Fact Sheet 2017–3047 

June 2017 Printed on recycled paper

National Water Quality Program

National Water-Quality Assessment Project

Overview of Water Quality

Principal Aquifer Studies (Burow 

and Belitz, 2014) are designed to evaluate 

groundwater used for public supply prior 

to any treatment. Groundwater quality is 

assessed by comparing concentrations to 

benchmarks established for drinking-water 

quality. Benchmarks and definitions of high, 

moderate, and low relative concentrations 

are discussed at the bottom of page 3. Water 

from 70 percent of the wells sampled did not 

have high concentrations of any contaminant 

measured.

Many inorganic constituents are 

naturally present in groundwater. The 

concentrations of inorganic constituents can 

be affected by natural processes as well as 

by human activities. One or more inorganic 

constituents with human-health benchmarks 

were detected at high concentrations in about 

30 percent of the study area and at moderate 

concentrations in about 28 percent.

Organic constituents derived from 

human activities are used in household, business, industrial, and agricultural products. 

They can enter the environment through 

normal usage, spills, or improper disposal. 

Organic constituents with human-health 

benchmarks were not detected at high or 

moderate concentrations in the study area.

Background

The Rio Grande aquifer system underlies an area of 29,000 square miles in Colorado, New Mexico, and Texas. 

The aquifer system is an important source of groundwater for public supply, ranking 18th in the Nation for this use and 

providing about 240 million gallons per day (Arnold and others, 2017a). The aquifer system is also used for domestic 

supply (ranked 17th for this use at about 18 million gallons per day) and irrigation (ranked 11th for this use at about 

867 million gallons per day; Arnold and others, 2017a). Land use overlying the aquifer system is mostly undeveloped 

(93 percent) with small amounts of agricultural (4 percent) or urban (3 percent) land. The overlying land area has a 

population of about 2 million people. The cities of Albuquerque, New Mexico, and El Paso, Texas, lie within the aquifer 

system boundaries.

The Rio Grande aquifer system consists of hydraulically interconnected basin-fill aquifers in about 20 alluvial 

basins extending along the Rio Grande Valley and nearby valleys (Robson and Banta, 1995; Wilkins, 1998). Basin-fill 

sediments are primarily unconsolidated to moderately consolidated gravel, sand, silt, and clay of Tertiary to Quaternary 

age. The thickness of basin deposits ranges greatly, from as little as 2,000 feet near El Paso, Texas, to as much as 30,000 

feet in the San Luis Valley of southern Colorado (Robson and Banta, 1995). Groundwater in the basin-fill aquifers is 

generally unconfined, but confining units are present in some basins (Anning and others, 2007). The climate is semiarid 

to arid, and precipitation in the valleys is mostly lost to evapotranspiration (Robson and Banta, 1995). Recharge occurs 

along mountain fronts from infiltration of mountain stream runoff and inflow from fractured bedrock (Robson and 

Banta, 1995; Bexfield and others, 2011; Thiros and others, 2014). Stream loss from the Rio Grande and (or) crop irrigation are also major sources of recharge to some basins (Robson and Banta, 1995; Bexfield and others, 2011). Discharge 

is dominated by evapotranspiration, particularly for topographically closed basins, although groundwater discharges 

naturally to the Rio Grande in some areas (Robson and Banta, 1995; Bexfield and others, 2011). In some basins, 

groundwater withdrawals for irrigation and (or) public supply have become a substantial discharge component, altering 

groundwater flow directions (Robson and Banta, 1995; Bexfield and others, 2011; Thiros and others, 2014).

Groundwater quality in the 

Rio Grande aquifer system was evaluated in 2014 by sampling 60 publicsupply wells that were spatially 

distributed across the aquifer system. 

Parts of the region are sparsely populated, and public-supply wells are 

not evenly distributed. Consequently, 

the study area was delineated by 

placing 5-kilometer (3.2-mile) buffers around existing public-supply 

wells; an equal-area grid was then 

defined within the buffered area. 

For this discussion, we define the 

study area as the depth zone used 

for public supply in the Rio Grande 

aquifer system in the buffered area. 

Water-quality data collected from 

the spatially distributed wells (6 in 

Colorado, 45 in New Mexico, and 9 

in Texas) are representative of water 

quality in the study area, following 

the approach described by Belitz and 

others (2010). This approach allows 

for the estimation of the percentage 

of the study area with concentrations that are high, moderate, and 

low with respect to constituent 

benchmarks. The accuracy of the 

estimates depends on the distribution 

and number of wells, not the size of 

the area (Belitz and others, 2010). 

The wells ranged from 25 to 2,000 

feet deep (averaging 643 feet deep) 

and were open to the aquifer across 

long depth intervals (averaging about 

295 feet). Samples were analyzed 

for a large number of water-quality 

constituents derived from natural and 

human sources.

Groundwater Quality in the Rio Grande 

Aquifer System, Southwestern United States

Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological 

Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water. The Rio Grande aquifer system constitutes one of the important aquifers being evaluated.

Albuquerque

Lubbock

UTAH COLORADO

KANSAS

ARIZONA

NEW

MEXICO

OKLAHOMA

TEXAS

110° 108° 106° 104° 102°

36°

34°

32°

30°

EXPLANATION

Rio Grande aquifer system boundary

Rio Grande aquifer system study

(buffered) area

City

Sampled well

Base from U.S. Geological Survey digital data, 1:1,000,000

Albers Equal-Area Conic projection

Standard parallels 29°

30' N. and 45°30' N.

Central meridian 96°

00' W.

North American Datum of 1983

Map area

MEXICO

El Paso

0 40 80 120 160 200 MILES

0 40 80 120 160 200 KILOMETERS

Sac17-7011_Overview_pies

Inorganic

constituents

Organic

constituents

CONSTITUENT CONCENTRATIONS

High Moderate Low or not detected

Values are a percentage of the study area with 

concentrations in the three specified categories. 

Percentages might not sum to 100 because of 

rounding.

42

30

28

100

U.S. Department of the Interior

U.S. Geological Survey
 

 

Results: Groundwater Quality at the Depth Zone Used for Public Supply in the 

Rio Grande Aquifer System

Inorganic Constituents With Human-Health Benchmarks 

Trace elements and major and minor ions are naturally present in the minerals of rocks, 

soils and sediments, and in the water that comes into contact with those materials. Samples 

were analyzed for 34 trace elements and major and minor ions, of which 19 have humanhealth benchmarks. Constituents from this group were detected at high concentrations in 

about 27 percent of the study area (the depth zone used for public supply) and at moderate 

concentrations in about 25 percent. Arsenic was the trace element most frequently detected 

at high concentrations (18 percent) and at moderate concentrations (22 percent). Fluoride, 

strontium, and uranium were detected at high concentrations in about 3 percent of the study 

area and at moderate concentrations in about 8, 2, and 7 percent of the study area, respectively. Manganese and molybdenum were not detected at high concentrations but occurred at 

moderate concentrations in about 2 and 3 percent of the study area, respectively.

Radioactivity is the release of energy or energetic particles during the spontaneous 

decay of unstable atoms. Humans are continuously exposed to small amounts of natural 

radioactivity. Most of the radioactivity in groundwater comes from the decay of isotopes 

of uranium and thorium that are naturally present in minerals in aquifer materials. Samples 

were analyzed for eight radioactive constituents, of which four have human-health benchmarks. Radioactive constituents were present at high levels in about 5 percent of the study 

area (gross-alpha activity and radon) and at moderate levels in about 10 percent of the study 

area.

Nutrients are naturally present at low concentrations in groundwater; high and moderate concentrations (relative to human-health benchmarks) generally result from human 

activities. Samples were analyzed for five nutrients, of which two have human-health benchmarks. Common sources of nutrients, aside from soils, include fertilizer applied to crops and 

landscaping, seepage from septic systems, and human and animal waste. The nutrient nitrate 

was detected at high concentrations in about 2 percent of the study area; nutrients were not 

present at moderate concentrations.

Inorganic Constituents and Field Measurements With 

Non-Health-Based Benchmarks

(Not included in water-quality overview charts shown on the front page)

Some constituents affect the aesthetic properties of water, such as taste, color, and 

odor, or can create nuisance problems, such as staining and scaling. The benchmarks used 

for these constituents were non-regulatory secondary maximum contaminant level (SMCL) 

benchmarks established for public drinking water. Some constituents, such as manganese 

and fluoride, have human-health benchmarks and SMCLs. Samples were analyzed for 11 

constituents that have SMCLs. One or more of these were present at high concentrations or 

values relative to the SMCL in about 42 percent of the study area and at moderate concentrations in about 28 percent.

Total dissolved solids (TDS) concentration is a measure of the salinity of the groundwater based primarily on concentrations of ions. All water naturally contains TDS as a result 

of the weathering and dissolution of minerals in rocks and sediments. Concentrations of 

TDS can be high because of natural factors or as a result of human activities, such as applications to the land surface of road salt, fertilizers, or other chemicals in urban or agricultural 

areas. Most of the study area had high (35 percent) or moderate (32 percent) concentrations 

of TDS. The ions chloride, fluoride, and sulfate were present at high concentrations in about 

10, 12, and 7 percent of the study area, respectively, and at moderate concentrations in 8, 10, 

and 23 percent, respectively. 

Anoxic conditions in groundwater (low amounts of dissolved oxygen) can result in the 

release of iron and manganese in minerals to the groundwater. Manganese was present at 

high concentrations relative to the SMCL in about 3 percent of the study area. Iron was not 

present at high concentrations relative to the SMCL. Iron and manganese were both present 

at moderate concentrations in about 2 percent of the study area.

Trace

elements and

major and minor

ions

Radioactive 

constituents

Nutrients

All non-health

benchmarks

27

25

48

30

85

98

28

42

5

10

2

INORGANIC CONSTITUENTS
High

Moderate

EX

Low or not detected

AMPLE Values are a percentage of the study area with concentrations 

in the three specified categories. Percentages might not sum to 

100 because of rounding.

PERCENTAGE OF STUDY AREA

Results: Groundwater Quality at the Depth Zone Used for Public Supply in the 

Rio Grande Aquifer System

Volatile Organic Compounds With Human-Health Benchmarks

Volatile organic compounds (VOCs) are present in many household, commercial, 

industrial, and agricultural products and are characterized by their tendency to volatilize (evaporate). Samples were analyzed for 90 VOCs, of which 38 have human-health 

benchmarks. No VOCs were detected at high or moderate concentrations in the study 

area.

Pesticides With Human-Health Benchmarks 

Pesticides, including herbicides, insecticides, and fumigants, are applied to crops, 

gardens and lawns, around buildings, and along roads to help control unwanted vegetation 

(weeds), insects, fungi, and other pests. Samples were analyzed for 227 pesticide compounds 

(pesticides and their breakdown products), of which 119 have human-health benchmarks. 

Pesticide compounds were not detected at high or moderate concentrations in the study area.

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

The USGS NAWQA Project uses benchmarks established for drinking water to provide 

context for evaluating the quality of untreated groundwater. The quality of water received 

by consumers can be different, because after withdrawal, groundwater may be treated prior 

to delivery. Federal regulatory benchmarks for protecting human health are used for this 

evaluation of water quality when available. Otherwise, non-regulatory human-health benchmarks and non-regulatory aesthetic benchmarks are used. Not all constituents analyzed have 

benchmarks and, thus, are not considered in this context. Out of 55 inorganic constituents 

and properties and 317 organic constituents, 24 and 157, respectively, have human-health 

benchmarks.

Concentrations are considered high if they are greater than a human-health benchmark (Toccalino and others, 2014) or SMCL. 

For inorganic constituents, concentrations are moderate if they are greater than one-half of a benchmark. For organic constituents, 

concentrations are moderate if they are greater than one-tenth of a benchmark; this lower threshold is used because organic constituents are generally less prevalent and have smaller concentrations relative to benchmarks than inorganic constituents (Toccalino and 

others, 2004).

Sac17-7011_Orangic_pies

Pesticides

VOCs

100

100

ORGANIC CONSTITUENTS

Constituent

Benchmark

Constituent

Benchmark

Type Value Type Value

Arsenic MCL 10 ppb Total dissolved solds (TDS) SMCL 500 ppm

Strontium MCL 4,000 ppb Fluoride SMCL 2 ppm

Fluoride MCL 4 ppm Sulfate SMCL 250 ppm

Uranium MCL 30 ppb Chloride SMCL 250 ppm

Nitrate as nitrogen MCL 10 ppm Manganese SMCL 50 ppb

Gross-alpha activity MCL 15 pCi/L pH SMCL 6.5–8.5

Radon-222 HBSL 4,000 pCi/L

Benchmark Type and Value for Selected Constituents

This table presents benchmarks for those constituents detected at high concentrations in the Rio Grande aquifer system. Benchmark types are regulatory U.S. Environmental Protection Agency (EPA) maximum contaminant levels (MCLs), non-regulatory health-based screening levels (HBSLs), and non-regulatory EPA secondary 

maximum contaminant levels (SMCLs). 

[Abbreviations: ppb, part per billion or microgram per liter (μg/L); ppm, part per million or milligram per liter (mg/L); pCi/L, picocurie per liter]
Spatial Distribution of Arsenic Concentrations 

Inorganic constituents with human-health benchmarks were present at high or moderate concentrations in 58 percent of the Rio Grande aquifer system study area. The most 

prevalent of these inorganic constituents was arsenic, which occurred at high or moderate 

concentrations in 40 percent of the study area; the distribution, however, varied from north 

to south. High or 

moderate concentrations of arsenic were 

present in 17 percent of 

the samples collected 

in the study area in 

Colorado; the sum of 

high and moderate concentrations increased 

to 36 and 34 percent in 

northern and southern 

New Mexico, respectively, and was greatest, 

78 percent, in Texas. 

These differences in 

water quality across the 

study area might reflect 

differences in the composition of rocks and 

sediments that compose 

the aquifers, aquifer 

flow-path characteristics, aquifer geochemical conditions, and 

other factors (Bexfield 

and others, 2011).

By MaryLynn Musgrove and Laura M. Bexfield

SELECTED REFERENCES

Anning, D.W., Bauch, N.J., Gerner, S.J., Flynn, M.E., Hamlin, S.N., Moore, S.J., Schaefer, D.H., Anderholm, S.K., and Spangler, L.E., 2007, Dissolved solids in basin-fill aquifers and streams in the southwestern United States (ver. 1.1, March 22, 

2010): U.S. Geological Survey Scientific Investigations Report 2006–5315, 168 p., https://pubs.usgs.gov/sir/2006/5315/.

Arnold, T.L., Bexfield, L.M., Musgrove, MaryLynn, Lindsey, B.D., Stackelberg, P.E., Barlow, J.R., DeSimone, L.A., Kulongoski, J.T., Kingsbury, J.A., Ayotte, J.D., Fleming, B.J., and Belitz, Kenneth, 2017a, Groundwater-quality data from the 

National Water-Quality Assessment Project, January through December 2014 and select quality-control data from May 

2012 through December 2014: U.S. Geological Survey Data Series 1063, 83 p., https://doi.org/10.3133/ds1063.

Arnold, T.L., Bexfield, L.M., Musgrove, MaryLynn, Lindsey, B.D., Stackelberg, P.E., Barlow, J.R., DeSimone, L.A., Kulongoski, J.T., Kingsbury, J.A., Ayotte, J.D., Fleming, B.J., and Belitz, Kenneth, 2017b, Datasets from groundwater-quality 

data from the National Water-Quality Assessment Project, January through December 2014 and select quality-control data 

from May 2012 through December 2014: U.S. Geological Survey data release, https://doi.org/10.5066/F7W0942N.

Belitz, K., Jurgens, B., Landon, M.K., Fram, M.S., and Johnson, T., 2010, Estimation of aquifer scale proportion using 

equal area grids—Assessment of regional scale groundwater quality: Water Resources Research., v. 46, no. 11, 14 p., 

doi:10.1029/2010WR009321.

Bexfield, L.M., Thiros, S.A., Anning, D.W., Huntington, J.M., and McKinney, T.S., 2011, Effects of natural and human factors 

on groundwater quality of basin-fill aquifers in the southwestern United States—Conceptual models for selected contaminants: U.S. Geological Survey Scientific Investigations Report 2011–5020, 90 p., https://pubs.usgs.gov/sir/2011/5020/.

Burow, K.R., and Belitz, K., 2014, Groundwater studies—Principal aquifer surveys: U.S. Geological Survey Fact Sheet 

2014–3024, 2 p., https://dx.doi.org/10.3133/fs20143024.

DeSimone, L.A., McMahon, P.B., and Rosen, M.R., 2014, The quality of our Nation’s waters—Water quality in principal aquifers of the United States, 1991–2010: U.S. Geological Survey Circular 1360, 151 p., https://dx.doi.org/10.3133/cir1360.

Robson, S.G., and Banta, E.R., 1995, Ground water atlas of the United States—Segment 2, Arizona, Colorado, New Mexico, 

Utah: U.S. Geological Survey Hydrologic Atlas 730–C, https://pubs.usgs.gov/ha/ha730/ch_c/index.html.

Thiros, S.A., Paul, A.P., Bexfield, L.M., and Anning, D.W., 2014, The quality of our Nation’s waters; Water quality in basin-fill 

aquifers of the southwestern United States—Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009: 

U.S. Geological Survey Circular 1358, 113 p., https://dx.doi.org/10.3133/cir1358.

Toccalino, P.L., Norman, J.E., Phillips, R.H., Kauffman, L.J., Stackelberg, P.E., Nowell, L.H., Krietzman, S.J., and Post, G.B., 

2004, Application of health-based screening levels to ground-water quality data in a state-scale pilot effort: U.S. Geological 

Survey Scientific Investigations Report 2004–5174, 64 p., https://pubs.usgs.gov/sir/2004/5174/.

Toccalino, P.L., Norman, J.E., and Schoephoester, K.M., 2014, Health-based screening levels for evaluating water-quality data: 

U.S. Geological Survey webpage, doi:10.5066/F71C1TWP, accessed at https://water.usgs.gov/nawqa/HBSL.

Wilkins, D.W., 1998, Summary of the southwest alluvial basins regional aquifer system in parts of Colorado, New Mexico, 

and Texas: U.S. Geological Survey Professional Paper 1407–A, 49 p., https://pubs.er.usgs.gov/publication/pp1407A.

Principal Aquifer Studies 

The USGS NAWQA Project has 

been assessing the quality of groundwater since 1991. The NAWQA studies 

include Land Use Studies (LUS), Major 

Aquifer Studies (MAS), and Principal 

Aquifer Studies (PAS). These three 

study types are based on sampling 

networks of wells distributed across an 

area of interest. The LUS networks typically consist of observation wells that 

are relatively shallow; MAS networks 

typically consist of domestic-supply 

wells that are intermediate in depth; 

and PAS networks typically consist of 

public-supply wells that are relatively 

deep. A national synthesis of shallow 

and intermediate depth groundwater 

quality was reported by DeSimone and 

others (2014). A regional synthesis of 

water quality for basin-fill aquifers in 

the southwestern United States, including the Rio Grande aquifer system, was 

reported by Thiros and others (2014). 

This Fact Sheet provides a summary 

of PAS data for 60 public-supply wells 

sampled in 2014 in the Rio Grande 

aquifer system (data available in Arnold 

and others, 2017b). 

The PAS assessments like this one 

allow for the comparison of constituent 

concentrations in untreated groundwater with benchmarks established for 

the protection of human health and for 

aesthetic qualities of drinking water and 

also provide a basis for comparison of 

groundwater quality among the principal aquifers. 

The data collected by the NAWQA 

Project include chemical analyses generally not available as part of regulatory compliance monitoring, including 

measurements at concentrations much 

lower than the levels used as humanhealth benchmarks, and measurement of 

constituents that can be used to trace the 

sources and movement of groundwater.

For more information

Technical reports and hydrologic data 

collected for the USGS NAWQA Project 

may be obtained from: 

NAWQA Chief Scientist

U.S. Geological Survey

12201 Sunrise Valley Drive, MS 413

Reston, VA 20192-0002

 Email: nawqapublicinfo@usgs.gov

WEB: https://water.usgs.gov/nawqa/

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20173047

Albuquerque

Lubbock

UTAH COLORADO

KANSAS

ARIZONA

NEW

MEXICO

OKLAHOMA

TEXAS

110° 108° 106° 104° 102°

36°

34°

32°

30°

0 40 80 120 160 200 MILES

0 40 80 120 160 200 KILOMETERS

MEXICO

El Paso

EXPLANATION

Pie diagrams show proportions of the 

study area with arsenic concentrations

Low

Moderate

High

!

Arsenic, in micrograms per liter (μg/L) 

Low, less than or equal to 5 μg/L

Moderate, greater than 5 μg/L

High, greater than human-health

benchmark of 10 μg/L

!

!

Rio Grande aquifer system boundary

Rio Grande aquifer system study (buffered) area

City

Colorado (6 wells)

Northern New Mexico (22 wells)

Southern New Mexico (23 wells)

Texas (9 wells)

83

17

64

27

9

65 17

17

33

44

22

Base from U.S. Geological Survey digital data, 1:1,000,000

Albers Equal-Area Conic projection

Standard parallels 29°

30' N. and 45°30' N.

Central meridian 96°

00' W.

North American Datum of 1983