U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Relations Among Rainstorm Runoff, 

Streamflow, pH, and Metal Concentrations, 

Summitville Mine Area, Upper Alamosa 

River Basin, Southwest Colorado, 1995–97

By Michael G. Rupert

Denver, Colorado

2001

01–4027

Prepared in cooperation with the

U.S. ENVIRONMENTAL PROTECTION AGENCY
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225 –0046
CONTENTS III

CONTENTS

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

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

Purpose and Scope....................................................................................................................................................... 4

Study Area Description................................................................................................................................................ 4

Methods of Investigation ............................................................................................................................................. 5

Annual and Seasonal Variation of Precipitation, Runoff, and Streamflow............................................................................ 6

Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations............................................................... 6

Effects of Rainstorm Runoff on Streamflow and pH................................................................................................... 6

Effects of Annual Variations in Streamflow on pH ........................................................................................... 9

Rainstorms in Different Parts of the Basin and Their Effects on Streamflow and pH ...................................... 9

Effects of Untreated Discharge from the Summitville Mine Impoundment................................................................ 11

Variations in pH of Water Discharging from Terrace Reservoir.................................................................................. 14

Comparison of pH Values to Water-Quality Standards and Toxicological Reference Values..................................... 15

Relation Between pH and Metal Concentrations......................................................................................................... 25

Summary and Conclusions .................................................................................................................................................... 31

References.............................................................................................................................................................................. 32

FIGURES

1. Map showing locations of study area, exposure areas, and water-quality sampling sites in the 

Alamosa River Basin................................................................................................................................................. 3

2. Boxplots showing change and duration of change in streamflow due to rainstorm runoff at selected sites, 

upper Alamosa River Basin, southwest Colorado, 1995–97..................................................................................... 8

3. Map showing streamflow and pH, August 30–31, 1997, upper Alamosa River Basin, southwest Colorado .......... 10

4. Boxplots showing change and duration of change in pH due to rainstorm runoff at selected sites, upper 

Alamosa River Basin, southwest Colorado, 1995–97............................................................................................... 12

5. Boxplots showing traveltimes of peak streamflow and low-pH front through selected segments of the 

Alamosa River and Wightman Fork, upper Alamosa River Basin, southwest Colorado, 1995–97 ......................... 14

6. Map and graphs showing streamflow and pH during July 9–15, 1997, upper Alamosa River Basin, 

southwest Colorado................................................................................................................................................... 16

7–18. Graphs showing:

7. Streamflow and pH at Alamosa River above Terrace Reservoir and Alamosa River below Terrace 

Reservoir, August 14 through September 30, 1996, upper Alamosa River Basin, southwest Colorado .......... 17

8. Federal water-quality standards, Toxicological Reference Values, and pH at Wightman Fork below 

Cropsy Creek, upper Alamosa River Basin, July 1, 1995, through September 30, 1997 ................................. 19

9. Federal water-quality standards, Toxicological Reference Values and pH at Wightman Fork at mouth, 

upper Alamosa River Basin, July 1, 1995, through September 30, 1997 ......................................................... 20

10. Colorado and Federal water-quality standards, Toxicological Reference Values, and pH at Alamosa River 

above Wightman Fork, upper Alamosa River Basin, July 1, 1995, through September 30, 1997.................... 21

11. Colorado and Federal water-quality standard, Toxicological Reference Values, and pH at Alamosa River 

below Jasper, upper Alamosa River Basin, July 1, 1995, through September 30, 1997................................... 22

12. Colorado and Federal water-quality standards, Toxicological Reference Values, and pH at Alamosa River 

above Terrace Reservoir, upper Alamosa River Basin, July 1, 1995, through September 30, 1997................. 23

13. Colorado and Federal water-quality standards, Toxicological Reference Values, and pH at Alamosa River 

below Terrace Reservoir, upper Alamosa River Basin, July 1, 1995, through September 30, 1997 ................ 24

14. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994–97, 

at Wightman Fork below Cropsy Creek, upper Alamosa River Basin, southwest Colorado............................ 26

15. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994–97, 

at Wightman Fork at mouth, upper Alamosa River Basin, southwest Colorado .............................................. 27

16. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994–97, 

at Alamosa River above Wightman Fork, upper Alamosa River Basin, southwest Colorado.......................... 28
IV CONTENTS

17. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994–97, 

at Alamosa River below Jasper, upper Alamosa River Basin, southwest Colorado......................................... 29

18. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994–97, 

at Alamosa River above Terrace Reservoir, upper Alamosa River Basin, southwest Colorado....................... 30

TABLES

1. Upper Alamosa River Basin streamflow-gaging station numbers, site identifiers, station names, elevations, 

and drainage areas ................................................................................................................................................... 4

2. Statistical summary of streamflow and pH at selected sites in the upper Alamosa River Basin, southwest 

Colorado, during July, August, and September 1995, 1996, and 1997................................................................... 7

3. Statistical summary of changes in streamflow and pH due to rainstorm runoff at selected sites in the 

upper Alamosa River Basin, southwest Colorado, during July, August, and September 1995, 1996, and 1997 ... 9

4. Statistical summary of traveltimes of peak streamflow and low-pH front through selected segments of the 

Alamosa River and Wightman Fork, upper Alamosa River Basin, southwest Colorado, during July, August, 

and September 1995, 1996, and 1997 ..................................................................................................................... 11

5. Statistical summary of duration of changes in streamflow and pH due to rainstorm runoff at selected sites 

in the upper Alamosa River Basin, southwest Colorado, during July, August, and September 1995, 1996, 

and 1997 .................................................................................................................................................................. 13

6. Changes in streamflow and pH due to rainstorms isolated to the reach upstream from the Alamosa River 

above Wightman Fork station during July, August, and September 1995, 1996, and 1997, upper Alamosa 

River Basin, southwest Colorado............................................................................................................................ 15

7. Percentage of pH measurements that did not meet Colorado and Federal water-quality standards and 

Toxicological Reference Values during July, August, and September 1995, 1996, and 1997, upper Alamosa 

River Basin, southwest Colorado............................................................................................................................ 17

CONVERSION FACTORS, VERTICAL DATUM, ACRONYMS, AND ABBREVIATED WATER-QUALITY UNITS

Multiply By To obtain

acre 0.4047 hectare

acre-foot (acre-ft) 1,233 cubic meter

cubic foot per second (ft3/s) 0.02832 cubic meter per second

foot (ft) 0.3048 meter

inch 25.4 millimeter

mile (mi) 1.609 kilometer

miles per hour (mi/h) 1.609 kilometers per hour

square mile (mi2

) 2.590 square kilometer

Degree Celsius (oC) may be converted to degree Fahrenheit (oF) by using the following equation:

oF = 9/5 oC+32

Degree Fahrenheit (oF) may be converted to degree Celsius (oC) by using the following equation:

o

C = 5/9 (oF–32)

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
CONTENTS V

Additional acronyms and site identifiers:

AR45.5 Alamosa River above Wightman Fork

AR41.2 Alamosa River below Jasper

AR34.5 Alamosa River above Terrace Reservoir

AR31.0 Alamosa River below Terrace Reservoir

EWI Equal width increment

EA Exposure area 

WF5.5 Wightman Fork below Cropsy Creek 

WFO.0 Wightman Fork at mouth

Abbreviated water-quality units:

mg/L milligrams per liter

μg/L micrograms per liter
Abstract 1

Relations Among Rainstorm Runoff, Streamflow, 

pH, and Metal Concentrations, Summitville Mine 

Area, Upper Alamosa River Basin, Southwest 

Colorado, 1995–97

By Michael G. Rupert

Abstract

The upper Alamosa River Basin contains 

areas that are geochemically altered and have 

associated secondary sulfide mineralization. 

Occurring with this sulfide mineralization are 

copper, gold, and silver deposits that have been 

mined since the 1870's. Weathering of areas with 

sulfide mineralization produces runoff with 

anomalously low pH and high metal concentrations; mining activities exacerbate the condition. 

Summer rainstorms in the upper Alamosa River 

Basin produce a characteristic relation between 

streamflow and pH; streamflow suddenly 

increases and pH suddenly decreases (commonly 

by more than 1 pH unit). This report evaluates 

changes in pH in the upper Alamosa River Basin 

during July, August, and September 1995, 1996, 

and 1997 to examine possible adverse environmental effects due to rainstorm runoff.

Ninety-three percent of the rainstorms 

occurring during 1995–97 produced runoff 

throughout the entire basin. Out of 54 storms, 

only 3 storms were isolated to the river reach 

upstream from the streamflow-gaging station 

Alamosa River above Wightman Fork, and only 

1 storm was isolated to the river reach between 

the streamflow-gaging stations Alamosa River 

below Jasper and Alamosa River above Terrace 

Reservoir.

Although most rainstorm runoff events 

occurred throughout the entire basin, pH changes 

were highest in parts of the basin that receive 

runoff from hydrothermally altered areas. The 

three principal altered areas within the basin are 

the Jasper, Stunner, and Summitville areas. Only 

limited mining occurred in the Stunner altered 

area, and yet significant decreases in pH values 

occur due to runoff from this area. Even after 

environmental restoration activities are completed 

at the Summitville Mine, the main stem of the 

Alamosa River may continue to be adversely 

affected by runoff from the Stunner and Jasper 

altered areas.

A comparison of measured pH with Federal 

and State of Colorado water-quality standards and 

Toxicological Reference Values indicates pH was 

too low to support aquatic life in many parts of 

the basin for extended periods of time. Added 

stresses from sudden decreases in pH due to rainstorm runoff compound the adverse effects. 

Discharge of effluent from the Summitville 

Mine impoundment can significantly decrease pH 

in the Alamosa River downstream to Terrace 

Reservoir. A release of only 3 cubic feet per 

second from the impoundment decreased pH by at 

least 1 standard unit at all downstream sites.

Low-flow years may pose a substantial risk 

to aquatic organisms within and downstream from 

Terrace Reservoir. During 1996, the basin had a 

low-flow year, and water storage and pool size of 

Terrace Reservoir were significantly reduced. The 

pH of water discharging from Terrace Reservoir 

was anomalously low during late August and 

September 1996, possibly due to geochemical 
2 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

interactions between sediment and the water 

column within the reservoir. 

In general, an inverse log-log relation exists 

between pH and the logarithm of dissolved metal 

concentrations, but the relations generally are not 

significant enough to confidently predict metal 

concentrations based upon measured pH values. 

INTRODUCTION

The Summitville Mine area of the upper 

Alamosa River Basin in Colorado (fig. 1) has been 

mined for copper, gold, and silver since the 1870's. 

Most mining operations at Summitville were on a relatively small scale until the mid-1980’s, when mining 

operations were expanded to include a large open-pit 

cyanide heap leach operation (Pendleton and others, 

1995). Cyanide and acid solutions were inadvertently 

discharged to Wightman Fork on numerous occasions 

during the late 1980's and early 1990's. Mining operations also exposed sulfide minerals to oxygen, causing 

acid-mine drainage and elevated metal concentrations 

in ground water and surface water at the mine site 

(Pendleton and others, 1995). In December 1992, the 

mining company declared bankruptcy and the U.S. 

Environmental Protection Agency (USEPA) initiated 

an Emergency Response Action to help reduce environmental effects from the mine. Since that time, the 

Summitville mine was declared a Superfund site and 

the USEPA and the Colorado Department of Public 

Health and Environment (CDPHE) have been taking 

remedial actions at the mine to help reduce adverse 

environmental effects.

Mining operations at Summitville are only one 

source of low pH and high metal concentrations in the 

Alamosa River Basin upstream from Terrace Reservoir (upper Alamosa River Basin). Significant natural 

sources of contaminants are Iron, Alum, Jasper, and 

Burnt Creeks. Kirkham and others (1995) inventoried 

219 mine openings and 130 mine dumps in the upper 

Alamosa River Basin, and estimated that nearly 11 

percent of the iron, 18 percent of the aluminum, and 1 

percent of the copper, manganese, and zinc in surface 

water upstream from the confluence of the Alamosa 

River with Wightman Fork are supplied by abandoned 

mines. The balance of these dissolved metals is from 

natural sources. Miller and McHugh (1994) identified 

five major altered areas in and adjacent to the upper 

Alamosa River Basin. Three of those altered areas are 

shown in figure 1.

Mining operations in the upper Alamosa River 

Basin, particularly at the Summitville Mine, are implicated in the elimination of trout populations in the 

Alamosa River and Terrace Reservoir. In 1889, the 

Alamosa River was known as a trout stream that 

contained Rio Grande cutthroat trout (Woodling, 

1995). As early as 1974, negative effects from acidmine drainage in the upper Alamosa River Basin were 

identified (Wentz, 1974). By 1985, the Rio Grande 

cutthroat had disappeared; however, reproducing 

brook trout were reported in an Alamosa River headwater reach (Woodling, 1995). From 1960 through 

1990, Terrace Reservoir was stocked with fingerling 

rainbow trout, which grew to catchable size and were 

harvested by anglers (Woodling, 1995). In the mid1980’s, the open-pit cyanide heap leach operation at 

Summitville, Colorado, was opened. Discharges from 

the facility increased the metal concentrations (primarily copper) in Wightman Fork and the Alamosa 

River downstream from the confluence with 

Wightman Fork. In June 1990, a fishkill in a privately 

owned pond supplied with irrigation water from the 

Alamosa River was confirmed by the Colorado Division of Wildlife (CDOW). On July 12, 1990, the 

CDOW concluded that no fish existed in Terrace 

Reservoir (Woodling, 1995). The copper concentration 

in the Alamosa River upstream from Terrace Reservoir 

was 1,270 μg/L (micrograms per liter) on June 23, 

1990, compared to a concentration of 30 μg/L in 1986. 

The rainbow trout 96-hour LC50 for copper (the 

concentration that will kill 50 percent of the test organisms in 4 days) is 52 μg/L.

Ortiz and others (1995) observed that the pH 

significantly decreased and dissolved metals significantly increased in response to localized rainstorms in 

the upper Alamosa River Basin. Therefore, a study 

was conducted during 1999 by the U.S. Geological 

Survey (USGS), in cooperation with the USEPA, to 

evaluate the relations among rainstorm runoff, streamflow, pH, and metal concentrations. Information from 

the study is intended to assist in the evaluation of acute 

and chronic effects of rainstorm runoff on pH in the 

Alamosa River and its tributaries. This evaluation 

provided information for the Draft Tier II Summitville 

Ecological Risk Assessment that the USEPA developed for the Summitville Superfund site. 
I

NTRODUCTION

3

EA-3c

EA-4

EA-5

EA-3b

EA-1

EA-2

EA-3a

EA-2

Base from

MILES

KILOMETERS

Figure 1. Locations of study area, exposure areas, and water-quality sampling sites in the Alamosa River Basin.
4 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

Purpose and Scope

The purpose of this report is to describe the 

relations among rainstorm runoff, streamflow, pH, and 

metal concentrations at six streamflow-gaging stations 

in the upper Alamosa River Basin. These stations 

(fig. 1, table 1) are: (1) Wightman Fork below Cropsy 

Creek (WF5.5, USGS station number 08235270), 

(2) Wightman Fork at mouth (WF0.0, USGS station 

number 08235290), (3) Alamosa River above 

Wightman Fork (AR45.5, USGS station number 

08235250), (4) Alamosa River below Jasper (AR41.2, 

USGS Station number 08235700), (5) Alamosa River 

above Terrace Reservoir (AR34.5; USGS Station 

number 08236000), and (6) Alamosa River below 

Terrace Reservoir (AR31.0, USGS station number 

08236500). The relations of streamflow and pH were 

based on data collected by continuous water-quality 

monitors during July 1 through September 30, 1995, 

1996, and 1997. The relations of dissolved metal 

concentrations and pH were based on nonstorm and 

storm samples collected during water years 1994–97 

(water years begin on October 1 and end on September 

30 and are designated by the calendar year in which 

they end). Annual precipitation data were evaluated 

for water years 1995, 1996, and 1997. Annual variations of precipitation in the basin were evaluated to 

examine their relation to rainstorm runoff and pH. 

Rainstorms occurring in different parts of the basin 

were evaluated to quantify effects due to runoff in 

those parts of the basin. The effects of discharge of 

effluent from an impoundment at the Summitville 

Mine was evaluated to observe pH changes in 

Wightman Fork and the Alamosa River. The pH values 

measured in Wightman Fork and the Alamosa River 

were compared to water-quality standards to evaluate 

potential effects on aquatic biota. 

Study Area Description

The upper Alamosa River Basin is in southwest 

Colorado (fig. 1). Elevations in the study area range 

from nearly 13,000 ft above sea level for the highest 

mountains to 8,400 ft near Terrace Reservoir. Annual 

precipitation ranges from approximately 40 inches in 

the highest mountains to approximately 12 inches near 

Terrace Reservoir (Miller and McHugh, 1994). Most 

of the high-elevation precipitation is in the form of 

snow.

The upper Alamosa River Basin extends from 

the Alamosa River headwaters to just downstream 

from Terrace Reservoir and has a drainage area of 

approximately 116 mi2. Terrace Reservoir stores water 

leaving the basin and serves as a trap for sediments. 

When constructed in 1912, Terrace Reservoir had a 

surface area of 300 acres and a storage capacity of 

approximately 8,110 acre-ft (Watts, 1996).

The upper Alamosa River Basin is located in the 

southeastern portion of the San Juan volcanic field 

(Bove and others, 1995). The bedrock is composed 

mostly of rhyolitic ash-flow deposits associated with 

the formation of the Platoro caldera complex. The 

bedrock of several areas in the basin is hydrothermally 

altered and contains sulfide minerals and precious 

metals. These altered areas (fig. 1) are identified easily 

in the field as tan and red bleached landscapes and are 

the ancient analogues of the active geyser basins at 

Yellowstone National Park in Wyoming (Bove and 

others, 1995). Runoff from mined areas and from 

Table 1. Upper Alamosa River Basin streamflow-gaging station numbers, site identifiers, station names, elevations, and 

drainage areas

[Exposure areas are segments of the basin that are being assessed in an Ecological Risk Assessment of the U.S. Environmental Protection Agency; 

mi2, square miles]

USGS 

station 

number

Site

identifier

Exposure

area Station name

Approximate 

elevation 

(feet above 

sea level)

Drainage 

area

(mi2)

08235270 WF5.5 Exposure Area 1 Wightman Fork below Cropsy Creek 11,090 4.44

08235290 WF0.0 Exposure Area 2 Wightman Fork at mouth 9,380 16.1

08235250 AR45.5 Exposure Area 3a Alamosa River above Wightman Fork 9,380 37.8

08235700 AR41.2 Exposure Area 3b Alamosa River below Jasper 9,030 76.3

08236000 AR34.5 Exposure Area 3c Alamosa River above Terrace Reservoir 8,610 107

08236500 AR31.0 Exposure Area 5 Alamosa River below Terrace Reservoir 8,380 116
INTRODUCTION 5

unmined altered areas adversely affects water quality 

and pH in the Alamosa River and its tributaries. Bove 

and others (1995) measured specific conductance and 

pH at more than 60 sites and observed a good correlation between the type and intensity of alteration and 

the degree to which surface water and spring water are 

degraded. The presence of old iron-oxide-cemented 

conglomerates many feet above modern streams indicates the generation of natural metal-rich, acidic 

drainage has been occurring for many thousands of 

years prior to mining in the area (Bove and others, 

1995).

The bedrock was altered through a complex 

series of geologic processes. The South Mountain 

volcanic dome was formed about 22.4 million years 

ago (Plumlee and others, 1995). As part of the domeforming cycle, additional magmatic material was 

intruded in the area beneath the dome. As the magmas 

crystallized, they released hot gases rich in sulfur 

dioxide. The gases rose along fractures and eventually 

condensed, producing fluids rich in sulfuric acid that 

extensively leached and altered the bedrock. 

Following the period of intense acid leaching, copperand gold-rich sulfide minerals were deposited in the 

highly altered dome rocks by hot hydrothermal fluids 

also derived from crystallizing magmas at depth 

(Plumlee and others, 1995). Sulfide minerals such as 

pyrite (FeS2), marcasite (FeS2), and enargite 

(Cu3AsS4) were deposited in these highly altered 

zones. Subsequent erosion exposed these sulfide 

minerals to well-oxygenated surface water and ground 

water, resulting in runoff with anomalously low pH 

values and anomalously high metal concentrations. 

Open-pit mining at Summitville exposed large quantities of sulfide minerals to oxygenated waters, creating 

adverse environmental effects much greater than those 

naturally occurring.

Acid-mine drainage is produced primarily by 

the oxidation of the mineral pyrite (FeS2). Pyrite 

oxidation is a complex process (Nordstrom and 

Alpers, 1999) that proceeds rapidly when pyrite and 

other sulfide minerals are exposed to oxygen and 

oxygenated water. Pyrite reacts with oxygen and water 

to form ferrous sulfate and sulfuric acid. The dissolved 

ferrous iron continues to oxidize and hydrolyze when 

the mine water is no longer in contact with pyrite, 

producing additional acidity. Iron- and sulfuroxidizing bacteria are known to increase reaction rates 

by several orders of magnitude (Nordstrom and 

Alpers, 1999). Acidic drainage occurs naturally in 

many locations in the upper Alamosa River Basin. 

Mining at Summitville substantially increased the 

oxidation rates by providing greater accessibility of air 

and oxygenated water through mine workings, waste 

rock, and tailings.

The USEPA has subdivided the upper Alamosa 

River Basin into five river reaches, which are termed 

Exposure Areas (EA) (fig. 1). These EAs were defined 

to enhance the ability to summarize environmental 

effects on biota within specific river reaches. These 

EAs were evaluated in the Draft Tier II Summitville 

Ecological Risk Assessment. Site WF5.5 is located at 

the downstream end of EA-1 and serves as a waterquality indicator site for EA-1. Site WF0.0 is a waterquality indicator site for EA-2, AR45.5 is a waterquality indicator site for EA-3a, AR41.2 is a waterquality indicator site for EA-3b, AR34.5 is a waterquality indicator site for EA-3c, and AR31.0 is a 

water-quality indicator site for EA-5 (fig. 1, table 1). 

Methods of Investigation

Stream-stage and pH were measured at 

six streamflow-gaging stations. Stream-stage data 

were measured every 15 minutes, and pH was 

measured every 30 minutes. These data were transferred by satellite and stored in the National Water 

Information System (NWIS) database operated and 

maintained by the USGS. Stream-stage data were 

converted to streamflow data by the NWIS database 

by using the stage/discharge relation established at 

each site. These data are available for download 

directly from the NWIS database (contact District 

Chief, U.S Geological Survey, Box 25046, Mail Stop 

415, Denver Federal Center, Denver, CO, 80225–

0046). For this study, streamflow and pH data for 30-

minute increments during July 1 through September 

30, 1995, 1996, and 1997 were analyzed using a 

variety of geographic-information-system, graphing, 

spreadsheet, and statistical software. 

Relations between pH and selected dissolvedmetal concentrations were evaluated by this study. 

There were insufficient data to specifically evaluate 

the effects of pH decreases on metal concentrations 

during storm events because only a subset of samples 

were collected during rainstorm runoff. To compile 

enough data for analysis, all dissolved-metal data 

collected throughout water years 1994–97 were used. 

Generally, composites of six samples in a 24-hour 
6 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

period were collected during the months of April 

through September. Because flow conditions were 

relatively stable in the fall and winter, equal width 

increment (EWI) grab samples were collected during 

October through March. 

Rainstorm runoff events were identified by 

observing streamflow records. Runoff events were 

identified when instantaneous streamflow increased at 

least 10 percent during a 1-hour period. Events identified using this method correlated well with daily 

precipitation data that have been collected since 

December 1992 at the Summitville Mine by USEPA 

contractors.

ANNUAL AND SEASONAL VARIATION 

OF PRECIPITATION, RUNOFF, AND 

STREAMFLOW

Annual and seasonal variations in precipitation 

affected streamflow in the upper Alamosa River 

Basin. Annual precipitation totals at the Summitville 

Mine for water years 1995, 1996, and 1997 were 

41.78, 17.12, and 72.81 inches, respectively. Water 

year 1996 was much drier than water years 1995 and 

1997, and precipitation in the summer months of 1996 

also was much less than in the summer months of 

1995 and 1997. July through September precipitation 

totals for 1995, 1996, and 1997 were l3.44, 7.24, and 

14.66 inches, respectively. The smaller amount of 

precipitation during water year 1996 is reflected in 

streamflow, where minimum, median, and maximum 

streamflows at all sites were much smaller in 1996 

(table 2) than in 1995 or 1997.

Annual and seasonal variations of precipitation 

in the upper Alamosa River Basin affected the magnitude and duration of rainstorm runoff. The number of 

rainstorm runoff events that occurred between July 1 

and September 30 was 17 in 1995, 18 in 1996, and 19 

in 1997. Although the number of runoff events was 

about the same each year, the magnitudes and durations differed among the three years. The changes in 

streamflow (difference between prestorm and peak 

flows) in 1996 were significantly less at all sites than 

in 1995 and 1997 (fig. 2). For instance, median 

changes in streamflow at AR45.5 due to rainstorm 

runoff were 30 ft3/s in 1995, 7.8 ft3/s in 1996, and 

42.5 ft3

/s in 1997 (table 3). Smaller changes in streamflow due to rainstorm runoff reflect the overall drier 

conditions in the summer of 1996.

The duration of change in streamflow due to 

rainstorm runoff on the main stem of the Alamosa 

River tended to be longer in 1996 than in 1995 and 

1997 (fig. 2; AR45.5, AR41.2, AR34.5). The generally 

longer durations of change in 1996 were related to less 

water in the Alamosa River; streamflow velocities 

were slower, and a longer time was needed for the 

peak streamflow to travel through the drainage system. 

The duration of change in streamflow tended to be less 

in 1996 at WF5.5 and WF0.0 possibly because of the 

much smaller drainage areas (table 1). 

RELATIONS AMONG RAINSTORM 

RUNOFF, STREAMFLOW, pH, AND 

METAL CONCENTRATIONS

Several factors affect streamflow, pH, and metal 

concentrations in the upper Alamosa River Basin. The 

following text evaluates the effects of rainstorm runoff 

on streamflow and pH, the effects of annual variations 

in streamflow on pH, the effects of rainstorms occurring in different parts of the basin, the effects of 

discharge of effluent from the Summitville Mine 

impoundment, the variations in pH of water 

discharging from Terrace Reservoir, a comparison of 

pH values to water-quality standards, and the relation 

between pH and metal concentrations. 

Effects of Rainstorm Runoff on 

Streamflow and pH

Rainstorm runoff in the upper Alamosa River 

Basin generally produces a characteristic relation 

between streamflow and pH. Generally, when streamflow increases due to rainstorm runoff, pH immediately decreases (fig. 3). Typically, when the rainstorm 

runoff ends, streamflow decreases to approximately 

near the prestorm discharge, and pH increases to 

approximately near the prestorm value. The sudden 

decrease in pH is probably due to runoff from hydrothermally altered areas (fig. 1) that are enriched in 

sulfide minerals. Between rainstorms, sulfide minerals 

in the soil and bedrock can oxidize. This oxidation can 

lower the pH and increase the metal concentrations in 

the sediments and interstitial water. When rainstorms 

occur in the oxidized areas, the runoff can carry the 

products of oxidized sulfide minerals into the drainage 
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 7

system, causing a decrease in the pH and an increase 

in the metal concentrations in local streams.

An example of the effects of rainstorm runoff on 

pH in the upper Alamosa River Basin is provided by 

data collected on August 30–31, 1997 (fig. 3), when a 

rainstorm produced runoff throughout the basin. As 

the streamflow increased, the pH immediately 

decreased at all sites. At AR45.5, the streamflow 

nearly doubled from about 40 ft3/s to about 80 ft3

/s; at 

the same time the pH decreased from about 6.5 to 

about 4.7. After the rainstorm, streamflow and pH 

returned to approximately the same values as before 

the rainstorm. As the peak streamflow moved downstream, the durations of the peak streamflow increased 

and the pH decreases were progressively longer. These 

longer durations may be the result of the stream water 

mixing with water from different parts of the basin.

The pH generally increases in a downstream 

direction (fig. 3, graph showing pH at all stations), 

probably as a result of dilution by runoff from unaltered areas. The increase probably does not occur as a 

result of reactions with materials in the stream channel 

because the bedrock in the downstream area is 

composed mostly of silicic volcanic rocks that have 

little buffering capacity. Also, the altered areas are 

located in the upper two-thirds of the basin (fig. 1), 

and runoff in the lower one-third of the basin is not 

affected by altered areas. Ward and Walton-Day 

(1995) observed that most dissolved-metal concentrations decrease and pH increases in a downstream 

direction. They indicated the changes in water quality 

Table 2. Statistical summary of streamflow and pH at selected sites in the upper 

Alamosa River Basin, southwest Colorado, during July, August, and September 1995, 

1996, and 1997

[WF5.5, Wightman Fork below Cropsy Creek; WF0.0, Wightman Fork at mouth; AR45.5, Alamosa River above 

Wightman Fork; AR41.2, Alamosa River below Jasper; AR34.5, Alamosa River above Terrace Reservoir; 

AR31.0, Alamosa River below Terrace Reservoir]

Year WF5.5 WF0.0 AR45.5 AR41.2 AR34.5 AR31.0

Minimum streamflow, in cubic feet per second

1995 1.6 3.6 32 39 30 67.9

1996 0.6 1.2 6.3 l0.4 19 11.8

1997 2.8 4.4 31 35 45 38.5

Median streamflow, in cubic feet per second

1995 6.7 12 61 91 104 177

1996 2.1 3.5 13 21.6 35 57

1997 6.1 11.7 59 81.9 91 100

 All 3 years 4.7 9.1 44 62 73 87.5

Maximum streamflow, in cubic feet per second

1995 51 164 218 889 920 812

1996 27 41 161 194 224 141

1997 30 79 236 350 464 290

Minimum pH, in standard units

1995 3.0 3.1 3.7 2.7 4.4 4.7

1996 2.8 3.4 3.0 3.4 4.1 4.3

1997 2.8 3.2 3.5 4.3 4.7 5.2

Median pH, in standard units

1995 3.6 4.3 5.9 4.8 5.6 5.6

1996 4.6 5.5 4.9 5.4 6.8 6.6

1997 4.1 4.9 6.5 5.9 6.7 6.7

All 3 years 4.1 4.9 6.0 5.3 6.5 6.2

Maximum pH, in standard units

1995 4.6 4.9 7.0 6.1 6.3 6.6

1996 6.7 6.6 7.2 7.0 7.3 7.1

1997 5.0 6.4 7.3 6.9 7.2 7.2
8 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

19

18

CHANGE IN STREAMFLOW,

 IN CUBIC FEET PER SECOND

17 18

19

Alamosa River above 

Wightman Fork (AR45.5)

1995

1996

1997

1995

1996

1997

17

1995

1996

1997

1995

1996

1997

Wightman Fork below

Cropsy Creek (WF5.5) Wightman Fork at mouth 

(WF0.0)

0

50

100

200

150

0

50

100

200

150

0

50

100

200

150

0

50

100

200

150

0

50

100

200

150

17

18 19

17 18

19

Alamosa River below

Jasper (AR41.2)

17

18 19

1995

1996

1997

Alamosa River above

Terrace Reservoir (AR34.5)

DURATION OF CHANGE IN STREAMFLOW,

IN HOURS

Alamosa River above 

Wightman Fork (AR45.5)

Wightman Fork below

Cropsy Creek (WF5.5) Wightman Fork at mouth 

(WF0.0)

17

18

19

17 17 18 18

19 19

17

17

18 18 19 19

Alamosa River below

Jasper (AR41.2)

Alamosa River above

Terrace Reservoir (AR34.5)

17

Interquartile range {

Number of rainstorm runoff events

75th percentile plus 1.5 times the 

interquartile range

75th percentile

Median (50th percentile)

25th percentile

25th percentile minus 1.5 times the

interquartile range

Individual data point

EXPLANATION

0

12

36

48

24

1995

1996

1997

0

12

36

48

24

1995

19961997

0

12

36

48

24

1995

1996

1997

0

12

36

48

24

1995

19961997

0

12

36

48

24

1995

1996

1997

Figure 2. Change and duration of change in streamflow due to rainstorm runoff at selected sites, upper Alamosa 

River Basin, southwest Colorado, 1995–97.
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 9

may be due to dilution or natural reactions (such as 

mineral precipitation and sorption) but stated additional data were needed for verification.

Relative streamflow is shown in figure 3 to 

illustrate how the peak streamflow moves through the 

basin; the magnitudes of streamflow at each site were 

made relative to each other in this graph to highlight 

the traveltime of the peak streamflow through the 

basin. For the rainstorm on August 30–31, 1997, 

approximately 0.3 day (approximately 7.2 hours) was 

needed for the peak streamflow to travel from AR45.5 

to AR34.5 (approximately 14.1 miles), with an 

approximate velocity of 2 mi/h. A statistical summary 

of the traveltimes of peak streamflow and the low-pH 

front throughout the basin for July through September 

in 1995, 1996, and 1997 is given in table 4.

Effects of Annual Variations in Streamflow on pH

The effects of annual variations in July through 

September precipitation on pH were evaluated. 

Minimum, median, and maximum pH had no apparent 

systematic relation to annual variations in streamflow 

(table 2). However, at most sites, the changes in pH 

(difference between prestorm and peak values) tended 

to be larger in 1996 than in 1995 and 1997 (table 3, 

fig. 4), presumably because of less water in the system 

(table 2) and, thus, less water available for dilution. It 

is unknown why the change in pH at AR45.5 was 

larger in 1997 (fig. 4). It also is unknown how the drier 

conditions in 1996 affected the pH of soils and interstitial water in the altered areas that are likely sources for 

low-pH waters in the basin.

With the exception of AR34.5, the variation 

in duration of change in pH due to rainstorm runoff 

between 1995, 1996, and 1997 was small (table 5, 

fig. 4). At AR34.5, the duration of change due to rainstorm runoff in 1996 was substantially longer. This 

longer duration of pH decrease could be detrimental to 

any aquatic life in the river.

The traveltimes of peak streamflow and the lowpH front as the peak moved through the basin also 

were calculated (table 4, fig. 5). Median traveltimes of 

peak streamflow and the pH front were slightly longer 

in 1996, presumably because of the lower streamflow 

that year.

Rainstorms in Different Parts of the Basin and 

Their Effects on Streamflow and pH

Ninety-three percent of the rainstorms identified 

using streamflow records produced runoff throughout 

the entire basin. During 1995–97, 54 rainstorms 

occurred, but only 3 storms were isolated to the reach 

upstream from AR45.5 and only 1 storm was isolated 

to the reach between AR41.2 and AR34.5.

Large decreases in pH were measured at 

AR45.5 during the three rainstorms that were isolated 

to the reach upstream from that site (table 6). These 

decreases ranged from 1 to 2.5 standard units and were 

thought to be due to runoff from the Stunner altered 

area (fig. 1). The pH decreased as much as 2.6 units at 

AR4l.2. For the most part, the pH was close to neutral 

Table 3. Statistical summary of changes in streamflow and 

pH due to rainstorm runoff at selected sites in the upper 

Alamosa River Basin, southwest Colorado, during July, 

August, and September 1995, 1996, and 1997

[WF5.5, Wightman Fork below Cropsy Creek; WF0.0, Wightman Fork 

at mouth; AR45.5, Alamosa River above Wightman Fork; AR41.2, 

Alamosa River below Jasper; AR34.5, Alamosa River above Terrace 

Reservoir]

Year WF5.5 WF0.0 AR45.5 AR41.2 AR34.5

Minimum change in streamflow, in cubic feet per second

1995 0 0 6 7 7

1996 0 0 2 0 2

1997 1 1.9 11 14 10

Median change in streamflow, in cubic feet per second

1995 9 11 30 36 51

1996 .9 2.2 7.8 11.5 12.5

1997 5.8 9 42.5 60 43

All 3 years 4.5 7 30 41 37

Maximum change in streamflow, in cubic feet per second

1995 42 150 135 150 115

1996 24 36 126 140 150

1997 25 72 200 310 260

Minimum change in pH, in standard units

1995 0 0 .1 .1 0

1996 0 0 0 0 0

1997 .2 .1 0 0 0

Median change in pH, in standard units

1995 .3 .3 .8 .25 .4

1996 .9 .9 1 .6 .6

1997 .8 .3 1.7 .5 .6

All 3 years .3 .4 1 .5 .5

Maximum change in pH, in standard units

1995 .6 .9 1.6 1 .9

1996 1.9 2.3 2.7 2.5 2.8

1997 1.5 1.2 3.1 2 1.7
10 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

Streamflow

pH

Streamflow-gaging station

EXPLANATION

RELATIVE STREAMFLOW

WF5.5

WF0.0

AR45.5

AR41.2

AR34.5

AUGUST 1997

30 31

 

 

STREAMFLOW, 

IN CUBIC FEET PER SECOND

pH, IN STANDARD UNITS

AUGUST 1997

ALAMOSA RIVER 

ABOVE WIGHTMAN FORK (AR45.5)

8070

60

5040

7.06.5

6.05.5

5.0

30 31

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

AUGUST 1997

WIGHTMAN FORK BELOW 

CROPSY CREEK (WF5.5)

5

2015

10

4.44.2

4.0

3.8

3.6

3.4

3.2

30 31

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

AUGUST 1997

WIGHTMAN FORK AT 

MOUTH (WF0.0)

7.5

25.022.521.017.515.012.510.0

5.1

5.0

4.9

4.8

4.7

4.6

30 31

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

AUGUST 1997

ALAMOSA RIVER 

BELOW JASPER (AR41.2)

90

8070

6050

110

100

6.05.9

5.85.7

5.6

5.5

5.4

30 31

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

AUGUST 1997

ALAMOSA RIVER ABOVE 

TERRACE RESERVOIR (AR34.5)

90

807060

110100 6.8

6.6

6.4

6.2

30 31

WF5.5

WF0.0

AR45.5 AR41.2

AR34.5

pH AT ALL STATIONS,

IN STANDARD UNITS 7

6

5

4

3

AUGUST 1997

30 31

 

Alamosa River

Alamosa River

Wightman Fork

Figure 3. Streamflow and pH, August 30

–31, 1997, upper Alamosa River Basin, southwest Colorado. 
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 11

by the time the peak streamflow reached AR34.5, but 

decreases in pH still occurred (table 6).

Only one rainstorm was isolated in the lowermost part of the basin. The small rainstorm caused a 

10 ft3/s (50 percent) increase in streamflow at AR34.5, 

but pH remained the same (6.3). No rainstorm-related 

changes in streamflow or pH occurred at any of the 

other sites. The lack of change in pH during the rainstorm runoff event suggests this part of the basin does 

not contribute significant low-pH runoff. No altered 

areas exist in this segment of the basin (fig. 1, EA-3c).

These results suggest that isolated rainstorms 

occurring downstream from Fern Creek (fig. 1) probably pose little risk of anomalously low pH. However, 

isolated rainstorms occurring in the upper Alamosa 

River Basin upstream from the confluence with 

Wightman Fork pose a substantial risk of contributing 

anomalously low pH runoff to the main stem of the 

Alamosa River, which can affect pH values in the 

Alamosa River for several miles downstream.

Effects of Untreated Discharge from the 

Summitville Mine Impoundment

There is an impoundment at the Summitvllle 

Mine which is used to capture and hold acid-mine 

drainage from the mine site. The impoundment waters 

have low pH values and high metal concentrations; the 

median pH value measured from May 1997 through 

October 1997 was 3.1 and the median dissolved 

copper concentration was 30,000 μg/L (Bruce 

Marshall, Rocky Mountain Consultants, written 

commun., 1999), which is more than 500 times the 96-

hour LC50 reported for rainbow trout. The impoundment waters are pumped to a treatment plant where the 

pH is raised and a large portion of the metal load is 

removed. The treated water then is discharged to 

Wightman Fork. However, sometimes untreated 

effluent from the impoundment has been discharged 

directly to Wightman Fork, such as during the mid- to 

late summer months of 1997 (July 10–12, July 23–25, 

August 7–8, and August 10–12) (Bruce Marshall, 

Rocky Mountain Consultants, written commun., 

1999). Releases on August 7–8 and August 10–12 

coincided with rainstorms, so the effects of the 

releases of untreated effluent on Wightman Fork and 

the Alamosa River could not be quantified; however, 

releases on July 10–12 (fig. 6) and July 23–25 

occurred at times of no precipitation.

Release of effluent from the Summitville Mine 

impoundment affects pH at all sites downstream from 

WF5.5 (fig. 6). At approximately 8:30 a.m. on July 10, 

1997, untreated effluent was released from the 

impoundment. This release increased the streamflow 

at WF5.5 from about 8 ft3/s to about 11 ft3 /s and 

decreased the pH from 4.2 to 3.2 (fig. 6). The release 

Table 4. Statistical summary of traveltimes of peak streamflow 

and low-pH front through selected segments of the Alamosa River 

and Wightman Fork, upper Alamosa River Basin, southwest Colorado, during July, August, and September 1995, 1996, and 1997

[WF5.5, Wightman Fork below Cropsy Creek; WF0.0, Wightman Fork at mouth; 

AR45.5, Alamosa River above Wightman Fork; AR41.2, Alamosa River below 

Jasper; AR34.5, Alamosa River above Terrace Reservoir; h, hour]

Year WF5.5 to 

WF0.0 AR45.5 to AR41.2 AR45.5 to AR34.5

Minimum traveltime of peak streamflow, in decimal days

1995 0.05 (1.2 h) 0.05 (1.2 h) 0.10 (2.4 h)

1996 .05 (1.2 h) .05 (1.2 h) .05 (1.2 h)

1997 .05 (1.2 h) .05 (1.2 h) .10 (2.4 h)

Median traveltime of peak streamflow, in decimal days

1995 .10 (2.4 h) .10 (2.4 h) .25 (6 h)

1996 .10 (2.4 h) .10 (2.4 h) .30 (7.2 h)

1997 .10 (2.4 h) .10 (2.4 h) .25 (6 h)

 All 3 

years

.10 (2.4 h) .10 (2.4 h) .25 (6 h)

Maximum traveltime of peak streamflow, in decimal days

1995 .20 (4.8 h) .45 (10.8 h) .70 (16.8 h)

1996 .35 (8.4 h) .20 (4.8 h) .55 (13.2 h)

1997 .15 (3.6 h) .25 (6 h) .35 (8.4 h)

Minimum traveltime of low-pH front, in decimal days

1995 .05 (1.2 h) .05 (1.2 h) .15 (3.6 h)

1996 .05 (1.2 h) .05 (1.2 h) .15 (3.6 h)

1997 .05 (1.2 h) .10 (2.4 h) .10 (2.4 h)

Median traveltime of low-pH front, in decimal days

1995 .20 (4.8 h) .18 (4.3 h) .33 (7.9 h)

1996 .25 (6 h) .20 (4.8 h) .40 (9.6 h)

1997 .15 (3.6 h) .15 (3.6 h) .35 (8.4 h)

All 3 

years

.20 (4.8 h) .18 (4.3 h) .35 (8.4 h)

Maximum traveltime of low-pH front, in decimal days

1995 .50 (12 h) .50 (12 h) 1.4 (33.6 h)

1996 .50 (12 h) .45 (10.8 h) 1.2 (28.8 h)

1997 .55 (13.2 h) .30 (7.2 h) .50 (12 h)
12 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

17

17

17

17 17

17

17

18

18

18

18

18

18

18

19

19

19

19

19

19

19

0

1

2

4

3

0

1

2

4

3

0

1

2

4

3

0

1

2

4

3

0

1

2

4

3

0

12

24

48

60

36

0

12

24

48

60

36

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

CHANGE IN pH, IN STANDARD UNITS DURATION OF CHANGE IN pH, IN HOURS

Alamosa River above 

Wightman Fork (AR45.5)

Alamosa River below

Jasper (AR41.2)

Alamosa River above

Terrace Reservoir (AR34.5)

Wightman Fork below

Cropsy Creek (WF5.5)

Wightman Fork below

Cropsy Creek (WF5.5)

Wightman Fork at mouth

(WF0.0)

Wightman Fork at mouth

(WF0.0)

17 18

19

0

12

24

48

60

36

1995

1996

1997

Alamosa River above 

Wightman Fork (AR45.5)

17

17

18

18

19 19

0

12

24

48

60

36

0

12

24

48

60

36

1995

1996

1997

1995

1996

1997

Alamosa River below

Jasper (AR41.2)

Alamosa River above

Terrace Reservoir (AR34.5)

17

Interquartile range {

Number of rainstorm runoff events

75th percentile plus 1.5 times the 

interquartile range

75th percentile

Median (50th percentile)

25th percentile

25th percentile minus 1.5 times the

interquartile range

Individual data point

EXPLANATION

Figure 4. Change and duration of change in pH due to rainstorm runoff at selected sites, upper Alamosa River 

Basin, southwest Colorado, 1995–97.
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 13

increased the streamflow at WF0.0 by about the same 

amount (3 ft3/s) and decreased the pH from 5 to about 

3.7. This release occurred in mid-July when snowmelt 

was still contributing to streamflow in the Alamosa 

River. Data from AR45.5 demonstrate a relation 

between streamflow and pH that is characteristic of 

snowmelt runoff; the streamflow displays a diurnal 

fluctuation, and pH closely mimics the variation in 

streamflow due to dilution by snowmelt (fig. 6). The 

diurnal fluctuations in streamflow are still apparent at 

AR41.2, but the diurnal fluctuations in pH are diminished. Instead, pH decreased by more than 1 standard 

unit due to the release of untreated effluent from 

the impoundment. Apparently, the untreated effluent 

was highly concentrated because the streamflow of 

Wightman Fork increased by only 3 ft3/s, and yet the 

pH at AR41.2, which had streamflow values ranging 

from approximately 95 ft3/s to approximately 

135 ft3/s, decreased by more than 1 unit. Data for 

AR34.5 demonstrated relations similar to those at 

Table 5. Statistical summary of duration of changes in streamflow and pH due to rainstorm runoff 

at selected sites in the upper Alamosa River Basin, southwest Colorado, during July, August, and 

September 1995, 1996, and 1997

[WF5.5, Wightman Fork below Cropsy Creek; WF0.0, Wightman Fork at mouth; AR45.5, Alamosa River above Wightman Fork; 

AR41.2, Alamosa River below Jasper; AR34.5, Alamosa River above Terrace Reservoir; %, percent; h, hour]

Year WF5.5 WF0.0 AR45.5 AR41.2 AR34.5

Minimum duration of change in streamflow, in decimal days

1995 0.10 (2.4 h) 0.10 (2.4 h) 0.15 (3.6 h) 0.20 (4.8 h) 0.20 (4.8 h)

1996 .10 (2.4 h) .10 (2.4 h) .15 (3.6 h) .05 (1.2 h) .20 (4.8 h)

1997 .05 (1.2 h) .05 (1.2 h) .25 (6 h) .25 (6 h) .10 (2.4 h)

Median duration of change in streamflow, in decimal days

1995 .3 (7.2 h) .35 (8.4 h) .40 (9.6 h) .45 (10.8 h) .48 (11.5 h)

1996 .23 (5.5 h) .30 (7.2 h) .48 (11.5 h) .45(10.8 h) .63 (15.1 h)

1997 .25 (6 h) .40 (9.6 h) .45 (10.8 h) .45 (10.8 h) .50 (12 h)

Maximum duration of change in streamflow, in decimal days

1995 .75 (18 h) .80 (19.2 h) .70 (16.8 h) .90 (21.6 h) 1.10 (26.4 h)

1996 .90 (21.6 h) 1.00 (24 h) l.40 (33.6 h) 1.80 (43.2 h) 2.80 (67.2 h)

1997 1.15 (27.6 h) 1.20 (28.8 h) 1.20 (28.8 h) 1.05 (25.2 h) 1.10 (26.4 h)

Minimum duration of change in pH, in decimal days

1995 .15 (3.6 h) .10 (2.4 h) .05 (1.2 h) .03 (0.7 h) .10 (2.4 h)

1996 .05 (1.2 h) .10 (2.4 h) .15 (3.6 h) .10 (2.4 h) .10 (2.4 h)

1997 .05 (1.2 h) .15 (3.6 h) .20 (4.8 h) .10 (2.4 h) .15 (3.6 h)

Median duration of change in pH, in decimal days

1995 .30 (7.2 h) .38 (9.1 h) .30 (7.2 h) .38 (9.1 h) .50 (12 h)

1996 .28 (6.7 h) .43 (10.3 h) .45(10.8 h) .43 (10.3 h) .75 (18 h)

1997 .30 (7.2 h) .38 (9.1 h) .40 (9.6 h) .35 (8.4 h) .53 (12.7 h)

Maximum duration of change in pH, in decimal days

1995 .75 (18 h) .60 (14.4 h) .60 (14.4 h) .65 (15.6 h) 1.00 (24 h)

1996 1.60 (38.4 h) 1.90 (45.6 h) .70 (16.8 h) 1.35 (32.4 h) 2.00 (48 h)

1997 .95 (22.8 h) 1.90 (45.6 h) 1.00 (24 h) .90 (21.6 h) .95 (22.8 h)

Number and percentage of rainstorms causing decreased pH values for greater than 4 hours

1995 13 (76%) 16 (94%) 14 (82%) 15 (88%) 15 (88%)

1996 15 (83%) 17 (94%) 16 (89%) 17 (94%) 17 (94%)

1997 14 (74%) 18 (95%) 19 (100%) 18 (95%) 18 (95%)

Number and percentage of rainstorms causing decreased pH values for greater than 96 hours

1995 0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)

1996 0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)

1997 0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)
14 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

17

17

17

17

17 17

18

0

4

8

12

16

20

24

0

4

8

12

16

20

24

0

4

8

12

16

20

24

0

4

8

12

16

20

24

0

4

8

12

16

20

24

0

4

8

12

16

20

24

18

18

18

18

18

19

19

19

19

19

19

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

1995

1996

1997

TRAVELTIME OF PEAK 

STREAMFLOW, IN HOURS

TRAVELTIME OF pH FRONT,

IN HOURS

From Wightman Fork below 

Cropsy Creek ( WF5.5) 

to Wightman Fork 

at mouth (WF0.0)

From Alamosa River above

Wightman Fork (AR45.5)

 to Alamosa River below

Jasper (AR41.2)

From Alamosa River above

Wightman Fork (AR45.5)

 to Alamosa River above

Terrace Reservoir (AR34.5)

17

Interquartile range

Number of rainstorm runoff events

75th percentile plus 1.5 times the 

interquartile range

75th percentile

Median (50th percentile)

25th percentile

25th percentile minus 1.5 times the

interquartile range

Individual data point

EXPLANATION

AR41.2; streamflow was more than 100 ft3/s, but pH 

decreased by more than 1 unit due to the release of 

untreated effluent from the impoundment.

Based upon data from July 10–12, 1997, and 

July 23–25, 1997 (which showed conditions similar to 

those described above), release of untreated effluent 

from the Summitville Mine impoundment may significantly decrease pH in the Alamosa River as far downstream as Terrace Reservoir.

Variations in pH of Water Discharging 

from Terrace Reservoir

The pH of water in, and discharging from, 

Terrace Reservoir during low-flow years may pose a 

significant risk to aquatic organisms. During the 

summer of 1996, the upper Alamosa River Basin had 

relatively low streamflow (table 2). Water storage 

and pool size of Terrace Reservoir also were relatively 

low; during August 1996, the storage was only 

Figure 5. Traveltimes of peak streamflow and low-pH front through selected segments of the Alamosa River and 

Wightman Fork, upper Alamosa River Basin, southwest Colorado, 1995–97.
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 15

2,091 acre-ft. In comparison, storage was 5,572 acre-ft 

during August 1997 (Craig Cotten, Colorado Department of Water Resources, written commun., 1999). 

During August 21–29, 1996, the upper basin received 

a series of relatively low-intensity, long-duration 

rainstorms. Shortly after the start of these rainstorms, 

pH at AR34.5 began a decrease from about 6.3 to 

about 4.7 on August 22, l996 (fig. 7). Approximately 

38 hours later, pH at AR31.0 began a decrease from 

about 6.7 to about 4.8. The distance between AR34.5 

and AR3l.0 is approximately 4.3 miles, so the travel 

velocity of the pH front through the reservoir was 

about 0.1 mi/h. On September 2, the pH at AR34.5 

increased to about 6.5, but the pH at AR31.0 remained 

less than 5.8 until near the end of September. During 

September 12–25, the basin also had several small 

rainstorms, and streamflow at AR34.5 ranged from 

about 25 ft3

/s to more than 40 ft3/s. During this time, 

pH at AR34.5 was never less than 6.0, but pH at 

AR31.0 ranged from about 4.3 to 5.4.

It is unclear why the pH remained anomalously 

low at AR3l.0 for such a long period of time, even 

though the pH at AR34.5 returned to more than 6.0. 

Geochemical interactions between sediment and the 

water column within the reservoir are a possible cause 

of the low pH. Water in the reservoir is well oxygenated and has limited microbial activity, so the most 

likely reactions or processes that can occur in the 

reservoir include oxidation and precipitation of ferric 

hydroxides, adsorption and desorption of metals, 

complexation of dissolved metals with ligands, and 

flocculation (Stogner and others, 1997). Ferric 

hydroxide formation is plausible and would help 

explain why the pH at AR3l.0 was lower than the pH 

at AR34.5. Horowitz and others (1996, p. 6) substantiated the possibility of ferric hydroxide formation; they 

reported the bottom of the reservoir to be covered by a 

thin (0.25- to 1.2-inch), soupy, fine-grained, redorange floc that they believed to contain large quantities of ferric hydroxide. Stogner and others (1997) 

noted that pH tended to decrease and metal concentrations tended to increase in a downstream direction in 

Terrace Reservoir. However, Stogner and others 

(1997, p. 21) suggested that oxidation and chemical 

precipitation of metal hydroxides probably were not 

dominant processes in the reservoir because most of 

the ferric hydroxide probably formed in the river 

upstream from the reservoir. A possible explanation 

may be that the relatively low water levels in Terrace 

Reservoir during the summer of 1996 allowed reservoir sediments to be exposed to the atmosphere and to 

be reworked due to downcutting by the Alamosa 

River, allowing for additional metal hydroxide formation. Regardless of the cause, these relations indicate 

that pH in the reservoir may remain significantly 

lowered following rainstorm runoff during low-flow 

years, thus hampering efforts to improve water quality. 

Additional study would be useful to help determine if 

geochemical interactions between the sediment and 

water column within Terrace Reservoir may hamper 

restoration of the reservoir and downstream reaches to 

fishable status, even after remediation efforts at the 

Summitville Mine are completed.

Comparison of pH Values to Water-Quality 

Standards and Toxicological Reference 

Values

The USEPA has established quality criteria for 

water for many constituents (U.S. Environmental 

Protection Agency, 1986). The freshwater chronic 

exposure criterion, which is long-term exposure under 

ambient conditions (not just storm events), for pH is 

6.5 to 9.0 standard units. At all sites examined in this 

Table 6. Changes in streamflow and pH due to 

rainstorms isolated to the reach upstream from the 

Alamosa River above Wightman Fork station during 

July, August, and September 1995, 1996, and 1997, 

upper Alamosa River Basin, southwest Colorado

[AR45.5, Alamosa River above Wightman Fork; AR41.2, Alamosa 

River below Jasper; AR34.5, Alamosa River above Terrace Reservoir; 

EA, exposure area; Q-begin, streamflow before rainstorm; Q-max, 

maximum streamflow due to rainstorm; pH-begin, pH value before

rainstorm; pH-min, minimum pH value due to rainstorm]

Date Qbegin

QmaxpHbeginpHmin

AR45.5 

9/01/95 45 53 6.2 5.2

7/12/96 40 90 6.5 4.5

7/25/96 20 31 6.5 4.0

AR41.2 

9/01/95 66 73 4.6 4.4

7/12/96 72 123 6.7 5.7

7/25/96 30 42 6.3 3.7

AR34.5

9/01/95 67 74 6.2 6.1

7/12/96 90 136 7.2 6.7

7/25/96 46 55 7.2 5.7
16 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

Streamflow

pH

Streamflow-gaging station

EXPLANATION

Alamosa River

Alamosa River

Wightman Fork

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

JULY 1997

WIGHTMAN FORK BELOW 

CROPSY CREEK (WF5.5)

9 10 11 12 13 14 15

9.59.08.58.07.57.06.56.05.5

11.511.010.510.0

4.24.0

3.8

3.63.4

3.2

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

JULY 1997

WIGHTMAN FORK 

AT MOUTH (WF0.0)

9 10 11 12 13 14 15

1816

14

12

108

5.0

4.5

4.0

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

JULY 1997

ALAMOSA RIVER 

BELOW JASPER (AR41.2)

9 10 11 12 13 14 15

959085

140135130

125120115

110

105100

6.26.0

5.8

5.6

5.4

5.2

 

 

pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND

JULY 1997

ALAMOSA RIVER ABOVE 

TERRACE RESERVOIR (AR34.5)

9 10 11 12 13 14 15

100

90

140130

120

110

7.06.5

6.0

 

 

STREAMFLOW, 

IN CUBIC FEET PER SECOND

pH, IN STANDARD UNITS

JULY 1997

ALAMOSA RIVER ABOVE 

WIGHTMAN FORK (AR45.5)

9 10 11 12 13 14 15

9590

85

8075

7065

115110

105100

6.86.7

6.6

6.5

6.4

 

Figure 6. Streamflow and pH during July 9

–15, 1997, upper Alamosa River Basin, southwest Colorado. 
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 17

 

 

pH, IN STANDARD UNITS pH, IN STANDARD UNITS

STREAMFLOW, 

IN CUBIC FEET PER SECOND STREAMFLOW, IN CUBIC FEET PER SECOND

 

 

 

 

 

ALAMOSA RIVER BELOW 

TERRACE RESERVOIR (AR31.0)

ALAMOSA RIVER ABOVE 

TERRACE RESERVOIR (AR34.5)

Streamflow

EXPLANATION

pH

10

20

30

40

50

60

70

10

20

30

40

50

60

70

7.0

6.0

6.5

5.5

5.0

4.5

7.0

6.0

6.5

5.5

5.0

4.5

August 14, 1996

September 1, 1996

September 15, 1996

September 30, 1996

August 14, 1996

September 1, 1996

September 15, 1996

September 30, 1996

Figure 7. Streamflow and pH at Alamosa River above Terrace Reservoir and Alamosa River 

below Terrace Reservoir, August 14 through September 30, 1996, upper Alamosa River 

Basin, southwest Colorado.
18 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

study, the pH was below 6.5 for extended periods of 

time (table 2, table 7, figs. 8–13). This indicates pH in 

many parts of the upper Alamosa River Basin do not 

meet the USEPA pH standard for aquatic life. 

The State of Colorado has established waterquality standards for most stream segments in the 

Alamosa River drainage (Colorado Department of 

Public Health and Environment, 1998). The waterquality standard for pH on the river segment near 

AR45.5 (EA-3a) varies depending on the time of year 

(fig. 10). Colorado water-quality standards for pH 

were exceeded on numerous occasions at AR45.5 and 

were exceeded continuously during most of August 

1996 (fig. 10). Colorado standards for pH were not 

established for the stream segments where WF5.5 and 

WF0.0 (EA-2) are located, but the Federal standard of 

6.5 was rarely met (figs. 8–9, table 7). The Colorado 

Table 7. Percentage of pH measurements that did not meet Colorado 

and Federal water-quality standards and Toxicological Reference Values 

during July, August, and September 1995, 1996, and 1997, upper Alamosa 

River Basin, southwest Colorado

[Measurements taken every 30 minutes during July, August, and September; WF5.5, 

Wightman Fork below Cropsy Creek; WF0.0, Wightman Fork at mouth; AR45.5, Alamosa River 

above Wightman Fork; AR41 2, Alamosa River below Jasper; AR34.5, Alamosa River above 

Terrace Reservoir; AR3 1.0, Alamosa River below Terrace Reservoir; TRV; Toxicological 

Reference Value; ne, standard not established]

Year WF5.5 WF0.0 AR45.5 AR41.2 AR34.5 AR31.0

Percentage of pH measurements less than Federal water-quality standard (pH 6.5)

1995 100 100 77 100 100 100

1996 99 98 87 86 23 44

1997 100 100 39 85 25 20

Percentage of pH measurements less than Colorado water-quality standard

(pH standard depends on specific reach)

1995 ne ne 1 100 100 100

1996 ne ne 42 86 23 44

1997 ne ne 1 85 25 20

Percentage of pH measurements less than TRV for chronic exposure, benthic

macroinvertebrates (pH = 6.5)

1995 100 100 77 100 100 100

1996 99 98 87 86 23 44

1997 100 100 52 85 25 20

Percentage of pH measurements less than TRV for acute exposure, 

benthic macroinvertebrates (pH = 5.38)

1995 100 100 22 99 31 35

1996 95 39 71 48 5 30

1997 100 91 4 16 5 1

Percentage of pH measurements less than TRV for chronic exposure,

adult rainbow trout (pH = 5.6)

1995 100 100 29 99 47 47

1996 96 56 73 58 6 35

1997 100 96 7 24 5 5

Percentage of pH measurements less than TRV for acute exposure, 

adult rainbow trout (pH = 4.2)

1995 97 46 1 1 0 0

1996 24 2 5 1 1 0

1997 54 7 1 0 0 0
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 19

Wightman Fork below Cropsy Creek (WF5.5)

pH, IN STANDARD UNI TS

1995 1996 1997

2

J A S O N D J F M A M J J A S O N D J F M A M J J A S

8

7

6

5

4

3

Federal chronic macro (pH = 6.5)

Chronic rainbow (pH = 5.6)

Acute macro (pH = 5.38)

Acute rainbow (pH = 4.2)

Figure 8. Federal water-quality standards, Toxicological Reference Values, and pH at Wightman Fork below Cropsy Creek (WF5.5), upper Alamosa River Basin, July 1, 1995, through September 30, 1997. [TRV, Toxicological Reference Value, Federal chronic macro, Federal waterquality standard and chronic TRV for benthic macroinvertebrates, pH = 6.5; chronic rainbow, chronic TRV for adult rainbow trout, pH = 5.6; acute macro, acute TRV for benthic macroinvertebrates, pH = 5.38; acute rainbow, acute TRV for adult rainbow trout, pH = 4.2]
20 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

pH, IN STANDARD UNI TS

8

7

6

5

4

3

2

Federal chronic macro (pH = 6.5)

Chronic rainbow (pH = 5.6)

Acute macro (pH = 5.38)

Acute rainbow (pH = 4.2)

Wightman Fork at mouth (WF0.0)

1995 1996 1997

J A S O N D J F M A M J J A S O N D J F M A M J J A S

Figure 9. Federal water-quality standards, Toxicological Reference Values and pH at Wightman Fork at mouth (WF0.0), upper Alamosa River Basin, July 1, 1995, through September 30, 1997. [TRV, Toxicological Reference Value; Federal chronic macro, Federal water-quality standard and chronic TRV for benthic macroinvertebrates, pH = 6.5; chronic rainbow, chronic TRV for adult rainbow trout, pH = 5.6; acute macro, acute TRV for benthic macroinvertebrates, pH = 5.38; acute rainbow, acute TRV for adult rainbow trout, pH = 4.2]
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 21

pH, IN STANDARD UNI TS

8

7

6

5

4

3

2

Federal chronic macro (pH = 6.5)

Coloradostandard

Chronic rainbow (pH = 5.6)

Acute macro (pH = 5.38)

Acute rainbow (pH = 4.2)

Alamosa River above Wightman Fork (AR45.5)

1995 1996 1997

J A S O N D J F M A M J J A S O N D J F M A M J J A S

Figure 10. Colorado and Federal water-quality standards, Toxicological Reference Values, and pH at Alamosa River above Wightman Fork (AR45.5), upper Alamosa River Basin, July 1, 1995, through September 30, 1997. [TRV, Toxicological Reference Value; Federal chronic macro, Federal water-quality standard and chronic TRV for benthic macroinvertebrates, pH = 6.5; chronic rainbow, chronic TRV for adult rainbow trout, pH = 5.6; acute macro, acute TRV for benthic macroinvertebrates, pH = 5.38; acute rainbow, acute TRV for adult rainbow trout, pH = 4.2; Colorado, Colorado water-quality standard for pH, varies depending on time of year]
22 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

pH, IN STANDARD UNI TS

8

7

6

5

4

3

2

Colorado, Federal, and chronic 

macro (pH = 6.5)

Chronic rainbow (pH = 5.6)

Acute macro (pH = 5.38)

Acute rainbow (pH = 4.2)

Alamosa River below Jasper (AR41.2)

1995 1996 1997

J A S O N D J F M A M J J A S O N D J F M A M J J A S

Figure 11. Colorado and Federal water-quality standard, Toxicological Reference Values, and pH at Alamosa River below Jasper (AR41.2), upper Alamosa River Basin, July 1, 1995, through September 30, 1997. [TRV, Toxicological Reference Value; Colorado, Colorado water-quality standard, pH = 6.5; Federal, Federal water-quality standard, pH = 6.5; chronic macro, chronic TRV for benthic macroinvertebrates, pH = 6.5; chronic rainbow, chronic TRV for adult rainbow trout, pH = 5.6; acute macro, acute TRV for benthic macroinvertebrates, pH = 5.38; acute rainbow, acute TRV for adult rainbow trout, pH = 4.2]
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 23

pH, IN STANDARD UNI TS

8

7

6

5

4

3

2

Colorado, Federal, and 

chronic macro (pH = 6.5)

Chronic rainbow (pH = 5.6)

Acute macro (pH = 5.38)

Acute rainbow (pH = 4.2)

Alamosa River above Terrace Reservoir (AR34.5)

1995 1996 1997

J A S O N D J F M A M J J A S O N D J F M A M J J A S

Figure 12. Colorado and Federal water-quality standards, Toxicological Reference Values, and pH at Alamosa River above Terrace Reservoir (AR34.5), upper Alamosa River Basin, July 1, 1995, through September 30, 1997. [TRV, Toxicological Reference Value; Colorado, Colorado water-quality standard, pH = 6.5; Federal, Federal water-quality standard, pH = 6.5; chronic macro, chronic TRV for benthic macroinvertebrates, pH = 6.5; chronic rainbow, chronic TRV for adult rainbow trout, pH = 5.6; acute macro, acute TRV for benthic macroinvertebrates, pH = 5.38p acute rainbow, acute TRV for adult rainbow trout, pH = 4.2]
24 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

pH, IN STANDARD UNI TS

8

7

6

5

4

3

2

Colorado, Federal, and 

chronic macro (pH = 6.5)

Chronic rainbow (pH = 5.6)

Acute macro (pH = 5.38)

Acute rainbow (pH = 4.2)

Alamosa River below Terrace Reservoir (AR31.0)

1995 1996 1997

J A S O N D J F M A M J J A S O N D J F M A M J J A S

Figure 13. Colorado and Federal water-quality standards, Toxicological Reference Values, and pH at Alamosa River below Terrace Reservoir (AR31.0), upper Alamosa River Basin, July 1, 1995, through September 30, 1997. [TRV.xicological Reference Value; Colorado, Colorado water-quality standard, pH = 6.5; Federal, Federal water-quality standard, pH = 6.5; chronic macro, chronic TRV for benthic macroinvertebrates, pH = 6.5; Chronic rainbow chronic TRV for adult rainbow trout, pH = 5.6; Acute macro, acute TRV for benthic macroinvertebrates, pH = 5.38; Acute rainbow, acute TRV for adult rainbow trout, pH = 4.2]
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 25

water-quality standard for pH in the river segments 

near AR41.2 (EA-3b), AR34.5 (EA-3c), and AR31.0 

(EA-5) is 6.5. The standard was seldom met at AR41.2 

throughout most of the record and often was not met at 

AR34.5 and AR31.0 (figs. 11–13, table 7).

Toxicological Reference Values (TRVs) have 

been established for water quality in the upper 

Alamosa River Basin (Camp Dresser and McKee 

Corporation, 1999). TRVs define the level below 

which a constituent or property of water may produce 

an adverse physiological effect to a particular aquatic 

species. The chronic TRV for benthic macroinvertebrates (pH = 6.5) was rarely met at WF5.5 and WF0.0 

(figs. 8–9, table 7). The acute TRV for benthic macroinvertebrates (pH = 5.38) and the chronic TRV for 

adult rainbow trout (pH 5.6) were rarely met at the 

Wightman Fork sites (WF5.5 and WF0.0) but were 

met more often at the Alamosa River Sites (AR45.5, 

AR41.2, AR34.5, and AR3l.0 (figs. 10–l3, table 7). 

The acute TRV for adult rainbow trout (pH = 4.2) was 

usually met at the Alamosa River sites but was rarely 

met at WF5.5. At WF0.0, the acute TRV for adult 

rainbow trout was met only about one-half the time in 

1995 but was usually met in 1996 and 1997. 

Aquatic organisms are sensitive to the duration 

of their exposure to decreased pH values. Most rainstorm runoff events resulted in decreased pH values 

for more than 4 hours (table 5). However, the individual rainstorm runoff events never resulted in 

decreased pH values for more than 96 hours, which is 

the largest time period typically used to determine 

acute exposure.

Relation Between pH and Metal

Concentrations

Relations between pH and streamflow, specific 

conductance, dissolved aluminum, dissolved copper, 

dissolved iron, and dissolved zinc concentrations were 

evaluated for all sites during 1994–97. In general, an 

inverse log-log relation exists between pH and the 

logarithm of dissolved metal concentrations 

(figs. 14–18), indicating that as pH decreases, metal 

concentrations generally increase. To quantify the 

extent to which each relation approximates a log-log 

relation and the amount of variation in metal concentrations that can be explained by pH, the coefficient of 

determination (r2) was calculated (Ott, 1993). A weak 

relation exists if r2 is near zero, and a strong relation 

exists as r2 approaches one. Most r2 values are less 

than 0.8. Spearman’s rho rank-order correlation coefficient (SR) (Ott, 1993, p. 465) was also calculated for 

each relation. The SR measures the monotonic association between pH and streamflow, specific conductance, or dissolved metal concentrations, even if the 

relation is not linear. A weak relation exists if SR is 

near zero and a strong relation exists as SR approaches 

one; a plus or minus sign indicates the slope of the 

relation. In many cases, SR values were larger than r2

values but were still less than ±0.8 (figs. 14–18). 

Although a general relation exists between pH and 

dissolved-metal concentrations, figures 14–18 show 

that a large range in metal concentrations were 

observed for the same pH value. For instance, when 

pH at AR41.2 was about 5, dissolved aluminum 

concentrations ranged from about 500 μg/L to about 

4,000 μg/L (fig. 17). These relations generally are not 

significant enough to confidently predict metal 

concentrations on the basis of measured pH values. 
26 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River

Basin, Southwest Colorado, 1995–97

pH, IN STANDARD UNITS

Wightman Fork below Cropsy Creek (WF5.5)

STREAMFLOW, 

IN CUBIC FEET PER SECOND

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS 

ETER PER CENTIMETER PER CENTIM

DISSOLVED ALUMINUM, 

S PER LITER IN MICROGRAMS PER LITER IN MICROGRAM

DISSOLVED COPPER,

S PER LITER IN MICROGRAMS PER LITER IN MICROGRAM

DISSOLVED IRON,

S PER LITER IN MICROGRAMS PER LITER IN MICROGRAM

DISSOLVED ZINC,

S PER LITER IN MICROGRAMS PER LITER IN MICROGRAM

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

r = 0.365SR = – 0.677

2

r = 0.406SR = – 0.702

2 r = 0.413

SR = – 0.712

2 r = 0.511

SR = – 0.786

2

r = 0.663SR = – 0.750

2

r = 0.222SR = 0.357

2

Figure 14. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994

–97, at Wightman Fork below Cropsy Creek 

(WF5.5), upper Alamosa River Basin, southwest Colorado. [r 2, coefficient of determination; SR, Spearman’s rho]
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 27

Wightman Fork at mouth (WF0.0)

pH, IN STANDARD UNITS

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

r = 0.157SR = – 0.493

2

r = 0.646SR = – 0.777

2

r = 0.514SR = – 0.664

2 r = 0.710

SR = – 0.806

2

r = 0.793SR = – 0.873

2 r = 0.008

SR = 0.128

2

STREAMFLOW, 

IN CUBIC FEET PER SECOND

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS 

PER CENTIMETER

DISSOLVED ALUMINUM, 

IN MICROGRAMS PER LITER

DISSOLVED COPPER,

IN MICROGRAMS PER LITER

DISSOLVED IRON,

IN MICROGRAMS PER LITER

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

Figure 15. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994

–97, at Wightman Fork at mouth (WF0.0), 

upper Alamosa River Basin, southwest Colorado. [r2, coefficient of determination; SR, Spearman’s rho]
28 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97 Alamosa River above Wightman Fork (AR45.5)

pH, IN STANDARD UNITS

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

r = 0.776SR = – 0.677

2 r = 0.713

SR = – 0.813

2 r = 0.627

SR = 0.745

2

r = 0.473SR = – 0.706

2 r = 0.568

SR = – 0.729

2 r = 0.729

SR = – 0.843

2

STREAMFLOW, 

IN CUBIC FEET PER SECOND

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS 

PER CENTIMETER

DISSOLVED ALUMINUM, 

IN MICROGRAMS PER LITER

DISSOLVED COPPER,

IN MICROGRAMS PER LITER

DISSOLVED IRON,

IN MICROGRAMS PER LITER

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

Figure 16. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994

–97, at Alamosa River above Wightman Fork (AR45.5), 

upper Alamosa River Basin, southwest Colorado. [r 2, coefficient of determination; SR, Spearman’s rho]
RELATIONS AMONG RAINSTORM RUNOFF, STREAMFLOW, pH, AND METAL CONCENTRATIONS 29

Alamosa River below Jasper (AR41.2)

pH, IN STANDARD UNITS

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

r = 0.575SR = – 0.751

2 r = 0.677

SR = – 0.768

2

r = 0.838SR = – 0.925

2

r = 0.649SR = – 0.805

2

r = 0.155SR = 0.428

2 r = 0.242

SR = – 0.480

2

STREAMFLOW, 

IN CUBIC FEET PER SECOND

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS 

PER CENTIMETER

DISSOLVED ALUMINUM, 

IN MICROGRAMS PER LITER

DISSOLVED COPPER,

IN MICROGRAMS PER LITER

DISSOLVED IRON,

IN MICROGRAMS PER LITER

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

Figure 17. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994

–97, at Alamosa River below Jasper (AR41.2), 

upper Alamosa River Basin, southwest Colorado. [r2, coefficient of determination; SR, Spearman’s rho]
30 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97 Alamosa River above Terrace Reservoir (AR34.5)

pH, IN STANDARD UNITS

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

10

3 0

10,000

100

100,000

1,000,000

1,000

4 5 6 7 8

r = 0.027SR = 0.137

2 r = 0.077

SR = – 0.264

2 r = 0.737

SR = – 0.827

2

r = 0.573SR = – 0.807

2 r = 0.437

SR = – 0.713

2 r = 0.704

SR = – 0.836

2

STREAMFLOW, 

IN CUBIC FEET PER SECOND

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS 

PER CENTIMETER

DISSOLVED ALUMINUM, 

IN MICROGRAMS PER LITER

DISSOLVED COPPER,

IN MICROGRAMS PER LITER

DISSOLVED IRON,

IN MICROGRAMS PER LITER

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

Figure 18. Relations between pH and streamflow, specific conductance, and selected metal concentrations, 1994

–97, at Alamosa River above Terrace Reservoir (AR34.5), 

upper Alamosa River Basin, southwest Colorado. [r2, coefficient of determination; SR, Spearman’s rho]
SUMMARY AND CONCLUSIONS 31

SUMMARY AND CONCLUSIONS

Rainstorm runoff can have a significant effect 

on pH of waters in the upper Alamosa River Basin. 

Frequently, when rainstorms occur in the basin, 

streamflow rapidly increases and pH rapidly 

decreases. In some cases, pH decreased by more than 

1 pH unit in response to rainstorm runoff. The sudden 

decreases in pH can have negative effects on aquatic 

life because the periods of low pH commonly last 

more than 4 hours.

The number of rainstorm runoff events that 

occurred between July 1 and September 30 was 17 in 

1995, 18 in 1996, and 19 in 1997. Rainstorm runoff 

events were identified by observing streamflow 

records. Runoff events were identified when instantaneous streamflow increased at least 10 percent during 

a 1-hour period. Although the number of events was 

about the same each year of the study, the magnitudes 

and effects on pH differed. The changes in streamflow 

due to rainstorm runoff were significantly less at all 

sites in 1996 than in 1995 and 1997 because the study 

area had overall lower flows in 1996. The changes in 

pH due to rainstorm runoff were larger in 1996 than in 

1995 and 1997 at most sites, possibly because of less 

water in the river and, thus, less water available for 

dilution.

Ninety-three percent of the rainstorms evaluated 

for this study produced runoff throughout the entire 

basin. During 1995–97, 54 storms occurred, but only 

3 storms were isolated to the reach upstream from 

site AR45.5, and only 1 storm was isolated to the 

reach between sites AR4l.2 and AR34.5. Although 

most runoff events occurred throughout the entire 

basin, pH changes were most significant in parts of the 

basin that receive runoff from hydrothermally altered 

areas. The three principal altered areas within the 

basin are the Stunner, Summitville, and Jasper areas. 

Only limited mining occurred in the Stunner altered 

area, and yet significant decreases in pH values occur 

due to runoff from this area. Even after environmental 

restoration activities are completed at the Summitville 

Mine, the main stem of the Alamosa River may 

continue to be adversely affected by runoff from the 

Stunner and Jasper altered areas.

Discharge of untreated effluent from the 

Summitville Mine impoundment can significantly 

decrease pH values in the Alamosa River as far downstream as Terrace Reservoir. On July 10, 1997, 

untreated effluent was released from the impoundment 

to Wightman Fork. Although the streamflow in 

Wightman Fork was increased by only 3 ft3/s, the pH 

in Wightman Fork decreased by about 1 pH unit. As 

the pH front moved down the river, pH decreased by 

about 1 unit at WF0.0, AR41.2, and AR34.5. The 

water stored in the impoundment apparently was 

highly concentrated because although streamflow 

increased by only 3 ft3/s, the pH at AR34.5 decreased 

by 1 pH unit with a streamflow of more than 100 ft3/s.

Rainstorm runoff from Terrace Reservoir may 

affect the pH of water in Terrace Reservoir and may 

pose a significant risk to aquatic organisms during 

low-flow years. During 1996, the basin had streamflows that were lower than during 1995 and 1997. 

Water storage and pool size of Terrace Reservoir also 

were lower than during 1995 and 1997. During August 

21–29, 1996, the basin experienced a series of relatively low-intensity, long-duration rainstorms. On 

August 22, 1996, pH at AR34.5 decreased from 

approximately 6.3 to approximately 4.7. Approximately 38 hours later, pH at AR3l.0 decreased from 

about 6.7 to about 4.8. On September 2, the pH at 

AR34.5 increased to approximately 6.5, but the pH at 

AR31.0 remained less than 5.8 through the end of 

September. During September 12–25, the basin also 

had some small rainstorms, and streamflow at AR34.5 

ranged from about 25 ft3

/s to more than 40 ft3/s. 

During this time, the pH at AR34.5 was never less 

than 6.0, but pH at AR31.0 ranged from 4.3 to about 

5.4. Geochemical interactions between sediment and 

the water column within the reservoir are a possible 

cause of the anomalously low pH. 

A comparison of pH with Federal and State of 

Colorado water-quality standards and Toxicological 

Reference Values (TRVs) indicates pH did not meet 

the standards or TRVs in many parts of the basin. The 

USEPA has established a freshwater chronic exposure 

criterion for pH of 6.5 to 9.0 units. At all sites examined in this study, the pH was less than 6.5 for 

extended periods of time. The State of Colorado has 

established pH standards for most stream segments in 

the Alamosa River drainage. These water-quality standards for pH were not met at AR45.5 on numerous 

occasions and were rarely met during most of August 

1996. The standard was seldom met at AR41.2 

throughout most of the record and was rarely met at 

AR34.5 and AR31.0. Toxicological Reference Values 

(TRVs) are pH values below which adverse physiological effects may occur to a particular aquatic species. 

The chronic TRV for benthic macroinvertebrates 
32 Relations Among Rainstorm Runoff, Streamflow, pH, and Metal Concentrations, Summitville Mine Area, Upper Alamosa River 

Basin, Southwest Colorado, 1995–97

(pH = 6.5) was rarely met at WF5.5 and WF0.0. The 

acute TRV for benthic macroinvertebrates (pH = 5.38) 

and the chronic TRV for adult rainbow trout (pH = 

5.6) were rarely met on the Wightman Fork (WF5.5 

and WF0.0) but were met more often at the Alamosa 

River sites (AR45.5, AR41.2, AR34.5, and AR31.0). 

The acute TRV for adult rainbow trout (pH = 4.2) was 

usually met at the Alamosa River sites but was rarely 

met at the Wightman Fork sites.

Relations between pH and streamflow, specific 

conductance, dissolved aluminum, dissolved copper, 

dissolved iron, and dissolved zinc concentrations were 

evaluated for all sites during 1994–97. In general, an 

inverse log-log relation exists between pH and the 

logarithm of dissolved-metal concentrations, but the 

relations generally are not significant enough to confidently predict metal concentrations on the basis of 

measured pH values. 

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