Groundwater Quality in the High Plains Aquifer: What 80 Public-Supply Wells Showed

The High Plains aquifer, its three subregions, and the wells sampled. USGS.

The aquifer

Groundwater supplies nearly 50 percent of the nation's drinking water. The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project tests the aquifers that matter most for it — among them the High Plains aquifer.

• Size: about 169,000 square miles under parts of eight states — Colorado, Kansas, Nebraska, New Mexico, Oklahoma, Texas, South Dakota and Wyoming — home to about 2 million people.
• Use: 13th in the nation for public-supply groundwater (about 390 million gallons a day in 2000) and first for irrigation, at far more — 17,000 million gallons a day. About 20 percent of the nation's irrigated farmland lies over it.
• Land above: 59 percent natural cover, including rangeland; 37 percent farmland; 4 percent urban or developed. The biggest cities are Amarillo, Lubbock and Midland, Texas, and Cheyenne, Wyoming; most people live in small towns and the countryside.

How it works

It spans enough country to be split into Northern, Central and Southern High Plains. Rain and snow decrease from east to west; temperatures rise from north to south.

• Rock: mostly unconfined, in Tertiary and Quaternary deposits near the surface forming six connected units. The largest, the Ogallala Formation, is mainly loose clay, silt, sand and gravel, cemented with carbonate in places, over older sedimentary bedrock.
• Recharge is scarce: summer heat and steady wind make evaporation among the nation's highest, and it outpaces precipitation over much of the area. What water does get in comes mainly from irrigation water soaking in; rain soaking in; storm and irrigation runoff through streambeds, playas and other low spots; and water rising from aquifers below.
• Discharge: mainly irrigation wells, flow to streams and deeper aquifers, flow out across the eastern edge, and evapotranspiration. Groundwater moves generally west to east.
• Falling water: pumping has far outpaced recharge, so water levels have dropped across parts of Kansas, Oklahoma, New Mexico and Texas.

The study

In 2015 and 2016 the USGS sampled 80 public-supply wells spread across the aquifer: 7 in Colorado, 11 in Kansas, 32 in Nebraska, 4 in New Mexico, 4 in Oklahoma and 5 in Wyoming in 2015, and 17 in Texas in 2016.

• The wells ran 54 to 812 feet deep (282 on average), open to the aquifer over 10 to 375 feet (about 84 on average).
• The study area is the depth zone used for public supply. Because the wells are evenly spread, the results can be read as percentages of that area — their accuracy depends on how many wells there are and how they're spread, not on the area's size.
• The water was tested untreated, before any treatment a utility might apply, so what reaches taps can differ.

How results are rated

Results are compared with benchmarks for drinking water — federal regulatory limits for health where they exist, otherwise non-regulatory health or aesthetic benchmarks. Human-health benchmarks exist for 28 of 55 inorganic constituents and properties and 170 of 310 organic ones.

• High: above a human-health benchmark or a secondary maximum contaminant level (SMCL).
• Moderate: for inorganic constituents, above one-half of a benchmark; for organic constituents, above one-tenth — a lower bar, since they are usually less common and further below their benchmarks.

Benchmarks for the constituents found at high levels (ppb = parts per billion, or micrograms per liter; ppm = parts per million, or milligrams per liter; pCi/L = picocuries per liter):

MCL: the EPA's regulatory maximum contaminant level. HBSL: a non-regulatory health-based screening level. SMCL: the EPA's non-regulatory secondary level.

What they found

Share of the study area at high, moderate and low levels. Percentages may not total 100 because of rounding. USGS.

81 percent of the wells had no constituent at a high level. Inorganic constituents with health benchmarks were high in about 19 percent of the study area and moderate in about 56 percent; organic ones were moderate in 3 percent.

Inorganic, with health benchmarks

• Trace elements and ions: 34 analyzed, 22 with health benchmarks (screening levels added aluminum, cobalt and iron in 2018). High in about 16 percent, moderate in about 50 percent.
• • Uranium and arsenic were most often high — 8 and 6 percent of the area.
  • Moderate: uranium about 15 percent, arsenic 44, fluoride 13.
  • Strontium was high or moderate in about 11 percent.
• Radioactivity: 8 constituents analyzed, 4 with benchmarks. Mostly from gross-alpha activity: high in about 1 percent, moderate in about 15. Most radioactivity in groundwater comes from the natural decay of uranium and thorium in aquifer minerals.
• Nutrients: 5 analyzed; nitrate and nitrite have benchmarks. Nitrate was high in about 3 percent and moderate in about 11. Beyond soils, sources include fertilizer, septic systems, and human and animal waste; earlier studies found more nitrate in shallow groundwater than in deeper, older water.

Organic

• Volatile organic compounds: 85 analyzed, 51 with benchmarks. Moderate in 1 percent, from trichloroethylene (TCE).
• Pesticides: 225 compounds, 119 with benchmarks. Moderate in 1 percent, from the herbicide atrazine.

Taste, color and scale

11 constituents with SMCLs — limits for taste, color, odor, staining and scaling — were analyzed. At least one was high in about 35 percent of the area and moderate in about 49.

• Total dissolved solids (TDS), a measure of saltiness, was high in 26 percent and moderate in 54. Weathering rock puts some in all water; road salt, fertilizers and other chemicals can raise it.
• Fluoride and sulfate were high in about 15 and 10 percent, moderate in 18 and 11.
• Manganese and iron, released where groundwater is low in oxygen, were high in about 11 and 1 percent.
• pH was above 8.5 (alkaline) in about 1 percent.

It depends where you are

TDS at each well, and the share of samples in each subregion with moderate or high TDS, arsenic, fluoride and uranium. USGS.

The most common problems were arsenic, fluoride and uranium among health benchmarks, and TDS among the rest — but not evenly:

• Southern High Plains: high TDS, arsenic and fluoride were most common here. Earlier studies linked them to salty water rising from the Dockum aquifer below.
• Northern High Plains: high and moderate uranium was more common here. Earlier work found uranium throughout the aquifer, tied to nitrate, especially in shallow water.

Differences in the rocks and sediments, flow paths and chemistry of the aquifer may explain the pattern.

About these surveys

NAWQA has studied groundwater quality since 1991, with three kinds of well network: Land Use Studies (shallow observation wells), Major Aquifer Studies (mid-depth household wells) and Principal Aquifer Studies (deeper public-supply wells, as here). They measure far below health benchmarks, and include tracers that show where groundwater comes from and how it moves — data routine compliance testing doesn't collect. The High Plains data are in a USGS data release; more at water.usgs.gov/nawqa.

Sources

Based on Groundwater Quality in the High Plains Aquifer, USGS Fact Sheet 2019–3055 (March 2020), National Water-Quality Assessment Project, U.S. Geological Survey; a work of the United States government in the public domain. The background section, charts, maps and benchmark table are taken from the fact sheet's PDF.