The Central Columbia Plateau of eastern Washington and western Idaho is some of the country's most important farmland — one of its top two producers of potatoes and wheat, a major grower of apples, and home to about 300,000 people. From 1992 to 1995 the U.S. Geological Survey studied its water as part of the National Water-Quality Assessment (NAWQA), and the headline was hard to miss: in about 20 percent of wells, nitrate exceeded the drinking-water standard, and in shallow wells nitrate levels were among the highest in the nation.
The cause was farming itself — and the most important finding may be how slowly the land answers when farming changes.
Most of the land in the Central Columbia Plateau is farmed. Image from the U.S. Geological Survey's report.
A dry land made wet
The climate is arid to semiarid. Only the Palouse subunit, with its thick windblown soil, gets enough rain for dryland farming and year-round streams. The Quincy-Pasco subunit farms because of the Columbia Basin Irrigation Project, which brings more than 2,500,000 acre-feet (800 billion gallons) of Columbia River water a year; irrigation there grew fast from 1955 to 1975 and raised the water table by tens to hundreds of feet. The North-Central subunit is mostly grazing land.
About 94 percent of all water withdrawn goes to irrigation, and 84 percent of drinking water comes from the ground.
Nitrate: fertilizer, water and depth
Fertilizer supplies 84 percent of the nitrate entering the plateau; feedlots, food-processing plants, septic tanks and treated wastewater matter only locally. Where fertilizer rates and irrigation are both highest — the Quincy-Pasco subunit — irrigation water soaking through the soil carries the nitrate down into shallow ground water. The regional average in shallow ground water is 6 milligrams per liter or more, against the EPA limit of 10; Columbia River water brought in for irrigation holds 1 or less. Drinking water high in nitrate is a potential health risk, especially for infants.
Depth protects. Deeper water is further from the fields, and where irrigation has raised the water table, newly recharged water tends to flow sideways into streams rather than down. So deep public-supply wells exceed the standard less often than shallow domestic wells.
Most public-supply and domestic wells over the nitrate limit are in the Quincy-Pasco subunit (wells sampled 1985–96). Image from the U.S. Geological Survey's report.
Nitrate is lower at greater depths, though some deep wells in irrigated areas are high. Image from the U.S. Geological Survey's report.
Why improvement is slow
Fertilizer use has leveled off since about 1985, and in some places nitrate may be leveling off too. In most areas it is still rising. Changes in ground water may take decades or longer to show — which the pesticide record also suggests: EDB, discontinued in the late 1970s, turned up more often in deeper wells, as if still moving down.
The survey found a cheap way to watch. In winter, when no irrigation water flows and storms are rare, the Quincy-Pasco wasteways carry mostly ground water, so a stream sample stands in for many wells. The trends differ from place to place:
| Site (winter base flow) | Nitrate trend |
|---|---|
| Frenchman Hills Wasteway | doubled, from about 3 to about 6 mg/L between 1966 and 1990, and apparently still rising |
| Crab Creek at Beverly | rising, but more slowly since 1980 |
| Crab Creek Lateral | fell from about 8 mg/L in 1966 to about 6 in 1991, as orchards — which need less nitrogen than row crops — rose from 9 to 29 percent of its cropland between 1974 and 1990 |
Three sites, three trends in winter nitrate, 1960–91. Image from the U.S. Geological Survey's report.
Pesticides, in wells and streams
Of 255 wells analyzed for 145 pesticides and volatile organic compounds, pesticides turned up in 60 percent of shallow wells and 46 percent of deeper public-supply wells — mostly herbicides, especially atrazine and its breakdown products — but generally at low concentrations. Only three exceeded a drinking-water standard or health advisory, in 2 percent of wells, and all three — dieldrin, EDB and 1,2-dichloropropane — are no longer used. No standard exists, however, for about 40 percent of the compounds analyzed.
In streams, pesticides were found at every site sampled but one, a forested headwater of the Palouse River. Six pesticides, mostly insecticides, sometimes exceeded criteria for protecting aquatic life, usually within a month or two of spraying. Carp in a small lake in the Quincy-Pasco area showed nervous-system changes that organophosphate insecticides are known to cause.
Soil, streams and fish
Over the last 100 years the Palouse has lost about 40 percent of its topsoil to erosion. Eroded soil carries long-banned compounds such as DDT with it: DDT's most persistent breakdown product exceeded sediment guidelines for aquatic life at 22 percent of the sites sampled. Irrigation method mattered — furrow irrigation erodes more than sprinklers, and DDT in streambeds and fish rose with the share of furrow-irrigated land. As sprinklers spread from the 1970s, the sediment Lower Crab Creek carried to the Columbia fell.
More furrow irrigation, more DDT in streambed sediment and fish. Image from the U.S. Geological Survey's report.
Streams also lost their shade. Most have no streamside vegetation left; canopy cover averages under 20 percent, and bank erosion is widespread. Water temperatures in urban and dryland Palouse streams exceeded the 22 °C level that protects aquatic life, and nutrient-fed plant growth drove oxygen at night below the 5 mg/L many fish need in the wastewater-dominated South Fork of the Palouse River. Streams that once held rainbow trout are now dominated by minnows; the spring-fed Crab Creek, cooler and cleaner, still supports trout.
Fish communities by land use: minnows and sculpin dominate. Image from the U.S. Geological Survey's report.
Sources
- U.S. Geological Survey, Water quality in the central Columbia Plateau, Washington and Idaho, 1992–95, Circular 1144, a contribution of the National Water-Quality Assessment Program.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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