Groundwater Quality in the Basin and Range Basin-Fill Aquifers of the Southwest

Groundwater provides nearly 50 percent of the nation's drinking water. The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in the aquifers that matter most for drinking water, among them the Basin and Range basin-fill aquifers of the Southwest.

The aquifers

These aquifers underlie 148,000 square miles of Nevada, California, Arizona, Utah and neighbouring states. They are the nation's fourth-largest source of public-supply groundwater, at about 1 billion gallons a day; tenth for domestic supply, at about 64 million gallons a day; and fourth for irrigation, at about 4.5 billion gallons a day. The land above is mostly undeveloped or rangeland (about 90 percent), with little farmland (about 4 percent) or urban land (about 3 percent), and about 12 million people live on it, in cities including Salt Lake City, Reno, Las Vegas and Phoenix.

The aquifers are mainly sediment-filled basins between mountain ranges; the fill is about 1,000 to 5,000 feet thick in many basins and thicker in some. Groundwater is mostly unconfined. In this arid region most rain and snow is lost to evapotranspiration; recharge comes mainly along mountain fronts, from mountain streams soaking in and from fractured bedrock, and in some basins from irrigation water and river seepage. Evaporation is the largest natural outflow. In some basins, pumping for irrigation or public supply has changed the natural direction of flow.

The survey

The study sampled 78 public-supply wells — 31 in Arizona, 17 in California, 1 in Idaho, 20 in Nevada and 9 in Utah. Because much of the region is sparsely populated and wells are unevenly spread, the study area was drawn as 20-kilometre (12.4-mile) buffers around existing public-supply wells, with an equal-area grid inside; the results describe the depth zone used for public supply within that area. The wells were 83 to 1,450 feet deep (575 feet on average) and open to the aquifer over long intervals (about 280 feet on average). Samples were analysed for a great many constituents from natural and human sources.

Results describe untreated groundwater, compared with drinking-water benchmarks: above a benchmark is high; above half a benchmark (a tenth, for organic constituents) is moderate.

Share of the study area with high, moderate and low concentrations. Inorganic constituents with health benchmarks: 20, 49 and 31 percent. Organic constituents: none high, 3 moderate, 97 low. USGS.

Constituents with health benchmarks

• Trace elements and ions (34 analysed, 19 with health benchmarks) were high in about 20 percent of the study area. Arsenic was the most common, high in 13 percent and moderate in 35 percent. Fluoride was high in about 7 percent; manganese, molybdenum and uranium in about 3 percent each.
• Radioactive constituents (8 analysed, 4 with benchmarks) were never high; moderate levels, mainly of gross alpha activity or radon, were found in about 15 percent.
• Nutrients (5 analysed, 2 with benchmarks) were never high. Nitrate was moderate in about 9 percent, much of it in the Sonoran Desert of southwestern Arizona and southeastern California, where nitrogen fixed by desert legumes may raise nitrate naturally.
• Volatile organic compounds (90 analysed, 38 with benchmarks) were never high and were moderate in 3 percent: the fumigant 1,2-dichloropropane and the solvent tetrachloroethylene.
• Pesticides (227 compounds analysed, 119 with benchmarks) were not found at high or moderate levels.

Constituents affecting taste, odour and scaling

Of 11 constituents with secondary (aesthetic) limits, one or more were high in about 36 percent of the study area and moderate in about 33 percent.

• Total dissolved solids, a measure of salinity, were high in 32 percent and moderate in 36 — perhaps reflecting natural dissolution of minerals and evaporation, as well as human changes to groundwater flow and excess irrigation water.
• Chloride and sulfate were high in about 12 and 13 percent, and moderate in 10 and 14.
• Fluoride exceeded its secondary limit in about 12 percent and was moderate in 8.
• Manganese exceeded its secondary limit in about 8 percent; iron was high in 1 percent and moderate in 3.
• pH was above 8.5 — alkaline — in about 6 percent.

Differences across the region

Health-benchmark inorganic constituents were high or moderate in 69 percent of the study area overall, but this ranged from 30 percent in Utah/Idaho to 82 percent in California, with Nevada (75 percent) and Arizona (71 percent) close to California. In California and Arizona — broadly the south — arsenic, fluoride, molybdenum, manganese and uranium all exceeded health benchmarks in some samples; in Nevada and Utah/Idaho, the north, only arsenic did, with no high or moderate fluoride or uranium. The differences may reflect the rocks and sediments that make up the aquifers, where land has been developed, and other factors.

By state grouping: the share of the study area with high, moderate and low health-benchmark inorganic constituents (pies), and high and moderate levels of each such constituent (bars). USGS.

The fact sheet gives the share of the study area with moderate levels of health-benchmark inorganic constituents as about 49 percent on its first page and in its chart, but about 40 percent in its results.

Sources

• MaryLynn Musgrove and Kenneth Belitz, Groundwater Quality in the Basin and Range Basin-Fill Aquifers, Southwestern United States, USGS Fact Sheet 2016–3080, September 2016. https://doi.org/10.3133/fs20163080
• USGS National Water-Quality Assessment Project: http://water.usgs.gov/nawqa/