are about 

25 ephemeral seeps and springs along the banks of the Red 

River and approximately 20 intermittent seeps and springs in 

tributary drainages on the north side of the river (South Pass 

Resources, Inc., 1995; Steffen, Robertson, & Kirsten, 1995; 

Robertson GeoConsultants, Inc., 2001b). Aluminum hydroxide 

often precipitates from springs downgradient from scar and 

mined areas on the north side of the Red River, affecting the 

color and turbidity of the river (Vail Engineering, Inc., 1989).

Study Site 9
Ground Water

There are four main types of water-bearing units present in the Red River Valley—fractured bedrock, waste-rock 

piles, debris fans, and Red River alluvium. Bedrock constitutes 

the largest volume of aquifer in the study area but probably 

contains only small amounts of ground water because of low 

porosity. Low hydraulic conductivity is controlled by fractures. Waste-rock piles and scars with associated debris fans 

are geochemically reactive, have high porosities, and have fast 

infiltration rates. Alluvium consists of alluvial aquifers that are 

restricted in areal extent, compared to bedrock aquifers, and 

have variable composition.

Streamflow and hillslope processes have been eroding the 

mountainous study area throughout the late Cenozoic because 

of uplift. Deposits of unconsolidated sediments are present 

in only specific locations and are relatively small in volume. 

Hillslope soils are thin and composed of materials eroded from 

adjacent upgradient slopes. Debris fans are composed of sediments rapidly shed from their respective catchments. Where 

the tributary catchments contain “erosion scars” the debris 

fans are large and active and contain both coarse- and finegrained, largely unsorted, debris-flow sediments. The chemistry of these sediments reflects the chemistry of their rapidly 

eroding and altered erosion scars. Sediments deposited by 

the Red River, in contrast, generally consist of well-washed, 

rounded sands, gravels, and cobbles and are composed of a 

mix of lithologies present in the entire Red River basin. The 

largest debris fans caused the Red River alluvium to aggrade 

behind the fans during the Quaternary. Thus, water flowing in 

the shallow alluvial aquifers likely passes alternately through 

Red River alluvium and debris-fan alluvium. Both the Red 

River alluvium and debris-fan alluvium are less than several 

hundred meters wide and less than 100 m thick.

Alluvial ground water is a calcium-sulfate type with 

magnesium commonly the second most abundant cation. 

Ground water downgradient from the waste-rock dumps and 

scars has acidic pH values and elevated metal concentrations 

compared to ground water upgradient from these altered areas. 

Bedrock ground waters are also calcium-sulfate type, but 

commonly of neutral pH. Most wells developed in the Red 

River Valley were installed to monitor water quality downgradient from mining operations (waste-rock dumps and tailings 

piles) and(or) scar areas. Wells installed during this study 

were located and developed for the purpose of measuring 

water levels and collecting water-chemistry data for a range 

of environments in the Red River Valley, similar to the mine 

environment, and to interpret the water/rock interactions under 

nonmining baseline conditions as a reference for pre-mining 

water quality for the mine site.

Hydrothermal alteration has produced substantial changes 

in mineralogy in the study area over relatively short distances, 

a common feature of hydrothermally altered terrains. Hence, 

both the mineralogy and the resultant water chemistry can 

change substantially over short distances. Such hydrogeochemical environments are highly heterogeneous, making the 

estimation of pre-mining ground-water chemistry a function of highly variable lithologies over an area the size of the 

mine site, rather than single fixed concentrations of various constituents.

Database for Speciation and Mineral 

Saturation Calculations

Computer software used for model applications continually undergoes modification with respect to both the primary 

coding and the databases. The two geochemical models used 

for speciation and mineral saturation calculations in this study 

were the WATEQ4F code (Ball and Nordstrom, 1991, which 

has been regularly updated since then) and the PHREEQC 

code (Parkhurst, 1995; Parkhurst and Appelo, 1999, which has 

also been regularly updated and enhanced). Both codes use 

the same thermodynamic database that is occasionally updated 

through critical evaluations (Nordstrom and others, 1990; Nordstrom and May, 1996; Ball and Nordstrom, 1998; Nordstrom 

and Archer, 2003; notes to recent versions of WATEQ4F, 

James Ball, written commun., 1995–present). To document the 

database used for the computations in this report, a compilation of the thermodynamic data has been tabulated in table 1–1 

of Appendix 1.

Mineral saturation indices are defined as the logarithm 

of the ratio of the ion-activity product, IAP, to the solubilityproduct constant, Ksp, for a given mineral:

The IAP is calculated from the speciation using the ionassociation model (see Nordstrom and Munoz, 1994), and 

the Ksp is calculated from data given in Appendix 1. Both the 

speciation and the Ksp values are temperature-dependent and 

are calculated for the temperature at which the sample was 

collected.

Mass-balance calculations were done with the 

BALANCE code (Parkhurst and others, 1982), which does not 

use any thermodynamic data.

Summary Conclusions from Previous 

Reports

The basic data, computations, and interpretations for this 

investigation are based on a series of studies on the relevant 

geology, hydrology, and geochemistry of the Red River Valley 

that ultimately led to the elucidation of processes governing 

water/rock interactions and the evolution of ground-water 

chemistry. This section summarizes the main results from 

these studies, which are tabulated in table 2. The diagram in 

figure 3 shows the main components of the investigation and 

10 Investigation 25—Summary Report

sp

IAP

K log

æ ö ç ÷ ç ÷ ç ÷

÷ ç ÷ è ø
their relationships to each other. It also shows how the individual reports on diverse tasks are grouped into major subject 

categories to lead to the final objective—pre-mining groundwater chemistry.

Historical Review of Water Quality

Numerous surface-water and ground-water quality studies were completed in the Red River Valley before the present 

investigation began. No compilation of these studies or evaluation of the quality of the data had been performed. It was 

unclear what some of the data sets indicated and how reliable 

they were. Consequently, one of the first tasks was to compile 

and evaluate these data and determine if it was possible to 

interpret the data sets.

Ground Water

Some 608 ground-water analyses were available for the 

time period 1992–2002. Ground waters were not analyzed 

prior to 1992 because monitoring ground-water quality for 

regulatory purposes was not required before that time. These 

analyses were compiled, evaluated, and reported by LoVetere 

and others (2004). They found that many of the analyses were 

incomplete in that not all the major ions were always reported, 

charge imbalances were occasionally too large, and fluoride 

concentrations were highly biased until there was a change in 

method (and laboratory) in 1997. Nevertheless, ion-ion plots 

were prepared for several constituents to see if there were any 

recognizable trends in the data. Sulfate concentrations covered 

a broad range from 100 to 8,000 mg/L. The highest concentrations were associated with waste-rock piles, and the highest 

concentrations of most other constituents tended to correlate 

with high sulfate concentrations. The strong positive correlation of calcium with sulfate reflects the influence of gypsum 

dissolution and calcite plus pyrite oxidation. The common 

occurrence of acidic waters with sulfate enrichment relative to 

the Ca:SO4

 molar ratio of gypsum (1:1) reflects the influence 

of pyrite oxidation on many of these ground waters.

Some concentrations were among the highest ever 

reported for those elements worldwide. For example, the highest beryllium concentration in ground water that Veselý and 

others (2002) reported was 160 μg/L, whereas the concentration was as high as 280 μg/L in ground water from the mine 

site. Some seeps had even higher concentrations. The highest 

concentration of cobalt reported by a National Academy of 

Sciences (1977) survey was 99 μg/L, whereas concentrations 

as high as 5 mg/L were reported from ground water likely 

associated with waste-rock piles.

Out of all these ground-water analyses, 328 were found 

to have acceptable charge imbalances (less than ±20 percent). Acceptable charge imbalances were a requirement for 

speciation and saturation index computations for calcite, 

gypsum, fluorite, rhodochrosite, manganite, and aluminum 

hydroxide. Saturation was reached for gypsum, calcite, fluorite, and aluminum hydroxide in many of the samples. These 

results provided the first indication of concentration limits on 

constituents from mineral-solubility limits. Further, the positive correlation of calcium with sulfate, and zinc with cadmium, especially for acid-sulfate waters, reflects the effect of 

acid dissolution on mobilization of metals and simple mineralogical sources for some constituents.

Red River Surface Water

Nearly 300 surface-water analyses were compiled for 

the time period 1965–2001 by Maest and others (2004). Most 

of these analyses were incomplete because the objective was 

to determine if concentrations of copper, zinc, cadmium, 

arsenic, and lead were found unacceptable for the Red River; 

the incomplete analyses resulted in unacceptable charge balances. Nevertheless, the analyses were evaluated for quality 

assurance/quality control (QA/QC) and usually met reasonable standards considered acceptable at the time of each 

study. An exception is the Melancon and others (1982) study, 

which reported high concentrations of arsenic that were never 

observed before or since and were considered unreliable. The 

most commonly sampled site for surface water was the Red 

River at the USGS Questa gage (site 08265000), next to the 

USDA Forest Service ranger station (fig. 1). Samples had been 

collected and analyzed from here since 1965, and trends in sulfate concentration were examined for the period 1965–2002. 

The variability in sulfate concentration overwhelmed any temporal trend until the data were discriminated on the basis of 

flow conditions. Three general flow conditions are recognizable—snowmelt, late summer (often monsoonal) rainstorms, 

and dry periods. Furthermore, solute concentrations varied 

differently for rising limb of snowmelt or rainstorm relative to 

falling limb of snowmelt or rainstorm conditions and varied 

differently for snowmelt and rainstorm. Hence, the hydrologic 

conditions were discriminated on the basis of rising limb of 

snowmelt, peak snowmelt, falling limb of snowmelt, rising 

limb of rainstorm, peak rainstorm, falling limb of rainstorm, 

and low flow (little or no change in flow). When samples were 

collected nearly simultaneously on the Red River at the Questa 

USGS gage and a site just upstream from the mine site from 

1965 to 2002 and only low-flow sulfate concentrations used, 

there appears to be an upward trend to about 1993 that would 

indicate some influence from the mine site. The trend is only 

indicative because insufficient data were collected in the late 

1960s and the early to mid-1970s to demonstrate this trend 

clearly. Furthermore, the observation that the increase in concentration occurs in the reach of the river that flows past the 

mine site does not necessarily mean that it is related to mining 

activities. The data do show very clearly the large variations in 

concentration that result from dynamic changes in the hydrologic conditions of the Red River Valley. The most striking 

change in water quality occurred on September 7, 1986, when 

a sudden drop in pH (from 7.4 to 3.8) occurred due to a storm.

Further trends in the water quality of the Red River 

were discussed by Verplanck and others (2006) with another 

Summary Conclusions from Previous Reports 11
12 Investigation 25—Summary Report

Table 2. Compilation of reports published within this project. 

[n/a, not applicable]

Series 

report no.

Title:

Questa Baseline and Pre-Mining Ground-Water Quality Investigation Reference

n/a Results of electrical surveys near Red River, New Mexico Lucius and others (2001)

n/a Mapped minerals at Questa, New Mexico, using Airborne Visible-Infrared 

Imaging Spectrometer (AVIRIS)

Livo and Clark (2002)

1. Depth to bedrock determinations using shallow seismic data acquired in the 

Straight Creek drainage near Red River, New Mexico

Powers and Burton (2004)

2. Low-flow (2001) and snowmelt (2002) synoptic/tracer water chemistry for the 

Red River, New Mexico

McCleskey and others (2003)

3. Historical ground-water quality for the Red River Valley, New Mexico LoVetere and others (2004)

4. Historical surface-water quality for the Red River Valley, New Mexico, 

1965–2001

Maest and others (2004)

5. Well installation, water-level data, and surface- and ground-water geochemistry in the Straight Creek drainage basin, Red River Valley, New Mexico, 

2001–2003

Naus and others (2005)

6. Preliminary brittle structural geologic data, Questa mining district, southern 

Sangre de Cristo Mountains, New Mexico

Caine (2003)

7. A pictorial record of chemical weathering, erosional processes, and potential 

debris-flow hazards in scar areas developed on hydrothermally altered rocks

Plumlee and others (in press)

8. Lake-sediment geochemical record from 1960–2002, Eagle Rock and Fawn 

Lakes, Taos County, New Mexico

Church and others (2005)

9. Historical seep, spring, underground, and tributary water quality for the Red 

River Valley, New Mexico

LoVetere and Nordstrom (written commun., 

2006) 

10. Geologic influences on ground and surface waters in the Red River watershed, 

New Mexico

Ludington and others (2004)

11. Geochemistry of alteration scars and waste piles Briggs and others (2003)

12. Geochemical and reaction-transport modeling based on tracer injection-synoptic 

sampling studies for the Red River, New Mexico, 2001–2002

Ball and others (2005)

13. Mineral microscopy and chemistry of mined and unmined porphyry molybdenum mineralization along the Red River, New Mexico—Implications for 

ground- and surface-water quality

Plumlee and others (2005)

14. Interpretation of ground-water geochemistry in catchments other than the 

Straight Creek catchment, Red River Valley, Taos County, New Mexico, 

2002–2003

Nordstrom and others (2005)

15. Methods and results of Phase II and III well installation and development and 

results of well logging, hydraulic testing, and water-level measurements in 

the Red River Valley, New Mexico, 2002–04

Blanchard and others (2007)

16. Quality assurance and quality control for water analyses McCleskey and others (2004)

17. Geomorphology of the shallow alluvial aquifer of the Red River Valley, 

New Mexico

Vincent (in press)

18. Characterization of brittle structures in the Questa caldera and speculation on 

their potential impacts on the bedrock ground-water flow system, Red River 

watershed, New Mexico

Caine (2007)

19. Leaching characteristics of composited materials from mine waste-rock piles 

and naturally altered areas near Questa, New Mexico

Smith and others (2007)

20. Water-chemistry trends of the Red River, Taos County, New Mexico, with data 

from selected seeps, tributaries, and snow, 2000–2004

Verplanck and others (2006)

21. Hydrology and water balance of the Red River Valley, New Mexico Naus and others (2006)

22. Ground-water budget for the Straight Creek drainage basin, Red River Valley, 

New Mexico

McAda and Naus (in press)

23. Quantification of mass loading from mined and unmined areas along the Red 

River, New Mexico

Kimball and others (2006)

24. Seismic refraction tomography for volume analysis of saturated alluvium in the 

Straight Creek drainage and its confluence with the Red River, Taos County, 

New Mexico

Powers and Burton (2007)

25. Summary of results and baseline and pre-mining ground-water geochemistry, 

Red River Valley, Taos County, New Mexico, 2001–2005 

Nordstrom, this volume.
Figure 3. Study components and their relationships for inferring pre-mining ground-water chemistry.

SURFACE-WATER 

HYDROLOGY/CHEMISTRY

Historical Data, Synoptic/Tracer, Quality 

Assurance/Quality Control, 

Mass Loadings, Reactive-Transport Modeling 

GEOMORPHOLOGY

Controls on Ground-Water 

Emergence 

GROUND-WATER HYDROLOGY

Historical Data, Well Installation, Water Levels, 

Water Chemistry, Water Budgets, Geochemistry

GEOCHEMISTRY

Controls on Water Chemistry 

BASELINE AND PGROUND-WATER 

Envir , 

-

and WasteGEOLOGY onmental Geology Mineralogy, Airborne Visible Infrared Imaging Spectrometry, Structure, Geophysics, Lake Sediments, Geochemistry/Leaching of Scars Rock Piles
example of the effect of a storm on the water quality of the 

Red River during September 2002. The pH at the USGS gage 

decreased from 7.8 to 4.8, and large increases in iron, aluminum, and manganese concentrations were evident. More 

recent effects of storms have been documented by Molycorp’s 

consultants. These storm-related changes and degradation of 

water quality in the Red River are caused by sudden flushing of acidity from the scars and debris fans, such as those at 

Hansen, Hottentot, and Straight Creek, by heavy rainstorms. 

During monsoonal rains, these scar-containing catchments 

mobilize large amounts of debris, dissolve soluble salts, and 

discharge subsurface waters containing high solute concentrations into the Red River.

Seeps, Springs, and Tributaries

Five hundred and thirty-six samples of seeps, springs, and 

tributaries were collected from 1992 to 2002 (LoVetere and 

Nordstrom, in press). The general range of solute concentrations is similar to those for historical ground-water analyses, 

but solutes achieve higher concentrations for seeps from 

waste-rock piles than those for ground water in waste-rock 

piles. For example, waste-rock seeps from Capulin Canyon 

and Goat Hill Gulch have sulfate concentrations in the range 

of 10,000 to 20,000 mg/L, whereas concentrations from well 

water in waste rock are usually less than 10,000 mg/L. These 

historical data, when combined with seeps sampled and analyzed for this investigation, provide a chemical profile of scardrainage seep chemistry. Values of pH for scar-drainage water 

were well constrained to 2.9 (±0.4), and sulfate concentrations 

were well constrained to 2,000 (±860) mg/L. These data are 

shown in histograms in the section on the chemistry of waters 

developed from scar weathering. The one exception to this 

scar-water chemistry trend is Bitter Creek, where seep samples 

were collected six times during 2000 by Robertson Geoconsultants, Inc. (2001a), and the sulfate concentrations ranged 

from 9,000 to 15,000 mg/L (values corrected from original 

report to comply with reasonable charge balances). The cause 

for the exceptionally high sulfate and other solute concentrations at Bitter Creek is not known and was not considered in 

this investigation.

The same general trends in solute concentrations found 

for the ground water at the Molycorp site apply to the seeps, 

springs, and tributaries. Mineral solubility controls by 

Al(OH)3

, gypsum, rhodochrosite, and fluorite provide upper 

limits to concentrations of Al, Ca, Mn, and F.

Geologic Investigations

Airborne Visible-Infrared Imaging Spectrometry

Airborne visible and infrared imaging spectrometry 

(AVIRIS) had been flown shortly before this investigation 

began so that data reduction and interpretation of the data 

were possible within the time constraints of this project. Livo 

and Clark (2002) completed AVIRIS maps for major minerals 

occurring at the surface for the study reach of the Red River 

Valley. These results provided a clear picture of the overall 

areal extent and composition of the hydrothermal mineralization for the valley and guidance for what minerals to seek during later investigations of mineralization in outcrops and surficial deposits. One of the distinguishing features of the AVIRIS 

study was the widespread extent of QSP and acid alteration in 

the valley. The QSP alteration has been largely weathered at 

the surface to an assemblage dominated by jarosite, gypsum, 

and kaolinite. Jarosite is indicative of acid-sulfate weathering 

with pH values in the range of 1–3.

Geologic Characterization of Weathering 

Processes

The development of surface- and ground-water composition begins with the geologic framework. The geology is both 

the physical and chemical medium through which the water 

flows and reacts. The overall perspective on the geology for 

the context of this investigation was reported by Ludington 

and others (2005). The most important aspect of this study 

was the geologic comparison of the chosen analog site with 

the mine site to answer the question of how satisfactory the 

Straight Creek catchment is as an analog. Descriptions of the 

mineralogy also provide the framework for the water/rock 

interactions that produce the observed ground-water chemistry.

Generally, the geologic analogy holds well. The scar at 

Straight Creek has similar topography, size, lithology, alteration and mineralogy, intensity and type of fracturing, geochemical abundance of elements, slope aspect and direction, 

and processes of physical erosion as the scars at the mine site, 

especially the larger scars in upper Sulphur Gulch and Goat 

Hill Gulch. An important difference between the Straight 

Creek analog site and the mine site is the Sulphur Gulch catchment. Sulphur Gulch eroded to deeper levels within the mineralized and altered zones than did Straight Creek (or Hansen or Hottentot Creeks). The depth that the erosional surface 

reached relative to the alteration zones is depicted schematically in figure 4. The erosion of Sulphur Gulch penetrates and 

intersects not only areas of extensive carbonate, fluorite, and 

molybdenite mineralization but exposes the intrusive stock. 

However, the mineralogy of this zone is well documented, 

which makes possible an estimation of the effect of mineral 

14 Investigation 25—Summary Report
Figure 4. Schematic geologic cross section of alteration zones with overlying dashed lines depicting the relative 

position of the erosional surface for five catchments along the Red River (from Ludington and others, 2005).

Summary Conclusions from Previous Reports 15
dissolution on water chemistry. The exposure of carbonates and fluorite, the low pyrite content, and the exposure 

of granitic stock and aplites will produce circumneutral-pH 

water with low iron and magnesium concentrations, but with 

high concentrations of manganese and fluoride. Geochemical 

maps from Robertson Geoconsultants, Inc. (2001b), indicate 

some metals could be more abundant in Sulphur and Goat Hill 

Gulches than in Straight Creek, Hansen Creek, or Hottentot 

Creek catchments. Also the Sulphur Gulch debris fan is much 

smaller than the Straight Creek debris fan because of its location on a bend in the stream channel where it likely has been 

washed downstream by high flows of the Red River.

Another important observation documented in Ludington 

and others (2005) is the coexistence of calcite and pyrite in 

veins within altered volcanic rocks. As the calcite and pyrite 

weather, they react in place to form crystals of gypsum which 

then dissolve into the infiltrating water. Gypsum crystals are 

abundant in the Red River Valley, and they are predominantly 

secondary in origin (formed from original “primary” minerals 

by weathering).

Mineralogical Characterization

General features of the mineralization, hydrothermal 

alteration, and host rock geology were reported in Ludington 

and others (2005). More detailed aspects of the mineralogy 

and mineral chemistry, especially the trace-element content 

of individual minerals, were reported in Plumlee and others 

(2005). The results of the mineral chemistry study identified the source minerals for trace elements of environmental 

concern, chiefly Mn, Pb, Zn, Cd, Co, Cr, Ni, Li, Be, and 

Cu. Optical microscopy, scanning electron microscopy with 

energy-dispersive X-ray analysis, electron probe microanalysis, and laser-ablation inductively coupled plasma mass spectrometry were used to identify individual mineral phases and 

their trace-element content. Trace-element determinations in 

bulk rock samples (Amalia Tuff, Tertiary andesite, quartz latite 

porphyry, and rhyolite) were also reported.

Most of the dissolved constituents have identifiable 

sources in specific minerals. Calcium is derived primarily 

from calcite and gypsum dissolution. Some calcium may 

be derived from plagioclase where it is locally abundant. 

Strontium and yttrium are predominantly associated with 

calcite. High-magnesium calcite and dolomite can be important sources of magnesium, but the widespread occurrence 

of chlorite and its fine-grain size indicate that it also could 

contribute magnesium to water bodies. The chlorites are also 

rich in iron and may contribute some iron to ground water. 

Pyrite is the dominant source for iron, cobalt, nickel, and, 

in combination with gypsum dissolution, sulfate. The most 

abundant source of copper is chalcopyrite, of lead is galena, 

and of zinc is sphalerite. The dominant source of cadmium 

is also sphalerite. The dominant source of fluorine is fluorite 

because of its widespread occurrence and rapid dissolution 

rate in acid water. Fluoride-rich biotites, chlorites, and illites 

do occur but likely contribute only minor amounts to ground 

water because of their slower dissolution rates and lower 

fluorine content. Rhodochrosite and manganese-rich calcites 

are the dominant source of manganese. There are multiple 

sources of aluminum and silica that include chlorite, kaolinite, 

smectite, feldspars, and to a lesser extent, illite. Lithium and 

beryllium do not seem to be identified with a distinct Li or Be 

mineral but rather are dispersed in lattices of several silicate 

minerals. Illites tend to carry high concentrations of lithium 

and beryllium in some samples. Chromium tends to be more 

concentrated in chlorites than any other phase although it is 

also somewhat dispersed. The elements cobalt, nickel, and 

chromium all tend to associate together, which is characteristic 

of a mafic source rock. Mafic rocks such as andesites are typically enriched in these elements, and ultramafic rocks are even 

more enriched in these same elements. Hence, the andesite is 

the likely original source for cobalt, nickel, and chromium, but 

hydrothermal alteration would likely have changed the initial 

mineralogical distribution of these elements in the andesite so 

that pyrite contains most of the cobalt and nickel, and chlorites 

contain most of the chromium.

Leaching Studies

Less than 2-mm-size composite samples from five mine 

waste-rock piles, nine erosional-scar areas, a less-altered site, 

and a tailings slurry pipe were analyzed for bulk chemistry and 

mineralogy and then subjected to two back-to-back leaching 

procedures. A 5-minute leach for easily soluble constituents 

(Hageman and Briggs, 2000) was followed by an 18-hour, 

end-over-end, rotating leach using deionized water. The purpose was to compare and contrast the leachability of acidity, 

metals, and sulfate from unmined scar areas with mine waste 

rock. The results showed that scar leachates have low pH values (less than 4.1). Under these low-pH conditions, many metals can be mobilized and remain in solution; however, anionic 

species (such as molybdenum) tend to be less mobile under 

acidic conditions. Waste-rock leachates generally have the 

same or higher pH values but still are acidic. Some increases 

in pH occurred with leaching time, which indicates there is 

some buffering capacity left in waste rock.

An important difference appeared in leachate composition for some of the waste-rock samples. Both Capulin and 

Sugar Shack West samples had lower pH, independent of 

leaching time, than the other waste-rock samples. Generally, 

metal concentrations in the waste-rock leachates do not exceed 

the upper range of those metal concentrations in the erosionalscar leachates. One notable exception is molybdenum, which 

is significantly higher in the waste-rock leachates compared 

with the scar leachates. The pH values in the waste-rock 

leachates span the range for some pH-dependent solubility 

and metal-attenuation reactions. Hence, as pH increased in 

the waste-rock leachates, concentrations of several metals 

decreased with increasing time and agitation. Beryllium was 

consistently higher as the pH of the leachate decreased, but 

most other constituents were not consistent except for calcium and sulfate. Capulin and Sugar Shack West samples had 

16 Investigation 25—Summary Report
consistently lower calcium and sulfate concentrations than the 

other waste-rock samples. This difference reflects the higher 

gypsum and calcite content of the Old Sulphur Gulch, Sugar 

Shack Middle, and Sugar Shack South rock piles compared 

to the Capulin and Sugar Shack West piles. The consistently 

higher sulfur stable isotope composition for the three rock 

piles (Old Sulphur Gulch, Sugar Shack Middle, and Sugar 

Shack South) also suggests the presence of some primary (or 

hypogene) gypsum. Indeed, the difference in calcium concentration is greater (in moles per liter) than the difference in sulfate concentration. That is, the three rock piles have about five 

times as much calcium as the other two (Capulin and Sugar 

Shack West) but only 3.5 times as much sulfate concentration. 

Hence, there is likely some calcite dissolution in addition to 

gypsum dissolution to account for this difference. Further, 

these interpretations would indicate that the wastes dumped 

at Capulin and Sugar Shack West are inherently depleted in 

calcite and gypsum relative to the other three waste piles, a 

situation that could be caused by (1) longer residence time and 

exposure to weathering after extraction or (2) the material had 

weathered more before extraction because it was closer to the 

surface. The data available on mine rock-pile production (Robertson Geoconsultants, Inc., 2000a) show that the development 

of Capulin and Sugar Shack West was nearly the same time 

period, at the beginning of open-pit operations (about 1965), 

whereas the other three began development about 1969 (Old 

Sulphur Gulch) or 1971 (Sugar Shack Middle and South). 

Therefore, the difference in leachate properties likely reflects 

the difference in the natural weathering profile of the area now 

occupied by the open pit. Capulin and Sugar Shack West rock 

piles contain a preponderance of shallow earth material containing more pyrite and less calcite and gypsum, whereas the 

other three sampled waste piles contain deeper, less oxidized 

rock with less pyrite and more carbonates.

Bedrock Structure and Ground-Water Flow

Fault zones, joints, and other fractures that might be permeable to ground-water flow were mapped in the vicinity of 

the southern Questa caldera boundary (Caine, 2003). Information on mapped fractures was then used to estimate the potential for bedrock to transmit ground water to the Red River by 

Caine (2007). Prior to this investigation, the bedrock through 

which the ground water flowed was thought to be impermeable and had not been considered a likely source of groundwater discharge to the Red River. Of particular importance 

was to determine whether bedrock discharge of ground water 

to the Red River on the north side and in the vicinity of the 

Columbine Park area adjacent to the mine site could explain 

some of the substantial solute-loading increase.

Pervasive, high-intensity fracturing was found along the 

southern portion of the Questa caldera. North-south-trending 

permeable fault zones run parallel to the presumed hydraulic 

gradient which, along with the joint network, makes bedrock 

transmission of ground water likely. The range of reasonable 

ground-water flow rates was constrained by a series of calculations that assumed hydraulic gradients consistent with the 

topography, permeabilities consistent with hydraulic aquifer 

tests in bedrock, and cross-sectional areas. The conclusion was 

that it was possible for ground-water flow to be conveyed by 

bedrock in this area. The concept that bedrock is impermeable 

in the vicinity of the caldera margin is not tenable. Bedrock 

in other areas away from the caldera margin is not likely to 

carry much ground water (unless other hydrothermal activity 

or tectonic activity produced similarly intensive jointing and 

faulting).

Lake-Sediment Chemistry

Sediments from two lakes in the study reach, Fawn Lakes 

and Eagle Rock Lake, were cored, dated with 137Cs, and analyzed for 34 elements. The lakes are manmade, having been 

dug as borrow pits for highway construction in about 1960. 

An abrupt change in the sediment element concentrations 

for Eagle Rock Lake occurred at the time of a major flood in 

1979 and caused significant changes in concentrations after 

1979 that suggest a new source of sediment. Comparisons of 

the post-1979 sediment-core chemistry with both mine wastes 

and pre-mining sediments from the vicinity of the mine site 

indicate that both are possible sources.

Seismic Profiles

Boundary conditions for ground-water flow in the 

Straight Creek analog site must include the bedrock/alluvium 

contact as it defines the lower boundary of the alluvial aquifer 

and it defines a permeability discontinuity between the alluvial 

and bedrock aquifers. Furthermore, definition of the bedrock 

contact would permit an estimate of the volume of alluvial 

material in the debris fan. Electrical geophysical methods did 

not detect a material property contrast between the bedrock 

and the alluvium (Lucius and others, 2001). Four geophysical 

seismic lines were run in 2002 (Powers and Burton, 2004), 

but bedrock reflection was still uncertain. Five more seismic 

lines were established, and seismic refraction tomography was 

employed, encompassing the Red River alluvium to complete the definition of the debris fan. Two different software 

packages were used for the tomography, and the results were 

compared for optimal definition of subsurface features. Powers 

and Burton (2007) reported on the refined seismic tomography 

for the Straight Creek debris fan and defined both the bedrock 

contact and the ground-water table. The results from these 

additional lines and refinements gave a much clearer picture of 

the cause of discontinuities in the ground-water table between 

wells SC–4A, SC–3A, and SC–6A. It revealed a drainage 

channel in bedrock that is located to the east of the surficial 

drainage channel. Well SC–6A is located in the bedrock channel, but SC–4A and SC–3A are not. For example, the water 

level in SC–6A was approximately 60 ft lower in elevation 

than SC–3A during the period of observation. Wells SC–3A 

and SC–4A are on an elevated bedrock surface that gives rise 

Summary Conclusions from Previous Reports 17
to perched water tables and explains the apparent water-table 

discontinuity. The bedrock surface also reveals a rough, tortuous outline of local heterogeneities that probably indicate large 

boulders incorporated into the debris fan. An example of the 

roughness of the bedrock surface is shown in figure 5 in which 

the contact between alluvium and bedrock is estimated by the 

contact between velocity contrast represented by the blue and 

red zones in the figure. The morphology of these heterogeneities makes it easier to understand how the Straight Creek 

alluvial ground water can remain separate from the Red River 

alluvial ground water. There could have been a small natural berm or boulder-packed divide that built up between the 

mouth of Straight Creek and the Red River to keep them spatially separate. Hardpan surfaces commonly form at contacts 

between acidic iron-rich water (such as Straight Creek) and 

circumneutral-pH water such as the ground water in the Red 

River (Blowes and others, 2003). A hardpan, possibly a ferricrete, could have formed at the contact or interface between 

Straight Creek alluvial water and Red River alluvial water to 

sustain separation of these waters.

Geomorphology

The geochemical and hydrologic processes in the Red 

River Valley occur within a complex terrain of evolving landforms. These landforms, lithologies, and associated hydraulic 

conductivities control surface-water and ground-water movement, ground-water discharge to surface water, and the spatial 

distribution of ground-water emergence. A geomorphological 

analysis by Vincent (in press) provided the necessary link 

between landforms and the spatial distribution of ground-water 

emergence into the Red River as found in the mass-loading 

profiles reported by Kimball and others (2006; following 

section). It also put the erosional processes of the Red River 

Valley into geologic context. Rates of scar erosion and debrisfan formation in the Red River Valley are among the fastest 

rates measured.

Ground-water emergence increases the solute mass 

loadings to the Red River and is not easily recognized as 

input from proximal catchments. The complexity of the link 

between ground-water flow and Red River flow was recognized by Vail Engineering, Inc. (1993, 2000) and shown quantitatively for the first time. Vincent (in press) demonstrated 

that debris fans, having lower hydraulic conductivity than the 

Red River alluvial sediments, but faster sediment-transport and 

deposition rates, would retard Red River alluvial ground-water 

flow and force Red River alluvial ground water to emerge 

into the water column in the debris fans. Debris fans can act 

as confining units, and the emerging ground water can change 

the chemistry of the water column intermittently and cause 

the ground-water table to drop well beneath the level of the 

riverbed on the downstream side of the fan. A substantial fraction of the ground water can emerge to become streamflow, 

the amount depending upon the overall properties of the debris 

fan. Narrowing of the canyon by bedrock can also restrict 

alluvial sediment transport and ground-water flow, forcing 

emergence of alluvial ground water. These geomorphic influences were in effect long before the onset of mining. Figure 6 

shows the Red River in longitudinal profile with the groundwater table between Sulphur Gulch and Bear Creek.

Figure 5. Example of a cross section for seismic line 3 showing bedrock (red), water (blue), and ground surface 

and unsaturated zone (yellow to beige) from seismic refraction tomography (from Powers and Burton, 2007).

18 Investigation 25—Summary Report Relative elevation, in meters

Velocity, in meters per second

Horizontal distance, in meters

Velocity depth model B, in meters per second
Hydrologic Investigations

Quality Control and Quality Assurance (QA/QC) of 

Water Analyses

Reliable water analyses are crucial to the objective of 

this investigation. Particular emphasis was placed on the 

QA/QC of all the analytical data. McCleskey and others 

(2004) reported on the QA/QC and found that for 72 blanks 

there was minimal to no contamination from processing, 

handling, and analyzing the samples. Occasionally different 

analytical methods were used to determine the same constituents in the same samples. These comparisons showed that 

for constituents more than 10 times the detection limit, the 

percent error was less than 8.5 for 95 percent of the samples. 

Charge imbalance was less than 12 percent for 98 percent of 

the samples. The range in all charge imbalances was ±16 percent. Spiked recoveries, standard reference water samples, and 

an interlaboratory comparison all performed well. Timing of 

filtration after sample collection for surface-water samples, 

effect of pore size on trace-element concentration, effect of 

sequential filtration on trace-element concentration, effect 

of filtration method (diameter of filter, and plate, capsule, 

syringe, or ultrafilter types), and ultrafiltration were compared 

as part of the QA/QC. The results showed that for concentrations greater than 0.05 mg/L, the various filtration procedures did not affect the determinations except for iron and 

aluminum. Below 0.05 mg/L, the metals mostly affected by 

filtration methods were iron, aluminum, and copper.

Synoptic and Tracer Studies of Red River

Ground-water flow travels from recharge to discharge 

along some flow path. The discharge endpoint for ground 

waters of the study reach is the Red River. Hence, it was 

necessary to obtain synoptic solute concentrations along the 

Red River with constant-flow tracer injection to quantify 

the discharge and determine portions of the reach receiving 

ground-water influx and the chemistry of that influx. Previous 

studies by Vail Engineering, Inc. (1993, 2000) and Robertson 

Geoconsultants, Inc. (2001b) of the water balance and sulfate 

load balance for the Red River showed that a substantial 

amount of ground water was discharged to the streamflow of 

the Red River just upstream from the Goat Hill debris fan. 

At least one-half of the solute load of this influx was thought 

to be derived from natural acidic scar drainage several miles 

Figure 6. Longitudinal cross section of the Red River along the reach of the mine site projected onto a two-dimensional plane with bedrock surface, water tables (light blue for August 21, 2001, and dark blue for January–February 

2002), projections of the debris-fan surface (red), and Red River streambed surface (from Vincent, in press).

Summary Conclusions from Previous Reports 19

2,470

2,450

2,430

2,410

Goat Hill Gulch

and debris fans

Sulpher Gulch Columbine Park debris fan

Bear

Creek

Streambed

(black line)

Thunder

Bridge

Well-washed

 Red River 

 gravel 

Columbine

Creek

Bedrock base of

alluvial aquifer

Debrisflow

sediment

Mill

reach

Goat Hill

reach

Cabin, Mine Shaft,

and Thunder

Bridge reaches

Portal

Springs

Inferred

2,390

Elevation, in meters 2,370

2,350

2,330

2,310

2,2900 1 2 3 4

Distance upstream, in kilometers

5 6 7 8
upstream (from Hansen Creek, Straight Creek, Hottentot 

Creek, and Bitter Creek scars).

Two synoptic/tracer studies were performed, one in 

August 2001 and the other in March–April 2002. The basic 

chemical data were published by McCleskey and others 

(2003), and the mass-loading curves for sulfate and other 

constituents and their interpretation were reported by Kimball 

and others (2006). In 2002 a study was planned for measuring 

the effect of snowmelt; however, there was no snowmelt peak 

recorded in the hydrograph for that year because of prolonged drought. The winter was warmer and drier than usual. 

It was the first year in 75 years of record that no snowmelt 

peak appeared.

The results revealed several step increases in discharge, 

concentrations, and mass loadings—(a) through the town of 

Red River, (b) near the boundary between the lower end of 

the Straight Creek debris fan and the upper end of the Hansen 

Creek debris fan, (c) near La Bobita, (d) just upstream from 

Sulphur Gulch debris fan, (e) Columbine Park–Cabin Springs 

area, (f) Thunder Bridge area (just upstream from the Goat 

Hill Gulch debris fan), and (g) just downstream from the 

Capulin Canyon mouth (figs. 5 and 7). Each of these changes 

in mass loading appears to be related to the occurrence of 

debris fans or a narrowing of the canyon walls by bedrock or 

areas of mineralization close to the Red River. The amount of 

mass-loading increase varied, depending on constituent and 

time of year. There were clear differences between the two 

studies caused by two events in 2002: (1) more ground-water 

pumping along the mine-site portion of the reach (for milling) 

and (2) drier climatic conditions. The pumping and dry conditions virtually eliminated the large increase in manganese 

loading in the Columbine Park area that was observed in the 

2001 study. By comparing and contrasting the two studies, the 

effect of pumping and seasonal trends on mass loading was 

clearly demonstrated.

The discharge of ground water to the Red River appears 

chemically analogous to an acid-base titration. The Red River 

begins upstream as a dilute, carbonate-buffered river and 

becomes less buffered as it moves through the study reach. 

Surface and alluvial ground water of the Red River upstream 

from the town of Red River (upstream from the study reach) 

has a more basic pH (higher than 8). As it moves down the 

valley, it receives acid influx from Bitter Creek, Hottentot 

Creek, Straight Creek, Hansen Creek, Sulphur Gulch, minewaste piles, Goat Hill Gulch, and Capulin Canyon. The acid 

influx gradually consumes the buffering capacity of the Red 

River, though not completely. Figure 7 shows the values of pH 

for the Red River decreased from 8.2 at the town of Red River 

to 7.9 at the Questa stream gage in August 2001. In March–

April 2002, the pH values decreased from 8.5 to 7.8 for the 

same locations.

Sulfate and soluble metal concentrations such as zinc, 

manganese, and total recoverable aluminum have increased 

continually with downstream transport. Figure 7 shows the 

changes in discharge, pH, and the concentrations of alkalinity, SO4

, Mn, Zn, total recoverable Al, and dissolved Al with 

downstream distance in the Red River from the synoptic/

tracer study of 2001. Abrupt decreases in pH and increases in 

SO4

, Zn, Mn, and total recoverable Al concentrations can be 

observed at three distinct locations, at Waldo Spring, Sulphur 

Gulch, and upstream from the Goat Hill debris fan. For manganese, total recoverable aluminum, and discharge, there is an 

additional increase in the Columbine Park–Cabin Springs area 

(plate 1) from ground-water emergence that did not appear in 

the 2002 synoptic-tracer study.

The results from the synoptic-tracer studies showed 

the complexity of the physical and chemical interactions of 

ground water with the Red River. Acidic ground waters with 

Figure 7. Longitudinal profiles of discharge, pH, alkalinity, 

dissolved sulfate, manganese, zinc, total recoverable aluminum, 

and dissolved aluminum with distance from the synoptic/tracer 

study of 2001 (from Nordstrom, 2005).

20 Investigation 25—Summary Report

0

20

40

60

80

100

120

0

0.2

0.4

0.6

0

20

40

60

80

100

120

5

6

7

8

9

0 5,000 10,000 15,000 20,000

0

0.5

1.0

1.5

2.0

2.5

SO4

Mn

Zn

Discharge

Alkalinity

pH

 pH, in standard units

Dissolved Al

Total Al

Goat Hill debris fan

Sulphur Gulch

Distance, in meters Waldo Springs

Alkalinity, in milligrams per liter as CaCO3

,

or discharge, in cubic feet per second Sulfate, in milligrams per liter Aluminum, in milligrams per liter

Manganese or zinc, in milligrams per liter
high solute concentrations from both natural processes and 

mine-related activities have entered the Red River alluvium 

and the Red River itself at distinct locations related to landform features, not at the juncture of each catchment with the 

Red River. These locations of ground-water emergence into 

Red River water are often 2 to 3 km (1 to 2 mi) downstream 

from the catchment junctures.

Reactive-Transport Modeling in the Red River

Reactive-transport modeling not only integrates quantitatively the combined processes of ground- and surface-water 

influx, but it also considers quantitatively the in-stream processes of oxidation, sorption, and mineral precipitation/dissolution. The basic constraining principles and theories are those 

of chemical thermodynamics, fluid flow, and mixing. Ball and 

others (2005) used the synoptic/tracer data of both 2001 and 

2002 to model mixing and in-stream chemical processes in the 

Red River with the OTIS/OTEQ code (Runkel, 1998; Runkel 

and Kimball, 2002).

In-stream reactive processes were necessary to model the 

behavior of hydrogen ion, iron, aluminum, copper, and zinc. 

Other constituents were sufficiently conservative to be modeled without reaction. Mineral precipitation (with formation 

of colloidal-size particles) can account for the low dissolved 

aluminum and iron concentrations in the Red River. Iron precipitation was simulated with ferrihydrite and Fe(OH)3

 solubility, and aluminum precipitation was simulated with microcrystalline to amorphous Al(OH)3

 solubility. Copper and zinc 

concentrations are kept low by sorption and were modeled by 

sorption onto hydrous ferric oxides. Aluminum hydroxide precipitates at pH values of about 5 until a pH of 7.5 is reached 

(Nordstrom and Ball, 1986; Ball and others, 2005). At this 

point, inorganic mineral precipitation is no longer effective 

because the aluminum concentration is virtually constant from 

pH 7.5 to 8.5 at a value much higher than that imposed by any 

Al(OH)3

 mineral solubility. Organic-aluminum complexing 

is proposed to explain the departure from mineral solubility 

controls (Ball and others, 2005).

Values of pH in the Red River oscillate between lower 

values resulting from inflow of acid water and higher values 

that appear to be caused by kinetically controlled degassing 

of carbon dioxide after acidification or possibly neutralization 

from reaction with carbonate minerals.

During this study, the chemical relationship among the 

inflow ground-water compositions (ground water discharging 

into the Red River), the Red River water compositions, and 

ground-water compositions from wells were examined. Inflow 

compositions can be obtained from (a) analyses of seeps and 

springs that are sampled next to the Red River (see McCleskey 

and others, 2003), (b) changes in constituent ratios, such as 

Ca/SO4

, upstream and downstream from emergent points, and 

(c) calculating the composition from the change in mass flux 

and discharge in the Red River upstream and downstream from 

emergent points.

Mixing lines, using conservative constituents, were found 

to reveal the chemical similarity between ground waters, 

inflow compositions, and changes in river-water composition. The mathematical basis for the mixing lines is given in 

the Appendix. An example is shown in figures 8 and 9 for (a) 

ground water upgradient from La Bobita (Straight, Hansen, 

and Hottentot Creeks), (b) the La Bobita well, (c) the Red 

River upstream from ground-water emergence, and (d) the 

Red River downstream from ground-water emergence. The 

reach of ground-water emergence can be seen in figure 7 in the 

vicinity of Waldo Springs from changes in metals concentrations. The other locations can be found on plate 1.

Two constituents, calcium and magnesium, are used for 

these examples. Constituent ratios to sulfate (such as calcium 

to sulfate ratio) plotted against sulfate concentration were 

found to reveal differences in the end-member water compositions. The calcium to sulfate ratio is plotted as a molar ratio 

so that the 1:1 molar ratio, representing pure gypsum dissolution, could be shown. Data plotting higher than the gypsum 

line are enriched in calcium from calcite dissolution that 

neutralizes acid drainage, indicating circumneutral-pH values. 

Data plotting lower than the gypsum line have additional 

sulfate from acid-sulfate waters derived from pyrite oxidation. The end members for the mixing line are the alluvial 

well water at SC–1A and the Red River upstream from the 

point of ground-water emergence. These two points anchor 

the mixing line with 100-percent source water represented 

by SC–1A composition and 0 percent represented by Red 

River at 7,200 m downstream from the town of Red River 

(distance points used in the synoptic/tracer study, McCleskey 

and others, 2003). All other data points are independent of the 

mixing-line calculation. The fact that all the water-chemistry 

data, with two exceptions (SC–5B and MMW–43A), lie along 

the mixing line suggests that the water mixing into the Red 

River near La Bobita is consistent with naturally acidic ground 

waters from scar areas as the source. Further, it suggests that 

the seepage water diluting SC–1A water to produce water 

compositions in wells SC–3A, SC–4A, and SC–6A are dilute 

and similar in composition to the Red River alluvial aquifer 

ground water.

Near La Bobita, part of the ground water emerges and the 

rest remains in the Red River alluvial aquifer. An important 

question is: How far downgradient from La Bobita can the 

same chemical signature be observed in the alluvial ground 

water? The next Red River alluvial well is MMW–17A. As 

shown in figures 8 and 9, its calcium and magnesium to sulfate 

ratios, respectively, are virtually identical to those for La 

Bobita (and other inflow waters). The next well water downstream from MMW–17A is that from MMW–43A. It shows 

a different signature from the mixing line for both calcium 

and magnesium to sulfate ratios at a much higher sulfate 

concentration than the La Bobita water, so it must be strongly 

affected by a different source composition of higher calcium, 

magnesium, and sulfate concentrations and higher ratios. The 

other exception to this mixing trend is water from well SC–5B 

(bedrock well). It was plotted to show that circumneutral-pH 

Summary Conclusions from Previous Reports 21
bedrock ground water in the Straight Creek catchment does 

not have the same chemical signature as the acid alluvial 

ground water. Its calcium to sulfate ratio is much higher than 

that for the alluvial ground water, and the magnesium to sulfate ratio is much lower than that for the alluvial ground water.

The well water at SC–7A also was used instead of 

SC–1A as the end-member source of acid water, and it makes 

no difference in the position of the mixing line. The observation that the Red River during mixing lies on the mixing line, 

as do the Waldo Springs discharge, La Bobita well water, 

and Straight Creek well-water compositions, indicates that 

the chemical signature is consistent with a flow path from 

the natural scar drainages to emergence in the Red River 

near La Bobita. This consistency in chemistry shows that the 

acid-scar drainages that become alluvial ground water do not 

mix with the alluvial ground water and emerge into the river 

until the narrowing of the canyon at La Bobita is reached. 

Well water from MMW–17A (just beyond the narrowing of 

the canyon) also is consistent with La Bobita, but the next 

downstream alluvial well, MMW–43A, has distinctly different chemistry. New sources of sulfate-rich water are entering 

the aquifer between MMW–17A and MMW–43A. The inflow 

compositions (seeps and springs near the banks of the river), 

from which one sample was chosen for the mixing calculations 

in the reactive-transport modeling, are also consistent with the 

mixing-line composition. The percent mixing indicates that 

less than 2 percent of SC–1A water was needed to mix with 

Red River water to cause the change in water chemistry of the 

Red River at La Bobita.

Diel, Storm Event, and Long-Term Trends in Red 

River

Variations in water chemistry at any one location can be 

caused by dynamic hydrologic events (snowmelt, rainstorms, 

floods), droughts, and anthropogenic activities (such as irrigation, water management with dams and canals, and wastewater 

discharge). Solar cycles regulate biological activity and consequently can affect water chemistry. During this investigation, 

water-chemistry variations in the Red River were observed 

by Verplanck and others (2006) and compared with historical 

Figure 8. Mixing line constructed between Red River upstream from ground-water emergence 

(7,200 meters) and well water from SC–1A, using calcium/sulfate molar ratios relative to sulfate 

concentrations. Other ground waters from Straight Creek, Hottentot Creek, Hansen Creek, La Bobita, 

MMW–17A, MMW–43A, and seeps in this reach are also plotted. Vertical bars indicate percentage of 

SC–1A water that has mixed with Red River (or similar) water (from Ball and others, 2005).

22 Investigation 25—Summary Report

0

0 500 1,000 1,500 2,000 2,500

0.5

1.0

1.5

2.0

50% 75%

25%

Red River at: 7,200 meters ; 7,800 meters; Inflows at: 7,270 meters; 7,300 meters;

7,457 meters (Waldo Springs); 7,588 meters : La Bobita;

Hansen; Hottentot; MMW-17A; MMW-43A; SC-8A; SC-7A; SC-6A;

SC-5A; SC-5B; SC-4A; SC-3A; SC-1A

Calcium/sulfate molar ratio

10%

EXPLANATION

; Most probable values for wells:

Sulfate, in milligrams per liter

Gypsum ratio
water-chemistry trends. They also reported on water-chemistry 

results from the Straight Creek and other scar drainages.

Water-chemistry variations in the Red River were found 

to be caused primarily by snowmelt and rainstorms and 

secondarily by length and degree of dry periods. Diel samples 

were collected during high flow (May 2003) and low flow 

(October 2003). Only at low flow and only for dissolved zinc 

and manganese was there a discernible diel cycle. Again, these 

diel trends were minor compared to snowmelt and rainstorm 

variations (Verplanck and others, 2006).

One series of samples collected before, during, and after 

the rainstorm of September 18, 2002, demonstrates the effect 

of natural scar drainage water on the water quality of the Red 

River at the Questa gage (Verplanck and others, 2006). In a 

matter of hours, discharge increased from 10 to 100 ft3

/s and 

the pH changed from 7.8 to 4.8 (fig. 10). The highest concentrations of iron, aluminum, zinc, and sulfate collected for 

the period 2001–2004 occurred during this storm. A greater 

change in chemistry of the Red River at the gage was found in 

the historical record (Maest and others, 2004) with a decrease 

in pH to 3.8.

Maest and others (2004) noted that when only the “lowflow” sulfate concentrations at the Questa gage were separated 

from the rest of sulfate concentration data set, there was an 

overall long-term trend of increasing concentration for the 

time period 1965–93 and then a small decrease after that time. 

More recent data from 2001 to 2004 from Verplanck and others (2006) were included to determine if the indication of a 

downward trend since 1993 was observed. These data indicate 

that there is no significant decrease in sulfate concentration 

since peak values were attained about 1993. The waterchemistry effects of drought years compared to wet years 

complicates the interpretation of discerning natural weathering 

processes from the effects of mining. A preliminary examination of the historical data suggests that prolonged dry periods 

can cause overall increases in solute concentration compared 

to wet periods because the relative proportion of surface 

flows, such as Columbine Creek and the portion of Red River 

upstream from the study reach, decreases more proportionally than the inflows of acidic ground water. Conversely, when 

surface-water flows increase, solute concentrations decrease 

except for momentary acid flushes, as shown in figure 10.

Figure 9. Mixing line constructed between Red River upstream from ground-water emergence 

(7,200 meters) and well water from SC–1A using magnesium/sulfate mass ratios relative to sulfate 

concentrations. Other ground waters from Straight Creek, Hottentot Creek, Hansen Creek, La Bobita, 

MMW–17A, MMW–43A, and seeps in this reach are also plotted (from Ball and others, 2005).

Summary Conclusions from Previous Reports 23

0

0.05

0.10

0.15

0.20

Red River at: 7,200 meters; 7,300 meters;

La Bobita;

Hansen; Hottentot; MMW 17A; MMW 43A; SC-8A; SC-7A; SC-6A;

SC-5A; SC-5B; SC-4A; SC-3A; SC-1A

Magnesium/sulfate mass ratio

7,800 meters; Inflows at: 7,270 meters;

7,457 meters (Waldo Springs); 7,588 meters; Most probable values for wells:

0 500 1,000 1,500 2,000 2,500

50% 75% 25%

10%

EXPLANATION

Sulfate, in milligrams per liter
Ground Water

Ground-water geochemistry is described in the “Interpretation of Ground-Water Geochemistry” section. All of the 

ground-water results are integrated to summarize weathering 

controls and mineral solubility controls on solute composition. Results of well installation and development and methods 

and results of water-level collection are found in Naus and 

others (2006) and in Blanchard and others (2007). Methods 

of sample collection for water analyses are found in Naus and 

others (2006) and QA/QC, which applies to all water analyses in this investigation, is found in McCleskey and others 

(2004). Sampling and preservation procedures are consistent 

with those found in the USGS National Field Manual for the 

Collection of Water-Quality Data (http://water.usgs.gov/owq/

FieldManual/).

Water Balances

Straight Creek

Considering the problems associated with aquifer tests on 

perched aquifers at Straight Creek and the inability to develop 

a ground-water model with such a spatially heterogeneous system, the effort was put into developing a ground-water budget 

(McAda and Naus, in press). Some 77 percent of precipitation 

was lost by evapotranspiration, leaving about 17 percent for 

ground-water infiltration in the debris fan and underlying regolith and about 5 percent for deeper bedrock ground water. This 

study indicated that the amount of ground-water flow to the 

Red River alluvium from Straight Creek is small, only about 

156 liters per minute (0.347 ft3

/s) or about 3 percent of the Red 

River alluvial flow at Straight Creek. If only debris-fan (and 

underlying regolith) ground water is considered, then ground 

water is only about 2 percent of Red River alluvial flow. This 

small proportion of flow adds to the difficulty of finding the 

mixing zone in the Straight Creek debris fan. This proportion 

of flow is also consistent with estimates from chemical data 

and mixing curves, indicating that scar drainage accounts for 

only about 2 percent of the water in the Red River near La 

Bobita where ground water emerges.

Estimates for ground-water emergence to the Red River 

near the La Bobita area have been obtained from several studies (Borland and others, 1990; Vail Engineering, Inc., 2000; 

Kimball and others, 2006), and their values range from 1.4 to 

5 ft3/s. This range is better than expected because different 

methods were used, the study was done in different water 

years, and the studies were done at different times of the year. 

These results show that ground-water emergence appears consistently in this part of the river, independent of water year.

Figure 10. Changes in discharge, pH, and sulfate and manganese concentrations at the 

U.S. Geological Survey Questa gage during the storm of September 17–18, 2002 (Verplanck 

and others, 2006).

24 Investigation 25—Summary Report

0

20

40

60

80

100

120

0

1

2

3

4

5

6

7

8

9

100

150

200

250

300

350

Discharge, in cubic feet per second

September 2002

Manganese

Discharge

Sulfate

pH

Sulfate concentration, in milligrams per liter

ph, in standard units and manganese concentration, in milligrams per liter

12 15 18 21 24
Red River Valley

A basinwide water balance was completed to achieve 

a better understanding of the relation between ground-water 

chemistry and flow of tributary basins to changes in flow and 

chemistry in the Red River (Naus and others, 2006). This 

study is another approach to understanding ground-water/ 

surface-water interactions in complex terrain and relates to 

the mixing lines shown for ground waters and surface waters 

(figs. 8 and 9), but is a more quantitative approach. Both 

approaches should be consistent with each other. This study 

also is consistent generally with previous studies such as the 

Borland and others (1990) seepage investigation, the Vail 

Engineering, Inc. (2000) flow and load-balance investigation, 

the Vincent (in press) geomorphological analysis, and the 

Kimball and others (2006) synoptic/tracer/loading investigation. A series of natural barriers to alluvial ground-water flow 

repeatedly causes ground-water emergence into the river (Vincent, in press), but the flow balance can vary with the seasons 

and with prolonged wet periods or prolonged dry periods. 

Each study was done at a different time period except for the 

Vincent (in press) study, which was done with data that corresponded to the same time period as the Kimball and others 

(2006) study. The Naus and others (2006) study was based on 

annual averages over long time periods. Vail Engineering, Inc. 

(2000), derived an October 1999 flow balance that included 

Red River discharge measurements and sulfate concentrations 

for a sulfate load balance.

Naus and others (2006) reexamined precipitation, 

evapotranspiration, and different regression equations previously used for estimating yields. Long-term mean annual 

precipitation for subbasins in the valley is estimated to range 

from 330 to 890 mm. Using this range of precipitation in 

empirical equations relating precipitation to elevation and 

catchment areas, three estimates were obtained for yields for 

each subbasin. Alternatively, yield was estimated by subtracting evapotranspiration estimates from precipitation and by the 

chloride balance method. Long-term mean annual catchment 

yields ranged from 45 to 52 ft3

/s. Ground water was shown to 

be 80 to 94 percent of total yield, depending on the amount of 

surface-water flow.

Integrating Water-Flow and Sulfate 

Mass-Load Balances for the 

Hottentot–La Bobita Reach

There is sufficient information to attempt a water and 

sulfate load balance in the reach upstream from the mine site 

(from Hottentot debris fan to the La Bobita area). The results 

shed light on the process of ground-water mixing and discharge to the Red River and the consequences for downstream 

transport of solutes from natural sources to the reach along the 

mine site.

McAda and Naus (in press) have estimated from seismic data and hydraulic conductivity that the ground-water 

flow in the Red River alluvium adjacent to the Straight 

Creek fan (at seismic line 7 in Powers and Burton, 2007) is 

11.65 ft3/s based on water levels in September 2003. This 

flow is assumed to be the ground-water flow before any water 

from the natural scar drainage (or adjacent catchment drainage) enters the Red River alluvium. The surface-water flow 

in the Red River measured by Kimball and others (2006) for 

the 2001 tracer study was 17.12 ft3

/s. Hence, the total flow 

was 28.8 ft3

/s, which compares well with the estimated yield 

(ground water plus surface water) for that location in the Red 

River Valley of 32.3 ft3

/s (based on data in Naus and others, 

2005). However, a better approximation of alluvial groundwater flow for August 2001, when the 2001 tracer study was 

obtained, would be 11.87 ft3

/s, assuming that conditions of 

August 2003 were similar to those of August 2001 (D.P. 

McAda, U.S. Geological Survey, written commun., 2005). 

Hence, the combined flow would be about 30 ft3

/s, only 7 percent different from the yield estimate. Similar estimates were 

made for water-level data that most closely approximate the 

conditions for the March/April 2002 synoptic/tracer.

The yield estimates can be used to calculate the 

yield from all catchments along the reach from Hottentot 

catchment to the end of ground-water emergence just downstream from the La Bobita campground. When the yields are 

summed from catchments 4–7 and 25–28 (catchment numbers from Naus and others, 2005), the result is 1.54 ft3

/s, of 

which about 0.89 ft3/s is primarily from scar drainage. Hence, 

of the total flow (32.3 ft3

/s), scar drainage base flow constitutes 2.8 percent. Of the ground-water flow in this section 

(13.19 ft3/s), the scar drainage constitutes 6.7 percent.

The results from the 2001 synoptic/tracer study by 

Kimball and others (2006) show two sections of ground-water 

emergence in this part of the study reach, one at about 4,800 to 

5,735 m (using the east end of the town of Red River as the 

zero point; Kimball and others, 2006) and the other at about 

5,765 to 8,700 m. The sum of those two flow increases is 

5.03 ft3/s. For the 2002 synoptic/tracer study, the sum of these 

flow increases is 3.65 ft3

/s. The alluvial outflow is simply the 

difference between ground-water emergence and the total flow 

before emergence. The resulting flow balance for the Red 

River alluvial aquifer along this reach is shown diagrammatically for both years in figure 11.

As a check on these calculations, an independent estimate of the alluvial outflow was obtained from Darcy’s law 

by considering the cross-sectional area of the canyon at the 

narrowing just downstream from the La Bobita campground 

(about 3,000 m2

), the hydraulic conductivity (about 0.3 cm/s), 

and the gradient (about 0.017 m/m by topography and about 

0.029 m/m with pumping in the mill reach). The range of 

alluvial outflow is then 5.4 ft3/s without pumping and 9.2 ft3/s 

with pumping (Kirk Vincent, written commun., 2005). Not 

only are the values estimated by difference within 50 percent 

of the Darcy calculation, but the estimate for 2002 when there 

was more pumping is within about 3 percent of the Darcy 

Integrating Water-Flow and Sulfate Mass-Load Balances for the Hottentote-La Bobita Reach 25
calculation. Consequently, a high degree of confidence can be 

attributed to the flow balances.

For the sulfate mass-load balance, it was assumed that the 

average sulfate concentration in the Red River alluvial ground 

water, before any water from the scar area mixed into it, was 

119 mg/L based on the concentration of sulfate in ground 

water from well SC–8A. This well appears to be unaffected by 

the acid alluvial ground water from Straight Creek and should 

contain Red River alluvial ground water. Hence, the sulfate 

mass-load inflow in the Red River alluvium, before it mixed 

with scar-drainage inflow, was 3,454 kg/day for conditions of 

August 2001.

Scar-drainage sulfate loads were calculated by taking 

the yields from each catchment (Naus and others, 2005) and 

multipying them by the median sulfate concentrations from 

each respective wel1. Catchments that have little or no scars 

and no wells are more difficult to assess. The catchment to the 

west of the Hansen scar drainage (number 7 in Naus and others, 2005) has a small amount of scar from Little Hansen, and 

the catchments on the south side of the Red River have some 

small scars, but most of these areas are without scars or wells. 

Seepages along the banks of the Red River near La Bobita 

have sulfate concentrations of about 400 mg/L (McCleskey 

and others, 2003; Ball and others, 2005), and a spring sampled 

to the west of Little Hansen contained 412 mg/L of sulfate. 

Hence, a sulfate concentration of 400 mg/L was assumed for 

these other catchments. The sulfate mass loads for each catchment are shown in table 3. The scar drainages (numbers 4–6) 

contribute 5 percent of the water flow in this reach of the Red 

River but 33 percent of the sulfate load. Fifty-four percent of 

the sulfate load [(3,454/6,426) × 100] is carried by the Red 

River alluvium as it enters this reach.

For the 2001 synoptic/tracer study, the increase in sulfate 

load in the Red River was 1,843 kg/day. For the 2002 study, 

the increase was 1,159 kg/day. The schematic sulfate massload balance is shown in figure 12.

From these calculations, the concentration of sulfate in the ground water of the Red River alluvium would 

increase from 119 to 196 mg/L. However, if the sulfate 

concentrations are calculated for the emerging ground water 

and the nonemerging ground water, a problem arises. The 

concentrations should be the same because they originate from 

the same source, but they are not. The emerging ground water 

contains 150 mg/L sulfate, whereas the nonemerging ground 

water contains 244 mg/L. This difference occurs because the 

emergent ground-water flow and mass load are derived from 

separate measurements than the inflow of ground water from 

the catchments. It could be argued that this concentration difference is not of consequence considering all the uncertainties 

in the data; however, the consequences for the load balance is 

quite serious. This model will be named Model I and is based 

on “best available data.”

A different model (Model II) can be derived by making 

the mass loadings and concentrations consistent with the concept that the concentrations of both emerging and nonemerging water originate from the same source and must have the 

same concentration of sulfate. This model is the “completely 

mixed model.” This model begins with data that are most certain, such as the emerging ground-water flow and loading, and 

adjusts the least certain data, such as the sulfate mass loading 

from the catchments, so that there are no inconsistencies in the 

concentrations. For this model, all the flows are kept the same 

as before, only the concentrations are changed. The loading 

for the alluvium is calculated by working backward from the 

emerging ground water. Using the loading of 1,843 kg/day for 

Figure 11. Water-flow balance for the Red River alluvial aquifer shown schematically for synoptic/tracer studies of 2001 and 2002 for the reach from Hottentot debris fan to La Bobita (from 

Kimball and others, 2006).

26 Investigation 25—Summary Report

11.87 ft3/s

1.54 ft3/s

2001

5.03 ft3/s

8.38 ft3/s

10.92 ft3/s

1.54 ft3/s 3.56 ft3

/s

8.90 ft3/s

2002

Alluvial inflow

Ground-water emergence

Alluvial outflow

13.41 ft3/s

12.46 ft3/s

Scar ground-water inflow
the sulfate load of the emerging ground water and adding to 

that the loading of the nonemerging ground water (the flow 

of the alluvial outflow ground water times 150 mg/L) gives 

4,914 kg/day for the alluvium sulfate load before emergence 

but after catchment waters have mixed. The flow and sulfate 

concentration of the alluvium before mixing with catchment 

yields are kept constant and, consequently, the sulfate mass 

load from the catchments must be found by difference to be 

1,460 kg/day. Rather than attempting to justify adjustment in 

every single catchment, they were all decreased proportionally. 

The consequent concentrations for the catchment yields from 

Model II are shown in table 4, and the schematic flow diagram 

is in figure 13.

In Model I the concentrations in the two outflows are 

inconsistent with each other; in Model II the concentrations 

remain constant in the outflows but inconsistent with measured concentrations in the inflow catchment water. A third 

model, Model III, assumes sulfate concentration is incorrect 

for the Red River alluvial inflow. Keeping Model I for the 

catchment sulfate inflows and Model II for the outflows, then 

Model III changes the concentration of the alluvial inflow so 

that everything else balances. For this mass flux to balance, 

the sulfate concentration has to be 67 mg/L (for 2001) and 

Table 3. Catchments that contribute ground water and sulfate loads to the Red River alluvium near natural scar drainages (Hottentot to La Bobita) for August 2001.

[ft3/s, cubic feet per second; mg/L, milligrams per liter; kg/day, kilograms per day; --, not available]

Catchment number Catchment Yield

(ft3

/s)

Sulfate concentration 

(mg/L)

Sulfate mass load (kg/

day)

4 Hottentot 0.30 913 670

5 Straight .24 1,300 763

6 Hansen .14 2,035 697

7 -- .21 400 205

25–28 -- .65 400 636

Total 2,972

Alluvium at Straight Creek Upstream 11.87 119 3,454

Grand total 6,426

Figure 12. Sulfate mass-load balance for the Red River alluvium shown schematically for the 

synoptic/tracer studies of 2001 and 2002 for the reach from Hottentot debris fan to La Bobita.

Integrating Water-Flow and Sulfate Mass-Load Balances for the Hottentote-La Bobita Reach 27

Scar ground-water inflow Ground-water emergence

2001

3,454 kg/day

2,972 kg/day 1843 kg/d

6,426 kg/day 4,583 kg/day

2002

3,178 kg/day

2,972 kg/day

6,150 kg/day

1,159 kg/day

4,991 kg/day

1,843 kg/day
41 mg/L (for 2002). The sulfate mass balance for Model III is 

shown in figure 14.

There are several sources of uncertainty that could be 

causing these inconsistencies. Concentrations for both the 

inflowing alluvial ground water and the inflowing catchment 

ground water may not be low enough. The waters may not 

be adequately mixed for this type of evaluation. Because of 

heterogeneities in the real system and the inability of the data 

to define these fine-scale gradients, the actual fluxes cannot be 

constrained much more than by using these three models.

It is important to realize that there is no logical basis or 

independent criterion to allow a best choice between these 

models without further data. The ground waters do not completely mix because of the tendency for acid ground water 

from the north catchments to hug the north side of the alluvial 

aquifer, and not enough is known about mixing to say at what 

point or along what reach it becomes mostly or completely 

mixed. Hence, Model II is not necessarily correct. Alternatively, Model I relies on complete mixing among catchment 

ground-water inflows and the Red River alluvial aquifer to 

calculate the mass balances, so it may not be correct either. 

What one can infer from this analysis is that about 20–40 percent of the sulfate loads in the alluvium are emerging in the 

La Bobita area. The remainder continues on downstream in 

the alluvial flow. In the reach from the mill site to Thunder 

Bridge, the emerging ground water contains sulfate loads 

that are severalfold greater than what remains in the alluvial 

ground water from the scar areas upstream from the mine site. 

Hence, new sources must have entered from the mine site to 

account for the load of sulfate entering in this reach of the Red 

River. These sources would be a mixture of loads from natural 

scars and from waste rock.

Vail Engineering, Inc. (2000), concluded that at least 

one-half of the sulfate load that emerged in the Columbine 

Park area was from natural scar drainage areas upstream from 

the mine site. Studies by Kimball and others (2006) indicate 

Table 4. Catchment yields, sulfate concentrations, and sulfate mass loads using Model II for 2001.

[ft3/s, cubic feet per second; mg/L, milligrams per liter; kg/day, kilograms per day; --, not available]

Catchment number Catchment Yield

(ft3

/s)

Sulfate concentration

(mg/L)

Sulfate mass load

(kg/day)

4 Hottentot 0.30 450 330

5 Straight .24 637 374

6 Hansen .14 998 342

7 -- .21 197 101

25–28 -- .65 197 313

Total 1,460

Alluvium at 

Straight Creek

Upstream 11.87 119 3,454

Grand total 4,914

Figure 13. Sulfate mass loads for Model II in which the sulfate mass loads of the 

catchments are back-calculated from the ground-water emergence mass loads.

28 Investigation 25—Summary Report

2001

3,454 kg/day

1,460 kg/day 1,843 kg/day

4,914 kg/day 3,071 kg/day

2002

3,180 kg/day

875 kg/day

4,055 kg/day

1,159 kg/day

2,896 kg/day

Scar ground-water inflow Ground-water emergence
that most of the emerging solute load near Columbine and the 

Goat Hill debris fan originate from the reach along the mine 

site (from the mill site to Thunder Bridge) for the time periods 

of August 2001 and March–April 2002. The analysis shown 

here helps to constrain this problem. Of the 4,055–6,426 kg/

day of sulfate (the total sulfate flux) that flows in the alluvium 

along the Hottentot-Straight-Hansen Creek scars, perhaps 

875–2,972 kg/day (22–73 percent) is directly derived from 

Hottentot-Straight-Hansen Creek scar drainages. About 20 

to 40 percent of the total emerges near La Bobita and the 

remainder (60–80 percent) continues downstream in the Red 

River alluvium. In other words, the ground-water flux leaving 

the La Bobita area after ground-water emergence is 8 to 9 ft3

/s, 

and the sulfate flux, depending on the year and time of year, is 

2,896 to 4,991 kg/day.

The water and sulfate flux in the ground water of the 

Columbine Park area (downstream from Columbine Creek) is 

not known, but Vincent’s (in press) analysis indicates that, for 

flows to balance under steady-state conditions, ground-water 

flow should be about 9.7 ft3

/s before ground water emerges 

near the Goat Hill fan and before pumping. About 8 ft3

/s of 

ground water emerges at Cabin Springs (about 0.8 ft3

/s) and 

just upstream from the Goat Hill fan (about 7.2 ft3

/s). These 

crude estimates indicate that 82 percent of the ground water is 

emerging here. From the mass-loading results, the emergent 

sulfate flux is about 4,300 kg/day in this same area. Using 

simple proportions, 100 percent of the sulfate flux would be 

5,244 kg/day, and the sulfate flux from upstream from the 

mine site would be 55 to 95 percent of this amount. However, sulfate concentrations at La Bobita are 400–500 mg/L, 

and they are the same at wells MMW–17A and MMW–28A 

(between Sulphur Gulch and La Bobita), but wells in the 

Red River alluvium downstream from Sulphur Gulch contain 

concentrations of sulfate that are higher, sometimes more than 

1,000 mg/L. Such a large increase in solute concentration 

indicates that the loading upstream from the mine site cannot account for most of the loading in the reach of the mine 

site. Further, there are ground-water inflows with high sulfate 

concentrations and ground-water emergences between the mill 

site and Goat Hill Gulch that must be included in the mass 

balances. Estimates can vary considerably because of large 

uncertainties in the data and because part of the variability in 

the data is caused by seasonal and climatic fluctuations and by 

the amount of ground-water pumping. Better measurements 

for hydraulic conductivity from aquifer tests, seismic profiles 

to define the alluvium geometry, and geoprobe measurements 

to define water levels and to collect samples for analysis along 

this reach of the Red River combined with water-quality data 

from existing wells and a detailed tracer study could likely 

define the sources and fate of solute flux in this area. Further 

work along these lines is needed. It must be remembered that 

simply stating that some portion of the increased sulfate loading in the Red River is derived from the mine-site reach does 

not indicate how much loading is from mining activities and 

how much is from natural scars and natural mineralized areas 

that exist on the mine site. A focused study might resolve 

this question.

Figure 14. Sulfate mass loads for Model III in which the sulfate mass load of the 

alluvial ground-water inflow is back-calculated from the ground-water emergence 

mass loads and the catchment inflows are kept constant.

Interpretation of Ground-Water Geochemistry 29

1,942 kg/day

2,972 kg/day 1,843 kg/day

4,914 kg/day 3,071 kg/day

2001

2002

1,083 kg/day

2,972 kg/day 1,159 kg/day

4,055 kg/day 2,896 kg/day

Scar ground-water inflow Ground-water emergence
Interpretation of Ground-Water 

Geochemistry

The Straight Creek analog site provided an opportunity 

to study in detail the hydrology, geology, and geochemistry of 

water/rock interactions in a Red River Valley catchment unaffected by mining activities. The geochemical processes determined from the Straight Creek study were then compared to 

other catchments unaffected by mining to ascertain the applicability of the Straight Creek results beyond the Straight Creek 

catchment. The valleywide comparison is fundamentally a 

scientific evaluation of the geochemical processes that were 

interpreted to occur in the Straight Creek catchment. From this 

comparison, generalizations can be formulated about water/

rock interactions given the known geologic and hydrologic 

properties of the valley. With this broader understanding of the 

water/rock interactions that led to the observed ground-water 

chemistry in the Red River Valley, the same concepts can be 

applied to the mine site with appropriate modifications that 

account for any geologic and hydrologic characteristics of the 

mine site that are substantially different from the other sites.

The Straight Creek catchment is shown in figure 15 with 

well locations designated. Well SC–9A was a temporary geoprobe installation from which no water samples were collected 

for analysis.

Median Concentrations from Monitoring Data

Water analyses from individual monitor wells showed 

relatively small variations over time with few exceptions. 

To represent averages of these solute concentrations the 

median value is used for purposes of graphical representation 

and interpretation. The median pH was obtained by taking 

the antilogarithm of the pH values, finding the median of the 

proton activities, then taking the logarithm of that median. 

The variance of the values around the median is shown by 

the mean absolute deviation (MAD, the absolute value of 

the mean of deviations about the median). The median is 

chosen instead of the mean because it is much less affected by 

outliers. Table 5 lists the medians and MADs for the monitoring data from this study. These values are used in the following sections that describe the general trends in water chemistry 

from which the pre-mining concentrations are derived.

The analyses from which table 5 is derived are in Naus 

and others (2005) and in Nordstrom and others (2005). There 

were two exceptions to the procedure previously mentioned 

for obtaining the median values. Wells SC–1B and SC–5B 

have evidence of contamination from unknown additives that 

were used to assist well development. The main contaminants 

are reflected in high and variable sodium, iron, and dissolved 

organic carbon (DOC) concentrations. The concentrations of 

these constituents tended to increase over time, reach a peak, 

and then decrease. The decreasing concentrations tended to 

reflect exponential decay, so we assume the last measured concentrations were closest to the true ground-water concentration 

before the additives were introduced.

Chemistry of Water Developed from Scar 

Weathering

During the course of these investigations, a notable consistency in the pH of scar-drainage water was observed. If just 

the pH values from water derived from scar weathering are 

compiled, excluding those which have traveled over, under, or 

through any sizable volume of debris fan or other rock material, the pH values are consistently near 3. Figure 16A shows 

a frequency distribution plot of pH values from a literature 

compilation of scar-seep chemistry (LoVetere and Nordstrom, 

in press). The mean and median are nearly the same, 2.97 and 

2.93, respectively, for 121 samples. The close correspondence 

of mean and median indicates a symmetrical distribution, and 

it is a necessary condition for normal distributions.

Similarly, the sulfate concentrations for scar drainage 

are also consistent with a mean of 2,014 mg/L and a median 

of 2,000 mg/L (fig. 16B). This consistency in sulfate concentrations for scar-drainage water helps to differentiate 

them from waste-rock drainage, which frequently is greater 

than 5,000 mg/L in sulfate concentration (LoVetere and 

Nordstrom, in press). However, two notes of caution must 

be applied. Waste-rock water compositions overlap with 

scar-drainage compositions (that is, they can be less than 

5,000 mg/L sulfate), and there is one scar-drainage example 

not sampled in the USGS study that has sulfate concentrations 

up to about 15,000 mg/L. This drainage is from the Bitter 

Creek scar and was considered outside the study area but 

was sampled by Robertson Geoconsultants, Inc. (2000a). It 

is unclear why the Bitter Creek water had such a high concentration. Evaporation or unusually high residence times are 

possibilities; because no isotopes or age dates were obtained, 

we could not determine why these waters are anomalous compared to all the others.

Water-Chemistry Classification for the Red River 

Valley Ground Water

Using medians to summarize the pH values and sulfate 

concentrations from all the wells and Straight Creek surface waters sampled in this study (fig. 17), the waters divide 

into three groups: Group I are dilute, neutral-pH, Ca-HCO3

type waters; Group II are neutral-pH, mineralized Ca-SO4

type waters; and Group III are acid, scar-derived, mineralized Ca-SO4

 type waters. The La Bobita well water is shown 

outside of Group III because it is a mixture of Group I with 

Group III waters. Group I waters are typical of surface and 

alluvial ground waters of the Red River upstream from Bitter 

Creek. They would be typical of water draining rocks with no 

more than propylitic alteration that are found as occasional 

outcrops along the north side of the valley and along areas 

30 Investigation 25—Summary Report
Figure 15. Location of observation wells in Straight Creek catchment and Advanced 

Waste-Water Treatment facilities.

Interpretation of Ground-Water Geochemistry 31

EXPLANATION

S C -9A

S C -8A

S C -7A

Base topography from U.S. Geological Survey Digital Elevation Model, 1:24,000

Base physiography from U.S. Geological Survey Digital Raster Graphic, 1:24,000

Other base features from U.S. Geological Survey Digital Orthophoto Quadrangle, 1:24,000

Universal Transverse Mercater projection, Zone 13

AWW T-1

Ad van ced Waste-Water

Treatm ent Plant F acilities AWW T-2

S C -5B

S C -5A

S C -3B S C -3A

S C -2B

S C -4A

S C -6A

S C -1B

S C -1A

Early observation well

Late observation well

0

0 100 200 METERS

500 1,000 FEET

Topographic contour — Shows altitude of land surface 

 in feet above the National Geodetic Vertical Datum 

 of 1929 (NGVD 29). Contour interval 40 feet. Horizontal 

 coordinate information is referenced to the North 

 American Datum of 1927 (NAD 27)

Advanced Waste-Water Treatment

 (AWWT) Plant Facilities
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight Creek 

drainage water and ground water analyzed in this study. 

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not determined; 

<, less than]

Sample ID

Straight Creek

n=14

SC–1A

n=15

SC–1B

n=14

Median MAD Median MAD Median MAD

pH (standard units) 2.98 0.13 3.64 0.04 6.75 0.06

pH Lab (standard units) 2.70 .06 3.15 .03 7.93 .06

Specific conductance (μS/cm) 3,085 274 2,790 59 3,300 119

Specific conductance Lab (μS/cm) 3,200 74 2,850 44 2,940 59

Temperature (°C) 7.7 5.2 7.5 .9 7.6 1.6

Eh, V .781 .060 .580 .021 .087 .160

DO (mg/L) 5.59 1.1 .3 .1 .5 .2

Constituent, mg/L

Ca 349 55 384 10 503 12

Mg 113 16 123 4 215 7

Na 8.08 1.5 16.7 1.7 66.1 12.3

K .753 .43 1.10 .13 11.2 3.56

SO4 2,030 460 2,100 59 1,850 104

Alkalinity as HCO3 --- --- --- --- 493 29

F 7.71 5.9 9.46 1.5 .740 .2

Cl 2.98 1.7 3.21 1.8 13.9 9.7

SiO2 74.2 15.8 94.7 3.6 25.4 2.1

Al 91.5 15.6 94.1 3.4 .012 .005

Fe 65.0 27.1 28.8 1.5 1

1.85 1.12

Fe(II) 2

.475 2.361 26.9 1.4 11.83 1.13

Li .181 .057 .217 .007 .159 .019

Sr .612 .115 .867 .021 11.0 .297

Ba 2

.002 2.002 .002 0 .008 .001

Mn 2.8 3.4 19.8 .7 6.07 .18

Zn 7.63 1.05 7.17 .25 2

.005 2.178

Pb 3

.012 3--- 3.009 3--- .0012 3---

Ni .727 .107 .730 .016 2.004 2.003

Cu 1.87 .28 .929 .044 <.0005 ---

Cd .039 .007 .038 .009 <.0002 ---

Cr .040 .005 .031 .006 <.0005 ---

Co .325 .046 .326 .015 3

<.0008 ---

Be .026 .004 .025 .003 2.001 20

DOC 1.44 .36 1.20 .1 1

6.5 .3

Sum cations (meq/L) 26.9 27.2 33.5

Sum anions (meq/L) 26.2 28.5 34.1

Speciated CI (%) 2.6 –4.8 –1.7

32 Investigation 25—Summary Report
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight Creek 

drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not determined; 

<, less than]

Sample ID

SC–2B

n=11

S–3A

n=14

SC–3B

n=14

Median MAD Median MAD Median MAD

pH (standard units) 6.44 0.10 3.31 0.06 5.88 0.10

pH Lab (standard units) 7.59 .34 3.41 .02 4.19 .22

Specific conductance (μS/cm) 2,420 104 2,375 67 2,850 89

Specific conductance Lab (μS/cm) 2,300 30 2,275 37 2,770 37

Temperature (°C) 8.3 1.8 7.9 .5 7.8 1.3

Eh, V .267 .049 .765 .016 .343 .046

DO (mg/L) .3 .2 4.8 .3 .6 .2

Constituent, mg/L

Ca 439 7 299 7 481 11

Mg 105 3 91 6 150 5

Na 22.1 1.8 16.5 1.8 33.0 2.5

K 3.39 .28 .79 .07 2.92 .59

SO4 1,480 59 1,600 44 1,920 82

Alkalinity as HCO3 144 26 --- --- 74 17

F 6.14 .61 6.82 .06 6.58 .70

Cl 3.91 .53 3.34 1.56 5.23 2.11

SiO2 1.6 .9 92.8 3.3 18.1 1.2

Al .636 .338 79.3 2.4 5.38 .33

Fe 33.2 .4 .549 .036 58.9 1.6

Fe(II) 33.2 1.1 .027 .012 58.9 1.4

Li .313 .024 .158 .016 .197 .022

Sr 2.32 .074 .334 .019 3.60 .133

Ba .006 .001 .002 0 .005 .001

Mn 17.3 1.2 15.3 .4 27.1 1.2

Zn 1.53 .28 5.44 .19 4.57 .25

Pb 3

.006 3--- <.0003 --- 3.011 3---

Ni .480 .028 .543 .022 .463 .019

Cu 2

.0005 2.0022 .818 .033 3<.0005 ---

Cd 2.001 20 .025 .003 2.002 20

Cr 2.008 20 .020 .002 2.006 2.003

Co .157 .024 .230 .019 .233 .018

Be .021 .003 .017 .001 .023 .004

DOC 1.55 .44 2.00 .30 2.70 .30

Sum cations (meq/L) 24.1 21.8 28.9

Sum anions (meq/L) 24.1 22.0 28.8

Speciated CI (%) .1 –.8 .2

Interpretation of Ground-Water Geochemistry 33
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight 

Creek drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not determined; <, 

less than]

Sample ID

SC–4A

n=15

SC–6A

n=5

S–5A

n=15

Median MAD Median MAD Median MAD

pH (standard units) 3.58 0.18 3.51 0.04 3.44 0.04

pH Lab (standard units) 3.20 .15 3.24 .03 3.45 .03

Specific conductance (μS/cm) 2,450 30 2,670 44 1,630 311

Specific conductance Lab (μS/cm) 2,470 119 2,640 0 1,570 178

Temperature (°C) 8.0 1.2 11.6 2.8 7.8 .7

Eh, V .604 .025 .664 .006 .755 .027

DO (mg/L) 1.7 1.1 .9 .4 .9 .4

Constituent, mg/L

Ca 296 10 365 6 163 25

Mg 114 10 114 10 61.4 13.5

Na 21.7 2.4 17.4 1.6 16.2 2.4

K 1.98 .39 1.11 .13 1.85 .09

SO4 1,690 59 1,880 74 957 62

Alkalinity as HCO3 --- --- --- --- --- ---

F 6.72 .43 8.78 .06 3.24 .68

Cl 5.16 1.24 3.10 .56 5.18 1.31

SiO2 73.6 8.2 93.2 5.2 76.3 3.9

Al 68.0 5.5 85.1 2.7 52.0 4.9

Fe 23.6 15.1 12.1 1.3 .450 .033

Fe(II) 21.2 14.9 1.4 1.5 .016 .013

Li .178 .015 .212 .007 .074 .018

Sr .649 .111 .430 .006 .507 .071

Ba .003 0 .002 0 2

.0008 2.0001

Mn 17.1 1.5 19.0 .3 7.64 1.36

Zn 5.28 .12 6.51 .30 2.67 .36

Pb .0021 3

--- 3.0009 3--- <.0003 ---

Ni .551 .018 .668 .053 .333 .077

Cu .362 .200 .639 .036 .215 .046

Cd .020 .003 .031 .001 .012 .006

Cr .013 .001 .021 .001 .008 .001

Co .234 .016 .277 .037 .136 .042

Be .016 .001 .018 .001 .008 .001

DOC 1.6 .3 1.40 .22 1.25 .67

Sum cations (meq/L) 22.9 25.3 14.2

Sum anions (meq/L) 23.7 25.2 13.8

Speciated CI (%) –3.5 .4 2.8

34 Investigation 25—Summary Report
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight 

Creek drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, 

microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not 

determined; <, less than]

Sample ID

SC–5B

n=15

SC–7A_1

n=4

SC–7A_2

n=4

Median MAD Median MAD Median MAD

pH (standard units) 7.52 0.12 3.90 0.05 3.95 0.04

pH Lab (standard units) 7.94 .13 3.10 .01 3.11 .01

Specific conductance (μS/cm) 2,480 89 1,505 22 1,395 82

Specific conductance Lab (μS/cm) 2,270 59 1,670 52 1,565 59

Temperature (°C) 8.1 .7 7.9 .7 9.5 2.4

Eh, V .014 .124 .566 .010 .556 .008

DO (mg/L) .2 .1 .4 .2 .3 0

Constituent, mg/L

Ca 548 34 187 5 162 2

Mg 37.4 4.9 6.2 1.3 5.1 1.5

Na 62.4 31.4 18.7 2.0 15.6 1.6

K 4.84 1.50 3.02 .11 2.67 .11

SO4 1,400 59 950 27 849 38

Alkalinity as HCO3 226 69 --- --- --- ---

F 1.06 .03 2.80 .37 2.40 .25

Cl 8.63 5.43 3.93 .41 4.24 .47

SiO2 21.5 2.5 58.1 1.3 57.6 .3

Al 2.004 2.004 36.4 1.6 33.3 1.0

Fe 4

.468 4.013 3.3 .6 29.5 .2

Fe(II) 4.462 4.004 3.2 .4 29.2 .5

Li .054 .013 .070 .002 .057 .004

Sr 8.33 .504 1.05 .052 .850 .032

Ba .025 .006 .004 .001 .005 0

Mn 2.68 .44 6.79 .33 5.62 .23

Zn <.005 --- 2.12 .10 1.82 .04

Pb <.0003 --- <.0003 --- <.0003 ---

Ni 2

.003 2.003 .244 .012 .219 .016

Cu <.0005 --- .060 .004 .056 .006

Cd <.0002 --- .007 0 .006 .001

Cr <.0005 --- .005 0 .004 0

Co <.0008 --- .100 .006 .092 .011

Be <.001 --- .006 .001 .005 .001

DOC 4

2.0 --- 1.31 .52 1.10 .44

Sum cations (meq/L) 24.6 14.9 13.0

Sum anions (meq/L) 24.2 14.1 12.7

Speciated CI (%) 1.7 5.6 1.8

Interpretation of Ground-Water Geochemistry 35
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight 

Creek drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, 

microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not 

determined; <, less than]

Sample ID

SC–7A_3

n=4

SC–7A_4

n=4

AWWT1

n=14

Median MAD Median MAD Median MAD

pH (standard units) 3.89 0.08 3.91 0.04 3.86 0.02

pH Lab (standard units) 3.11 .02 3.12 .01 3.07 .04

Specific conductance (μS/cm) 1,370 52 1,345 52 1,470 22

Specific conductance Lab (μS/cm) 1,480 22 1,495 30 1,630 30

Temperature (°C) 1.5 4.2 1.4 4.4 9.9 1.0

Eh, V .561 .004 .560 .015 .555 .052

DO (mg/L) .3 .1 .3 .1 .4 .2

Constituent, mg/L

Ca 151 3 150 8 158 3

Mg 5.7 .8 48.8 1.8 53.6 2.3

Na 15.5 1.3 17.3 2.2 16.6 .7

K 2.82 .22 2.83 .26 2.77 .25

SO4 821 39 815 26 893 23

Alkalinity as HCO3 --- --- --- --- --- ---

F 2.55 .15 2.55 .07 2.60 .16

Cl 4.33 .51 4.44 .42 5.56 1.16

SiO2 58.8 .9 56.7 1.9 62.4 1.7

Al 31.4 2.0 31.0 1.4 36.3 .7

Fe 27.2 .6 26.9 1.2 33.4 .9

Fe(II) 26.8 .3 26.7 .4 33.3 1.0

Li .056 .003 .056 .002 .059 .002

Sr .766 .023 .754 .056 .758 .024

Ba .005 0 .005 .001 .005 0

Mn 5.30 .19 5.22 .44 5.90 .15

Zn 1.82 .05 1.85 .02 2.06 .04

Pb <.0003 --- <.0003 --- 3

.0003 3---

Ni .199 .022 .211 .021 .265 .026

Cu .065 .012 .067 .013 <.0005 ---

Cd .006 .001 .005 .001 .006 0

Cr .004 0 .004 0 .082 .007

Co .082 .004 .087 .005 .102 .010

Be .005 0 .005 .001 .005 .001

DOC 1.15 .36 1.05 .30 1.25 .37

Sum cations (meq/L) 12.4 12.3 13.3

Sum anions (meq/L) 12.4 12.4 13.4

Speciated CI (%) .3 –.4 –.6

36 Investigation 25—Summary Report
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight 

Creek drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, 

microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not determined; <, less than]

Sample ID AWWT2

n=1

SC–8A

n=5

HANSEN

n=4

Median MAD Median MAD

pH (standard units) 6.78 6.63 0.01 3.79 0.07

pH Lab (standard units) 7.93 8.02 .06 3.75 0

Specific conductance (μS/cm) 3,110 384 24 2,795 15

Specific conductance Lab (μS/cm) 2,860 365 13 2,700 15

Temperature (°C) 8.7 6.9 .9 8.4 .4

Eh, V .351 .524 .154 .731 .012

DO (mg/L) .54 6.1 .7 5.2 .3

Constituent, mg/L

Ca 406 52 4 457 7

Mg 266 11 1 98 4

Na 50.0 5.7 .4 16.3 2.7

K 4.74 1.01 .12 3.80 .34

SO4 1,830 119 9 2,035 7

Alkalinity as HCO3 353 64 1 --- ---

F 4.07 .26 .01 3.60 .10

Cl 5.03 2.93 .10 1.87 .40

SiO2 21.0 12.9 .7 55.4 1.3

Al .0068 .007 .002 8.6 3.4

Fe .101 2

.001 20 .064 .011

Fe(II) .076 <.002 --- 2

.009 2.006

Li .177 .004 .001 .091 .004

Sr 7.38 .31 .043 2.82 .178

Ba .006 .022 .003 .003 0

Mn 5.7 .002 3

--- 11.9 1.0

Zn .528 2

.037 2.024 2.73 .13

Pb <.008 <.0003 --- 3

.0009 3---

Ni .211 .004 0 .584 .019

Cu <.0005 <.0005 --- .114 .007

Cd <.001 2

.0002 2.0046 .006 0

Cr <.0005 <.0005 --- .003 0

Co .115 <.0008 --- .177 .016

Be .005 <.001 --- .016 .001

DOC 2.00 .70 .15 .70 .18

Sum cations (meq/L) 32.1 3.4 26.3

Sum anions (meq/L) 31.4 3.3 27.4

Speciated CI (%) 2.0 4.1 –4.2

Interpretation of Ground-Water Geochemistry 37
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight 

Creek drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not determined; 

<, less than]

Sample ID

Hottentot

n=5

La Bobita

n=4

CC–2A

n=5

Median MAD Median MAD Median MAD

pH (standard units) 2.99 .01 4.35 0.22 6.05 0.13

pH Lab (standard units) 2.59 .01 4.24 .00 4.94 .18

Specific conductance (μS/cm) 1,670 59 900 0 1,520 15

Specific conductance Lab (μS/cm) 1,930 44 856 9 1,400 59

Temperature (°C) 7.5 .0 8.6 3.0 16.0 .9

Eh, V .630 .006 .578 .067 .348 .110

DO (mg/L) .2 .1 3.7 .4 5.1 .9

Constituent, mg/L

Ca 86.9 2.2 111 4 225 38

Mg 39.9 3.0 32.1 1.0 13.2 2.4

Na 8.14 .56 13.0 .4 34.5 6.8

K 2.06 .18 1.81 .39 6.87 1.02

SO4 913 13 467 2 816 50

Alkalinity as HCO3 --- --- --- --- 64 10

F 4.10 .06 2.65 .22 19.07 1.45

Cl 1.81 .24 5.00 .73 2.98 .28

SiO2 92.1 1.3 33.4 .4 26.7 2.8

Al 62.3 4.7 12.3 .7 5.42 .13

Fe 87.7 2.5 .037 .020 33.6 4.0

Fe(II) 85.6 2.5 .010 .010 33.6 4.0

Li .074 .016 .034 .005 .039 .004

Sr 2

.010 2.006 .814 .012 .825 .147

Ba 2

.001C 20 .022 .002 .005 .001

Mn 7.99 .77 2.67 .10 4.9 9.6

Zn 3.70 .31 .96 .06 3.96 2.22

Pb <.0003 --- .0009 0 3

.0007 3---

Ni .345 .009 .133 .002 2

.028 2.005

Cu .118 .006 .031 .002 2

.0005 2.0004

Cd .005 0 .004 0 .003 0

Cr .012 0 .006 .007 2

.0007 2.0003

Co .123 .016 .019 .001 .016 .004

Be .011 0 .006 .001 .078 .009

DOC 1.10 0 1.25 .36 1.35 .37

Sum cations (meq/L) 13.7 8.1 12.8

Sum anions (meq/L) 13.0 7.8 14.5

Speciated CI (%) 5.5 4.3 –12

38 Investigation 25—Summary Report
Table 5. Median values for pH, temperature, specific conductance (SC), redox potential, dissolved oxygen (DO), dissolved solute 

concentrations, dissolved organic carbon (DOC), cation- and anion-equivalent sum, and speciated charge imbalance for Straight 

Creek drainage water and ground water analyzed in this study.—Continued

[ID, identifier; CI, charge imbalance; D.L., detection limit; MAD, median absolute deviation; meq/L, milliequivalents per liter; Lab, laboratory; μS/cm, 

microsiemens per centimeter; mg/L, milligrams per liter; n, number of analyses; V, volts; °C, degrees Celsius; Eh, redox potential; %, percent; ---, not determined; <, less than]

Sample ID

CC–2B

n=4

CC–1B

n=4 Ranger station

n=1

Median MAD Median MAD

pH (standard units) 6.91 0.11 7.16 0.16 6.50

pH Lab (standard units) 7.82 .04 8.06 .24 ---

Specific conductance (μS/cm) 1,770 30 640 30 490

Specific conductance Lab (μS/cm) 1,525 7 464 35 ---

Temperature (°C) 8.7 .7 9.7 3.1 8.8

Eh, V .329 .099 .449 .046 .450

DO (mg/L) 1.0 .9 .3 .2 5.55

Constituent, mg/L

Ca 376 19 96 2 71.8

Mg 21 1 13 0 15

Na 4.8 5.4 23.0 1.6 8.5

K 4.56 .27 1.90 .16 1.32

SO4 852 13 119 7 223

Alkalinity as HCO3 303 3 242 4 45.8

F 2.00 .02 1.35 .23 .92

Cl 2.59 .16 6.24 .53 5.00

SiO2 23.2 .2 14.2 .7 17.6

Al .011 .004 .005 .001 .087

Fe 1.33 1.01 2

.032 2.013 .142

Fe(II) 1.26 1.01 2

.027 2.017 .135

Li .070 .010 .023 .001 .004

Sr 4.73 .059 1.27 .082 .509

Ba .013 .001 .026 .003 .048

Mn 5.01 .38 .219 .028 .014

Zn 3

--- 3--- 3--- 3--- .035

Pb <.0003 --- <.0003 --- <.0003

Ni 2

.002 2.001 2.002 2.002 .005

Cu <.0005 --- 2.0007 2.0004 .0010

Cd <.0002 --- <.0002 --- .0002

Cr 2

.0007 2.0002 <.0005 --- <.0005

Co 2.002 20 <.0008 --- <.0007

Be <.001 --- <.001 --- <.001

DOC 1.20 .27 2.15 .07 .87

Sum cations (meq/L) 17.7 6.48 4.55

Sum anions (meq/L) 17.9 6.20 4.91

Speciated CI (%) –.9 4.5 –7.6

1

Last four sampling trips used.

2

Values below D.L. set equal to D.L. 

3

Highly variable results.

4

Last two sampling trips used.

Interpretation of Ground-Water Geochemistry 39
of the south side. Indeed these waters are typical of shallow 

recharge water in many geologic settings.

The common occurrence of scar water with pH averaging 

close to 3.0 can be explained as water that has developed from 

pyrite oxidation with relatively small amounts of precipitated 

hydrous ferric oxides, gypsum dissolution, and minor amounts 

of aluminosilicate dissolution. Figure 18 shows a simulation of 

pyrite oxidation alone (Nordstrom, 2004) using the computer 

code PHREEQC (Parkhurst and Appelo, 1999). The pH of 

the resultant solution is plotted as a function of the amount of 

pyrite oxidized under four scenarios: (1) no oxidation of the 

resultant acid iron-sulfate solution, (2) oxidation to an acid 

ferric-sulfate solution but no precipitation of hydrous ferric 

oxides, (3) oxidation with precipitation of ferrihydrite, and 

(4) oxidation with precipitation of goethite. The important 

point about this diagram is to notice the crossover points at 

pH values of 3.3 and 2.4 because they represent inflection 

points or regions of buffering capacity. That is, if only pyrite is 

weathered there will be a tendency for the resultant solutions 

to reach pH values of 2.4–3.3 depending on how much iron is 

oxidized and precipitated. The median pH value of 3 for scar 

leachates lies in between these values although more toward 

the value of 3.3, which reflects oxidation but little or no precipitation of hydrous ferric oxides (HFO). Our observations of 

scar-water chemistry also show that these waters contain fully 

oxidized iron, but precipitated HFO seems to be minimal. If 

aluminosilicate dissolution was substantial, then the pH would 

be different. In scars, the mineral assemblage is primarily QSP. 

The quartz is insoluble, and sericite (or illite) does not weather 

substantially (as evident in the low concentrations of potassium). Gypsum dissolution also has no significant effect on 

these pH values. The simulation indicates that we understand 

the main compositional features of water derived from natural 

scar weathering.

Geochemical Mass Balance on Straight Creek 

Drainage

A simple mass-balance calculation to estimate the amount 

of minerals dissolved and precipitated to produce the Straight 

Creek scar-drainage water is derived by using the inverse modeling method of Plummer and others (1983), also described 

by Parkhurst (1997), Bricker and others (2004), Nordstrom 

(2004), and Bowser and Jones (2002). Assuming the initial 

water is rainwater, which can be approximated by pure water, 

and the final water is the median water-chemistry analysis for 

Straight Creek drainage water from table 5, and using idealized mineral compositions based on petrography and X-ray 

diffractometry (Plumlee and others, 2005), the results are 

two models shown in table 6. For example, for the dolomite 

model, dolomite or magnesian calcite could be substituted for 

chlorite as a source of magnesium, but the amount of dolomite 

that is required (4.64 mmol/kgH2O) seems unreasonable in this 

environment because dolomite is not abundant at Straight 

Creek. Chlorite is abundant, however (Ludington and others, 

2005; Plumlee and others, written commun., 2004), and the 

mass balance in the dolomite model has more dolomite dissolving than gypsum, which does not seem reasonable. From 

these two models it is clear that gypsum and pyrite dissolution 

dominate the weathering reactions. Dissolution of chlorite, 

kaolinite, and oligoclase will raise the pH slightly from 

that predicted in figure 18. This neutralization capacity is overcome by the greater amount of pyrite oxidized in the chlorite model shown in table 6 (6.2 mmol/kgH2O) compared to 

the range indicated by figure 18 (0.5 – 3 mmol/kgH2O). Using 

dolomite for a magnesium source instead of chlorite potentially develops more neutralization capacity, but the mass balance requires more pyrite to be oxidized with the resultant pH 

about the same. Unfortunately, these two models are mutually 

exclusive, whereas magnesium probably is derived from both 

sources. Laboratory and field data demonstrate that calcite 

and dolomite dissolution rates (Brown and Glynn, 2003) are at 

least a couple of orders of magnitude faster than chlorite dissolution rates (Brandt and others, 2003) so that a dolomite model 

would be preferred.

Figure 16. (A) Frequency distribution plot of pH for scar-drainage water in the Red River Valley and (B) frequency distribution 

plot of sulfate concentrations for scar-drainage water in the 

Red River Valley (excluding Goat Hill water).

40 Investigation 25—Summary Report

1 2 3 4 5 6 7 0

5

10

15

20

25

30

35

 n = 121

 Mean = 2.97

 Median = 2.93

 Standard deviation = 0.36

Frequency distribution

pH, in standard units

Gaussian distribution

(A)

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

0

5

10

15

20

25

30

35

Gaussian distribution

Sulfate concentration, in milligrams per liter

n = 114

Mean = 2,014 

Median = 2,000

Standard deviation = 862

(B)
The water chemistry evolves according to the presumed 

flow paths shown in figure 19 with the largest mass transfer, 

as noted by Robertson Geoconsultants, Inc. (2001b) and Shaw 

and others (2003), in the scar area. Several observations have 

shown that the Straight Creek surface water is the main input 

flow and source of constituents that appear in both the alluvial 

and bedrock ground waters as described by Shaw and others 

(2003). The Straight Creek drainage sinks into the debris-flow 

fan almost all year round (the only exception is during the rare 

intense monsoon rainstorm when the overland flow actually 

reaches the Red River). The median chemistry of Straight 

Creek surface water is nearly identical to the median chemistry 

for water in the most upgradient well (SC–1A). Water levels 

in SC–1A fluctuate with the change in discharge of Straight 

Creek, and no other obvious sources are visible. The main 

change between Straight Creek water and SC–1A water is 

iron reduction along with some copper and chromium removal 

(also probably by reduction) and some gain in silica.

Water in well SC–6A is closer in chemical composition 

to those in SC–1A than water in SC–3A and SC–4A. Furthermore, the ground-water ages from youngest to oldest are 

SC–1A to SC–6A to SC–5A to AWWT1 and SC–7A (Naus 

and others, 2005; Nordstrom and others, 2005). Water from 

SC–3A and SC–4A is apparently much younger than any 

of these. These results indicate mixing with younger water. 

Indeed, water from SC–4A appears to be SC–3A water with a 

small amount of admixed SC–3B water that produces a greater 

age (than SC–3A) and an increase in solute concentrations. 

This unexpected result indicates how bedrock aquifers and 

debris-flow aquifers can mix to some extent even though their 

permeabilities appear to be quite different. Water from SC–8A 

is not shown on the diagram, and it appears to be Red River 

alluvial aquifer water unaffected by acidic scar-drainage water. 

From the reported data, there appears to be little or no hydraulic connection between SC–7A and SC–8A.

Water chemistry from the La Bobita well has much in 

common with the water chemistry at Hansen and Straight 

Creek debris-fan ground water; its composition is consistent 

with a diluted version of these waters from mixing with some 

Red River alluvial water and with removal of some iron, silica, 

and aluminum. This result is consistent with the tracer study 

that showed an increase in solute concentrations and loads in 

this reach of the Red River near La Bobita.

Figure 17. Plot of pH against median sulfate concentration for all ground waters and Straight 

Creek surface water. Group I are dilute, nonmineralized, Ca-HCO3 type waters; Group II are mineralized, carbonate-buffered, Ca-SO4

 type waters; and Group III are acid, scar-derived, Ca-SO4 type 

waters.

Geochemical Controls on Solute Concentrations 41

0 500 1,000 1,500 2,000 2,500 3,000

1

2

3

4

5

6

7

8

9

Group III

Group II

Straight Creek , SC1A , SC1B , SC2B, SC3A, SC3B, SC4A ,

SC6A , SC5A , SC5B , AWWT1, AWWT2 , SC7A , SC8A ,

CC1A , CC1B, CC2A , CC2B, Hansen , Hottentot , La Bobita ,

Ranger station 

pH, in standard units

Sulfate concentration, in milligrams per liter

Group I

EXPLANATION
Geochemical Controls on Solute 

Concentrations

Trends in Specific Conductance

Specific conductance correlates well with sulfate concentration for most surface and ground waters within the Red 

River Valley because the waters are dominated by dissolution 

of gypsum and pyrite, and sulfate is the dominant anion almost 

everywhere. In terms of equivalents per liter, sulfate is 50 to 

99 percent of the anion equivalents for all samples, and sulfate 

approaches 50 percent for the most dilute, upstream samples in 

the Red River with pH values around 8. The correlation of specific conductance with sulfate concentrations for water in the 

Straight Creek catchment is shown in figure 20A, and other 

ground waters sampled in this study are shown in figure 20B. 

For Straight Creek surface and alluvial ground waters, the 

equation of best linear fit is:

Specific conductance (microsiemens per centimeter) = 

1.197(SO4

, milligrams per liter) + 391.

The linear fit is a reasonable approximation in the range 

of about 400 to 3,000 mg/L. The linear fit is not a good fit 

at very low or very high sulfate concentration. As higher 

concentrations are reached, the extent of ion pairing increases, 

which effectively decreases the rate of increase of specific 

Figure 18. Decrease in pH with the incremental oxidation of pyrite to (a) an acid ferrous sulfate 

solution with no dissolved iron oxidation, (b) oxidation of the dissolved iron sulfate solution but no 

precipitation of hydrous ferric oxides, and (c) oxidation with precipitation of ferrihydrite. 

Table 6. Weathering mass balances of minerals that can 

account for the median chemistry of Straight Creek scardrainage water.

[--, not available]

Mineral phase 

at Straight Creek 

(idealized chemical formula in 

parentheses)

Millimoles of mineral 

transferred per kilogram 

of water; positive = mass 

mineral dissolved,

negative = mass mineral 

precipitated

Chlorite 

model

Dolomite 

model

Gypsum (CaSO4

•2H2O) 8.51 3.78

Pyrite (FeS2

) 6.21 8.66

Chlorite (Mg5

Al2Si3O10[OH]8) .93 --

Dolomite (CaMg[CO3

]

2

) -- 4.64

Kaolinite (Al2

Si2O5[OH]4) .47 1.40

Oligoclase (Na0.8Ca0.2Al1.2Si2.8O8

) .44 .44

Fluorite (CaF2

) .20 .20

Sphalerite (ZnS) .11 .11

Illite (K0.6Mg0.25Al2.3Si3.5O10[OH]2

) .032 .032

Chalcopyrite (CuFeS2

) .029 .029

Goethite (FeOOH) –5.08 –7.40

Silica (SiO2

) –3.82 –2.89

42 Investigation 25—Summary Report

(a) Pyrite oxidized, no Fe(II) oxidized

Crossover pH = 3.26

10-2

1

2

pH, in standard units 3

4

10-1 10

Pyrite oxidized, in millimoles per kilogram of water

102 103 1

Crossover pH = 2.39

(b) Pyrite oxidized, FE(II) oxidized

(c) Pyrite oxidized, FE(II) oxidized

 and precipitated
Figure 19. Evolutionary flow paths based on changes in ground-water chemistry with downstream gradient. Arrows point in the direct

gradient and more dilute water. Thicker arrows indicate higher solute concentrations. The double-headed arrows shown for interactionand SC–3B and SC–4A and SC–3A reflect the fact that bedrock water chemistry of SC-3B influences the chemistry of SC–4A but to a lesthat from SC–3A.

Rainwater

S C -1B 

AWWT 2

SC-5A

Straight CreekScar Drainage 

SC-1A SC-6A

SC-3A

SC-3B

 SC-5B

SC-4A

 ?

 ?

 ?
conductance per unit increase in sulfate concentration. Acid 

sulfate water of even higher sulfate concentration than shown 

here clearly shows a tendency to plateau with increasing concentration. However, an analysis of this refinement is beyond 

the scope of this study.

A more important trend is the bias of other ground-water 

samples shown in figure 20B toward the upper confidence 

limit. This bias is caused by ground waters, such as SC–1B 

and SC–5B, which have circumneutral pH and high alkalinity 

values. These ground waters have higher specific conductance 

44 Investigation 25—Summary Report

Figure 20. Plot of specific conductance against sulfate concentrations for (A) Straight Creek 

surface and alluvial ground waters and (B) median values for ground waters and Straight Creek 

surface water.

0

1,000

2,000

3,000

4,000

0 500 1,000 1,500 2,000 2,500 3,000

0

1,000

2,000

3,000

4,000

R2= 0.97

 Linear fit, 

Straight Creek Upper and lower 95-percent prediction limit

, , 

SC1A

, 

SC1B

, 

SC2B

, 

SC3A , , SC3B SC4A

,

, , , , , ,

SC6A

, 

SC5A

, 

SC5B

, 

AWWT1

, 

AWWT2

, 

SC7A

, 

SC8A

CC1A CC1B CC2A CC2B Hansen Hottentot La Bobita,

Ranger station 

Sulfate concentration, in milligrams per liter

EXPLANATION

Specific conductance, in microsiemens per centimeter

(A)

(B)
than the best linear fit to the sulfate-dominated waters in 

figure 18(a) because dissolved inorganic carbon is contributing 

substantially to the anion equivalents and to the conductivity 

of these solutions. Nevertheless, all samples except those from 

SC–1B, SC–5B, and CC–2B are at or within the prediction 

limits that express a 95-percent confidence for the variation 

in the data from the best fit. Because dilute waters may have 

a substantially higher proportion of free ions than the highsulfate ground waters, this correlation should not be used for 

sulfate concentrations less than about 400 mg/L such as those 

for the Red River.

The correlation of specific conductance with sulfate 

concentrations for the Red River surface water (McCleskey 

and others, 2004; Verplanck and others, written commun., 

2004) has been obtained separately from Red River ground 

water and is shown in figure 21. The correlation coefficient of 

0.97 demonstrates how well the specific conductance responds 

to changes in sulfate concentration and can be used to predict 

the sulfate concentration for dilute water samples. The best fit 

equation for this line is:

Specific conductance (microsiemens per centimeter) = 

1.46(SO4

, milligrams per liter) + 179, 

which should give an estimate of the sulfate concentration 

from measured conductivity within about ±20 percent at the 

95-percent confidence level.

Geochemical Controls on Dissolved Sulfate 

Concentrations

There are only two substantial sources of dissolved 

sulfate in surface and ground waters of the Red River Valley— 

gypsum dissolution and pyrite oxidation. If both the gypsum and the pyrite were formed under similar hydrothermal 

conditions, then these sources could be differentiated based on 

stable isotope compositions of the dissolved sulfate (Seal and 

others, 2000; Seal, 2003). Unfortunately, the gypsum is secondary; that is, it has formed from the weathering of pyrite and 

calcite (Ludington and others, 2005). Under these conditions 

gypsum inherits nearly the same Σ34S value as the pyrite. The 

Σ18O composition of the gypsum will also be the same as that 

in sulfate derived from pyrite oxidation so that stable sulfate 

isotope data of dissolved sulfate does not distinguish between 

these sources. However, chemical data can differentiate these 

sulfate sources, and it was used to do that in the mass-balance 

calculations.

The dissolved sulfate concentration in Red River Valley 

ground water should be dependent on the residence time of 

the ground water as well as on the source of the sulfate. The 

longer the water is in contact with gypsum and the longer that 

water and oxygen contact pyrite surfaces, the higher the concentration of sulfate should be if there were no other processes 

(up to the solubility limit). As a consequence, one might 

predict that there would be higher sulfate concentrations the 

Geochemical Controls on Solute Concentrations 45

Figure 21. Correlation between specific conductance and sulfate concentration for Red River 

surface-water samples from this study.

0

200

400

600

800

0 100 200 300 400 500

 Linear fit

Upper and lower 95-percent 

prediction limit

Sulfate concentration, in milligrams per liter

R2

= 0.97

Specific conductance, in microsiemens per centimeter
longer ground water has resided in a debris fan derived from a 

scar, but this prediction would be incorrect. As demonstrated 

in the Straight Creek debris-fan ground water, there is uniform 

dilution from the uppermost well in the fan to the lowermost 

well (Naus and others, 2005). Sulfate concentrations in water 

from well SC–1A are more than 2,000 mg/L and decrease 

to about 800 mg/L in the lower portion of well SC–7A, less 

than one-half of the original input concentration. This trend is 

shown in figure 22 in which the sulfate concentrations of the 

Straight Creek alluvial ground waters are plotted relative to the 

elevation of the bottom of the screened well. The main alluvial 

ground-water flow follows the contact between the bedrock 

and the alluvium. This “bedrock surface” has an identifiable channel, as shown from seismic studies, and would be 

expected to carry the major portion of the alluvial flow. That 

channel shows that well SC–6A is located within the deeper 

portion of the channel, whereas wells SC–3A and SC–4A are 

not. Wells SC–3A and SC–4A are in a perched bedrock position relative to the main channel, and consequently, their water 

tables are perched with respect to that in well SC–6A. The 

positions of wells SC–3A and SC–4A explain why their solute 

concentrations are less than those in well SC–6A even though 

they are close together and at the same ground-surface elevation. They are in a perched position to the side of the drainage 

where they would receive more dilution from dilute-water 

seepage along the side of the canyon, whereas well SC–6A 

receives more direct flow from water in the scar area and by 

water in well S–1A without as much dilution. By the time 

ground water reaches wells SC–5A and SC–7A, it is diluted 

with Red River alluvial ground water. Regardless of the source 

of diluting water, continual dilution is apparent all the way to 

the Red River alluvium. Samples from SC–8A appear to have 

little or no acid water. It is likely to be representative of uncontaminated Red River ground water. These flow paths have 

been shown schematically with the use of arrows in figure 22.

Similar trends are apparent in the other debris fans receiving scar-drainage water (Naus and others, 2005; Nordstrom 

and others, 2005). The source of this dilution appears to be 

more dilute ground water seeping in from the sides of the 

drainage. Such seepage would be expected based on the steep 

topography of the canyon. Dilution from the east tributary of 

Straight Creek and from vertical infiltration during snowmelt 

and floods might also contribute, but this dilution is not likely 

to be the main source of dilution because snowmelt and floods 

46 Investigation 25—Summary Report

Figure 22. Relation between sulfate concentrations of Straight Creek well waters and elevation of the bottom of 

the screened interval for each well.

2,500

2,550

2,600

2,650

2,700

2,750

500 1,000 1,500 2,000 2,500

SC1A , SC4A , SC6A , , SC3A

SC5A , SC7A

Sulfate concentration, in milligrams per liter

Elevation of the bottom of well screen, in meters 

EXPLANATION

General flow direction of ground water
are more episodic and infrequent events compared to side 

seepage. For this reason, key constituent concentrations are 

plotted against sulfate concentrations in subsequent sections of 

this report to reveal the extent to which they change either by 

dilution (conservative and nonreacting) or by reaction (nonconservative). Hydrogeochemistry cannot predict the exact 

flow path of ground water, nor the exact minerals along that 

flow path, nor the exact dissolution rate for those conditions 

in the aquifers of the Red River Valley. Hence, empirical data 

(direct measurements) on ground-water chemistry and weathering conditions are used for those conditions that are not 

defined by mineral solubility limits. It is not strictly empirical 

because the main minerals that weather to form the groundwater composition and their relative weathering rates are 

known. Also, the change in mineral abundance with location 

and trace-element content of those minerals is known.

The regular dilution trends found for specific conductance relative to sulfate concentrations in Straight Creek 

surface and ground waters (figs. 20 and 22) were so linear 

that it was useful to plot every inorganic solute concentration 

relative to sulfate concentration. These linear correlations 

served several purposes: (1) They distinguished conservative 

constituents (those undergoing dilution only) from nonconservative, or reactive, constituents (those undergoing mineral 

precipitation/dissolution or redox reactions), (2) they identified 

constituents added or removed from the Straight Creek surface 

drainage upon entering the debris-fan aquifer, and (3) they 

provided a framework for a comparison of solutes in other 

catchments and, ultimately, a reference framework for estimating pre-mining ground-water solute concentrations at the mine 

site. The linear equations and their correlation coefficients are 

in table 1–2 of Appendix 1.

Geochemical Controls on Dissolved Iron 

Concentrations

Dissolved iron concentrations, particularly in surface 

and ground waters of the Red River Valley, are complicated 

functions of both pH and redox (reduction-oxidation) chemistry. Water of low pH usually contains high iron concentration, 

but iron can precipitate from solution if it is oxidized and the 

pH is about 2.5 or higher, thus lowering the concentration. 

The primary source of dissolved iron in the Red River Valley 

Geochemical Controls on Solute Concentrations 47

Figure 23. (A) Iron concentrations plotted against sulfate concentrations for all ground waters and Straight Creek surface 

water. (B) Ferrous iron:total dissolved iron molar ratio plotted against sulfate concentrations for all ground waters and 

Straight Creek surface water.

0 500 1,000 1,500 2,000 2,500 3,000

0

0.25

0.50

0.75

1.00

0 0 500 1,000 1,500 2,000 2,500 3,000

0.1

1.0

10

100

1,000

Ferrous iron : total dissolved iron ratio

(B)

Straight Creek , SC1A, SC1B, SC2B, SC3A, SC3B, SC4A, SC6A,

SC5A, SC5B, AWWT1, AWWT2, SC7A, SC8A, CC1A, CC1B

CC2A, CC2B, Hansen, Hottentot, La Bobita, Ranger station 

Iron concentration, in milligrams per liter

(A)

EXPLANATION

Sulfate concentration, in milligrams per liter
is from the oxidation of pyrite. Hence, the concentrations of 

iron in surface and ground waters depend on (1) the amount 

of pyrite oxidized, (2) the extent to which the dissolved iron 

is oxidized, (3) any mineral-precipitation reactions that can 

provide an upper limit to iron concentrations, and (4) any 

changes in pH caused by other reactions such as carbonate dissolution or feldspar dissolution. As shown in figure 23A, iron 

concentrations cover a large range of values (less than detection to 300 mg/L) and do not reflect a predictable correlation 

with sulfate concentrations although, qualitatively, the iron 

concentrations do decrease generally with decreasing sulfate 

concentration.

The iron concentrations for well SC–1A average slightly 

less than those of Straight Creek, but the sulfate concentrations 

average about the same. Hence, Straight Creek drainage water 

can be the input for iron mass flux to the alluvial ground water 

if some iron is precipitated. Iron precipitation would seem reasonable because most of the dissolved iron in Straight Creek is 

already oxidized (see fig. 23B), and the larger mineral surface 

area in the debris fan should encourage precipitation.

Figure 23B shows the dissolved Fe(II)/Fe(total) redox 

ratio discriminates the ground water into primarily two groups, 

those that are largely reduced (SC–1A, SC–1B, SC–2B, 

SC–3B, SC–4A, SC–5B, SC–7A, AWWT1, CC–1B, CC–2A, 

CC–2B, and Hottentot) and those that are largely oxidized 

(Straight Creek drainage, SC–3A, and SC–5A). Very few 

waters sampled have intermediate Fe(II/III) redox ratios (La 

Bobita, CC–1A, AWWT2, and Hansen).

Ferric Iron Concentrations

Figure 24A shows that when ground-water concentrations of ferric iron are plotted against pH there is a substantial 

decrease in concentration by two orders of magnitude as the 

pH increases from 2.5 to 4. This decrease is far greater than 

that afforded by ground-water dilution alone and is strong 

evidence for precipitation of iron minerals.

The most generally applicable control on the upper limit 

of iron concentrations in oxic waters is HFO precipitation, 

represented by the formula Fe(OH)3

. Although the mineral 

that precipitates is not a single phase but often a mixture of 

schwertmannite, ferrihydrite, and microcrystalline goethite, 

all with slightly different stoichiometries from Fe(OH)3

, it is 

more straightforward to model it as Fe(OH)3

 or ferrihydrite 

precipitation. Determining the concentration of dissolved, 

oxidized iron (ferric iron) is challenging because precipitated 

HFO particles are commonly colloidal in size and are not easily filtered with 0.45 or even 0.1 μm pore size. Colloidal ferric 

iron particles then become included as part of the dissolved 

ferric iron concentrations, biasing the results. Consequently, 

this study compared the measured redox potential, or Eh, with 

the calculated Eh because the measured Eh will respond only 

to the truly electroactive, and, thus, dissolved activities of 

ferrous and ferric iron. Where the comparisons were in good 

agreement, the concentrations were then used to calculate 

the saturation indices (SI) for HFO (Nordstrom and others, 

2005). The results plotted in figure 24B show the SI for HFO 

relative to pH for all the samples that met the Eh comparison criteria. This figure indicates that the ferrihydrite (HFO) 

saturation does provide an upper limit for iron concentrations 

in oxidized water. Figure 24C shows that as the dissolved iron 

concentrations increase there is a consistent plateau of the 

ferrihydrite saturation indices, independent of iron concentrations. At low pH, jarosite saturation is reached (fig. 25) and 

may provide a control on ferric iron concentrations.

Ferrous Iron Concentrations

Anoxic waters can maintain high concentrations of ferrous iron at any pH value, but a limit would be reached by 

siderite saturation in the presence of carbonate buffering and at 

neutral pH. Figure 26A shows that as alkalinity concentrations increase, the concentrations of ferrous iron decrease, 

an expected common-ion effect if siderite solubility limits 

ferrous iron concentrations. Figures 26B and 24C show the 

SI values for siderite relative to pH and to calcium concentrations. As with all carbonates, equilibrium solubility requires 

near-neutral pH values. A trend that will be recurring with the 

carbonate equilibrium mineral solubilities and demonstrated 

in figure 26C is that a uniform approach to saturation is seen 

with increasing calcium concentration. Siderite has not been 

found as a primary hydrothermal mineral. The preferred 

hypothesis is that with continued dissolution of known carbonates, primarily calcite and possibly some ankerite, the high 

ferrous iron concentrations eventually reach siderite saturation 

and begin to precipitate as thin coatings on carbonate minerals. 

Within reasonable uncertainties, siderite appears to provide an 

upper limit for ferrous iron concentrations.

Consequently, iron concentrations in ground water can 

vary considerably depending on the amount of pyrite weathering and the amount of dissolved ferrous iron oxidation. 

Surface water originating in scar areas contains high concentrations of iron that are rapidly oxidized, but most ground 

water contains predominantly reduced iron, depending on 

proximity to the water table. Some alluvial ground waters are 

well aerated and contain nearly all ferric iron. The solubility 

of HFO provides an upper limit to ferric iron concentrations 

at moderately acidic to neutral pH values (3.5–8), jarosite 

provides an upper limit to ferric iron concentrations at low pH 

(1–3), and siderite solubility provides an upper limit to ferrous 

iron concentrations at circumneutral pH.

Geochemical Controls on Dissolved Manganese 

Concentrations

Dissolved manganese concentrations behave somewhat 

similarly to dissolved iron concentrations in that manganese 

occurs in a more-soluble reduced form, manganous ion, and 

a less-soluble oxidized form, manganic ion. Manganese, 

however, is more easily reduced in anoxic water than iron and 

is generally more mobile. Hence, manganese concentrations 

48 Investigation 25—Summary Report
Geochemical Controls on Solute Concentrations 49

Figure 24. (A) Ferric iron concentrations plotted against pH for all ground waters and Straight Creek surface water. 

Ferrihydrite saturation index plotted against (B) pH and (C) iron for all ground waters and Straight Creek surface water 

fulfilling the Eh measurement requirements described in Naus and others (2005) and in Nordstrom and others (2005). The 

logKsp = 4.9 and 3.0 represent the upper and lower bounds, respectively, on the solubility of HFO (Nordstrom and others, 

1990).

2 3 4 5 6 7 -7

-6

-5

-4

-3

-2

-1

0

1

2

0 50 100 150 200 250

-7

-6

-5

-4

-3

-2

-1

0

1

2

2 3 4 5 6 7 8 0.001

0.01

0.1

1.0

10

100

1,000

logKsp = -1.0

logKsp= 3.0

Ferrihydrite saturation index

pH, in standard units

logKsp = 4.9

LogKsp = -1.0

LogKsp = 3.0

LogKsp = 4.9

Ferric iron concentration, in milligrams per liter

Iron concentration, in milligrams per liter

(B) (C)

(A)
show a good correlation with sulfate concentrations as shown 

in figure 27A for the Straight Creek surface and alluvial 

ground waters. Figure 27A includes a 95-percent confidence 

for the prediction limits about the linear best fit. Statistics of 

best linear fit for all the correlation lines shown in this paper 

are provided in table 1–2 in the Appendix and were originally 

derived from the Straight Creek study (Naus and others, 2005). 

Water from SC–8A is circumneutral pH, dilute, Red River 

alluvial ground water and would not necessarily correlate in 

metal concentration with the acidic alluvial ground waters 

of Straight Creek. Hence, it was included in the linear fit if it 

was consistent with acidic ground-water data, and otherwise 

it was not used. The best fit line with the confidence intervals and median values of the Straight Creek alluvial ground 

waters were then used to compare with the median values of 

other ground waters (Straight Creek bedrock ground waters 

and Hansen Creek, Hottentot Creek, La Bobita, and Capulin 

Canyon ground waters) and Straight Creek drainage water 

(fig. 27B). Manganese concentrations in the Straight Creek 

surface water are nearly the same as those from SC–1A. Most 

other constituents show the same correspondence and provide 

evidence that Straight Creek is the main flow input for the 

Straight Creek alluvial ground water.

Most of the other ground waters have manganese concentrations that agree well with the Straight Creek alluvial 

ground-water trend, but some important differences must 

be noted. Water from well CC–2A contains considerably 

more manganese than the Straight Creek trend, and water 

from well SC–3B contains slightly more manganese than the 

trend. Water from CC–2A (Capulin Canyon) also is anomalously high in beryllium and fluoride concentrations. No 

other outstanding anomalies have been observed for CC–2A 

ground water. It would seem reasonable to conclude that there 

is more rhodochrosite, fluorite, and beryllium mineralization 

found locally at CC–2A (and rhodochrosite only at SC–3B) 

than in the Straight Creek catchment. The lack of other solute 

anomalies at CC–2A probably reflects the circumneutral pH 

conditions, which should reduce the aqueous mobility of elements such as aluminum, copper, cobalt, and nickel. The high 

reduced iron, high sulfate, and elevated zinc concentrations 

indicate there is sulfide mineralization in this part of Capulin 

Canyon but not enough oxidation of pyrite to lower the pH 

values substantially.

Another departure from the Straight Creek manganesesulfate trend in alluvial ground water is the anomalously 

low manganese concentrations for waters from wells SC–1B, 

SC–5B, AWWT2, and Hansen. This consistent trend of lower 

manganese concentrations for ground water with circumneutral pH and substantial carbonate alkalinity (not Hansen) 

indicates that aqueous saturation is reached with respect to 

rhodochrosite. Furthermore, this trend shows that although 

manganese concentrations are limited by rhodochrosite saturaFigure 25. Saturation indices for potassium jarosite plotted relative to pH.

50 Investigation 25—Summary Report

2 3 4 5

-10

-8

-6

-4

-2

0

2

4

6

8

10

Straight Creek , SC1A , SC3A , SC4A, SC6A ,

SC5A , AWWT1, SC7A, Hansen , Hottentot ,

La Bobita, Hansen surface water , Hottentot surface water

K-Jarosite saturation index

pH, in standard units

EXPLANATION
Geochemical Controls on Solute Concentrations 51

Figure 26. (A) Ferrous iron plotted against alkalinity. Siderite saturation index plotted against (B) pH, (C) calcium concentrations, and (D) dissolved inorganic carbon concentrations for all ground waters and Straight Creek surface water.

0 100 200 300 400 500 600

-3

-2

-1

0

1

2

3

4 5 6 7 8 9

-3

-2

-1

0

1

2

3

0 0.005 0.010 0.015 0.020

-3

-2

-1

0

1

2

3

0 100 200 300 400 500 600

-10

0

10

20

30

40

50

60

70

Siderite saturation indexSiderite saturation index

Disordered

Crystalline

C

Siderite saturation index

pH, in standard units

Disordered

Crystalline

Disordered

Crystalline

D

Ferrous iron concentration, in milligrams per liter

Alkalinity, as milligrams per liter bicarbonate

Calcium concentration, in milligrams per liter Dissolved inorganic carbon concentration, in molality 

(C) (D)

(A) (B)
tion, the dissolution of other carbonate minerals, such as calcite, keeps the alkalinity elevated. The manganese concentrations are plotted with respect to alkalinity in figure 28A and, 

similar to ferrous iron concentrations, demonstrate a commonion effect of decreasing manganese with increasing alkalinity. 

Figures 28B and 28C show the saturation indices for rhodochrosite from all well waters relative to the total dissolved 

inorganic carbon (DIC) concentrations and the pH, respectively. Saturation with respect to disordered rhodochrosite is 

maintained for many of the waters over a wide range of DIC, 

supporting the concept that rhodochrosite saturation limits the 

manganese concentrations in ground water. The SI values of 

rhodochrosite as a function of pH demonstrate that this 

solubility control is only operative at circumneutral pH values (greater than 6.5). The pH effect holds for most all carbonate mineral solubilities. Because the water from the Hansen 

well is acidic, the relatively low manganese concentrations 

cannot be a result of rhodochrosite solubility equilibrium. It is 

apparent that the relative abundances of soluble manganeserich minerals such as rhodochrosite and manganiferous calcite 

(or dolomite) (Plumlee and others, 2005) vary substantially 

in the Red River Valley and can be expected to vary according to the alteration zones outlined by Ludington and others (2005). If the Hansen catchment is more distal from the 

thermal core of the alteration, and the core was more centered 

near the Straight Creek catchment or to the east of Straight 

Creek, then there should be less manganese mineralization 

in the weathering zone of the Hansen catchment relative to 

the Straight Creek or Hottentot catchments. It is noteworthy 

that, although the Hansen well water contains lower manganese to sulfate ratios than the waters from the Straight Creek 

alluvial wells, the concentration of manganese is higher than 

that in Hottentot well water and in waters from wells SC–5A, 

SC–7A, and AWWT1. These observations indicate that the 

ratio of rhodochrosite to pyrite available to weathering can 

vary significantly between some catchments. In the CC–2A 

Figure 27. Manganese concentrations plotted against sulfate concentrations for (A) Straight Creek surface and alluvial ground waters and (B) median manganese and sulfate concentrations for all ground waters and Straight Creek 

surface water. Linear fit and prediction limits at 95-percent probability for Straight Creek data only were used for both 

A and B (from Naus and others, 2005; Nordstrom and others, 2005).

52 Investigation 25—Summary Report

0 500 1,000 1,500 2,000 2,500 3,000

0

10

20

30

40

50

60

0 500 1,000 1,500 2,000 2,500 3,000

0

10

20

30

40

50

60

Manganese concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit

Straight Creek

, , 

SC1A

, 

SC1B

, , , , 

SC2B

, 

SC3A

, 

SC3B

, 

SC4A SC6A

SC5A , 

, 

SC5B

, 

AWWT1

, 

AWWT2

, 

SC7A

, 

SC8A

,

CC1A CC1B

CC2A

, , , , , , , , 

CC2B Hansen Hottentot La Bobita Ranger station 

Sulfate concentration, in milligrams per liter

EXPLANATION

(A) (B)
Geochemical Controls on Solute Concentrations 53

Figure 28. (A) Dissolved manganese concentrations plotted against alkalinity. Rhodochrosite saturation index 

plotted against (B) dissolved inorganic carbon concentrations and against (C) pH for all ground waters and Straight 

Creek surface water.

0 0.005 0.010 0.015 0.020

-6

-5

-4

-3

-2

-1

0

1

2

3

4 5 6 7 8 9

-6

-5

-4

-3

-2

-1

0

1

2

3

0 100 200 300 400 500 600

0

10

20

30

40

50

60

Rhodochrosite saturation index

Disordered

Crystalline

pH, in standard units

Disordered

Crystalline

Manganese concentration, in milligrams per liter

Alkalinity, as milligrams per liter bicarbonate

Dissolved inorganic carbon concentration, in molality

(B) (C)

(A)
well, the rhodochrosite to pyrite ratio is high, whereas in the 

Hansen catchment the ratio is low. The ratios represented by 

the Straight Creek alluvial ground waters are probably average 

values between these extremes.

Important generalizations can be drawn from these results 

for the behavior of manganese mobility in ground water of 

the Red River Valley. First, manganese is readily available in 

the weathering zone from the common occurrence of rhodochrosite or manganiferous calcite. Second, for circumneutral 

pH water with adequate alkalinity, rhodochrosite equilibrium 

provides an upper limit to manganese concentrations. Third, 

manganese to sulfate ratios can vary substantially because of 

the variability of abundance of rhodochrosite to pyrite in the 

weathering zone.

Geochemical Controls on Dissolved Aluminum 

and Silica Concentrations

Although the State of New Mexico has a ground-water 

quality standard for aluminum and not for silica, these two 

constituents must be discussed together because they have 

geochemical similarities. They both change solubility with pH, 

but for different reasons, and they tend to react with each other 

to form clay minerals.

Aluminum

The concentration and speciation of dissolved aluminum 

in natural waters is primarily controlled by pH. Under acid 

conditions, aluminum is highly soluble, and at circumneutral 

pH, aluminum is insoluble. This behavior is directly related to 

the value of the first hydrolysis constant, -log K1

 = pK1 = 5.0. 

When the pH of an acid, aluminum-rich solution increases and 

approaches the pK1

, aluminum begins to hydrolyze and precipitate, generally in the pH range 4.5–5.0 (Nordstrom and Ball, 

1986). Not only does the amount of hydrolysis increase with 

increasing pH values, but the rate of hydrolysis also increases 

in this pH range (Hem and Roberson, 1990). In acid sulfate 

water, the actual precipitate is more of an aluminum hydroxysulfate compound rather than a pure aluminum hydroxide 

(Bigham and Nordstrom, 2000) because of the high concentrations of sulfate. This geochemical reaction has often been 

observed in surface water but has not been as well documented 

for ground water. Blowes and others (2003) report a buffering 

of tailings water pH under mildly acidic conditions that they 

ascribe to aluminum hydrolysis at a pH of about 4.5.

These concepts can be further evaluated because of the 

numerous reliable aluminum analyses from this study that 

span a wide range of pH. Figure 29A plots aluminum concentration against sulfate concentration from the alluvial wells 

in Straight Creek, excluding wells AWWT1 and SC–7A. The 

linear fit correlates well with a correlation coefficient of R2

 = 

0.93. Hence, aluminum is behaving conservatively for these 

waters. The analyses from AWWT1 and SC–7A were not 

included because they deviate noticeably from the linear fit, as 

can be seen in figure 29B. La Bobita ground waters are also 

below the correlation line and their pH values average 4.35, 

close to the range where aluminum hydrolysis begins. Bedrock 

ground waters, circumneutral in pH, are also considerably 

lower in aluminum concentration. These results are consistent 

with aluminum concentrations governed by pH. This sensitivity to pH is seen most clearly in figure 29C in which aluminum concentrations decrease rapidly as pH increases to 4.

Possible solubility controls by amorphous to microcrystalline Al(OH)3

 and by alunite, KAl3(SO4)

2

(OH)6, are 

shown by the saturation indices plotted in figures 30A–30D. 

The logarithm of the speciated free aluminum ion, log aAl

3+, 

is plotted against pH in figure 30A to compare with similar 

figures for other sites (for example, Nordstrom and Ball, 1986; 

Driscoll and others, 1984). At low pH (below 4.5), a linear 

trend is apparent with a shallow slope, parallel to the linear 

fit for the Straight Creek alluvial ground waters (Naus and 

others, 2005) but with a slightly lower intercept. These results, 

along with the excellent correlation of aluminum with sulfate 

concentrations in figure 29A, indicate conservative behavior 

of aluminum for these acid waters. Conservative behavior 

does not mean necessarily that minerals such as alunite are not 

forming under these conditions; rather, if alunite is forming, 

the amount of mass transfer is too small to affect the aqueous 

concentrations of aluminum and sulfate.

At circumneutral pH values, the aluminum decreases 

within the field-temperature range of crystalline gibbsite to 

amorphous Al(OH)3

. Figure 30B shows the saturation indices 

for crystalline gibbsite to amorphous Al(OH)3

 plotted as a 

function of pH. Again, a simple mixing line with conservative 

behavior is indicated for the tightly correlated values at low 

pH. A plateau of SI values is observed at circumneutral pH. 

These results show that hydrolysis and precipitation of a form 

of hydrolyzed aluminum controls aluminum concentrations as 

described by Nordstrom and Ball (1986). Aqueous aluminum 

behaves conservatively at low pH values and reacts rapidly at 

high pH values to form an insoluble precipitate. The same process is seen along the banks of the Red River where white precipitates form as a result of mixing of low-pH, aluminum-rich 

seepage water with the neutral-pH water of the river. There is a 

noteworthy difference between surface water and ground water 

for the transition pH from conservative to nonconservative 

behavior. For surface water the transition pH is consistently in 

the range of 4.5 to 5.5 (fig. 30A; Nordstrom and Ball, 1986). 

Using the data from Straight Creek (Naus and others, 2005) 

as shown in figure 29B, the transition pH is lower, closer to 4. 

Wells SC–7A and AWWT1 consistently show lower aluminum concentrations relative to sulfate (fig. 29B) than all the 

other alluvial waters in Straight Creek, and these waters have 

pH values in the range of 3.8–4.0. These data indicate that the 

transition pH to nonconservative behavior for aluminum is 

more than 0.5 of a pH unit lower than that observed in surface 

water. This difference might be expected because of the higher 

available surface area of solids contacting water in aquifers 

compared to sediments in surface water. Another possibility 

is that hydrous aluminosilicate (or amorphous aluminosilicate 

54 Investigation 25—Summary Report
Geochemical Controls on Solute Concentrations 55

Figure 29. Dissolved aluminum concentrations plotted against sulfate concentrations for (A) Straight 

Creek surface and alluvial ground waters, (B) median aluminum and sulfate concentrations for all ground 

waters and Straight Creek surface water, and (C) dissolved aluminum concentrations plotted against pH. 

Linear fit and prediction limits at 95-percent probability for Straight Creek data only were used for both A 

and B (from Naus and others, 2005; Nordstrom and others, 2005).

0 500 1,000 1,500 2,000 2,500 3,000

0

20

40

60

80

100

120

0 500 1,000 1,500 2,000 2,500 3,000

0

20

40

60

80

100

120

2 3 4 5 6 7 8

25

0

50

75

100

125

150

Aluminum concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

(A) and (B): Linear fit, Upper and lower 95-percent prediction limit; (C): All data plotted

Straight Creek SC1A, SC1B

, 

SC2B

, 

SC3A

, 

, , , SC3B, , 

, 

SC4A

, 

SC6A,

SC5A

, 

SC5B

, 

AWWT1

, 

AWWT2

, 

SC7A

, 

SC8A CC1A CC1B,

CC2A

, 

CC2B

, 

Hansen Hottentot La Bobita Ranger station 

EXPLANATION

Aluminum concentration, in milligrams per liter

pH, in standard units

(B)

(C)

(A)
Figure 30. (A) The log of the activity of free Al3+ plotted against pH for all ground waters and Straight Creek surface water. Solubility limits for amorphous aluminum hydroxide (maximum value for temperature range and crystalline gibbsite 

(lowest value for temperature range) are shown. (B) Amorphous aluminum hydroxide saturation index plotted against pH 

for all ground waters and Straight Creek surface water. Alunite saturation index normalized to the total stoichiometric 

coefficient plotted against (C) pH, and (D) aluminum concentration.

56 Investigation 25—Summary Report

(C) (D)

(A) (B)

2 3 4 5 6 7 8

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

2 3 4 5 6 7 8

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

2 3 4 5 6 7 8 9

-3

-2

-1

0

1

2

3

0 20 40 60 80 100 120 140 160

-3

-2

-1

0

1

2

3

Amorphous

Al(OH)3

(5°C)

Gibbsite (12°C)

Log aAl3+

pH, in standard units

pH, in standard units

Linear fit for

Straight Creek 

alluvial ground water

Amorphous aluminum hydroxide saturation index

Amorphous Al(OH)3

Gibbsite, 12°C

Aluminum concentration, in milligrams per liter

Alunite saturation index normalized to the total stoichiometric coefficient
gel) might be forming. This concept will be considered further 

in the next section.

Alunite SI values are plotted as a function of pH and aluminum concentration in figures 30C and D, respectively. The 

SI values are normalized to the total stoichiometric coefficient 

(for alunite, the total stoichiometric coefficient is 12; Nordstrom, 1999). Alunite is a stable mineral phase at low pH (<6; 

Nordstrom, 1982) and high sulfate concentrations that might 

provide a solubility limit. However, the linear, tightly clustered 

values at low pH that are not parallel to the horizontal line 

of constant saturation index do not reflect a solubility limit. 

Instead, the data reflect the same conservative mixing mentioned previously. Supersaturation is achieved at circumneutral 

pH, but alunite is not stable under these conditions. Again, 

though alunite may be forming, it does not provide an obvious 

solubility limit.

Silica

Silica concentrations in most ground waters are derived 

from dissolution of aluminosilicate minerals during weathering combined with some silica or clay mineral precipitation 

when concentrations are sufficiently high. Hence, under acid 

weathering conditions, both aluminum and silica can come 

from the same source, but dissolved silica is already fully 

hydrolyzed as Si(OH)4

 for all pH values in waters of the Red 

River Valley. Equilibrium thermodynamics and the known 

solubility of silica indicate that silica concentrations should 

not change as a function of pH; however, in figure 31A they 

clearly do. The silica concentrations decrease substantially as 

pH increases to 4, and then they decrease slightly or not at all. 

There is a kinetic explanation for this behavior. Dissolution 

of silica from silicate minerals easily occurs at low pH, but 

the silica precipitation rate is a slow process at low pH. Upon 

silicate mineral dissolution at low pH, a mixture of silica polymers and monomers appear in solution. The polymers dissociate to monomers, and the rate of the process is pH-dependent. 

It is much slower at pH 3 than at circumneutral pH values 

(Dietzel, 2000; Icopini and others, 2005). Silica precipitation 

requires polymerization of the individual silica molecules into 

chains and clusters, and polymerization rates are also dependent on pH (Iler, 1979). Again, low pH retards polymerization 

and precipitation. The result is that silica concentrations can 

be high in acid waters and above solubility limits with respect 

to stable silica or aluminosilicate mineral phases.

The aluminum and silica concentrations plotted in figure 31B correlate well, although not in a linear fashion and not 

for the same reason (although interactions between dissolved 

silica and dissolved aluminum are well established; Exley 

and others, 2002). Aluminum increases with decreasing pH 

because of increased dissolution rate for aluminosilicate minerals with increased acidity. Silica increases with decreasing 

pH because of the decreasing polymerization rate with increasing acidity. The silica concentration plotted against sulfate 

concentration for Straight Creek alluvial well waters except 

SC–7A and AWWT1 in figure 31C shows a dilution trend 

with sulfate. In the flow path that can be inferred from figure 

31D, the silica increases in concentration from Straight Creek 

drainage water to SC–1A and gets diluted from SC–1A to 

SC–5A; then silica is attenuated, apparently by precipitation, 

and diluted flowing to AWWT1 and SC–7A. The attenuation 

indicated for silica is very similar to the attenuation pattern 

indicated for aluminum, and because these waters approach a 

pH of 4, this water chemistry might be amenable to the formation of a hydrous aluminosilicate colloid (Exley and others, 

2002) or an aluminosilicate (clay mineral or gel) precipitation. If the saturation indices for various silica minerals are 

considered (fig. 32A), the high silica concentrations at low pH 

appeared to have reached a plateau that is the solubility limit 

of amorphous silica. At higher pH (5.5–8) a lower plateau of 

silica appears to have been reached that is slightly higher than 

chalcedony solubility and could represent some microcrystalline silica phase. Saturation indices for kaolinite and halloysite 

are shown in figure 32B (normalized by the stoichiometry). 

This figure shows that at circumneutral pH values the water 

chemistry has reached the range of halloysite to kaolinite solubility, and these SI values provide further evidence that some 

type of clay mineral (or gel) is forming in this pH range. Kaolinite is found in the weathering profile and with hydrothermal 

minerals. It is difficult to determine its origin without much 

more detailed work and isotopic studies. Halloysite has not 

been identified, but its saturation index is used to approximate 

fine-grained, poorly crystalline kaolinite. Because of the difficulty in eliminating all particles of colloidal aluminosilicates 

during filtration, it is not known whether the supersaturation 

observed for kaolinite and halloysite is real or an artifact.

Geochemical Controls on Dissolved Fluoride 

Concentrations

Fluoride concentrations showed a linear correlation with 

sulfate concentrations for the alluvial Straight Creek ground 

waters, indicating a dilution trend as shown in figure 33A. 

Little difference in fluoride concentration is apparent between 

Straight Creek drainage water and SC–1A and the other alluvial ground waters as seen in figure 33B. Possibly some small 

addition of fluoride from fluorite dissolution is indicated. 

Most of the other ground waters follow this trend, but there are 

some notable exceptions (fig. 33B). Waters from well CC–2A 

are anomalously high in fluoride relative to sulfate, consistent 

with the high manganese and relatively high sulfate concentrations that indicate mineralization near this well. Fluoride 

concentrations from Straight Creek bedrock ground waters 

SC–1B, SC–5B, and AWWT2 and from alluvial Hansen 

ground waters are low or depleted relative to sulfate compared 

to the Straight Creek alluvial ground waters. This depletion 

indicates a possible solubility control by fluorite. Similar 

correlation trends are seen in figures 33C and 33D in which 

fluoride concentrations are plotted against calcium concentrations for Straight Creek alluvial ground waters and all other 

well waters, respectively. The depletion in fluoride relative to 

Geochemical Controls on Solute Concentrations 57
Figure 31. (A) Silica concentrations plotted against pH for all ground waters. (B) Silica concentrations plotted 

against aluminum concentrations for all ground waters. For samples having a pH value less than 4, silica concentrations plotted against sulfate concentrations for (C) Straight Creek alluvial ground water; and (D) median silica 

and sulfate concentrations for all ground waters and Straight Creek surface water. Linear fit and prediction limits 

at 95-percent probability for Straight Creek data only were used for both A and B (from Naus and others, 2005; 

Nordstrom and others, 2005).

58 Investigation 25—Summary Report

25

50

75

100

125

0

25

50

75

100

125

20

40

60

80

100

120

0

20

40

60

80

100

120

A and B: All ground-water data

C and D: Linear Fit, Upper and Lower 95 Percent Prediction Limit

 Straight Creek, SC1A, SC1B, SC2B, SC3A, SC3B, SC4A,

 SC6A, SC5A, SC5B, AWWT1, AWWT2, SC7A, SC8A,

 CC1A, CC1B, CC2A, CC2B, Hansen, Hottentot, La Bobita

 Ranger Station 

EXPLANATION

(A) (B)

(C) (D)

2

0 500 1,000 1,500 2,000 2,500 3,000 0 500 1,000 1,500 2,000 2,500 3,000

3 4 5 6 7 8 0 20 40 60 80 100 120

pH, in standard units Silica concentration, in milligrams per liter

Silica concentration, in milligrams per liter

Aluminum concentration, in milligrams per liter

Silica concentration, in milligrams per literSilica concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter Sulfate concentration, in milligrams per liter
calcium is even more apparent in figure 33D, indicating that 

fluorite solubility equilibrium is a likely candidate for controlling the upper limit to fluoride concentrations in circumneutral 

pH ground waters.

Fluoride concentrations are plotted against calcium concentrations for all ground waters in figure 34A with two different symbols reflecting waters of low pH (<4.5) and high pH 

(>4.5). The trend of decreasing fluoride concentrations with 

increasing calcium concentrations is apparent for the higher 

pH ground waters and supports an upper fluoride concentration limit dictated by fluorite equilibrium mineral solubility. 

Saturation indices for fluorite plotted against pH in figure 34B 

confirm that fluorite solubility limits are reached at pH values 

of 6–7.5. A problematic trend in figure 34B is the degree of 

supersaturation up to nearly 1.5 orders of magnitude. Because 

dissolved aluminum is known to complex with large quantities of fluoride (Tagirov and others, 2002), fluorite saturation 

indices are plotted against aluminum in figure 34C. This plot 

clearly shows that high aluminum concentrations maintain 

strong undersaturation for fluorite solubility consistent with 

a high degree of complexing even though the concentrations 

of fluoride are more than 1 mg/L. Figure 34D details the 

low concentration range of aluminum from figure 34C and 

indicates that the samples that are supersaturated generally are 

those with low but detectable aluminum concentrations. There 

could be errors in the speciation calculations for this range of 

solute concentrations that would explain the supersaturation, 

or supersaturation could be real with precipitation inhibited by 

an unknown complex or other kinetic reasons.

Dissolved fluoride concentrations are plotted against 

dissolved aluminum concentrations in figure 35 with low pH 

samples shown in solid circles and high pH samples shown 

in open circles. This figure provides a further clue to the 

fluorite supersaturation effect because the high pH samples 

do not correlate with aluminum. Further, the highest fluoride 

concentrations at high pH are those that are supersaturated in 

figures 34C and 34D. This lack of correlation might have been 

considered as analytical errors, but that possibility is discounted because analyses were checked by two different methods, ion chromatography and ion-selective electrode potentiometry, passing rigorous laboratory quality control. Another 

possibility is that some unknown complexing, not accounted 

for in the speciation computations, is causing the supersaturation effect at high fluoride concentration. For the purposes 

of this study it is sufficient to note that fluorite saturation is 

reached and can limit fluoride concentrations in these waters.

Molling (1989) found that micas formed during the 

hydrothermal alteration of the volcanics were enriched in 

fluoride, up to 6 weight percent. This amount of rock fluoride 

could be comparable to or even greater than that contained 

in fluorite mineralization and could, therefore, be a source of 

fluoride to ground water during weathering. However, fluoride 

in the hydroxyl lattice site of phyllosilicates is released by 

high-pH water, not acid water; at low pH, fluoride tends to 

be adsorbed onto clay minerals (Harrington and others, 2003; 

Geochemical Controls on Solute Concentrations 59

Figure 32. (A) Silica saturation index plotted against pH, and (B) kaolinite saturation index plotted against pH.

2 3 4 5 6 7 8

-3

-2

-1

0

1

2

3

2 3 4 5 6 7 8 9 10 -3

-2

-1

0

1

2

3

Kaolinite

Kaolinite saturation index

pH, in standard units

Halloysite

Silica saturation index

Chalcedony

Amorphous silica

Quartz

(A) (B)
60 Investigation 25—Summary Report

Figure 33. Fluoride concentrations plotted against sulfate concentrations for (A) Straight Creek alluvial 

ground waters and (B) median fluoride and sulfate concentrations for all ground waters and Straight 

Creek surface water. Fluoride concentrations plotted against calcium concentrations for (C) Straight 

Creek and alluvial ground water and (D) median concentrations for all ground waters and Straight Creek 

surface water. Linear fit and prediction limits at 95-percent probability for Straight Creek data only were 

used for all plots (from Naus and others, 2005; Nordstrom and others, 2005).

 Linear fit, Upper and lower 95-percent prediction limit

Straight Creek SC1A

, 

, , , , , , , , , SC1B

, 

SC2B

, 

SC3A

, 

SC3B

, 

SC4A

,

SC6A

, 

SC5A

SC5B , 

, 

AWWT1

, 

AWWT2

, 

SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

(C) (D)

(A) (B)

0 100 200 300 400 500 600

0

5

10

15

20

0 100 200 300 400 500 600

0

5

10

15

20

0 500 1,000 1,500 2,000 2,500 3,000

0

5

10

15

20

0 500 1,000 1,500 2,000 2,500 3,000

0

5

10

15

20

Calcium concentration, in milligrams per liter

Fluoride concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

EXPLANATION
Geochemical Controls on Solute Concentrations 61

Figure 34. (A) Fluoride concentrations plotted against calcium concentrations. Fluorite saturation index plotted against (B) 

pH, (C) aluminum concentrations, and (D) aluminum concentrations less than 10 milligrams per liter for all ground waters and 

Straight Creek surface water.

0 2 4 6 8 10 -8

-6

-4

-2

0

2

0 50 100 150 200 -8

-6

-4

-2

0

2

0 2 4 6 8 10 -8

-6

-4

-2

0

2

Fluoride less than 1 mg/L

Fluoride greater than 1 mg/L

Fluoride less than 1 mg/L

Fluoride greater than 1 mg/L

Fluorite saturation index

Fluorite saturation index

pH, in standard units

0 100 200 300 400 500 600 -2

0

2

4

6

8

10

12

14

16

18

20

22

pH less than 4.5

pH greater than 4.5

Fluoride concentration, in milligrams per liter

Calcium concentration, in milligrams per liter

Aluminum concentration, in milligrams per liter

(C) (D)

(A) (B)
Zhu and others, 2006). Fluorite dissolves so readily in sulfuric 

acid solutions that it can be safely assumed to be the main 

source of dissolved fluoride. The reaction of sulfuric acid 

with fluorite to produce hydrofluoric acid has been known 

for a long time for etching glass; the reaction was discovered 

by Heinrick Schwanhard in 1670 (Emsley, 2001) and by Carl 

Wilhelm Scheele a few decades later (Ihde, 1984).

Geochemical Controls on Dissolved Calcium 

Concentrations

Dissolved calcium concentrations are derived primarily 

from gypsum dissolution and secondarily from calcite dissolution. These are the two most soluble calcium minerals in the 

Red River Valley, and they are abundantly and widely distributed (Ludington and others, 2005; Livo and Clark, 2002). The 

alluvial ground waters in the Straight Creek catchment show 

a dilution trend as seen in figure 36A. The best linear fit, with 

95-percent confidence intervals from figure 36A, are plotted 

in figure 36B along with the median values for waters from all 

wells (table 5) and found to follow a similar trend except for 

bedrock Hansen and CC–2A. Straight Creek drainage waters 

also correlate well with the alluvial ground waters, indicating that the Straight Creek surface water can be the input for 

the alluvial water with no substantial additions nor depletions of calcium. Bedrock ground waters should have higher 

calcium:sulfate ratios because these waters have additional 

calcium from calcite (and possibly dolomite) dissolution that 

gives them circumneutral, carbonate-buffered pH values. A 

few wells even have water that has higher calcium:sulfate 

ratios than that from pure gypsum dissolution (figs. 36C and 

36D) such as CC–1A, CC–1B, and CC–2B (fig. 33B). Figure 

36D shows how the calcium:sulfate molar ratio varies with 

pH. At low pH values the ratio is low, indicating the preponderance of sulfate from pyrite oxidation. For ratios that approach 1 (indicative of stoichiometric gypsum dissolution), the 

pH must be about 6 or higher, and the highest ratios are only 

reached at pH values above 7. These trends all support the 

contentions described previously. In figure 36D the two lower 

horizontal dashed lines indicate the range found for waterchemistry dominance by gypsum dissolution based on isotopic 

compositions in the upper Animas River drainage basin (Nordstrom and others, in press). The trends seen for the Red River 

Valley data are consistent with those seen in the upper Animas 

River in that the marked shift to higher pH values occurs at a 

calcium:sulfate ratio of about 0.6. This range is where gypsum is more dominant as a source of sulfate over sulfate from 

pyrite oxidation and a more dominant source of calcium than 

that from calcite dissolution.

Gypsum is one of the most common minerals within 

mineralized areas of the study reach between the town of Red 

River and the USGS gaging station. Gypsum forms naturally 

through weathering from reaction between hydrothermal 

calcite and pyrite that commonly are present together in veins 

(Ludington and others, 2005). The widespread occurrence of 

gypsum in mineralized areas, scars, and debris fans combined 

with its relatively high solubility is reflected in the SI values 

for gypsum plotted in figures 37A, B, and C. In figure 37A 

the gypsum SI values are plotted relative to the calcium 

concentrations. The SI values are all close to saturation, and 

they progress continuously toward saturation with increasing 

calcium concentrations. Unlike calcite SI values that can be 

62 Investigation 25—Summary Report

Figure 35. Fluoride concentrations plotted in relation to aluminum concentrations for all 

ground waters and Straight Creek surface water.

0 20 40 60 80 100 120

0

2

4

6

8

10

12

14

16

18

20

22

pH less than 4.5

pH greater than 4.5

Aluminum concentration, in milligrams per liter

Fluoride concentration, in milligrams per liter
Geochemical Controls on Solute Concentrations 63

Figure 36. Calcium concentrations plotted against sulfate concentrations for (A) Straight Creek surface and alluvial ground waters and (B) for median ground-water concentrations and Straight Creek 

surface water. Linear fit and prediction limits at 95-percent probability for Straight Creek data only 

were used for both A and B (from Naus and others, 2005; Nordstrom and others, 2005). (C) Ground 

waters and Straight Creek surface water, all data. (D) Calcium:sulfate molar ratio plotted against pH 

for all ground waters and Straight Creek surface water. Dotted lines indicate the region of increasing 

dominance by gypsum dissolution (0.6 to 1.1 in calcium:sulfate molar ratio) and from calcium:sulfate 

molar ratio of 1.1 to 2 indicates increasing contribution of calcite dissolution to the calcium:sulfate 

ratio (based on Nordstrom and others, 2007). 

0 500 1,000 1,500 2,000 2,500 3,000 0

100

200

300

400

500

600

0 500 1,000 1,500 2,000 2,500 3,000 0

100

200

300

400

500

600

2 3 4 5 6 7 8 0

0.5

1.0

1.5

2.0

2.5

0 500 1,000 1,500 2,000 2,500 3,000 0

100

200

300

400

500

600

Calcium concentration, in milligrams per liter

Gypsum dissolution

(A) and (B): Linear fit, Upper and lower 95-percent prediction limit

Straight Creek

, 

, , , , , , , , , SC1A SC1B

, 

SC2B

, 

SC3A SC3B

, , 

SC4A

, 

SC6A SC5A

SC5B

, 

AWWT1

, 

AWWT2 SC7A

, 

, 

, 

SC8A

,

CC1A CC1B

CC2A CC2B Hansen Hottentot La Bobita Ranger station 

(C) and (D): All data plotted

Sulfate concentration, in milligrams per liter

Gypsum dissolution

Gypsum solubilty 

range for Straight 

Creek bedrock wells

EXPLANATION Calcium:sulfate molar ratio

pH, in standard units

Equimolar calcite + gypsum

Range of water chemistry 

dominated by gypsum dissolution

Sulfate concentration, in milligrams per liter

Gypsum solubilty 

range for Straight 

Creek bedrock wells

(C) (D)

(A) (B)
64 Investigation 25—Summary Report

Figure 37. Gypsum saturation index plotted against (A) calcium concentrations, (B) sulfate concentrations, 

(C) the calcium:sulfate molar ratio for Straight Creek ground water and surface water. (D) Gypsum saturation 

index plotted against calcite saturation index.

0 500 1,000 1,500 2,000 2,500 3,000 -3

-2

-1

0

1

0 100 200 300 400 500 600

-3

-2

-1

0

1

0 0.5 1.0 1.5 2.0

-3

-2

-1

0

1

-3 -2 -1 0 1 2

-3

-2

-1

0

1

pH less than 4

Gypsum saturation index

pH greater than 4

Calcium : sulfate molar ratio

(A), (B), and (C): all data plotted

(D): SC1B, SC2B, SC3B, SC5B, AWWT2, SC8A, CC1A ,

CC1B, CC2A, CC2B, Ranger station 

Calcite saturation index

EXPLANATION

Calcium concentration, in milligrams per liter Sulfate concentration, in milligrams per liter

(C) (D)

(A) (B)
supersaturated, gypsum SI values achieve equilibrium saturation and do not become supersaturated. The regular approach 

to gypsum saturation with increasing calcium concentration 

indicates the simple dependence on a single source for calcium, which is calcite. Whenever the calcium concentrations 

are at or in excess of 450 mg/L, the water can be expected to 

be at gypsum saturation. The gypsum SI values are plotted 

with respect to sulfate concentrations in figure 37B and do 

not show as regular a progression to saturation as when plotted against calcium concentrations. At sulfate concentrations 

greater than about 1,300 mg/L, gypsum saturation is reached, 

but not consistently for all waters.

In figure 37D, the SI values for gypsum and calcite are 

shown. Whereas gypsum provides a consistent solubility limit, 

calcite is supersaturated when gypsum is saturated but not 

when gypsum is undersaturated. The calcite supersaturation 

occurs for water high in magnesium and sulfate, both known 

to be inhibitors of calcite precipitation. Another possible 

reason for supersaturation is that the calcite in the Red River 

Valley is known to contain variably substituted magnesium, 

manganese, and iron. This solid substitution for calcium in 

calcite can also lead to apparent supersaturation when the saturation indices are computed with respect to pure calcite.

The SI values for calcite are plotted against pH in figure 38, showing that calcite saturation is reached only at pH 

values above 6.5, and supersaturation tends to occur at pH values above 7.2. Between the pH values of 6.5 and 7.2, calcite 

equilibrium solubility is maintained.

Geochemical Controls on Dissolved Magnesium 

Concentrations

Sources of dissolved magnesium in ground water include 

dolomite, magnesian calcite (and other magnesian carbonates), 

and chlorite. Carbonates are more soluble and weather more 

easily than silicates; hence, they are more likely the source of 

dissolved magnesium. Dissolution of chlorites cannot be discounted, however, because they are fine grained and abundant.

Concentrations of magnesium and sulfate correlate well 

for Straight Creek alluvial ground waters (fig. 39A), but notable deviations are observed when concentrations from ground 

waters in other catchments are compared to the same correlation (fig. 39B). Neither gain nor loss in magnesium is needed 

when comparing the Straight Creek drainage water with water 

from well SC–1A and the other Straight Creek alluvial ground 

waters. When magnesium concentrations are plotted against 

calcium concentrations for Straight Creek alluvial ground 

waters the correlation is obvious (fig. 39C) but not for all 

other well waters (fig. 39D). This variation in magnesium 

concentrations was seen in historical ground-water analyses 

(LoVetere and others, 2004) and seems to be related to sources 

and limitations on calcium and sulfate concentrations that do 

not apply to magnesium concentrations. Gypsum dissolves, 

which increases calcium and sulfate concentrations but does 

not affect magnesium concentrations for well waters SC–5B, 

CC–2A, and CC–2B. Gypsum equilibrium solubility limits 

calcium concentrations but not magnesium concentrations for 

Geochemical Controls on Solute Concentrations 65

Figure 38. Calcite saturation index plotted against pH for all ground waters and Straight 

Creek surface water.

5.0 5.5 6.0 6.5 7.0 7.5 8.0

-3

-2

-1

0

1

2

3

Calcite saturation index

pH, in standard units
66 Investigation 25—Summary Report

Figure 39. Magnesium concentrations plotted against calcium concentrations for (A) Straight Creek 

surface and alluvial ground waters and (B) median concentrations for all ground waters and Straight Creek 

surface water. Magnesium concentrations plotted against sulfate concentrations for (C) Straight Creek 

surface and alluvial ground waters and (D) all ground waters and Straight Creek surface water. Linear fit 

and prediction limits at 95-percent probability for Straight Creek data only were used for all plots (from Naus 

and others, 2005; Nordstrom and others, 2005).

0 500 1,000 1,500 2,000 2,500 3,000

0

50

100

150

200

250

300

0 500 1,000 1,500 2,000 2,500 3,000

0

50

100

150

200

250

300

Magnesium concentration, in milligrams per liter

0 100 200 300 400 500 600

0

50

100

150

200

250

300

0 100 200 300 400 500 600

0

50

100

150

200

250

300

Calcium concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit

Straight Creek SC1A

, 

SC1B

, 

SC2B

, , , 

SC3A SC3B SC4A

,

SC6A

, 

SC5A

SC5B , 

, 

, , , , , 

, 

, , , , 

AWWT1

, 

AWWT2

, 

SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

Sulfate concentration, in milligrams per liter

EXPLANATION

(A) (B)

(C) (D)
well waters AWWT2, SC–3B, and CC–1B. Dolomite has been 

found in hydrothermally altered areas (Ludington and others, 2005; Plumlee and others, 2005), and a plot of dolomite 

saturation indices as a function of calcium concentrations does 

show saturation achieved at the highest calcium concentrations 

(fig. 40).

Geochemical Controls on Dissolved Strontium 

Concentrations

Unlike most other elements, strontium does not correlate 

with sulfate concentrations (fig. 41A). Instead, strontium concentrations are low and constant until the highest concentrations of calcium are reached (fig. 41). As shown in figure 36D, 

the highest Ca:SO4

 molar ratios are indicative of increasing 

contribution of calcite dissolution relative to gypsum dissolution as a source for dissolved calcium. These results show 

that the main source of strontium is from the dissolution (or 

recrystallization) of calcite. Chemical analyses of carbonate 

minerals from the Red River Valley show that detectable strontium concentrations can range from 0.04 weight percent to as 

high as 0.6 weight percent (as SrO; Plumlee and others, 2005). 

These concentrations would correspond to Sr:Ca molar ratios 

of 0.0005 to 0.006. Molar ratios for Sr:Ca in ground water in 

the study area range from 0.00003 to 0.010. The highest Sr:Ca 

molar ratios are similar for carbonate minerals obtained from 

drill cuttings and for ground water. Indeed, the highest Sr:Ca 

molar ratios in ground water are found in Straight Creek wells 

SC–1B and SC–5B, and the highest Sr:Ca molar ratios in 

carbonate minerals are found in well cuttings from these same 

wells (Plumlee and others, 2005). These well waters also have 

reached saturation with respect to both gypsum and calcite. It 

would appear that high-Sr calcites are dissolving upgradient 

from SC–1B in the bedrock under anoxic conditions and that 

they have largely weathered out of the exposed bedrock and 

colluvial material that contribute solutes to the Straight Creek 

alluvial ground waters. Furthermore, gypsum is produced from 

the reaction of calcite with pyrite in the oxidizing part of the 

weathering zone, and it seems reasonable to assume that the 

gypsum will not incorporate as much strontium at low temperature as the original hydrothermal calcite (Glynn, 1991). 

Release of trace elements during recrystallization is a process 

used in chemical engineering to purify soluble materials. For 

example, ground waters from well SC–1B have neutral pH 

values, are at saturation with respect to gypsum and calcite, 

and have the highest Sr:Ca ratios. However, ground waters 

from SC–1A have only 20 percent less calcium but 92 percent 

less strontium. The chemistry indicates that most of the calcium in SC–1A comes from gypsum dissolution whereas calcium in SC–1B must come from both calcite and gypsum dissolution under anoxic conditions (no pyrite oxidation). Hence, 

strontium can be an indicator of calcite dissolution. But this 

hypothesis creates a dilemma. If the high sulfate concentration 

in SC–1B waters is derived solely from gypsum dissolution 

then there is insufficient calcium to balance sulfate, which 

is in excess of calcium by more than 13 meq/L. This excess 

indicates the bedrock ground waters originated with chemistry 

similar to Straight Creek surface water with acid pH values 

that infiltrated bedrock, became anoxic, and was neutralized 

by strontium-rich calcite. If the median sulfate concentration 

for Straight Creek is decreased to the median sulfate concentration for SC–1B (see table 5) and the calcium concentration 

for Straight Creek is decreased proportionally by the same 

amount, and that amount of calcium is assumed to result 

from gypsum dissolution, then 8 mmol/L of calcium out of 

12.6 mmol/L for SC–1B must come from gypsum dissolution 

and the other 4.6 mmol/L must come from calcite dissolution. 

In this case, the Sr:Ca molar ratio for that calcite would have 

to be 0.027, or about 4.5 times the highest molar ratio found 

in any carbonate mineral. This result would indicate that either 

calcites with higher Sr:Ca molar ratios are present than have 

been analyzed, that some strontianite occurs in the carbonate 

minerals, or that there is another source of strontium.

Strontianite SrCO3 and celestine SrSO4

 saturation indices 

are plotted against calcium concentrations in figures 41C 

and D, respectively. Strontium concentrations reach celestine 

saturation as a solubility limit whereas strontianite SI values 

appear to reach a limit at about 1 order of magnitude undersaturated. This limit was shown by Plummer and others (1990) to 

be a result of the phase rule and the fact that calcite, gypsum, 

and celestine all reach saturation (see Naus and others, 2005). 

Celestine, therefore, could be another source of strontium in 

the study area, but none has been identified in the mineralogical study (Plumlee and others, 2005).

Geochemical Controls on Solute Concentrations 67

Figure 40. Dolomite saturation index plotted against 

calcium concentrations.

0 100 200 300 400 500 600 -8

-6

-4

-2

0

2

Dolomite saturation index

Calcium concentration, in milligrams per liter

Disordered

Crystalline
68 Investigation 25—Summary Report

Figure 41. (A) Plot of strontium concentrations against sulfate concentrations for all ground waters and 

Straight Creek surface water. (B) Plot of strontium concentrations against calcium concentrations for 

all ground waters and Straight Creek surface water. (C) Strontianite saturation index relative to calcium 

concentrations for ground water and Straight Creek surface water. (D) Celestine saturation index relative to 

calcium concentrations for all ground waters and Straight Creek surface water.

0 500 1,000 1,500 2,000 2,500 3,000

0

2

4

6

8

10

12

0 100 200 300 400 500 600

0

2

4

6

8

10

12

0 100 200 300 400 500 600

-5

-4

-3

-2

-1

0

1

2

0 100 200 300 400 500 600

-4

-3

-2

-1

0

1

2

Strontium concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

Strontianite saturation index

Celestite saturation index

Calcium concentration, in milligrams per liter Calcium concentration, in milligrams per liter

(C) (D)

(A) (B)
Geochemical Controls on Solute Concentrations 69

Figure 42. (A) Barium concentrations plotted against sulfate concentrations for all ground waters and Straight Creek 

surface water. Barite saturation index plotted against (B) barium concentration and (C) pH for all ground waters and 

Straight Creek surface water. (D) Dissolved (0.45-micrometer filtration) barium concentrations plotted against total recoverable barium concentrations for all ground waters and Straight Creek surface water; those data with SI for barite greater 

than +0.2 are shown with open circles, all others with filled circles.

2 3 4 5 6 7 8 9

-2

-1

0

1

2

0 0.01 0.02 0.03 0.04 0.05 0.06 -2

-1

0

1

2

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.02 0.04 0.06 0.08

0

0.02

0.04

0.06

0.08

Barite saturation index

pH, in standard units

Barite saturation index

Sulfate concentration, in milligrams per liter Barium concentration, in milligrams per liter

 Barium concentration, in milligrams per liter

Barite saturation index less than 0.2 

Barite saturation index greater than 0.2

Dissolved barium concentration, in milligrams per liter

Total barium concentration, in milligrams per liter

(C) (D)

(A) (B)
Geochemical Controls on Dissolved Barium 

Concentrations

The most common source of barium in mineralized areas 

is barite, BaSO4

, because it commonly occurs as a gangue 

mineral. A gangue mineral is an accessory mineral that formed 

with the mineral deposit but does not have any economic 

value.

Barium concentrations in acid-sulfate water are often near 

detection limits because barite reaches solubility equilibrium, 

and the common-ion effect from sulfate, along with the low 

solubility of barite, minimizes barium concentrations. Solubility limits readily explain the lack of correlation in figure 42A 

in which barium concentrations are plotted with respect to 

sulfate concentrations. The only apparent trend is the tendency 

for barium concentrations to decrease at the higher sulfate 

concentrations, consistent with the common-ion effect for 

solubility control. In figure 42B the SI values for barite are 

plotted as a function of barium concentrations. Although the 

SI values level out to a plateau that would indicate a solubility limit, the values also are supersaturated by up to an order 

of magnitude. There are at least four possible reasons for this 

supersaturation. Freshly precipitated barite can be so fine 

grained that there is a grain-size, or surface-area, effect on the 

equilibrium solubility (Balarew, 1925; Hina and Nancollas, 

2000). A related possibility is that colloidal particles of barite 

are not adequately filtered during field filtration. A third possibility is that the precipitating barite has a variable composition 

caused by solid substitution (for example, Felmy and others, 

1993). A further, and less likely, possibility is that insufficient 

barium complexing is included in the speciation computations. 

Other research indicates that colloidal barite that can occasionally pass through filter membranes is the main cause (H.E. 

Taylor and D.K. Nordstrom, U.S. Geological Survey, oral 

commun., 2004).

When barite SI values are plotted relative to pH, as shown 

in figure 42C, no pH limitation is apparent. Barite provides a 

solubility limit to barium concentrations for a wide range of 

pH values, although most of the supersaturated values occur in 

the pH range 5.5–8.

In figure 42D, dissolved barium concentrations are 

plotted relative to total recoverable barium concentrations 

with open circles designating those samples that are supersaturated with respect to barite (SI values greater than +0.2). 

Nearly all the supersaturated values are those with the highest 

barium concentrations, and many of these have total recoverable barium concentrations greater than dissolved barium 

concentrations. This trend lends support to the possibility 

that fine-grained or colloidal barite particles are a cause for 

apparent supersaturation, but the other possibilities cannot be 

discounted at this time.

Geochemical Controls on Dissolved Zinc 

Concentrations

The most abundant and most easily weathered mineral 

that provides a source of zinc to ground water is the mineral 

sphalerite, ZnS.

For the Straight Creek alluvial ground waters, zinc 

concentrations correlate well with sulfate concentrations 

(fig. 43A), and there is little difference between Straight 

Creek drainage-water composition and alluvial ground-water 

composition. Upon comparing the Straight Creek alluvial 

ground-water zinc concentrations with those of other ground 

waters in figure 43B, no enriched anomalies are observed, but 

several examples of depletions in zinc are noteworthy. The 

wells depleted in zinc relative to sulfate for the Straight Creek 

alluvial ground waters are those of neutral pH and one acidic 

ground water, Hansen (probably less sphalerite mineralization because it is more distal from the hydrothermal source). 

There is no identifiable mineral solubility equilibrium that has 

been reached for zinc, although the possibility of a mineral 

solubility limitation such as hydrozincite [Zn5(CO3)

2

(OH)6] or 

hemimorphite [Zn4Si2O7(OH)2•H2O], both of which would be 

stable under neutral to high pH, cannot be excluded. Empirically, zinc concentrations do not appear to exceed 2 mg/L for 

ground water of circumneutral pH.

Another element distinctive of mineralized areas is manganese, and zinc and manganese are both soluble enough to be 

found in ground water and surface water. Figure 43C shows a 

plot of zinc relative to manganese for the Straight Creek surface and alluvial ground waters. The correlation is the strongest of all elements (R2

 = 0.99) with little difference between 

the concentrations for Straight Creek surface water and those 

for ground water in well SC–1A. Other ground waters have 

similar zinc and manganese concentrations (fig. 43D) except 

for waters from wells SC–2B, SC–3B, and CC–2A.

Geochemical Controls on Dissolved Cadmium 

Concentrations

Cadmium concentrations also correlate well for the 

Straight Creek alluvial ground waters with little difference 

between Straight Creek surface water and water from well 

SC–1A (fig. 44A). The comparison for other ground waters is 

substantially similar to that for zinc and sulfate concentrations 

with depleted cadmium concentrations in circumneutral pH 

ground waters (fig. 44B). This similarity would be expected 

because the source of cadmium is sphalerite (Plumlee and 

others, 2005). The Zn:Cd weight ratios in sphalerite are often 

in the range of 50–200 (Fleischer, 1955). The ratios of Zn:Cd 

can vary with alteration zone, probably reflecting changing 

conditions with distance from the hydrothermal source, competition from other trace elements, and possibly differences in 

source abundance. Crystallochemical considerations such as 

ionic radius and bonding coordination also affect the amount 

of substitution.

70 Investigation 25—Summary Report
Geochemical Controls on Solute Concentrations 71

Figure 43. Zinc concentrations plotted against sulfate concentrations for (A) Straight Creek surface and 

alluvial ground waters and (B) median values for all ground and surface waters. Zinc concentrations plotted 

against manganese concentrations for (C) Straight Creek surface and alluvial ground waters and (D) median 

values for all ground and surface waters.

0 500 1,000 1,500 2,000 2,500 3,000

0

2

4

6

8

10

12

5000 1,000 1,500 2,000 2,500 3,000

0

2

4

6

8

10

12

0 10 20 30 40 50

0

2

4

6

8

10

12

0 10 20 30 40 50

0

2

4

6

8

10

12

Zinc concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

Manganese concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit,

Straight Creek

, , , 

SC1A SC1B SC2B

, 

SC3A

, 

SC3B

, 

SC4A

,

SC6A

, 

SC5A

, 

, , , , , , , , , 

SC5B AWWT1

, , 

AWWT2

, 

SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

EXPLANATION

(C) (D)

(A) (B)
72 Investigation 25—Summary Report

Figure 44. Cadmium concentrations plotted against sulfate concentrations for (A) Straight Creek surface and 

alluvial ground waters and (B) all ground and surface waters. Cadmium concentrations plotted against zinc 

concentrations for (C) Straight Creek surface and alluvial ground waters and (D) all ground waters and Straight 

Creek surface water.

500

5000 1,000 1,500 2,000 2,500 3,000

0

0.02

0.04

0.06

0.08

0.10

0 500 1,000 1,500 2,000 2,500 3,000

0

0.02

0.04

0.06

0.08

0.10

0 2 4 6 8 10

0

0.01

0.02

0.03

0.04

0.05

0.06 0.06

0 2 4 6 8 10

0

0.01

0.02

0.03

0.04

0.05

Cadmium concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

Zinc concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit,

Straight Creek

, , , 

SC1A SC1B SC2B

, 

SC3A

, 

SC3B

, 

SC4A

,

SC6A

, 

SC5A

, 

, , , , , , , , , 

SC5B AWWT1

, , 

AWWT2

, 

SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

EXPLANATION

(C) (D)

(A) (B)
Two samples of sphalerite from Molycorp’s open pit, 

which were analyzed for cadmium, gave Zn:Cd weight ratios 

of 130 and 170, whereas three sphalerites from Straight Creek 

basin gave ratios of 54, 60, and 62 (Plumlee and others, 2005). 

The cadmium is much more enriched in Straight Creek sphalerites than in the open-pit sphalerites; however, the limited 

sampling makes it difficult to reach definitive conclusions.

From figure 44C the Zn:Cd weight ratio for the Straight 

Creek acidic, alluvial ground waters is about 180. This ratio 

also applies to other ground waters as shown in figure 44D 

except for waters from wells SC–3B, CC–2A, and Hottentot. Waters from CC–2A and Hottentot are slightly enriched 

in zinc relative to sulfate compared to the Straight Creek 

trend (fig. 44B) and depleted in cadmium relative to sulfate 

compared to the Straight Creek trend, suggesting cadmiumdepleted sphalerites occur in these areas.

For cadmium, there are no substantial enrichments 

compared to the Straight Creek linear best-fit trend, only a few 

depletions. Consequently, the Straight Creek trend provides an 

upper limit to cadmium concentrations relative to sulfate and 

zinc concentrations.

Geochemical Controls on Dissolved Copper 

Concentrations

The primary natural source of copper in the Red River 

Valley is the mineral chalcopyrite, CuFeS2

 (Plumlee and others, 2005). There are minor to trace amounts of other copper 

sulfide minerals combined with tin, bismuth, and tellurium, 

but these cannot contribute much copper to the overall mass 

balance and transport of copper.

Although copper concentrations do follow a general 

dilution trend for the Straight Creek alluvial ground waters 

(fig. 45A), there is a poor correlation for other ground waters 

and a large decrease in copper concentration in going from 

the median Straight Creek copper concentration to the median 

copper concentration in water from well SC–1A (fig. 45B). 

This large decrease is caused by reduction processes. All of 

the dissolved iron in Straight Creek is in the ferric iron or 

oxidized form, but on entering the debris fan (well SC–1A) the 

dissolved iron is reduced to the ferrous iron form (fig. 23B). 

The same driving force for reduction would affect the redox 

state of the dissolved copper. This rapid reduction would 

precipitate copper out as cuprite (Cu2

O) or elemental copper, 

or by replacement in sulfide minerals in the debris fan, such 

as the replacement of sphalerite by covellite (CuS) and pyrite 

by chalcocite (Cu2

S) in a process known as supergene enrichment (Garrels and Christ, 1965; Krauskopf and Bird, 1995). 

For circumneutral-pH ground water, both strong adsorption 

and supergene enrichment would lower low copper concentrations in ground water such as shown in figures 45B and 45D. 

The strong tendency for copper sorption at moderately acidic 

pH values (4.5–6) is well established (for example, Krauskopf, 

1956; Pickering, 1979; Dzomback and Morel, 1990).

An interesting observation can be made regarding the 

copper concentration data from well SC–6A. In the five water 

samples collected, the copper concentrations varied from 

0.77 mg/L (the first sample taken) to 0.37 mg/L (the last sample taken). This concentration range is the largest for any well 

water for which the sulfate concentration is virtually constant. 

An examination of the data reveals that the iron redox species 

also vary widely with the copper concentrations. The highest 

copper concentration corresponds to the highest concentration (3.88 mg/L) of ferric iron and the largest percentage of 

ferric iron in the total dissolved iron (88 percent), indicating 

a strongly oxidized water, whereas the lowest copper concentration corresponds to the highest ferrous iron concentration 

(12 mg/L or 80 percent of the total), indicating a strongly 

reduced water. This trend of varying copper concentration 

according to the changing redox conditions over time within 

the same well water also supports the importance of redox in 

controlling copper concentrations.

Copper, like cadmium, tends to correlate well with zinc 

for the acidic ground waters of the Straight Creek alluvial 

aquifer (fig. 45C) and poorly for all ground waters (fig. 45D). 

The copper is higher in the Straight Creek alluvial ground 

waters than in any of the other ground waters, and this fact 

raises the question of whether Straight Creek as an analog site 

does not bias the conclusions for pre-mining copper concentrations in the direction of values that are too high. Information 

available at this time is insufficient to reach such a conclusion. 

Chalcopyrite is too ubiquitous in QSP alteration to observe 

any spatial differences in chalcopyrite mineralization (Plumlee 

and others, 2005). Hence, the upper limits to copper concentrations (relative to sulfate) are taken as those defined by the 

Straight Creek trend.

Geochemical Controls on Dissolved Nickel and 

Cobalt Concentrations

The source for nickel and cobalt in the Red River Valley 

is primarily pyrite (Plumlee and others, 2005). When a single 

mineral is the only substantive source of two or more constituents and those constituents dissolve congruently and are 

not affected by precipitation or sorption processes, then they 

should be highly correlated in the water chemistry. Furthermore, nickel and cobalt have one and only one stable oxidation state in natural waters, as divalent cations. The trivalent 

oxidation state of cobalt is not stable in aqueous solution, and 

it oxidizes water to form oxygen (Maki and Tanaka, 1985).

Nickel concentrations, for the Straight Creek alluvial 

ground waters, correlate strongly with sulfate concentrations (fig. 46A), and the Straight Creek median concentration is nearly identical to the median concentration for water 

from well SC–1A (fig. 46B). The other ground waters are 

represented in figure 46B, which continues to show a strong 

correlation, but only for acidic ground waters. Ground water of 

circumneutral pH is depleted in nickel. A similar correlation 

can be found for cobalt; but instead of displaying an almost 

Geochemical Controls on Solute Concentrations 73
74 Investigation 25—Summary Report

Figure 45. Copper concentrations plotted against sulfate concentrations for (A) Straight Creek surface and 

alluvial ground waters and (B) all ground and surface waters. Copper concentrations plotted against zinc 

concentrations for (C) Straight Creek surface and alluvial ground waters and (D) all ground waters and Straight 

Creek surface water.

0 500 1,000 1,500 2,000 2,500 3,000

0

0.5

1.0

1.5

2.0

2.5

0 500 1,000 1,500 2,000 2,500 3,000

0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10

0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10

0

0.5

1.0

1.5

2.0

2.5

Sulfate concentration, in milligrams per liter

Copper concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit,

Straight Creek SC1A

, 

, , , , , , , , , 

, 

SC1B

, 

SC2B

, 

SC3A SC3B

, , , 

SC4A

,

SC6A

, 

SC5A

SC5B

, 

AWWT1

, 

AWWT2

, 

SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

Zinc concentration, in milligrams per liter

EXPLANATION

(C) (D)

(A) (B)
Geochemical Controls on Solute Concentrations 75

Figure 46. Nickel concentrations plotted against sulfate concentrations for (A) Straight Creek surface and 

alluvial ground waters and (B) all ground and surface waters. Cobalt concentrations plotted against nickel 

concentrations for (C) Straight Creek surface and alluvial ground waters and (D) all ground waters and 

Straight Creek surface water.

0 500 1,000 1,500 2,000 2,500 3,000

0

0.2

0.4

0.6

0.8

1.0

1.2

0 500 1,000 1,500 2,000 2,500 3,000

0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.2 0.4 0.6 0.8 1.0 1.2

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1.0 1.2

0

0.1

0.2

0.3

0.4

0.5

0.6

Sulfate concentration, in milligrams per liter

Nickel concentration, in milligrams per liter Cobalt concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit,

Straight Creek

, 

SC1A

, 

, , , , , , , , , SC1B

, 

SC2B

, 

SC3A

, 

SC3B SC4A

,, , ,

SC6A SC5A

SC5B AWWT1

, , 

AWWT2

, 

SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

Nickel concentration, in milligrams per liter

EXPLANATION

(C) (D)

(A) (B)
identical correlation, the correlation between cobalt and nickel 

is shown in figure 46C for Straight Creek surface and ground 

waters, and the median values for all ground waters are shown 

in figure 46D. These plots portray the strongest correlation 

(R2 = 0.98) found for any two constituents except for the correlation for zinc and manganese.

For pyrite samples collected on the mine site, highly variable concentrations of cobalt were found, up to 0.58 weight 

percent, but nickel concentrations were mostly below detection 

(Plumlee and others, 2005). Pyrite samples from the Straight 

Creek catchment, however, averaged about 0.1 weight percent 

each in both cobalt and nickel. If the nickel and the cobalt in 

Straight Creek pyrite dissolved congruently and there were no 

attenuation processes downgradient in acid waters, the Co:Ni 

weight ratios in the water should be close to unity for the 

Straight Creek alluvial ground waters. Although the concentrations of these two elements are in the same range, the cobalt 

concentrations are typically about one-half of those for nickel 

(the slope of the linear fit is 0.44). Attenuation processes in 

these acid waters are unlikely for these elements. Hence, there 

are two possibilities for the lack of better agreement between 

the element ratios in the mineral and in the surface and ground 

waters. The first possibility is that limited sampling and 

analysis for trace elements in pyrite do not provide enough 

data to delineate the full range and meaningful average value. 

The other possibility could be that there is another source for 

nickel that could increase nickel concentrations over those for 

cobalt. Trace-element determinations for andesite collected in 

the Straight Creek catchment do indicate that nickel concentrations are higher than cobalt concentrations by a factor 

of 2 to 3. Whether this increased ratio is caused by pyrite or 

some other mineral is unknown. Nevertheless, the agreement 

between the Co:Ni ratios in pyrite and in the acidic ground 

water indicates, as hypothesized, that pyrite oxidation is the 

primary source of cobalt and nickel in ground water. Further, 

because most bedrock ground waters are anoxic and cannot 

oxidize pyrite, cobalt and nickel concentrations would have to 

be much lower than concentrations in acid ground waters.

Geochemical Controls on Dissolved Chromium 

Concentrations

Chromium originally resided in mafic minerals of 

the unaltered andesite that might include spinels, olivines, 

pyroxenes, biotites, and amphiboles. Hydrothermal alteration converted these minerals to chlorite, illite, and epidote. 

The chromium should mostly reside in the chlorite, probably 

substituting for trivalent iron and aluminum. Analyses of 

chlorites tend to support this hypothesis (Plumlee and others, 

2005) with chromium concentrations in the range of 0.002 to 

0.008 weight percent. In chlorites found in ultramafic rocks, 

chromium concentrations can average 0.3 weight percent 

(Oze and others, 2004). Consequently, dissolved chromium in 

ground water might be an indicator of chlorite dissolution. The

 three elements, chromium, cobalt, and nickel, also are 

commonly found together and in increasing concentration as 

rock types change in composition from andesitic to ultramafic. 

In natural waters, dissolved chromium can exist in two possible oxidation states—the reduced trivalent cation (Cr3+) and 

the oxidized hexavalent anion (CrO4

2–). Trivalent chromium is 

much less soluble than hexavalent chromium because of the 

low solubility of chromium (III) hydroxide. Water containing oxidized iron, such as Straight Creek surface drainage, 

would contain hexavalent chromium, and water containing 

reduced iron, such as the alluvial and bedrock ground waters 

in Straight Creek, would contain trivalent chromium.

Although chromium concentrations in the Straight Creek 

alluvial ground waters correlate with sulfate concentrations 

(fig. 47A), there is a decrease in chromium concentrations 

in ground water between Straight Creek drainage water to 

ground water from well SC–1A (fig. 47B). This decrease 

in concentration would be consistent with the change from 

oxidizing conditions to reducing conditions that are reflected 

in the Fe (II/III) ratio and in the decrease in copper concentrations. Further decreases in concentration would be expected 

for bedrock ground waters of circumneutral pH because of a 

longer residence time under anoxic conditions. This concept 

is corroborated by figure 47B in which the well waters with 

the lowest concentrations are in bedrock. The only enriched 

chromium anomaly is found in water from well AWWT1, the 

wastewater-treatment plant observation well. This concentration is so anomalous that it is highly likely to be from contamination from the use of chrome-plated parts or stainless steel 

from the well construction that would slowly dissolve in an 

acidic water. Hence, the upper limit of chromium concentrations relative to sulfate in acidic ground waters can be defined 

by the Straight Creek trend. Substantially lower concentrations 

are expected for anoxic ground water, especially if they are 

circumneutral in pH.

The correlation between chromium and nickel for 

the Straight Creek acid alluvial ground waters is shown in 

figure 47C with a correlation coefficient of 0.76. The same 

correlation accounts for other ground waters (fig. 47D) and 

supports the contention that chromium, nickel, and cobalt are 

strongly associated, even in altered rock

Geochemical Controls on Dissolved Lithium, 

Sodium, and Potassium Concentrations

Lithium is present in rocks and alteration minerals of the 

Red River Valley at higher concentrations than normally found 

in the same rocks lacking mineral deposits and hydrothermal 

alteration. It, along with beryllium, is one of the signature 

elements for molybdenum deposits. The minerals source for 

lithium is more dispersed than most other elements, and it is 

present in higher concentrations in some illites (sericite) than 

most other silicate phases that have been analyzed (Plumlee 

and others, 2005).

76 Investigation 25—Summary Report
Geochemical Controls on Solute Concentrations 77

Figure 47. Chromium concentrations plotted against sulfate concentrations for (A) Straight Creek surface and 

alluvial ground waters and (B) all ground and surface waters. Chromium concentrations plotted against nickel 

concentrations for (C) Straight Creek surface and alluvial ground waters and for (D) all ground waters and 

Straight Creek surface water.

0 0.5 1.0 1.5

0

0.02

0.04

0.06

0.08

0.10

0 0.5 1.0 1.5

0

0.02

0.04

0.06

0.08

0.10

0 500 1,000 1,500 2,000 2,500 3,000

0

0.02

0.04

0.06

0.08

0.10

0 500 1,000 1,500 2,000 2,500 3,000

0

0.02

0.04

0.06

0.08

0.10

Nickel concentration, in milligrams per liter

Chromium concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit,

Straight Creek

, 

, , , , , , , , 

, 

, , 

SC1A SC1B

, 

SC2B

, , , , , 

SC3A SC3B SC4A SC6A

SC5A

, 

SC5B AWWT1 AWWT2

, 

SC7A

, 

SC8A CC1A CC1B

CC2A CC2B Hansen Hottentot La Bobita Ranger station

Sulfate concentration, in milligrams per liter

EXPLANATION

(C) (D)

(A) (B)
78 Investigation 25—Summary Report

Figure 48. Lithium concentrations plotted against sulfate concentrations for (A) Straight Creek surface and 

alluvial ground waters and (B) all ground and surface waters. Potassium concentrations plotted against sodium 

concentrations for (C) Straight Creek surface and alluvial ground waters and for (D) all ground waters and 

Straight Creek surface water.

0 500 1,000 1,500 2,000 2,500 3,000

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 500 1,000 1,500 2,000 2,500 3,000

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 10 20 30 40 50 60 70

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 0

2

4

6

8

10

12

Lithium concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

Potassium concentration, in milligrams per liter

Straight Creek surface and 

alluvial ground water

(A) and (B): Linear fit, Upper and lower 95-percent prediction limit

Straight Creek SC1A SC1B SC2B SC3A SC3B SC4A SC6A

, 

, , , , , , , , , 

, , , , , , , , , 

, , , 

SC5A

SC5B AWWT1 AWWT2 SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

Sodium concentration, in milligrams per liter

EXPLANATION

Straight Creek surface and 

alluvial ground water

(C) (D)

(A) (B)
As shown in figure 48A, lithium correlates well with 

sulfate for the Straight Creek alluvial ground waters. That correlation also accounts for most of the lithium concentrations 

for the other ground waters (fig. 48B). One lithium-enriched 

water is from SC–2B, and two lithium-depleted waters are 

Hansen and SC–5B.

Sodium and potassium concentrations, although higher 

than lithium concentrations, are still low compared to most 

ground waters (Hem, 1985). The main source minerals for 

sodium are feldspars such as albite and other plagioclase 

feldspars. The main source minerals for potassium are illite 

(sericite) and potassium feldspars (orthoclase, adularia).

Figure 48C shows a plot of potassium relative to sodium 

concentrations for all ground waters, with filled circles designating the Straight Creek surface and ground waters. The 

concentration range for potassium is much lower than for 

sodium, which is typical for nearly all ground waters because 

sodium minerals are more soluble than their potassium 

counterparts. For example, albite (NaAlSi3

O8) is more soluble 

and weathers more readily than orthoclase (KAlSi3

O8). These 

two elements do generally occur together, indicating that 

increased weathering still maintains a roughly constant ratio 

for these elements. Figure 48D shows the median values for all 

well waters.

Geochemical Controls on Dissolved Beryllium 

Concentrations

Beryllium is another element found in high concentrations in areas of molybdenum ore deposits. The mineral beryl 

(Be3

Al2Si6O18), although rare, has been found underground at 

the mine site (Bruce Walker, Molycorp, Inc., oral commun., 

2003). The formation of emeralds, a particular green form of 

beryl, has been found to form at high temperature (greater than 

300oC) and from a brine with high fluoride concentration, at 

least for the deposits in Colombia (Banks and others, 2000). 

Wood (1991, 1992) has shown the importance of Be-F complexing for the speciation and transport of beryllium in hydrothermal fluids. Hence, beryllium should reflect this strong 

association with fluorine in water derived from the weathering 

of such deposits. More commonly, beryllium is found as a 

dispersed element with higher concentrations in some illites 

(Plumlee and others, 2005). Consequently, lithium, beryllium, 

and fluoride concentrations should correlate in acidic ground 

waters in the Red River Valley.

Beryllium correlates well with sulfate for the Straight 

Creek alluvial ground waters (fig. 49A), and that correlation 

holds generally for other ground waters (fig. 49B) with some 

exceptions. Water from well CC–2A is anomalously high in 

beryllium, and at Straight Creek ground waters from wells 

SC–1B, SC–5B, and AWWT2, all bedrock wells, are anomalously low in beryllium. It should be noted here that some 

ground waters monitored on the mine site have beryllium 

concentrations up to a few hundred micrograms per liter, and 

these are likely to be the highest concentrations ever reported 

for ground water (compare to Veselý and others, 2002).

The anomalously high concentration of beryllium at 

well CC–2A is associated with anomalously high fluoride 

concentration and might be caused by both abundance and 

by complexing between beryllium and fluoride. This relation 

can be seen in figures 50A and 50B. The correlation between 

beryllium and fluoride in the Straight Creek waters is quite 

good (R2

 =0.73 compared to 0.84 for the correlation with 

sulfate), but the correlation in figure 50B shows that high 

beryllium in CC–2A is expected based on the high fluoride 

concentration. Consequently, in interpreting the weathering 

of beryllium, the concentration of fluoride must also be taken 

into account.

In figures 50C and 50D, the correlations of beryllium 

with lithium are shown. The correlation coefficient for the 

Straight Creek alluvial ground waters is as strong as most of 

the other correlations (R2

 = 0.87).

Pre-Mining Ground-Water Chemistry at 

Molycorp’s Questa Mine Site

Building on the information gained from the Straight 

Creek analog study and the additional information gained from 

wells located at Hottentot Creek, Hansen Creek, La Bobita 

Campground, and Capulin Canyon, the pre-mining groundwater chemistry at the mine site is derived next. For this analysis, the median values of water-chemistry constituents from 

the monitoring wells are used.

The primary constraints on chemical constituents for 

ground water in mineralized, but unmined, areas are (1) 

correlations found between constituents for the Straight 

Creek natural analog site, (2) applicable equilibrium mineralsolubility limitations, and (3) exceptionally high concentrations of constituents found at other undisturbed sites besides 

Straight Creek that can be explained by unusual geochemical 

circumstances.

Pre-Mining Ground Waters at Molycorp’s 

Questa Mine

Molycorp’s Questa mine site consists of three main 

drainages—Capulin Canyon, Goat Hill Gulch, and Sulphur 

Gulch. Sulphur Gulch is further divided into three tributary 

drainages—Spring Gulch, Blind Gulch, and Sulphur Gulch 

(fig. 51). There are four smaller, unnamed drainages between 

Goat Hill Gulch and Sulphur Gulch along the north side of 

the Red River that also must be considered. There are other 

smaller drainages to the east of the Sulphur Gulch drainage 

and within the mine-site boundaries that are small enough to 

be neglected and will not be considered further.

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 79
80 Investigation 25—Summary Report

Figure 49. Beryllium concentrations plotted against sulfate concentrations for (A) Straight 

Creek surface and alluvial ground waters and (B) all ground and surface waters.

0

0.02

0.04

0.06

0.08

0.10

0 500 1,000 1,500 2,000 2,500 3,000

0

0.02

0.04

0.06

0.08

0.10

Sulfate concentration, in milligrams per liter

Beryllium concentration, in milligrams per liter

 Linear fit, 

Straight Creek Upper and lower 95-percent prediction limit

, , 

, , 

SC1A

, 

SC1B

, 

SC2B SC3A

, , 

SC3B SC4A

,

, , , , , ,

SC6A

, 

SC5A

, 

SC5B

, 

AWWT1

, 

AWWT2

, 

SC7A

, 

SC8A

CC1A CC1B CC2A CC2B Hansen Hottentot La Bobita,

Ranger station 

EXPLANATION

(A)

(B)
Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 81

Figure 50. Beryllium concentrations plotted against fluoride concentrations for (A) Straight Creek alluvial ground waters and (B) all ground and surface waters. Beryllium concentrations plotted against lithium 

concentrations for (C) Straight Creek alluvial ground waters and for (D) all ground waters and Straight Creek 

surface water.

0 5 10 15 20

0

0.02

0.04

0.06

0.08

0.10

0 5 10 15 20

0

0.02

0.04

0.06

0.08

0.10

0 0.1 0.2 0.3 0.4

0

0.02

0.04

0.06

0.08

0.10

0 0.1 0.2 0.3 0.4

0

0.02

0.04

0.06

0.08

0.10

Beryllium concentration, in milligrams per liter

Fluoride concentration, in milligrams per liter

Lithium concentration, in milligrams per liter

 Linear fit, Upper and lower 95-percent prediction limit,

Straight Creek

, , , , 

, , , , , , , , , 

, 

SC1A SC1B SC2B SC3A

, , , , , 

SC3B SC4A SC6A SC5A

SC5B AWWT1

, , 

AWWT2 SC7A SC8A CC1A CC1B CC2A CC2B

Hansen Hottentot La Bobita Ranger station 

EXPLANATION

(C) (D)

(A) (B)
Each of the three main drainages is considered below 

in sequence from west to east of the mine-site property. 

Pre-mining solute concentrations for the mine site are 

based on the hydrogeochemical processes found to control 

the concentrations at the Straight Creek analog site modified 

by the differences observed between Straight Creek and minesite mineralogy, lithology, and hydrology. These differences 

are primarily related to the position of the weathering surface 

with respect to the sequence of hydrothermal alteration zones 

that change with distance from the intrusion center (Ludington 

and others, 2005).

Capulin Canyon

Capulin Canyon is composed mostly of Amalia Tuff of 

rhyolitic composition with an exposure of andesite appearing 

in the upper part of the canyon and a mineralized zone in the 

lower part of the canyon. A portion of this mineralized zone 

forms a scar on the east side of the catchment. At the head of 

the canyon, waste piles reside from the open-pit operation, and 

their leachates were allowed to discharge into the canyon until 

the early 1990s when a collection impoundment contained 

these waters and a pump system discharged them to Goat Hill 

Gulch. This complex lithology with overlying waste rock from 

mining explains the wide range of ground-water compositions that have been recorded. It also means that there is not 

a simple pre-mining ground-water composition that can be 

assigned to Capulin Canyon.

Although Capulin Canyon contains waste-rock piles that 

have been weathering and leaching solutes into this drainage 

since the late 1960s, the waste-rock leachates were diverted 

with a collection system that was constructed beginning in 

1992. In light of this diversion, it was thought possible that 

ground water monitored downgradient from the impoundment diversion in the canyon might be dominated by natural 

processes with little effect from mine wastes. Of the three 

main drainages in the mine area, Capulin Canyon has seen the 

least impact from mining and mineral processing activities 

and has been the least affected by the ground-water cone of 

depression around the open pit, underground mines, and the 

pumped alluvial aquifers. Consequently, plots of water chemistry were prepared with the specific objectives of determining 

82 Investigation 25—Summary Report

Figure 51. Topographic map of mine site with watershed boundaries, scar areas, debris fans, and Red River alluvium.

0

Base data from U.S. Geological Survey Digital Raster Graphic

as developed from 1982 aerial photography

Molycorp mine boundary

Watershed boundary

EXPLANATION

Debris fan

Alteration scar

Red River alluvium

0.5 1.0 MILE

0 0.5 1.0 KILOMETER
(1) whether the chemistry of the waste-pile leachates could be 

distinguished from leachates from natural scar weathering and 

(2) the range of water chemistry from natural scar weathering 

in this canyon.

Those plots (figs. 52–54) reveal that the impoundment 

water collecting waste-rock leachates has generally higher 

concentrations of most constituents than the water derived 

from natural scar weathering. Sulfate concentrations are 

commonly 8,000 to 18,000 mg/L in the impoundment water 

compared to less than 2,500 mg/L for scar-weathering water. 

The pH values for both impoundment water and scar water are 

consistently low, between 2.5 and 3.5 (fig. 52B). It is unclear 

how much of the higher concentration is caused by evaporation compared to greater dissolution rate of minerals. Both 

factors could be significant.

Trace and minor elements, especially high beryllium 

and fluoride concentrations, seem characteristic of waste-pile 

leachates. Figures 53A and B plot the lower concentration 

range of fluoride against calcium and against sulfate, respectively. The impoundment waters shown in parallel figures 53C 

and 53D show the much higher concentrations of fluoride 

contained in the impoundment waters.

Figures 54A–54D show the low and high ranges of 

concentrations for zinc and manganese relative to sulfate and 

how much higher the concentrations in the impoundment are 

than in the ground waters and scar leachate seep waters. Figures 55A–55D show the low and high ranges of concentrations 

for nickel and cobalt relative to sulfate. Again, the highest 

concentrations are in the impoundment water. The strong correlation between cobalt and nickel is consistent with a single 

source for these elements, that is, pyrite (figs. 55B and 55D).

There is a distinct possibility that systematic sampling 

for water and sulfate stable isotopes in this canyon could 

reveal more quantitatively the differences between waste-pile 

weathering and natural-scar weathering and the influence of 

evaporation, but that remains to be studied. The chemical data 

do show that (1) alluvial well MMW–23A may contain some 

water derived from waste-pile leachates, (2) bedrock well 

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 83

Figure 52. Plot of pH against sulfate concentration for (A) seeps and ground waters in Capulin Canyon with less 

than 3,000 milligrams per liter sulfate concentration and (B) same waters including those surface-water impoundment 

waters with greater than 3,000 milligrams per liter sulfate concentration.

0 3,000 6,000 9,000 12,000 15,000 18,000 21,000

1

2

3

4

5

6

7

8

9

0 500 1,000 1,500 2,000 2,500 3,000

1

2

3

4

5

6

7

8

9

 Seep water near scar in Capulin Canyon

 Surface-water impoundment sample from 

 waste-rock drainage in Capulin Canyon

 Alluvial ground water near scar in Capulin 

 Canyon (MMW–2)

Field pH, in standard units

EXPLANATION

Sulfate concentration, in milligrams per liter

Bedrock ground water near scar in Capulin 

 Canyon (MMW–3)

Alluvial ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23A) 

Bedrock ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23B)

(A) (B)
84 Investigation 25—Summary Report

Figure 53. (A) Plot of fluoride concentrations (less than 60 milligrams per liter) against calcium concentrations for Capulin Canyon waters. (B) Plot of fluoride concentrations (less than 60 milligrams per liter) against 

sulfate concentrations (less than 5,000 milligrams per liter). (C) Same plot as figure 53A but showing the 

entire range of concentrations. (D) Same plot as figure 53B but showing the entire range of concentrations.

0 100 200 300 400 500 600 700 0

20

40

60

80

100

120

140

160

0 1,000 2,000 3,000 4,000 5,000 0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 0

10

20

30

40

50

60

0 2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 0

20

40

60

80

100

120

140

160

Calcium concentration, in milligrams per liter

Fluoride concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter

(C) (D)

(A) (B)

 Seep water near scar in Capulin Canyon

 Surface-water impoundment sample from 

 waste-rock drainage in Capulin Canyon

 Alluvial ground water near scar in Capulin 

 Canyon (MMW–2)

EXPLANATION

Bedrock ground water near scar in Capulin 

 Canyon (MMW–3)

Alluvial ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23A) 

Bedrock ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23B)
Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 85

Figure 54. (A) Plot of zinc against manganese concentrations (less than 120 milligrams per liter) for Capulin 

Canyon waters. (B) Plot of zinc against sulfate concentrations (less than 5,000 milligrams per liter) for Capulin 

Canyon waters. (C) Same plot as figure 54A but showing the entire range of concentrations. (D) Same plot as 

figure 54B but showing the entire range of concentrations.

0 20 40 60 80 100 120

0

5

10

15

20

25

0 2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000

0

20

40

60

80

100

120

140

160

180

200

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000

0

4

8

12

16

20

0 100 200 300 400 500

0

10

20

30

40

50

60

70

80

90

100

110

120

Sulfate concentration, in milligrams per liter

Zinc concentration, in milligrams per liter

Manganese concentration, in milligrams per liter

(C) (D)

(A) (B)

 Seep water near scar in Capulin Canyon

 Surface-water impoundment sample from 

 waste-rock drainage in Capulin Canyon

 Alluvial ground water near scar in Capulin 

 Canyon (MMW–2)

EXPLANATION

Bedrock ground water near scar in Capulin 

 Canyon (MMW–3)

Alluvial ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23A) 

Bedrock ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23B)
86 Investigation 25—Summary Report

Figure 55. (A) Plot of nickel against sulfate concentrations (less than 5,000 milligrams per liter) for Capulin 

Canyon waters. (B) Plot of cobalt against nickel concentrations (less than 1.4 milligrams per liter) for Capulin 

Canyon waters. (C) Same plot as figure 55A but showing the entire range of concentrations. (D) Same plot as 

figure 55B but showing the entire range of concentrations.

0 1,000 2,000 3,000 4,000 5,000

0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12

0

1

2

3

4

5

6

0 2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0

0.1

0.2

0.3

0.4

0.5

Nickel concentration, in milligrams per liter

Sulfate concentration, in milligrams per liter Nickel concentration, in milligrams per liter Cobalt concentration, in milligrams per liter

(C) (D)

(A) (B)

 Seep water near scar in Capulin Canyon

 Surface-water impoundment sample from 

 waste-rock drainage in Capulin Canyon

 Alluvial ground water near scar in Capulin 

 Canyon (MMW–2)

EXPLANATION

Bedrock ground water near scar in Capulin 

 Canyon (MMW–3)

Alluvial ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23A) 

Bedrock ground water near waste-rock drainage 

 in Capulin Canyon (MMW–23B)
MMW–23B appears to contain uncontaminated ground water, 

(3) alluvial well MMW–2 appears to be strongly influenced 

by scar drainage although influence from waste-rock leachates 

cannot be ruled out completely, (4) bedrock well MMW–3 is 

indicative of water in natural but mineralized bedrock, and (5) 

wells CC–1B, CC–2A, and CC–2B, located away from wasterock influence, reflect water chemistry from both unmineralized and mineralized rock, indicating effects of mineralized 

rock on water chemistry away from obvious scar areas. 

Historical samples of ground-water chemistry were collected 

with objectives other than attempting to discern waste-water 

signatures with those from natural weathering. Consequently, 

it is difficult to assign specific pre-mining ground-water quality parameters in this catchment.

The range of concentrations for dissolved constituents 

in Capulin Canyon is much larger than expected because part 

of the canyon is largely unmineralized and other parts are 

strongly mineralized. Nonetheless, the following conclusions 

can be drawn:

1. The ground waters in Capulin Canyon can be divided into 

those alluvial waters unaffected by scar leachate (represented by CC–1B, CC–2A, and CC–2B), those affected 

by scar leachate (represented by MMW–2), and bedrock 

ground waters.

2. Analytical results for waters from uncontaminated Capulin Canyon alluvial aquifers reflect a large range of water 

compositions that tend to be high in concentration relative 

to New Mexico ground-water quality standards. The range 

of water-quality parameters is given in table 7 for the 

constituents of concern.

3. The range of values of ground-water chemistry demonstrates that Capulin Canyon contains mineralized tuff and 

andesite buffered by carbonate minerals where the pyrite 

content is low.

4. Most important, the main scar is located in the lower 

portion of the canyon and, consequently, acid waters with 

elevated metal concentrations dominate the water chemistry before entering the Red River alluvial system.

5. These concentrations are within or close to the range of 

concentrations found in the debris-flow aquifer at Straight 

Creek with two notable exceptions—fluoride and beryllium. The higher concentrations of these two elements 

found in the Capulin ground waters are likely a result of 

the more intense alteration and higher content of fluorite. 

Also, beryllium and fluoride tend to correlate well both 

in the chemistry of these alteration zones and in waters 

derived from these rocks. The abundance of these elements in the water is no doubt related to both abundance 

in the rocks and the strong aqueous complexation between 

fluoride and beryllium under acidic conditions.

6. The values for Capulin Canyon alluvium upgradient 

from the scar are based on the CC wells, bedrock wells 

at Straight Creek, and mineral solubility controls where 

applicable (table 7). The values for Capulin Canyon alluvium downgradient from the scar are based on MMW–2 

and mineral solubility controls (table 8). The values for 

bedrock are based on MMW–3 and mineral solubility 

controls (table 9).

The bedrock ground waters in Capulin Canyon are 

buffered to circumneutral pH values and have substantially 

lower metal concentrations. The analytical data provided for 

bedrock wells MMW–23B and MMW–3 did not always have 

low enough detection limits for the purpose of quantifying 

trace-element concentrations of concern. Sulfate, manganese, 

cobalt, and fluoride are at concentrations greater than the 

ground-water quality standards. Sulfate could be as high as 

2,000 mg/L and fluoride as high as 5 mg/L based on the available monitoring data.

Ground water from MMW–23B is substantially more 

dilute than that from MMW–3, which reflects the effect of 

mineralized zones in the lower part of the drainage. Sulfate 

concentrations for MMW–23B are about 250 mg/L and dissolved solids about 550 mg/L compared to average concentrations of 1,500 and 2,500 mg/L, respectively, for MMW–3. 

Ground water in MMW–23B would also be more dilute 

because it is closer to recharge, whereas ground water in 

MMW–3 is closer to discharge.

Goat Hill Gulch

Goat Hill Gulch has exposures of Amalia Tuff in the 

upper half of the catchment and Tertiary andesite in the lower 

half with an exposure of biotite-rich granite appearing near the 

middle. Large quantities of scar are exposed in steep canyons, 

and a large debris fan has aggraded and deposited sediment 

across part of the Red River. This catchment is dominated by 

QSP alteration. Physical erosion has been very rapid in the 

recent past such that unweathered pyrite can still be found in 

downgradient exposures of debris.

The weathering surface for Goat Hill Gulch with respect 

to the position of the alteration zones is very similar to that of 

Straight Creek as estimated by Ludington and others (2005), 

but the size of the debris fan is much larger. Consequently, the 

estimated values of pre-mining ground-water chemical constituents and properties should be no less in concentration than 

those in the Straight Creek study. For this catchment we have 

used the highest concentrations of constituents in the Straight 

Creek study, those measured in waters from well SC–1A; however, beryllium and fluoride concentrations should be as high 

as those in Capulin Canyon. The results are listed in table 10. 

If the concentration of a constituent from SC–1A was less than 

the highest concentration from Capulin alluvial water, then the 

Capulin value was used to replace it (for example, iron and 

manganese).

Following the development of the parameters in table 10, 

corroboration of the water chemistry in Goat Hill Gulch debris 

fan unaffected by mining became possible with water-quality 

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 87
Table 7. Range of values for water-quality constituents of concern for Capulin Canyon alluvial ground waters unaffected by scar 

drainage.

[Values exceeding ground-water quality standards (table 1) shown in bold; mg/L, milligrams per liter; msc, mineral solubility control; calc’n, calculation; 

<, less than or equal to; <, less than; WATEQ4F speciation code, Ball and Nordstrom, 1991]

Water-quality constituent 

(Capulin alluvium above scar)

Range, in mg/L 

(except pH) Source

Aluminum (Al) 0.005 – 5 CC wells, SC–1B, SC–5B, msc

Beryllium (Be) 0.001 – 0.08 CC wells, SC–1B, SC–5B

Cadmium (Cd) <0.003 CC wells, SC–1B, SC–5B

Chromium (Cr) <0.0007 CC wells, SC–1B, SC–5B

Cobalt (Co) <0.016 CC wells, SC–1B, SC–5B

Copper (Cu) 0.0007 CC wells, SC-1B, SC-5B

Fluoride (F) 1 – 20 CC wells, SC–1B, SC–5B, msc

Iron (Fe) 0.03 – 34 CC wells, SC–1B, SC–5B, msc

Lead (Pb) <0.001 CC wells, SC–1B, SC–5B

Manganese (Mn) 0.2 – 41 CC wells, SC–1B, SC–5B, msc

Nickel (Ni) <0.028 CC wells, SC–1B, SC–5B

Sulfate (SO4

) 100 – 850 CC wells, msc

Zinc (Zn) <4 CC wells, SC wells

Dissolved solids 500 – 1,700 CC wells; WATEQ4F calc’n

pH (standard units) 6 – 7.5 CC wells

88 Investigation 25—Summary Report

Table 8. Range of values for water-quality constituents of 

concern for Capulin Canyon alluvial ground waters affected by 

scar drainage.

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter; msc, mineral solubility control; calc’n, calculation; <, less than; WATEQ4F speciation code, Ball and Nordstrom, 1991] 

Water-quality constituent 

(Capulin alluvium below 

scar)

Range, in mg/L 

(except pH) Source

Aluminum (Al) 8 – 100 MMW–2 well, msc

Beryllium (Be) 0.015 – 0.08 MMW–2 well

Cadmium (Cd) 0.004 – 0.04 MMW–2 well

Chromium (Cr) <0.007 MMW–2 well 

Cobalt (Co) 0.1 – 0.30 MMW–2 well

Copper (Cu) 0.05 – 0.4 MMW–2 well

Fluoride (F) 10 – 30 MMW–2 well

Iron (Fe) 20 – 50 MMW–2 well

Lead (Pb) 0.003 – 0.010 MMW–2 well

Manganese (Mn) 20 – 50 MMW–2 well

Nickel (Ni) 0.2 – 0.8 MMW–2 well

Sulfate (SO4

) 1,700 – 2,200 MMW–2 well, msc

Zinc (Zn) 4 – 10 MMW–2 well

Dissolved solids 2,400 – 2,700 MMW–2 well, 

WATEQ4F calc’n

pH (standard units) 3.7 – 5.5 MMW–2 well

Table 9. Range of values for water-quality constituents of 

concern for Capulin Canyon bedrock ground waters.

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter; msc, mineral solubility control; calc’n, calculation; <, less than; WATEQ4F speciation code, Ball and Nordstrom, 1991]

Water-quality 

constituent 

(Capulin bedrock)

Range, in mg/L 

(except pH) Source

Aluminum (Al) 0.01 – 1 MMW–3, msc

Beryllium (Be) 0.001 – 0.0025 MMW–3

Cadmium (Cd) 0.004 – 0.007 MMW–3

Chromium (Cr) 0.0005 – 0.005 MMW–3

Cobalt (Co) 0.07 – 0.10 MMW–3

Copper (Cu) 0.001 – 0.005 MMW–3

Fluoride (F) 2 – 5 MMW–3, msc

Iron (Fe) 0.05 – 1 MMW–3, msc

Lead (Pb) <0.001 MMW–3

Manganese (Mn) 3 – 40 MMW–3, msc

Nickel (Ni) 0.01 – 0.2 MMW–3

Sulfate (SO4

) 1,000 – 2,000 MMW–3, msc

Zinc (Zn) 0.005 – 0.5 MMW–3

Dissolved solids 2,000 – 2,900 MMW–3, 

WATEQ4F calc’n

pH (standard units) 6 – 7 MMW–3
data from wells MMW–44A and MMW–42A. These wells 

were developed and sampled in 2002 and after. They are 

located in the toe of the Goat Hill Gulch debris fan downgradient from the subsidence area that provides a sink for 

waste-rock leachates that come from both Goat Hill Gulch 

and Capulin Canyon. Waste waters from mining activities are 

effectively diverted underground (to the underground mines) 

by this subsidence area. Water from these wells should be 

unaffected by mining activities, and evidence to support this 

claim comes from four lines of evidence. First, sampling and 

analysis for more than 100 anthropogenic organic compounds 

(including solvents, petroleum products, phenols) were made 

in these ground waters, and no detectable concentrations were 

found (Molycorp, Inc., written commun., 2004). Second, 

nitrate concentrations were detected but at low concentrations 

(less than1 mg/L). Third, the concentrations of major and trace 

solutes closely resemble those in ground water from areas of 

similar pH but known not to be disturbed by mining activities. Introduction of a substantial amount of contaminants 

into the lower Goat Hill Gulch debris fan would have changed 

the chemistry of the ground water in some obvious manner. 

Fourth, if anthropogenic activities have caused the introduction of enough fluid or contaminants to change the groundwater chemistry, it would have to have been an obvious or 

intentional spill in this gulch that should have been acknowledged in the record and known to regulatory agencies. Such 

an event has not happened according to inquiries made to both 

Molycorp and regulatory agencies. Hence, it seemed reasonable to compare the pre-mining concentrations in table 10 with 

results from wells MMW–42A and MMW–44A. Water from 

these wells was not sampled or analyzed by the USGS, but a 

QA/QC report (McCleskey and others, 2004) demonstrated 

that comparisons of USGS results with those from Molycorp 

were of high quality. The comparison is given in table 11.

A few noteworthy differences appear in this comparison. 

Iron, manganese, and dissolved-solids concentrations are substantially lower in waters from wells MMW–42A and MMW–

44A, and aluminum and copper concentrations are higher in 

these waters. Other constituent concentrations are remarkably 

close to the estimated concentrations. The reason that iron and 

manganese concentrations are lower and copper concentrations are higher has to do with the difference in redox properties between waters from SC–1A and those from MMW–42A 

and MMW–44A. Waters from SC–1A are reduced (dissolved 

oxygen concentrations are near detection and usually less than 

0.5 mg/L), and waters from MMW–42A and MMW–44A 

are oxidized (dissolved oxygen concentrations range from 

1.4 to 4 mg/L). Oxidation will remove iron and manganese 

from solution and not remove copper from solution. Reduction 

will remove copper from solution. Hence, concentrations of 

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 89

Table 10. Preliminary values for water-quality constituents of 

concern for alluvial ground waters in Goat Hill.

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter]

Water-quality constituent 

(Goat Hill alluvium)

Values, in 

mg/L 

(except pH)

New Mexico 

standard, in mg/L 

(except pH)

Aluminum (Al) 100 5.0

Beryllium (Be) .08 1

.004

Cadmium (Cd) .04 .01

Chromium (Cr) .03 .05

Cobalt (Co) .4 .05

Copper (Cu) 1.0 1.0

Fluoride (F) 30 1.6

Iron (Fe) 50 1.0

Lead (Pb) .009 .05

Manganese (Mn) 50 .2

Nickel (Ni) .75 .2

Sulfate (SO4

) 2,000 600.0

Zinc (Zn) 10 10.0

Dissolved solids 2,900 1,000

pH (standard units) 3–4 6–9

1

 Federal standard, not New Mexico standard.

Table 11. Estimated preliminary values of pre-mining groundwater quality constituents for Goat Hill Gulch debris fan 

compared to the range of values from ground waters in wells 

MMW-42A and MMW-44A.

[mg/L, milligrams per liter]

Water-quality 

constituent 

(Goat Hill alluvium)

Estimated 

values, in

mg/L 

(except pH)

Range of values for wells 

MMW–42A and MMW–

44A, in mg/L (except pH)

Aluminum (Al) 100 130–350

Beryllium (Be) .08 0.03–0.06

Cadmium (Cd) .04 0.04–0.1

Chromium (Cr) .03 0.03–0.6

Cobalt (Co) .4 0.3–0.5

Copper (Cu) 1.0 3–6

Fluoride (F) 30 17–28

Iron (Fe) 50 1.5–5

Lead (Pb) .009 0.001

Manganese (Mn) 50 20–41

Nickel (Ni) .75 0.4–1.1

Sulfate (SO4

) 2,000 1,300–3,100

Zinc (Zn) 10 5.5–10

Dissolved solids 2,900 2,400–4,300

pH (standard units) 3–4 2.8–4.6
redox-sensitive species in Straight Creek ground waters cannot 

be used to estimate pre-mining concentrations in Goat Hill 

Gulch ground waters. Iron concentrations can be estimated 

by assuming full oxidation and precipitation as hydrous ferric 

oxide in the pH range given (3–4). The ground water closest 

in overall composition would be that from well SC–3A, which 

has a pH of about 3.3, dissolved iron predominantly as ferric 

iron, and sulfate concentrations of about 1,600 mg/L. Median 

iron and manganese concentrations in SC–3A are 0.5 and 

15 mg/L, respectively. These values provide a lower bound 

to the measured values from MMW–42A and MMW–44A. 

Concentrations of copper in SC–3A are about 0.8 mg/L, but 

some of the copper has been removed by reduction so that 

this value should also be a lower bound. Water from SC–3A 

has dissolved-solids concentrations of about 2,300 mg/L, 

closer to those of MMW–42A and MMW–44A than those of 

the estimated pre-mining concentration. These comparisons 

indicate that some adjustments are needed for the values of 

pre-mining ground-water quality constituents of the Goat Hill 

Gulch debris fan.

The final estimated values of ground-water quality 

constituents and properties for the Goat Hill Gulch debris fan 

were based on the preliminary values in table 11 with the following adjustments. Solute concentrations from ground waters 

of MMW–42A are more dilute than those from MMW–44A 

and are likely to have mixed with some Red River alluvial 

ground water because of the location of this well. Therefore, 

ground-water concentrations from MMW–44A were used 

as an upper limit. The pH range was left unchanged because 

it accurately represents the range that occurs naturally for 

ground waters in the Red River Valley of this water composition. The final estimated values are listed in table 12.

Sulphur Gulch

Sulphur Gulch, mistakenly named for the bright yellow 

coatings of ferrimolybdite resembling sulfur, has an erosional 

surface that cuts deep into the alteration of the molybdenum 

ore body, exposing a small portion of the intrusive body (aplite 

dikes and granitic stockwork). This catchment is probably 

least analogous to the Straight Creek alluvial system, but there 

may be some analogy with respect to the bedrock conditions at 

Straight Creek. Perhaps the most important aspect of Sulphur 

Gulch is the striking change in lithology from the upper part 

of the drainage to the lower part. At the head of the drainage, 

a scar is exposed containing QSP alteration and from which 

acid waters develop. However, at lower elevations the drainage 

cuts deeper into a thick section of fluorite-carbonate veining 

that can neutralize acid water and establish a circumneutral pH 

with low metal concentrations except for manganese, iron, calcium, magnesium, and strontium (Ludington and others, 2005, 

their fig. 6C; this report, fig. 2). Fluoride concentrations might 

also be elevated during neutralization, but because of the 

abundant amount of calcium and the consequent common-ion 

effect on the solubility of fluorite, concentrations of fluoride 

are relatively lower.

Sulphur Gulch is composed of three relatively comparable subcatchments—upper Sulphur Gulch, Blind Gulch, 

and Spring Gulch. The boundaries of these subcatchments are 

shown in figure 56. Sulphur Gulch lithology is dominated by 

a large scar of QSP alteration. Blind Gulch contains Tertiary 

granite stock, Tertiary andesite, and Proterozoic metamorphic rocks (fig. 57). Granite and andesite dominate the Blind 

Gulch outcrop with potassic alteration, carbonate-fluorite 

veining, and propylitic alteration dominating the alteration 

assemblages. Spring Gulch is a downdropped block of intense 

propylitic alteration. “AVIRIS data (Livo and Clark, 2002) 

maps of carbonate and epidote signals indicate that the most 

intense areas of propolytic alteration along the lower Red 

River occur in the downdropped Spring Gulch block east of 

the mine site... .” (Ludington and others, 2005). Hence, water 

draining upper Sulphur Gulch will be an acid-sulfate type 

similar to water draining the Straight Creek scar; water draining Blind and Spring Gulches will be a neutral-pH, carbonatebuffered type. Blind Gulch drainage mixes with upper Sulphur 

Gulch drainage, then that mixture mixes with drainage from 

Spring Gulch. If the relative proportions and compositions of 

these drainage waters can be estimated, then these mixtures 

can be simulated with a geochemical code to estimate resultant water chemistry of the drainage water that infiltrates and 

dominates the ground-water chemistry of the Sulphur Gulch 

debris fan.

90 Investigation 25—Summary Report

Table 12. Estimated final values for water-quality constituents 

of concern for alluvial ground waters in Goat Hill.

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter]

Water-quality constituent 

(Goat Hill alluvium)

Values, in mg/L 

(except pH)

New Mexico 

standard, in mg/L 

(except pH)

Aluminum (Al) 350 5.0

Beryllium (Be) .06 1

.004

Cadmium (Cd) .1 .01

Chromium (Cr) .6 .05

Cobalt (Co) .5 .05

Copper (Cu) 6 1.0

Fluoride (F) 28 1.6

Iron (Fe) 5 1.0

Lead (Pb) .001 .05

Manganese (Mn) 41 .2

Nickel (Ni) 1.1 .2

Sulfate (SO4

) 3,100 600.0

Zinc (Zn) 10 10.0

Dissolved solids 4,300 1,000

pH (standard units) 3–4 6–9

1

Federal standard, not New Mexico standard.
The proportion of water discharge, or yield, from upper 

Sulphur, Blind, and Spring Gulches has been estimated as 

35 percent, 25 percent, and 40 percent, respectively. These 

proportions were determined by using estimates of precipitation and evapotranspiration (Naus and others, 2006). The 

water compositions from each catchment were estimated as 

follows. Upper Sulphur Gulch drainage-water composition 

is assumed to be nearly identical to Straight Creek drainagewater composition. Waters from Blind and Spring Gulches 

are represented by a water issuing from the propylitic zone in 

the area west of Little Hansen (Verplanck and others, 2006). 

These water compositions, listed in table 13, proportions 

of mixing, and appropriate mineral solubility controls were 

used as input to the PHREEQCi code (Parkhurst and Appelo, 

1999) using the same thermodynamic data listed in table 1–1 

of Appendix 1. Saturation indices were checked to see if 

supersaturation occurred with respect to amorphous aluminum hydroxide and ferrihydrite, Fe(OH)3

. These are the most 

soluble forms for aluminum and iron minerals and the most 

likely ones to precipitate upon mixing under oxidizing conditions. After mixing, the water enters the alluvial aquifer of 

the debris fan, and it is assumed that the iron is reduced. The 

aluminum concentration that results from using amorphous 

aluminum hydroxide as the precipitating phase (22.8 mg/L) 

seems high relative to the trend of aluminum with pH shown 

in figure 29C. Consequently, a range of reasonable aluminous 

mineral phases was substituted for amorphous aluminum 

hydroxide to determine the effect of different solubilityproduct constants (Ksp) on the aluminum concentration. The 

results, listed in table 14, also affect the pH and iron concentrations. The water that is most analogous to this simulated 

mixture is the La Bobita ground water. Two samples with the 

closest pH values (4.45 and 4.54) from La Bobita were used 

as additional constraints for aluminum and iron concentrations 

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 91

Figure 56. Topographic map of the three subcatchments that constitute Sulphur Gulch—upper Sulfur Gulch, Blind 

Gulch, and Spring Gulch. 

Base data from U.S. Geological Survey Digital Raster Graphic

as developed from 1962 aerial photography, 1:24,000 scale

Subcatchment boundary

Erosion scar

EXPLANATION

0 1,200 2,400 FEET

0 250 500 METERS
92 Investigation 25—Summary Report

Figure 57. Geologic map of the three subcatchments that constitute Sulphur Gulch—upper Sulfur 

Gulch, Blind Gulch, and Spring Gulch.

EXPLANATION 0 0.5 MILE

0 0.5 1.0 KILOMETER

N
Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 93

Table 13. Compositions of water used for the reactive mixing calculations.

[Mixture composition is derived from two-step serial mixing calculations using PHREEQC, with mixing proportions indicated in 

column headings; specific conductance and dissolved-solids values were calculated using WATEQ4F; values of zero used for mixing 

calculations for concentrations less than method detection limit; °C, degrees Celsius; μS/cm, microsiemens per centimeter; ppm, parts 

per million; Eh, redox potential; V, volts; mg/L, milligrams per liter; <, less than; --, not calculated]

Catchment

Analog

Upper Sulphur Gulch

Straight Creek

Blind and Spring Gulches

West of Little Hansen 

Spring

Straight Creek : 

West of Little Hansen 

mixture

Proportion initial mixture 0.53 : 0.47

Proportion final mixture 0.57 : 0.43

Temperature, °C 7.7 9.5 8.96

pH 3.0 7.71 5.45

Specific conductance, μS/cm 3,776 1,080 1,898

Total dissolved solids, ppm 2,810 886 1,327

Eh, V .780 .449 .443

Constituent, mg/L

Calcium 349 176 229

Magnesium 113 39.1 61.5

Sodium 8.1 22.6 18.2

Potassium .75 2.38 1.89

Sulfate (SO4) 12,070 412 913

Alkalinity as HCO3 <1 213 31.6

Fluoride 7.70 2.00 3.73

Silica 74.2 19.1 35.8

Aluminum 91.5 <.002 22.7

Iron (total) 65.0 <.002 .16

Iron (II) .50 <.002 .15

Manganese 20.8 <.001 6.30

Copper 1.87 <.0005 .57

Nickel .727 <.002 .21

Zinc 7.63 <.004 2.31

Cadmium .039 <.0005 .012

Lead .012 <.0001 .004

Chromium .040 <.0005 .012

Cobalt -- -- --

Beryllium .078 <.001 .025

1

Initial SO4 concentration of 2,030 mg/L adjusted to charge balance the solution.
and set the lower bound. Trace elements such as Co, Ni, Cr, 

Zn, and Cd were treated conservatively and simply diluted in 

the appropriate proportions. Beryllium was the only element 

that was treated differently from the others. Beryllium was 

assumed to be present in a higher concentration in the original 

upper Sulphur Gulch drainage water than in the Straight Creek 

drainage water because of the closer proximity of Sulphur 

Gulch to beryllium-enriched alteration of the Questa ore body. 

The original concentration before mixing was assumed to be 

equal to the highest concentration found in the USGS study 

for unmined areas, that from well CC–2A (0.078 mg/L). It was 

then mixed conservatively to obtain the final concentration.

The final mixed composition in table 13, assumed to be 

similar to the type of water that entered the Sulphur Gulch 

debris fan, was used to provide the concentrations for constituents of concern listed in table 15. All values were rounded off 

to the nearest two digits except when the concentration was 

less than 1 mg/L, and then they were rounded off to the nearest digit.

Sugar Shack Catchments

The area of the Sugar Shack waste-rock piles facing the 

Red River (areas marked 1, 2, and 3 in fig. 51; areas marked 

8, 9, and 10 on plate 1) contain natural scars buried under 

waste rock. Before mining, the only aquifer of note would be 

the relatively impermeable bedrock. With waste-rock piles in 

place, they now have the potential to form overlying unconsolidated aquifers, although the greater portion of them are unsaturated (Molycorp, oral and written commun., 2002–2005). 

The question is often posed as to how much of the mobilized 

contaminants from this area originate from natural scars and 

how much from waste-rock leachates. It must be noted again 

that the aim of this USGS study was to determine what were 

the pre-mining ground-water quality concentrations, not to distinguish between what is being released currently from natural 

scars and what is being released currently from waste-rock 

piles where the two are intermixed in the same small catchment. Some of our results may be helpful in reaching those 

conclusions, but they are not the goal of this study.

These three catchments are not the same geologically. 

The two on the sides (1 and 3 in fig. 51; 8 and 10 on plate 

1; now designated “outside catchments”) have scars that are 

nearly the size of that in Straight Creek, but the total catchment area is much less. The catchment in the middle has 

a larger area than the other two and has no scar mapped 

except for rather small areas of scar overlap from the other 

catchments that could be within the errors of mapping accuracy. The middle catchment is also more deeply incised than 

the other catchments, which suggests it may be a “healed 

scar.” The term “healed scar” is meant to designate an area 

where a scar previously existed that no longer exists because 

something has changed to allow vegetation to develop and the 

erosion rate has slowed. The east forks of Straight Creek and 

Hottentot Creek may be examples of “healed scars.” Geologically, a healed scar is an area where the erosion has incised 

the QSP-altered bedrock enough that the rock now exposed 

to weathering has a decreased pyrite content. The weathering 

and erosion may have removed enough rock to get below the 

main zone of QSP alteration, or it may be simply a QSP zone 

of lower pyrite content. In either situation, the acid production has either stopped or decreased sufficiently to allow 

vegetation to grow.

The two outside catchments can be assumed to drain 

water of similar chemistry to Straight Creek. Based on the 

median values from table 5, the water-quality constituents of 

concern are listed for these catchments in table 16.

94 Investigation 25—Summary Report

Table 14. pH, aluminum, and iron concentrations calculated by 

assuming different aluminous minerals precipitate upon mixing 

in the Sulphur Gulch catchment.

[Log Ksp, logarithm of the solubility-product constant; mg/L, milligrams per 

liter; T, total; a, amorphous; μc, microcrystalline; c, crystalline]

Phase Log Ksp pH Al, in 

mg/L

Fe(T), 

in 

mg/L

Al(OH)3

(a) 10.8 5.45 22.8 0.16

Gibbsite (μc) 9.35 4.95 20.4 .18

Gibbsite (c) 8.11 4.52 19.5 .24

Kaolinite 7.435 4.18 18.8 .43

Basaluminite (a) 26.3 4.68 18.2 .20

Basaluminite (c) 22.7 4.33 17.4 .31

Alunite –1.4 5.68 23.8 .16

Table 15. Water-quality constituents of concern for lower 

Sulphur Gulch drainage and debris fan.

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter]

Water-quality constituent 

(Sulphur Gulch alluvial ground water)

Range, in

mg/L (except pH)

Aluminum (Al) 12–23

Beryllium (Be) 0.03

Cadmium (Cd) 0.01

Chromium (Cr) 0.01

Cobalt (Co) 0.09

Copper (Cu) 0.6

Fluoride (F) 3.7

Iron (Fe) 0.01–0.4

Lead (Pb) 0.004 

Manganese (Mn) 6.3

Nickel (Ni) 0.2

Sulfate (SO4

) 913 

Zinc (Zn) 2.3

Dissolved solids 1,327

pH (standard units) 4.2–5.7
The middle catchment is likely mineralized, but to a 

lesser extent than the outside catchments. An estimate of the 

chemistry of this water can be obtained as follows. The east 

fork of Straight Creek is thought to be a “healed” scar; that 

is, it probably contained a scar at one time, but weathering 

has eroded through the pyrite-rich zone and now vegetation 

can grow. There is rarely any drainage from the east fork of 

Straight Creek. On one occasion a small flow of water was 

found, and the pH and specific conductance were measured 

at 3.45 standard units and 973 μS/cm, respectively. These 

data can be used as an analog for the middle catchment water 

chemistry. The correlation between specific conductance and 

sulfate concentrations for Straight Creek surface and ground 

waters gives a sulfate concentration of 486 mg/L for the east 

fork of Straight Creek. Utilizing the correlation equations in 

table 1–2 of Appendix 1 and the estimated sulfate concentration, the concentrations of other constituents were estimated 

(first number of range in table 17). Consideration of yield 

estimates from east fork drainage would indicate that water 

mixing from this drainage with the scar drainage of Straight 

Creek to produce the ground water sampled in SC–1A would 

have to contain sulfate concentrations to 1,500 mg/L. Hence, 

the same correlations were used to derive the upper limit of 

concentrations of other constituents in table 17.

Bedrock Ground Water

Bedrock ground waters on the mine site can contain a 

large range of solute concentrations depending on the lithology and type of alteration, on whether in a predominantly 

recharge or predominantly discharge zone, and on depth 

(affects redox parameters and degree of mixing with shallow ground water or surface water). For this discussion, only 

bedrock in the saturated zone and lying under some debris or 

soil/sediment cover will be considered. Bedrock ground waters 

have already been considered for Capulin Canyon and will 

not be considered further. The bedrock ground waters under 

consideration here would include the area underlying Sulphur 

Gulch and Sugar Shack catchments. Results from the Straight 

Creek bedrock ground-water investigation will be considered 

for analogous characteristics.

Of the four wells that were completed in bedrock in the 

Straight Creek catchment (SC–1B, SC–2B, SC–3B, SC–5B) 

problems were encountered with two wells, SC–2B and 

SC–3B. Well SC–2B was supposed to be an alluvial well, but 

water was not reached until bedrock was penetrated because it 

was located too close to the side of the canyon. When the well 

was completed, about one-third of the screened interval was 

accessible to the air and was open to oxidation. It cannot be 

considered further. Well SC–3B gave water-chemistry results 

that appeared to have some similarities with alluvial ground 

water and may have been slightly affected by being in a mixing zone. Though these irregularities exist for well SC–3B, it 

may be representative of some types of bedrock ground waters 

on the mine site. A common characteristic of waters from 

wells SC–1B and SC–5B is that they have reached aqueous 

Pre-Mining Ground-Water Chemistry at Molycorp’s Questa Mine Site 95

Table 16. Water-quality constituents of concern for two Sugar 

Shack catchments (8 and 10 on plate 1).

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter]

Water-quality constituent

(Sugar Shack outside catchments)

Values, in

mg/L (except 

pH)

Aluminum (Al) 92

Beryllium (Be) .03

Cadmium (Cd) .04

Chromium (Cr) .04

Cobalt (Co) .3

Copper (Cu) 2

Fluoride (F) 8

Iron (Fe) 65

Lead (Pb) .01

Manganese (Mn) 21

Nickel (Ni) .7

Sulfate (SO4

) 2,030

Zinc (Zn) 8

Dissolved solids 2,800

pH (standard units) 3.0

Table 17. Water-quality constituents of concern for middle 

Sugar Shack catchment (area 9 on plate 1).

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter; <, less than]

Water-quality 

constituent (Sugar 

Shack catchments)

Values, 

in mg/L 

(except pH)

Source

Aluminum (Al) 33–72 Table 1–2 Appendix 1

Beryllium (Be) 0.02–0.07 Table 1–2 Appendix 1

Cadmium (Cd) <0.001–0.02 Table 1–2 Appendix 1

Chromium (Cr) < 0.005–0.02 Table 1–2 Appendix 1

Cobalt (Co) 0.07–0.25 Table 1–2 Appendix 1

Copper (Cu) 0.4–1.4 Based on Straight Creek 

surface-water correlation; Nordstrom and 

others (2005) 

Fluoride (F) 1.4–6.8 Table 1–2 Appendix 1

Iron (Fe) 6–50 Based on two Straight 

Creek samples of 525 

and 1510 mg SO4

/L

Lead (Pb) <0.0007 Table 1–2 Appendix 1

Manganese (Mn) 2–14 Table 1–2 Appendix 1

Nickel (Ni) 0.2–0.6 Table 1–2 Appendix 1

Sulfate (SO4

) 500–1,500 See text

Zinc (Zn) 0.7–5 Table 1–2 Appendix 1

Dissolved solids 700–2,000 Based on two Straight 

Creek samples of 525 

and 1,510 mg SO4

/L

pH (standard units) 3.5 Measurement, see text
saturation with respect to siderite, rhodochrosite, calcite, celestine, and gypsum. Waters from SC–5B and SC–2B also have 

reached saturation with respect to fluorite. Ground water from 

SC–3B has only reached saturation with respect to siderite, 

fluorite, and gypsum.

The mineralogy of the alteration zones occurring in 

bedrock at the mine site indicate that the mineral dissolution 

and saturation levels found in Straight Creek bedrock ground 

waters apply to the mine site. Gypsum, rhodochrosite, calcite, 

and fluorite saturation are often reached in bedrock ground 

waters on the mine site (wells MMW–34B and MMW–35B; 

LoVetere and others, 2004). Sufficient similarity in mineralogy and water chemistry exists to utilize the data from SC–1B 

and SC–5B as analogs for pre-mining bedrock ground-water 

concentrations on the mine site with the exception noted earlier that beryllium, manganese, and iron concentrations seem 

to be higher on the mine site than at the Straight Creek site. 

The results are listed in table 18. Caution must be applied in 

using these results because some bedrock ground waters may

be oxidizing or may mix with alluvial ground waters of lower 

pH so that a large range of “bedrock” ground-water chemistry is possible. The results in table 18 are meant to represent 

the larger proportion of bedrock ground-water chemistry, but 

certainly not the full range.

Summary

In April 2001, the U.S. Geological Survey (USGS) and 

the New Mexico Environment Department (NMED) began a 

cooperative study to infer the pre-mining ground-water quality at the Molycorp molybdenum mine site in the Red River 

Valley of northern New Mexico. This study was prompted by 

discussions between Molycorp and NMED and, as described 

in DP1055 (Discharge Permit 1055), indicated the USGS 

should execute a study of natural background (pre-mining) 

concentrations in ground water at the mine site. This report 

summarizes the results of this study with the estimates of premining ground-water quality.

The Red River Valley lies along a volcanic caldera 

boundary, is highly mineralized, and contains highly mineralized ground water. It is within this terrain that molybdenite 

mineralization was discovered and has provided an economic 

source of molybdenum for 85 years. It has been known that 

naturally acidic ground water occurs in this valley and may 

be widespread. It is an area where natural concentrations of 

metals are high enough to be environmentally deleterious, and 

this background level needs to be better defined so as to apply 

justifiable regulatory standards.

Most of the rock exposed to weathering both on the mine 

site and east of the mine site along the north flank of the Red 

River consists of two Tertiary volcanic flows—a rhyolitic 

tuff overlying an andesite. The Tertiary andesite (and the tuff 

to some extent) has been hydrothermally altered to varying 

degrees. The most intense alteration, quartz-sericite-pyrite 

(QSP), contains substantial pyrite and weathers quickly to 

form iron-rich acid-sulfate waters that hasten physical weathering. This QSP alteration forms scar areas of rapid debris 

flow, keeping rock outcrops denuded of vegetation. Catchments containing scars also contain acid ground waters in the 

debris fan that eventually are neutralized by the carbonatebuffered water in the Red River alluvial aquifer.

The exact flow path of ground water, the exact composition of reactive minerals and their surfaces encountered 

during ground-water flow, and the exact dissolution rate of 

every important reactive mineral for any specific site are not 

predictable quantities. Just the mineral dissolution rate alone 

is a complex function of surface area, surface composition, 

temperature, pH, and solution composition. Therefore, direct 

measurements of the spatial and temporal variations of water 

chemistry are necessary. However, once the data are obtained, 

a quantitative and deterministic interpretation that can be 

generalized for similar sites is possible. This was the approach 

used in this study.

The key to the determination of pre-mining ground-water 

quality at the mine site was to study the geological, hydrological, and geochemical processes at a proximal analog site 

that has not been disturbed by mining. The Straight Creek 

catchment was chosen for this purpose; it contains the same 

geologic and hydrologic features as the mine site and differs primarily in not having an erosional surface that cuts as 

96 Investigation 25—Summary Report

Table 18. Water-quality constituents of concern for the range 

of pre-mining bedrock ground waters exclusive of those in 

Capulin Canyon.

[Values exceeding ground-water quality standards (table 1) shown in bold; 

mg/L, milligrams per liter; <, less than or equal to]

Water-quality 

constituent

(bedrock ground waters)

Values, in

mg/L 

(except pH)

Source

Aluminum (Al) 0.004–0.01 SC–1B, SC–5B

Beryllium (Be) 0.001–0.08 SC–1B, SC–5B, 

MMW–35B, CC–2A

Cadmium (Cd) ≤0.0001 SC–1B, SC–5B

Chromium (Cr) ≤0.007 SC–1B, SC–5B

Cobalt (Co) 0.02 SC–1B, SC–5B

Copper (Cu) ≤0.003 SC–1B, SC–5B

Fluoride (F) 1–3 SC–1B, SC–5B, 

MMW–35B

Iron (Fe) 0.5–2 SC–1B, SC–5B

Lead (Pb) ≤0.001 SC–1B, SC–5B

Manganese (Mn) 2–6 SC–1B, SC–5B, 

MMW–35B

Nickel (Ni) 0.001–0.005 SC–1B, SC–5B

Sulfate (SO4

) 1,400–1,900 SC–1B, SC–5B

Zinc (Zn) ≤0.005 SC–1B, SC–5B

Dissolved solids 2,000–3,500 SC–1B, SC–5B

pH (standard units) 6.5–7.8 SC–1B, SC–5B
deep into the hydrothermally altered zones as the mine site. 

Twenty-seven reports detail the geological, mineralogical, 

hydrological, and geochemical characteristics of the analog 

site and other areas in the Red River Valley, including the mine 

site, that help to infer the physical and chemical properties of 

ground water at the mine site before mining took place. These 

27 reports include surface mapping of mineralogy by Airborne Visible-Infrared Imaging Spectroscopy (AVIRIS); environmental geology of the Red River Valley; mineralogy and 

mineral chemistry to identify the source of dissolved constituents of concern; geophysical studies on depth to ground-water 

table and to bedrock; bedrock fractures and their potential 

influence on ground-water flow; lake-sediment chemistry; 

leaching studies of scar and waste-rock materials; compilations of historical surface-water and ground-water quality; 

synoptic/tracer studies with mass loadings in the Red River; 

temporal trends in Red River water quality; reactive-transport 

modeling of the Red River; hydrology and water balance for 

the Red River Valley; ground-water geochemistry of wells in 

areas unaffected by mining; and quality assurance and quality 

control of water analyses. Specific studies aimed at elucidating ground-water processes at the Straight Creek analog site 

include electrical surveys; high-resolution seismic surveys; 

age-dating with tritium/helium; water budget, ground-water 

hydrology, and geochemistry; and comparison of mineralogy 

and lithology to that of the mine site.

Eleven wells were installed and monitored, surface-water 

drainage was monitored, and two existing wells were monitored in the Straight Creek catchment. The contact between 

the Straight Creek debris fan and bedrock was outlined with 

high-resolution seismic profiles modeled tomographically. The 

bottom of the debris fan lies on a highly irregular surface, and 

ground water in the debris fan is acidic with pH values of 3–4, 

carrying high concentrations of dissolved sulfate and metals. 

In the underlying bedrock, the ground water has neutral pH, 

and most trace elements are in low concentration, but iron, 

manganese, and sulfate remain at high concentration.

The Straight Creek debris fan is fed primarily by Straight 

Creek surface water of low pH (2.5–3) derived from the 

weathering of QSP-altered bedrock with a high pyrite content 

(up to 10 percent). This surface water contains only oxidized 

iron, but after it infiltrates the fan, it becomes reduced with the 

removal of some copper and the addition of silica and sodium. 

This ground water is diluted to one-half of its original concentration from canyon seepage and from mixing with Red River 

alluvial ground water at the toe. Some aluminum and silica are 

removed in the most diluted waters (near pH 4). Most dissolved constituents in these alluvial ground waters are diluted 

proportionally with sulfate, as shown by significant linear 

correlations with sulfate. Exceptions include barium, which is 

limited by barite solubility, and strontium, which appears to be 

derived from carbonate mineral dissolution. These same linear 

correlations held for most elements in other catchments containing naturally acidic ground water. These trends provided a 

constraint on mineral weathering under acidic conditions.

Mineral solubility controls are effective in limiting the 

concentrations of nine constituents. These controls are manifested in two ways—by the common-ion effect, which shows 

up in plots of key dissolved constituents, and by plots of 

saturation indices. Ferric iron is limited by the solubility of a 

hydrous ferric-oxide phase (HFO), and ferrous iron is limited 

by the solubility of siderite. Manganese is limited by the solubility of rhodochrosite. Aluminum and silica are limited by 

the solubility of amorphous aluminum hydroxide, a hydrous 

aluminosilicate colloid or clay, and amorphous silica, depending on pH and concentration. Calcium is limited by the 

solubility of gypsum or calcite, depending on pH. Magnesium 

concentrations approach dolomite solubility saturation for a 

few waters. Fluoride is limited by the solubility of fluorite. 

Barium is limited by the solubility of barite. Solubility limits 

for calcite, siderite, rhodochrosite, and fluorite are reached 

at circumneutral pH values. Solubility limits for HFO are 

reached at pH values of 2.5 and higher. Solubility limits for 

aluminum are reached at pH values of 4 and higher for ground 

water and 5 or higher for surface water. Gypsum and barite 

solubilities are independent of pH.

The metal-sulfate correlation trends for acidic ground 

waters at Straight Creek generally agreed well with data from 

ground waters in other catchments unaffected by mining 

activities and provided a framework to delineate ranges of 

concentrations for each catchment on the mine site where no 

solubility control exists. The geochemical constraints that 

define solute concentrations had to be applied separately to 

bedrock and alluvial aquifers and, in the example of Capulin 

Canyon, had to be applied to different parts of the catchment 

because of hydrothermal alteration and mineralization that 

changes substantially with distance. Exceptions to the correlation trends were noted for beryllium, fluoride, and manganese because examples were found where the pH is too low 

(about 6.0) for any solubility control.

Sulfate loading from natural scar areas upstream from the 

mine site (Hottentot, Straight, and Hansen Creeks) has been 

thought to be a substantial source of sulfate loading into the 

Red River during ground-water emergence in the Columbine 

Park area. Using data of yields of water flow from the appropriate catchments, a water flow balance was derived for the area 

of the upstream scars. Estimates and measurements of sulfate 

concentrations for these ground waters were then combined 

with the water-flow balance to complete a sulfate-load balance 

for this limited section of the river. The results indicate that 

sulfate from scar drainages continues downstream in the Red 

River alluvium to the mine site and might account for more 

than 50 percent of the sulfate loading that emerges into the Red 

River in the Columbine Park reach if no sulfate fluxes entered 

the Red River alluvium from the mine site. However, increases 

in sulfate concentration occur in the alluvial ground water 

along the mine-site reach, which suggests substantial sulfate 

fluxes from the mine site. This investigation has not attempted 

to measure sulfate fluxes from the mine site nor to resolve how 

much of the sulfate loading is from mining activities relative to 

that from natural scars on the mine site. Further data are needed 

to determine the sources and amounts of these loads.

Summary 97
The results demonstrate that ground-water quality standards in the State of New Mexico can be exceeded by natural 

conditions in the Red River Valley. Standards are commonly 

exceeded by tenfold with manganese, iron, fluoride, and sulfate. Manganese can exceed the standards by as much as 250 

times. The high manganese concentrations are caused by the 

common occurrence and high solubility of rhodochrosite and 

manganiferous calcite in acidic water. The high natural concentrations of other constituents were caused by the particular 

geological and hydrological conditions that exist in the Red 

River Valley. Extensive hydrothermal alteration has emplaced 

pyrite, sphalerite, chalcopyrite, calcite, rhodochrosite, fluorite, and illite with enriched concentrations of cobalt, nickel, 

cadmium, beryllium, and chromium in addition to iron, zinc, 

copper, sulfur, and manganese that are major components of 

these minerals. Cobalt and nickel are primarily found in the 

pyrite, cadmium in the sphalerite, beryllium in the illite, and 

chromium in the chlorite.

This investigation took a scientific approach to determine 

the sources of constituents and properties of regulatory concern (Al, Be, Cd, Co, Cr, Cu, F, Fe, Mn, Ni, Pb, SO4

, Zn, and 

pH) and hydrogeochemical processes affecting their transport. 

The objective was to infer pre-mining ground-water chemistry 

at an active mine site. The results have shown that the objective is achievable but greatly complicated by the small-scale 

heterogeneities in geology, mineralogy, and hydrology. Such 

heterogeneities increase the number of measurements needed, 

the level of detail that must be interpreted, and the uncertainties in the final estimates. The approach used including metalsolute concentration trends relative to sulfate concentrations, 

flux or mass-loading estimates, and the generalizations based 

on mineral solubility controls should be transferable to other 

sites where estimates of natural water/rock interactions in 

mineralized landscapes are required.

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104 Investigation 25—Summary Report
Appendix 1

Appendix 1105
Table 1–1. Thermodynamic data used by WATEQ4F for modeling aqueous speciation and mineral solubility.

[References are: 1, Ball and Nordstrom (1991); 2, Nordstrom and others (1990); 3, Nordstrom and May (1996); 4, Baron and Palmer (1996); ∆H, enthalpy of 

reaction, in kilocalories per mole; K, equilibrium constant for the reaction]

Aqueous 

species Reaction ∆H Log K Reference

OH– H2O  H+ + OH– 13.362 –14.00 2

MgOH+ Mg2+ + H2

O  MgOH+ + H+ 15.952 –11.44 2

MgF+ Mg2+ + F–  MgF+ 3.2 1.82 2

MgCO3

° Mg2+ + CO3

2–  MgCO3o

 Log KMgCO3

°

 = 0.991 + 0.00667T

2.713 2.98 2

MgHCO3

+ Mg2+ + HCO3–  MgHCO3+

 Log KMgHCO3

+ = –59.215 + 2537.455/T +2 0.92298Log10(T)

.79 1.07 2

MgSO4

° Mg2+ + SO4

2–  MgSO4o 4.55 2.37 2

CaOH+ Ca2+ + H2

O  CaOH+ + H+ 0 –12.78 2

CaHCO3

+ Ca2+ + HCO3

–  CaHCO3+

 Log KCaHCO3

+ = 1209.12 + 0.31294T – 34765.05/T – 478.782Log10(T)

2.69 1.106 2

CaCO3

° Ca2+ + CO3

2–  CaCO3

°

 Log KCaCO3

°

 = –1228.732 – 0.299444T + 35512.75/T + 485.818Log10(T)

3.545 3.224 2

CaSO4

° Ca2+ + SO4

2–  CaSO4o 1.65 2.3 2

CaF+ Ca2+ + F–  CaF+ 4.12 .94 2

NaCO3

– Na+

 + CO3

2–  NaCO3– 8.91 1.27 2

NaHCO3

° Na+ + HCO3

–  NaHCO3

° 0 –.25 2

NaSO4

– Na+

 + SO4

2–  NaSO4– 1.12 .7 2

NaF° Na+

 + F–  NaF° 0 –.24 2

KSO4

– K+

 + SO4

2–  KSO4– 2.25 .85 2

AlOH2+ Al3+ + H2O  AlOH2+ + H+

 Log KAlOH2+ = 4.771 –2899.05/T

13.24 –5 3

Al(OH)2

+ Al3+ + 2H2

O  Al(OH)2

+

 + 2H+

 Log KAl(OH)2

+ = 88.5 – 9391.6/T – 27.121Log10(T)

26.9 –10.1 3

Al(OH)3

° Al3+ + 3H2O  Al(OH)3° + 3H+ 42.16 –16.8 3

Al(OH)4

– Al3+ + 4H2

O  Al(OH)4

–

 + 4H+

 Log KAl(OH)4

– = 40.875 –10908.4/T – 11.041Log10(T)

43.38 –22.99 3

AlF2+ Al3+ + F–  AlF2+ 1.06 7.0 3

AlF2

+ Al3+ + 2F–  AlF2+ 1.98 12.7 3

AlF3

° Al3+ + 3F–  AlF3° 2.16 16.8 3

AlF4

– Al3+ + 4F–  AlF4– 2.2 19.4 3

AlSO4

+ Al3+ + SO42–  AlSO4+ 2.29 3.5 3

Al(SO4

)

2

– Al3+ + 2SO42–  Al(SO4

)

2

– 3.11 5.0 3

FeOH+ Fe2+ + H2

O  FeOH+ + H+ 13.2 –9.5 2

Fe(OH)3

– Fe2+ + 3H2

O  Fe(OH)3

–

 + 3H+ 30.3 –31 1

FeHCO3

+ Fe2+ + HCO3–  FeHCO3+ 0 2.0 2

106 Investigation 25—Summary Report
Table 1–1. Thermodynamic data used by WATEQ4F for modeling aqueous speciation and mineral solubility.—Continued

[References are: 1, Ball and Nordstrom (1991); 2, Nordstrom and others (1990); 3, Nordstrom and May (1996); 4, Baron and Palmer (1996); ∆H, enthalpy of 

reaction, in kilocalories per mole; K, equilibrium constant for the reaction]

Aqueous 

species Reaction ∆H Log K Reference

FeSO4° Fe2+ + SO4

2–  FeSO4o 3.23 2.25 1

Fe(OH)2

° Fe2+ + 2H2O  Fe(OH)2° + 2H+ 28.6 –20.5 2

FeOH2+ Fe3+ + H2

O  FeOH2+ + H+ 10.4 –2.19 2

FeSO4

+ Fe3+ + SO42–  FeSO4+ 3.91 4.04 2

FeCl2+ Fe3+ + Cl–  FeCl2+ 5.6 1.48 2

FeCl2

+ Fe3+ + 2Cl–  FeCl2+ 0 2.13 2

FeCl3

° Fe3+ + 3Cl–  FeCl3

o 0 1.13 2

Fe(OH)2

+ Fe3+ + 2H2

O  Fe(OH)2

+

 + 2H+ 17.1 –5.67 2

Fe(OH)3

° Fe3+ + 3H2O  Fe(OH)3° + 3H+ 24.8 –12.56 2

Fe(OH)4

– Fe3+ + 4H2

O  Fe(OH)4

–

 + 4H+ 31.9 –21.6 2

FeF2+ Fe3+ + F–  FeF2+ 2.7 6.2 2

FeF2

+ Fe3+ + 2F–  FeF2+ 4.8 10.8 2

FeF3

° Fe3+ + 3F–  FeF3

o 5.4 14 2

Fe(SO4

)

2

– Fe3+ + 2SO42–  Fe(SO4

)2

– 4.6 5.38 2

Fe2

(OH)2

4+ 2Fe3+ + 2H2

O  Fe2(OH)2

4+ + 2H+ 13.5 –2.95 2

Fe3

(OH)4

5+ 3Fe3+ + 4H2

O  Fe3(OH)4

5+ + 4H+ 14.3 –6.3 2

H3SiO4

– H4

SiO4°  H3SiO4

–

 + H+ Log KH3

SiO4

– = –302.3724 –.050698T + 

15669.69/T – 1119669/T2

 + 08.18466Log10(T)

6.12 –9.83 2

H2

SiO4

2– H4

SiO4°  H2SiO4

2– + 2H+

 Log KH2

SiO4

2– = –294.0184 –0.07265T + 

11204.49/T – 1119669/T2

 + 108.18466Log10(T)

17.6 –23 2

SiF6

– H4

SiO4° + 4H+ + 6F–  SiF6

2– + 4H2

O –16.26 30.18 1

SrOH+ Sr2+ + H2

O  SrOH+ + H+ –13.29 2

SrHCO3

+ Sr2+ + HCO3

–  SrHCO3+ Log KSrHCO3+ = –3.248 + 0.01486T 6.05 1.18 2

SrCO3

° Sr2+ + CO3

2–  SrCO3o

 Log KSrCO3° = –1.019 + 0.012826T 5.22 2.81 2

SrSO4

° Sr2+ + SO4

2–  SrSO4o 2.08 2.29 2

BaOH+ Ba2+ + H2

O  BaOH+ + H+ –13.47 2

BaHCO3

+ Ba2+ + HCO3

–  BaHCO3+

 Log KBaHCO3

+ = –3.0938 + 0.013669T 5.56 .982 2

BaCO3

° Ba2+ + CO3

2–  BaCO3o

 Log KBaCO3° = 0.113 + 0.008721T 3.55 2.71 2

BaSO4

° Ba2+ + SO4

2–  BaSO4o 2.08 2.29 2

MnCl+ Mn2+ + Cl–  MnCl+ 0 .61 2

MnCl2

° Mn2+ + 2Cl–  MnCl2° 0 .25 2

MnCl3

– Mn2+ + 3Cl–  MnCl3– 0 –.31 2

MnOH+ Mn2+ + H2

O  MnOH+ + H+ 14.4 –10.59 2

Mn(OH)3

– Mn2+ + 3H2

O  Mn(OH)3

–

 + 3H+ 0 –34.8 1

Appendix 1 107
Table 1–1. Thermodynamic data used by WATEQ4F for modeling aqueous speciation and mineral solubility.—Continued

[References are: 1, Ball and Nordstrom (1991); 2, Nordstrom and others (1990); 3, Nordstrom and May (1996); 4, Baron and Palmer (1996); ∆H, enthalpy of 

reaction, in kilocalories per mole; K, equilibrium constant for the reaction]

Aqueous 

species Reaction ∆H Log K Reference

MnO4

– Mn2+ + 4H2

O  MnO4

–

 + 8H+ 5e– 176.62 –127.824 1

MnO4

2– Mn2+ + 4H2

O  MnO4

2– + 8H+

 + 4e– 150.02 –118.44 1

MnF+ Mn2+ + F–  MnF+ 0 .84 2

MnSO4

° Mn2+ + SO4

2–  MnSO4

° 3.37 2.25 2

MnHCO3

+ Mn2+ + HCO3–  MnHCO3+ 0 1.95 2

CuCl2

– Cu+

 + 2Cl–  CuCl2

– 1.23 8.22 1

CuCl3

2– Cu+

 + 3Cl–  CuCl3

2– 1.91 8.42 1

CuCO3

° Cu2+ + CO3

2–  CuCO3

° 0 6.73 1

Cu(CO3

)

2

2– Cu2+ + 2CO32–  Cu(CO3

)

2

2– 0 9.83 1

CuCl+ Cu2+ + Cl–  CuCl+ 8.65 .43 1

CuCl2

° Cu2+ + 2Cl–  CuCl2° 10.56 .16 1

CuCl3

– Cu2+ + 3Cl–  CuCl3– 13.69 –2.29 1

CuCl4

2– Cu2+ + 4Cl–  CuCl42– 17.78 –4.59 1

CuF+ Cu2+ + F–  CuF+ 1.62 1.26 1

CuOH+ Cu2+ + H2

O  CuOH+ + H+ 0 –8 1

Cu(OH)2

° Cu2+ + 2H2O  Cu(OH)2° + 2H+ 0 –13.68 1

Cu(OH)3

– Cu2+ + 3H2

O  Cu(OH)3

–

 + 3H+ 0 –26.9 1

Cu(OH)4

2– Cu2+ + 4H2

O  Cu(OH)4

2– + 4H+ 0 –39.6 1

Cu2(OH)2

2+ 2Cu2+ + 2H2

O  Cu2(OH)2

2+ + 2H+ Log KCu2(OH)22+ = 2.497 – 3833/T 17.539 –10.359 1

CuSO4

° Cu2+ + SO4

2–  CuSO4

° 1.22 2.31 1

CuHCO3

+ Cu2+ + HCO3–  CuHCO3+ 0 2.7 1

ZnCl+ Zn2+ + Cl–  ZnCl+ 7.79 .43 1

ZnCl2

° Zn2+ + 2Cl–  ZnCl2° 8.5 .45 1

ZnCl3

– Zn2+ + 3Cl– ZnCl3– 9.56 .5 1

ZnCl4

2– Zn2+ + 4Cl–  ZnCl42– 10.96 .2 1

ZnF+ Zn2+ + F–  ZnF+ 2.22 1.15 1

ZnOH+ Zn2+ + H2

O  ZnOH+ + H+ 13.4 –8.96 1

Zn(OH)2

° Zn2+ + 2H2O  Zn(OH)2° + 2H+ 0 –16. 9 1

Zn(OH)3

– Zn2+ + 3H2

O  Zn(OH)3

–

 + 3H+ 0 –28.4 1

Zn(OH)4

2– Zn2+ + 4H2

O  Zn(OH)4

2– + 4H+ 0 –41.2 1

ZnOHCl° Zn2+ + H2O + Cl–  ZnOHCl° + H+ 0 –7.48 1

ZnSO4

° Zn2+ + SO4

2–  ZnSO4

° 1.36 2.37 1

Zn(SO4

)

2

2– Zn2+ + 2SO42–  Zn(SO4

)

2

2– 0 3.28 1

ZnHCO3

+ Zn2+ + HCO3–  ZnHCO3+ 0 2.1 1

ZnCO3

° Zn2+ + CO3

2–  ZnCO3

° 0 5.3 1

Zn(CO3

)2

2– Zn2+ + 2CO32–  Zn(CO3

)

2

2– 0 9.63 1

108 Investigation 25—Summary Report
Table 1–1. Thermodynamic data used by WATEQ4F for modeling aqueous speciation and mineral solubility.—Continued

[References are: 1, Ball and Nordstrom (1991); 2, Nordstrom and others (1990); 3, Nordstrom and May (1996); 4, Baron and Palmer (1996); ∆H, enthalpy of 

reaction, in kilocalories per mole; K, equilibrium constant for the reaction]

Aqueous 

species Reaction ∆H Log K Reference

HCO3

– H+

 + CO3

2–  HCO3–

 Log KHCO3

– = 107.8871 + 0.03252849T – 5151.79/T + 563713.9T2

 – 

38.92561Log10(T)

–3.561 10.329 2

H2

CO3° H+ + HCO3

–  H2

CO3°

 Log KH2CO3°

 = 356.3094 + 0.06091964T – 21834.37/T + 1684915T2 – 

126.8339Log10(T)

–2.177 6.352 2

HSO4

– H+

 + SO4

2–  HSO4–

 Log KHSO4–

 = –56.889 + 0.006473T + 2307.9/T + 19.8858Log10(T)

3.85 1.988 2

HF° H+

 + F–  HF°

 Log KHF° = –2.033 + 0.012645T + 429.01/T

3.18 3.18 2

HF2

– H+

 + 2F–  HF2

– 4.55 3.76 2

H2

F2° 2H+ + 2F–  H2F2° 0 6.768 1

Mineral phase Reaction ∆H Log K Reference

K–Jarosite KFe3

(SO4)

2

(OH)6 + 6H+  K+ + 3Fe3+ + 2SO4

2– + 6H2

O –45.0 –11.0 4

Ferrihydrite Fe(OH)3

 + 3H+  Fe3+ + 3H2O 0 4.891 2

Siderite (c) FeCO3  Fe2+ + CO3

2– –2.48 –10.89 2

Siderite (d) FeCO3  Fe2+ + CO3

2– 0 –10.45 2

Rhodochrosite MnCO3  Mn2+ + CO3

2– –1.43 –11.13 2

Al(OH)3(a) Al(OH)3

 + 3H+  Al3+ + 3H2O –26.5 10.8 2

Gibbsite (c) Al(OH)3

 + 3H+  Al3+ + 3H2O –24.5 9.35 2

Gibbsite (c) Al(OH)3

 + 3H+  Al3+ + 3H2O –22.8 8.11 2

K–Alunite KAl3

(SO4)2(OH)6 + 6H+  K+ + 3Al3+ + 2SO4

2– + 6H2

O –50.25 –1.4 2

SiO2(a) SiO2

 + 2H2O  H4SiO4°

 Log KAmorphous Silica = –0.26 – 731/T

3.34 –2.71 2

Chalcedony SiO2

 + 2H2O  H4SiO4°

 Log KChalcedony = –0.09 – 1032/T

4.72 –3.55 2

Quartz SiO2

 + 2H2O  H4SiO4°

 Log KQuartz = 0.41 – 1309/T

5.99 –3.98 2

Fluorite CaF2  Ca2+ + F–

 Log KFluorite = 66.348 – 4298.2/T – 2571Log(T)

4.69 –10.6 2

Calcite CaCO3  Ca2+ + CO3

2–

 Log KCalcite = –171.9065 – .077993T + 2839.319/T + 71.595Log(T)

–2.297 –8.48 2

Gypsum CaSO4

·2H2O  Ca2+ + SO4

2– + 2H2

O

 Log KGypsum = 68.2401 – 3221.51/T – 25.0627Log(T)

–.109 –4.58 2

Dolomite CaMg(CO3

)

2  Ca2+ + Mg2+ + 2CO3

2– –9.436 –17.09 2

Strontianite

SrCO3  Sr2+ + CO3

2–

 Log KStrontianite = 155.0305 – 7239.594/T – 56.58638Log(T) –.40 –9.271 2

Celestite

SrSO4  Sr2+ + SO4

2–

 Log KCelestite = 14805.9622 – 2.4660924T +756968.533/T – 40553604/T2

+ 5436.3588Log(T)

–1.037 –6.63 2

Barite

BaSO4  Ba2+ + SO4

2–

 Log KBarite = 136.035 – 7640.41/T – 48.595Log(T) 6.35 –9.97 2

Appendix 1 109
110 Investigation 25—Summary Report

Table 1–2. List of analytic equations for linear best fits of correlated data in the form of y=mx + b.

[R2 is the correlation coefficient]

y–axis 

parameter

x–axis 

parameter equation R2 Data used for linear regression

aluminum sulfate y = 0.0385x + 14.5 0.93 Straight Creek, SC1A, SC3A, SC6A, SC5A

beryllium fluoride y = 0.0026x – 0.0013 0.85 SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A

beryllium lithium y = 0.1058x – 0.0005 0.87 SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A

beryllium sulfate y = 0.00005x – 

0.0046

0.84 SC1A, SC3A, SC6A, SC5A

cadmium sulfate y = 0.00002x – 

0.0103

0.96 Straight Creek, SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A, SC8A

cadmium zinc y = 0.0056x – 0.0031 0.92 Straight Creek, SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A, SC8A

calcium sulfate y = 0.17x + 14.2 0.95 Straight Creek, SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A, SC8A

chromium sulfate y = 0.00002x – 

0.0107

0.75 Straight Creek, SC1A, SC3A, SC6A, SC5A, SC7A, SC8A

chromium nickel y = 0.0558x – 0.0063 0.76 Straight Creek, SC1A, SC3A, SC6A, SC5A, SC7A, SC8A

cobalt nickel y = 0.444x – 0.0058 0.98 Straight Creek, SC1A, SC6A, SC5A, AWWT1, SC7A, SC8A

copper sulfate y = 0.0006x – 0.288 0.82 SC1A, SC3A, SC6A, SC5A

copper zinc y = 0.151x – 0.136 0.84 SC1A, SC3A, SC6A, SC5A

fluoride sulfate y = 0.0049x – 0.970 0.85 SC1A, SC3A, SC6A, SC5A

fluoride calcium y = 0.0244x – 0.260 0.87 SC1A, SC3A, SC6A, SC5A

lithium sulfate y = 0.0001x – 0.041 0.84 SC1A, SC3A, SC6A, SC5A

magnesium sulfate y = 0.0049x – 0.970 0.95 SC1A, SC3A, SC6A, SC5A

magnesium calcium y = 0.272x + 14.3 0.95 SC1A, SC3A, SC6A, SC5A

manganese sulfate y = 0.0111x – 3.06 0.97 Straight Creek, SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A, SC8A

nickel sulfate y = 0.0004x – 0.0289 0.95 Straight Creek, SC1A, SC3A, SC6A, SC5A

silica sulfate y = 0.0158x + 63.8 0.75 SC1A, SC3A, SC6A, SC5A

specific conductance sulfate y = 1.46x + 179 0.97 Straight Creek, SC1A, SC3A, SC5A, AWWT1, SC7A, SC8A

specific conductance sulfate y = 1.20x + 391 0.97 Red River

zinc sulfate y = 0.0041x – 1.28 0.96 Straight Creek, SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A, SC8A

zinc manganese y = 0.371x – 0.140 0.99 Straight Creek, SC1A, SC3A, SC6A, SC5A, AWWT1, SC7A, SC8A
Appendix 2. Mathematical derivation of curves for mixing lines shown in 

figures 8 and 9.

The derivation for the mixing lines shown in figures 8 and 9 is based on three important assumptions:

1. Constituents are soluble and conservative (do not react on mixing).

2. There are only two dominant end–member compositions (1 and 2) to produce the mixture (3).

3. The end–member compositions are sufficiently different to discern mixing.

From conservation of mass:

M1 + M2 = M3, where M = mass and M3 = mass of mixture

Because M= CQ, where C = concentration and Q = discharge

C1Q1 + C2Q2 = C3Q3

and

Q1 + Q2 = Q3

or

Q1/Q3 + Q2/Q3 = 1

and

 x + (1–x) = 1

so that

C1x + C2(1–x) = C3

or

C2 + C1x – C2x = C3

In terms of element ratios:

(C1/C4)x + (C2/C4)(1–x) = C3/C4

For figures 8 and 9, the proportion, x, is directly related to the sulfate concentration, C4, and the element ratio, C3/C4

changes as a function of x, the mixing proportion. The ratios C1

/C4 and C2/C4 are constant for the particular time of sampling 

because they are the conservative end–members. 

Appendix 2 111
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