Groundwater Quality in the Columbia Plateau Basalt Aquifers, Northwestern U.S.

Groundwater supplies nearly 50 percent of the nation's drinking water. The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Project checks the quality of groundwater in aquifers that are important drinking-water sources — among them the Columbia Plateau basaltic-rock aquifers.

The aquifers

They underlie about 42,000 square miles of Washington, Oregon and Idaho, home to about a million people, including Spokane, the Tri-Cities (Richland, Kennewick and Pasco), Coeur d'Alene and Lewiston. They rank 19th in the nation for public supply, at about 223 million gallons a day. The land above is mostly natural cover (64 percent) and farmland (33 percent), with a little urban and developed land (4 percent).

Dry land, big farms. Most of the plateau is semiarid — 7 to 15 inches of rain a year in its centre — yet it supports a $6-billion-a-year farm industry that leads the nation in apples, hops and several other crops, and holds about 7 percent of the nation's irrigated land, watered by wells and river diversions. Demand for water, for farming, development and ecosystems, is high.

Stacked lava. The plateau is a basin between the Cascade Range and the Rocky Mountains, filled with basalt and sediment. The main aquifers are the flood basalts of the Columbia River Basalt Group: lava flows that erupted 17 to 6 million years ago, each 10 to more than 300 feet thick and in places at least 14,000 feet in total. Productive basalt units — the largest and thickest being the Grande Ronde Basalt — are separated by less permeable layers and flow interiors that confine the water. Only the basalt aquifers were sampled; the basin-fill aquifers above them, which rank 67th for public supply, were not.

Groundwater flows from the plateau's high margins toward the Columbia and Snake Rivers. The Cascades recharge the west; the Rocky and Blue Mountains recharge the east and south. Irrigation has moved water levels locally by up to hundreds of feet — up where irrigation water soaks in, down where pumping is heavy.

The Columbia Plateau aquifer system and the wells sampled. USGS.

How it was sampled

In 2016 the USGS sampled 60 public-supply wells spread across the aquifers — 35 in Washington, 20 in Oregon and 5 in Idaho. The study covers the depth used for public supply, and the results estimate the percentage of that area with high, moderate or low concentrations; accuracy depends on the number and spread of wells, not the size of the area. Wells were 91 to 2,434 feet deep (average 604) and usually drew water over long open intervals of 5 to 1,297 feet (average about 293). Water was tested before any treatment.

How "high" and "moderate" are defined

Concentrations are compared with drinking-water benchmarks: EPA's regulatory maximum contaminant levels (MCLs), non-regulatory health-based screening levels (HBSLs), and EPA's non-regulatory, aesthetic secondary levels (SMCLs). 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 an SMCL.
• Moderate: above half a benchmark for inorganic constituents, or above one-tenth for organic ones, which are generally rarer and lower relative to benchmarks.

Water that reaches consumers may differ, because it may be treated after pumping.

Overview

Water from 92 percent of the wells had no contaminant at a high concentration.

Share of the study area with high (yellow), moderate (green) or low or undetected (blue) concentrations. USGS.

• Inorganic constituents with human-health benchmarks: high in about 8 percent of the study area, moderate in about 10 percent.
• Organic constituents with human-health benchmarks: none at high or moderate concentrations.

Inorganic constituents

Inorganic constituents by group. USGS.

• Trace elements and major and minor ions (34 analysed, 22 with health benchmarks, including aluminium, cobalt and iron since 2018): high in about 5 percent — specifically arsenic — and moderate in about 7. Fluoride was the most common at moderate levels (3 percent).
• Radioactivity, mostly from the natural decay of uranium and thorium in aquifer minerals (8 analysed, 4 with benchmarks): high in about 3 percent and moderate in about 2, as gross-alpha activity and radon.
• Nutrients (5 analysed, 2 with benchmarks) — mainly from fertiliser, septic systems and human and animal waste: nitrate was high in about 2 percent and moderate in about 7.

Taste, colour and scaling

Eleven constituents with SMCLs — which concern taste, colour, odour, staining and scaling — were measured. At least one was high in about 20 percent of the study area and moderate in about 38.

• Total dissolved solids (TDS), a measure of salinity, from natural weathering or from road salt, fertiliser and other chemicals: high in 7 percent, moderate in 37.
• Fluoride: high in about 3 percent, moderate in about 8. Sulfate: high or moderate in about 5.
• Manganese and iron, released where groundwater has little oxygen: manganese high in about 10 percent and moderate in about 13; iron high or moderate in about 10.
• pH above 8.5 (alkaline, outside the 6.5–8.5 range) in about 7 percent.

Benchmarks for the constituents found at high levels

ppb: parts per billion (micrograms per litre); ppm: parts per million (milligrams per litre); pCi/L: picocuries per litre.

Organic constituents

Organic constituents. USGS.

• Volatile organic compounds (85 analysed, 51 with benchmarks): none at high or moderate concentrations.
• Pesticides (225 compounds and breakdown products analysed, 119 with benchmarks): none at high or moderate concentrations.

Where the high values are

Wells with high (yellow), moderate (green) and low (blue) concentrations of inorganic constituents: health-based benchmarks (top) and non-health-based benchmarks (bottom). USGS.

• Most wells with moderate or high values of constituents with health benchmarks were in Washington and Idaho; only one sample in Oregon exceeded a health-based benchmark.
• Constituents with non-health benchmarks were elevated more often (20 percent high, 38 moderate), and high values were likewise more common in the northern, central and eastern parts of the aquifers in Washington and Idaho.
• An earlier assessment found the aquifers vulnerable to contaminants from intensive farming, such as nitrate, and to naturally occurring constituents — both greatest in the shallower parts.

About these studies

NAWQA has assessed groundwater since 1991 with three kinds of well network: Land Use Studies (shallow observation wells), Major Aquifer Studies (mid-depth domestic wells) and Principal Aquifer Studies (deeper public-supply wells), like this one. They compare untreated groundwater with benchmarks and between principal aquifers, and include analyses not usually part of regulatory monitoring — at far lower concentrations than benchmarks, and of substances that trace where groundwater comes from and how it moves.

Sources

• Musgrove, M., 2020, Groundwater quality in the Columbia Plateau basaltic-rock aquifers, northwestern United States: U.S. Geological Survey Fact Sheet 2019–3058. https://doi.org/10.3133/fs20193058 · https://pubs.usgs.gov/publication/fs20193058
• Data: Arnold and others, 2020b, U.S. Geological Survey data release, https://doi.org/10.5066/P9W4RR74
• The background, the sampling, the overview figures, the benchmark table and the maps come from the fact sheet's PDF; the web version lacks them.
• The fact sheet's table gives iron's SMCL as "300 ppm"; the EPA's secondary standard for iron is 300 micrograms per litre — 300 ppb.
• Rewritten in hubnx's own words.