Ground water was sampled in a 900-square-mile agricultural area in the Pasco Basin, which includes parts of eastern Benton County and western Franklin County, Washington, to determine distributions of nitrate and fluoride. Additional data were obtained to determine if fertilizers, irrigation water, septic systems, and naturally occurring nitrate are sources of nitrate in ground water. Limited sampling also was done to determine if pesticides were present in the ground water.

Nitrate concentrations in ground water ranged from less than 0.1 to 100 milligrams per liter as nitrogen, and median concentrations of nitrate nitrogen in ground water were 3.2 and 6.7 milligrams per liter for Benton and Franklin Counties, respectively. In Franklin County, where a large percentage of the land is used for irrigated agriculture, nitrate nitrogen concentrations in water from 31 percent of sampled wells were equal to or greater than the maximum contaminant level for drinking water of 10 milligrams per liter. In Benton County, nitrate concentrations in water from about 10 percent of the sampled wells exceeded the maximum contaminant level.

Nitrate concentrations in ground water at some locations in Franklin County have increased by as much as two orders of magnitude since the early 1950's. Historical data generally were not available to evaluate changes of nitrate concentrations in ground water in Benton County, except for the area around the town of Finley. A comparison of data collected during this study with data collected during 1976-77 indicate that nitrate concentrations in ground water of the Finley area probably have not changed over the intervening period.

Applied nitrogen fertilizers are a major source of nitrate in ground water at many locations in the study area. Surface water used for irrigation does not contain sufficient nitrate to cause elevated concentrations in ground water. Instead, canal seepage, which makes up about 50 percent of the ground-water recharge in the study area, tends to dilute the nitrate present in ground water.

Septic systems in the Finley area of Benton County are a source of nitrate in ground water, but analyses of data and results of a numerical model analysis of nitrate concentrations in the unconfmed ground-water system indicate that they are not the primary source of nitrate in ground water in this area.

Naturally occurring nitrate may be a source of nitrate in ground water underlying Badger Coulee in Benton County. Average masses of natural nitrate per unit volume of sediment in two boreholes in Badger Coulee were equivalent to 2,590 and 964 pounds of nitrogen, respectively, in a block of sediments 50 feet thick underlying an acre of land. At most other locations in the study area, the amount of natural nitrate in ground water is probably small compared with nitrate from anthropogenic sources.

Fluoride concentrations in ground water in the study area ranged from less than 0.1 to 4.7 milligrams per liter; the median concentration was 0.5 milligram per liter. The concentration of fluoride in water from only two of 143 wells equalled or exceeded 2.0 milligrams per liter, which is the secondary maximum contaminant level for drinking water. Both are deep wells open to the Saddle Mountains Basalt in Franklin County. Large concentrations of fluoride in deep ground waters of the Pasco Basin are apparently the result of natural conditions in the deeper basalt aquifers.

One or more pesticide compounds were detected in 10 of 29 ground-water samples, which were analyzed for selected chlorophenoxy acid herbicides, triazine herbicides, carbamate insecticides, organophosphorus insecticides, and a few other types of pesticides. The sampling locations did not represent a random distribution, but instead, most were wells open to unconfined, shallow ground water in irrigated areas. The pesticides found include the herbicides atrazine, dicamba, metribuzin, picloram, and 2,4,5-T. Also present were aldicarb sulfone and aldicarb sulfoxide, which are degradation products of the insecticide aldicarb. Except for metribuzin, pesticide concentrations were at or near the analytical reporting limits. In all instances, the concentrations of pesticides detected were below the health advisory levels that are issued by the U.S. Environmental Protection Agency