Central Columbia Plateau - Yakima River Basin NAWQA Study - Publications

Nitrate Concentrations in Ground Water of the Central Columbia Plateau

USGS Open-File Report 95-445

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The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program is designed to assess the status of and trends in the quality of the Nation's water resources, and to gain a better understanding of the natural and human factors that affect water quality. The Central Columbia Plateau is one of 60 NAWQA study units (major river basins and parts of aquifer systems) located throughout the Nation. In the Central Columbia Plateau, nitrate concentrations for 19% of the 573 wells shown below exceed the U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) for drinking water. These concentrations include USGS samples from 1942-94, although 93% of the data are from 1980-94. Where more than one analysis was available for a well, this document refers to the mean concentration as the nitrate concentration for the well.

Map showing concentrations of nitrate in wells

Land use practices are the dominant influence over the distribution and concentration of nitrate in ground water.

Nitrogen fertilizers applied to fields are the primary source of nitrate in shallow ground water. Nitrogen fertilizers not used by crops can be carried to the underlying aquifer by water percolating through the soil. In the arid Central Columbia Plateau, irrigation water carries nitrate into shallow ground water. Irrigated agriculture is consequently associated with high nitrate concentrations and high frequency of contamination of ground water in the study area.

Agriculture covers 8,000 square miles, or 61%, of the study area. Range land covers an additional 4,000 square miles (31%) of the study area. The remaining 1,000 square miles (8%) is largely forest or water (see map below). Patterns in land use, climate, geology, and hydrology divide the study area into natural subunits suitable for comparison. Different land uses and irrigation practices in each subunit account for much of the variation of nitrate concentration across the study area (see table below).

Map showing major land uses in the
     Central Columbia Plateau

Quincy-Pasco subunit

The Quincy-Pasco area is one of the most productive agricultural areas in the country.

99% of water used in the study area supports agricultural irrigation in the dry southwestern region -- mainly in Grant, Franklin, and Adams Counties.

CBIP influence: The Columbia Basin Irrigation Project (CBIP) brings more than 2,500,000 acre-feet of water per year from the Columbia River, through Banks Lake to the Quincy-Pasco subunit.

Pie charts showing percentages of different land uses in the
                Quincy-Pasco subunit, and percentages of nitrate concentrations
                at several levels

North-Central subunit

This subunit is characterized by channels of exposed basalt and thin, poor soil. The climate is arid, and deep ground water is the only source of irrigation water. Non-irrigated agriculture in this subunit is much like adjoining areas of the Palouse subunit.

The North-Central subunit's deep water table makes pumping expensive, but irrigated agriculture is possible in the more arable lands bordering the CBIP. High nitrate concentrations appear in the shallower ground water surrounding the CBIP.

Pie charts: percentages of different land uses in the North-Central
	  subunit, and percentages of nitrate concentrations at several

Palouse subunit

Hills of fine-grained wind-blown sediments, and a relatively moist climate, support non-irrigated agriculture across much of the Palouse.

Pie charts: percentages of different land uses in the Palouse
             subunit, and percentages of nitrate concentrations at several

Percentages of wells with nitrate concentrations exceeding the EPA MCL

                               Domestic and     All
                               public supply*   wells
         County** Adams            18%          8%
                  Douglas          30%         14%
                  Franklin         33%         33%
                  Grant             9%          7%
                  Whitman           4%          5%
         Subunit  Palouse           4%          6%
                  Quincy-Pasco     29%         28%
                  North-Central    15%          9%
                  Study area       21%         19%

         *Percentages are similar for Washington Department
          of Health Class A public supply wells.
         **Percentages were not calculated for Latah, Lincoln,
           Nez Perce, and Spokane Counties, which fall partly
           outside the study area.


Recharge is water moving into the ground-water system.

Recharge from precipitation and irrigation may carry nitrogen compounds from the soil into the aquifer, often resulting in elevated nitrate concentrations in shallow wells.

Map showing rates of recharge across the Central Columbia
  • Water from the Columbia Basin Irrigation Project (CBIP) gives the arid Quincy-Pasco subunit the highest rate of recharge in the study area.

  • In the North-Central subunit, the expense of pumping deep ground water reduces the amount of irrigation contributing to recharge.

  • The Central Columbia Plateau's highest rate of precipitation is more than 20 inches per year in the Palouse subunit.

In parts of the Quincy-Pasco subunit, water-table altitudes have been raised by several hundred feet, with both positive and negative effects. Numerous wetlands and springs have sprung up, providing new habitat for waterfowl; but along the Columbia River landslides have occurred and subsurface drains are required to prevent waterlogging of soil.

Recharge moves nitrate primarily into shallow ground water.

Agricultural fertilizers are the main source of nitrate in ground water.

Man-made fertilizers became widely available after World War II and quickly came into use across the Nation. Today, nitrogen fertilizers (commonly nitrate or ammonium compounds) are used in large quantity in most agricultural settings.

Map showing fertilizer application rates, by county
  • The Quincy-Pasco subunit's high annual rate of fertilizer application coincides with high nitrate concentrations in the subunit's shallow ground water (see map of nitrate concentrations).
The extent and intensity of agriculture in the Central Columbia Plateau make high annual rates of fertilizer application a particularly strong influence over water quality. Other sources of nitrogen, such as livestock production, food processing wastes, and sewage, are lesser influences at a regional scale.

Nitrogen, ammonia, and ammonium fertilizers break down into nitrate.

High nitrate concentrations may be an indicator of the presence of pesticides.


Nitrate concentrations are generally lower at greater depths.

Graph showing relation between nitrate concentration and depth to sample
  • In the Central Columbia Plateau, 26% of wells less than 300 feet deep have nitrate concentrations exceeding the EPA MCL of 10 mg/L.

  • Only 8% of wells deeper than 300 feet have nitrate concentrations exceeding the MCL.

Some nitrate may be transformed into other compounds as it is carried through the ground-water system. Mixing also decreases nitrate concentrations, as water with higher nitrate concentrations enters deep ground-water systems farther from agricultural influences.

Nitrate concentrations at the water table may vary greatly.

Ground water moves along flow paths that vary from a few feet to hundreds of miles (shown with arrows, below).

Cross-section of ground-water flow
  • Shallow flow paths tend to be influenced by land use practices at the surface, while water in deeper flow paths is farther from human influences.

  • Deeper flow paths also have much longer travel times, in most cases predating modern land use practices.

Water from different sources increases the variability of nitrate concentrations in shallow ground water. In the Quincy-Pasco subunit, recharge water carrying nitrate along shallow paths mixes with canal water from the CBIP. Recharge water carrying nitrate into deeper flow systems mixes with deeper, older water.

Nitrate concentrations in the regional ground-water system have generally increased since the 1950's.

Graph showing nitrate concentrations at Ringold
	     Springs, 1955-1995
  • Nitrate concentrations in the regional ground-water system vary greatly but have generally increased, due to increased irrigation and use of nitrogen fertilizers.

  • Single wells may show high variable trends in nitrate concentration. A good illustration of regional trends is Ringold Springs, a ground water discharge point integrating an area of several square miles.

Human Health Effects of Nitrate

A June 1995 Washington State Department of Health Fact Sheet states:

For more health information, contact your water utility or county health agency, or the Washington State Department of Health: 1 (800) 521-0323.

USGS Open-File Report 95-445, by Sarah J. Ryker and Joseph L. Jones

Based on Water-Resources Investigations Report 94-4258.


Greene, K.E., Ebbert, J.C., and Munn, M.D., 1994, Nutrients, suspended sediment, and pesticides in streams and irrigation systems in the Central Columbia Plateau, in Washington and Idaho, 1959-1991: U.S. Geological Survey Water-Resources Investigations Report 94-4215, 64+ p.

Jones, J.L., and Wagner, R.J., 1995, Ground-water quality of the Central Columbia Plateau in Washington and Idaho: Analysis of available nutrient and pesticide data, 1942-1992: U.S. Geological Survey Water-Resources Investigations Report 94-4258, 119 p.

Washington State Department of Health, June 1995, Nitrate in drinking water: Washington State Department of Health Fact Sheet, 2 p.

For further technical information contact:

     Project Chief
     Central Columbia Plateau - Yakima River Basin NAWQA Study
     1201 Pacific Avenue - Suite 600
     Tacoma, WA 98402
     Phone: (206) 593-6530 x2687
January 1996, Second printing

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Last modified: Thu Jan 22 15:23:21 1998