Public-supply well

Domestic well

Monitoring well

Coarse-grained sediment

Consolidatedrock

Fine-grained sediment

Water Quality of the Southwest Basin-Fill AquifersNational Water-Quality Assessment Program

By Susan Thiros

Contaminants in water from one of every three drinking-water wells sampled are a potential human-health concern

Water from about 42 percent of domestic wells and 26 percent of public-supply wells in the Southwest contained at least one contaminant at a concentration that

was greater than its human-health benchmark.

Arsenic and uranium derived from natural sources are a potential drinking-water concern

0 5 10 15

Uranium

Nitrate

Arsenic Southwest Basin-Fill Aquifers

Percentage of samples from drinking-water wellswith concentrations greater than human-health benchmarks

Aquifers nationwide

The Southwest Principal Aquifers study area encompasses much of California, Nevada, and parts of Utah, Arizona, New Mexico, and Colorado. Over 46.6 million people, 15 percent of the Nation’s population, reside in this arid and semiarid area. Demands for irrigation and drinking water have substantially increased groundwater withdrawals and irrigation return flows—these changes have increased the migration of contaminants from natural and human sources to depths used to supply drinking water in many basin-fill aquifers. Groundwater quality and the effects of contaminant movement are of critical importance to the 1.4 million people in mostly rural areas who obtain their water from unregulated domestic wells.

WY

CA

NV

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UTCO

WA

OR ID

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NM

Artificial recharge and groundwater withdrawals for irrigation and public supply are mobilizing contaminants to basin-fill aquifersExcess irrigation water and groundwater withdrawals can increase the movement of nitrate from fertilizers and manure, volatile organic compounds and pesticides used in urban and agricultural areas, and dissolved solids to deeper parts of basin-fill aquifers pumped for drinking water. Surprisingly, artificial recharge and groundwater withdrawals are mobilizing naturally-occurring uranium from aquifer sediments—water sampled from one in four domestic wells in the eastern Central Valley, California exceeds the MCL. Nitrate concentrations exceeded its MCL in samples from almost 4 percent of public-supply wells and 18 percent of domestic wells in the Southwest.

Land and water development in combination with natural factors continues to increase dissolved-solids concentrations in groundwater Concentrations of dissolved solids in deep wells used for public supply are typically low, but substantial increases in concentrations over time in some areas of the Southwest raises questions about the long-term viability of the groundwater resource for drinking water. Dissolved solids, which can impart an unpleasant taste to water, exceeded its secondary MCL in one of every four samples from drinking-water wells (19 percent of public-supply wells and 31 percent of domestic wells).

A statistical model of arsenic in groundwater estimates that concentrations equal to or greater than the human-health benchmark are likely to occur in almost one-half of the basin area in the Southwest.

0 1,000 2,000 3,000 4,000 5,000

Concentration of dissolved solids, in milligrams per liter

Public-supply wells (252 samples)

Domestic wells (312 samples)

Agricultural monitoring wells (210 samples)

Urban monitoring wells (199 samples)

19%

31%

45%

64%

Secondary drinking-water standard for dissolved solids is 500 milligrams per liter.Number next to line is the percentage of samples that exceeded the standard.

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Understanding the factors that affect arsenic and other natural contaminants in groundwater can help water suppliersprioritize areas for new groundwater development and reduce treatment costs.

Racetrack Playa, Death Valley, California

The volcanic and granitic rocks in the Southwest are natural sources of arsenic and uranium in groundwater. Water sampled from 19 percent of public-supply wells and 14 percent of domestic wells in the Southwest exceeded the MCL for arsenic. Exposure to arsenic and uranium con-tributes to an increased risk of cancer. Overall, contaminants in basin-fill aquifers from natural sources are a greater concern for drinking water than are contaminants from human sources.

Irrigation well in Central Valley, California

For more information: http://water.usgs.gov/nawqa/studies/praq/swpa

Arsenic, uranium, and nitrate exceeded their respective U.S. Environmental Protection Agency Maximum Contaminant Levels (MCLs) more than twice as frequently in samples from Southwest drinking-water wells than in groundwater samples collected nationwide. The widespread detection of contaminants in domestic wells and the lack of regulation underscores the need for public education on where contaminants are likely to occur and testing/treatment options.

NEVADA

OREGONIDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Paci f i c

Ocean

Southwest Principal Aquifersstudy area boundary

EXPLANATIONPredicted arsenic concentration,

in micrograms per liter

< 1.0 1.0 to < 5.0

5.0 to < 10.0 10.0

Areas not modeled (mostly mountains)

≥

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Dieldrin

Dibromochloro-propane (DBCP)

Nitrate

Uranium

Arsenic

0 5 10 15 20 25 30Percentage of groundwater samples with concentrations greater than human-health benchmarks

Urban monitoring wellsAgricultural monitoring wellsDomestic wellsPublic-supply wells

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