Central Columbia Plateau - Yakima River Basin NAWQA Study - Publications

Sampling Irrigation Wasteways, Surface and Subsurface Drains to Monitor Ground-Water Quality in the Quincy-Pasco Basins, Washington

By James C. Ebbert and Joseph L. Jones

Abstract

In the Quincy-Pasco Basins, trends in concentrations of nitrate and other dissolved constituents in ground water can be monitored by sampling base flows in irrigation wasteways and surface drains during the winter months. During winter, when irrigation water is not delivered to farms and storms large enough to produce runoff are infrequent, water in wasteways and surface drains is derived entirely from ground-water discharges. Therefore, the concentration of any dissolved constituent in the water represents an average of concentrations in those discharges. Thus, a sample collected in winter from a wasteway or surface drain can substitute for many samples from individual wells.

Discharges from subsurface drains also can be sampled to assess the quality of ground water. Subsurface drains, which are installed beneath cropland to maintain the water table below the root zone, collect mostly shallow ground water moving along short flowpaths, whereas ground water that enters wasteways and surface drains moves along flowpaths of widely varying lengths. Wasteways and surface drains typically integrate a larger volume of ground water than subsurface drains.

The U.S. Geological Survey found that mean concentrations of nitrate, chloride, and the pesticides atrazine, metribuzin, and simazine were similar among samples from monitoring wells, subsurface drains, and wasteways and surface drains sampled during winter; in many instances these concentrations were not statistically different. Concentrations in samples from wells usually had the greatest variance because these samples represent only a small part of the aquifer. Because waters in wasteways and surface drains represent the largest volumes of ground water, concentrations in these samples usually had the least variance.

In most parts of the Quincy-Pasco Basins, concentrations of nitrate in base flows in wasteways and surface drains have increased since the 1960s - indicating an increase in nitrate concentrations in ground water. For example, from 1966 through 1990, base-flow concentrations of nitrate in Frenchman Hills Wasteway doubled from about 3 to about 6 milligrams per liter. However, not all trends are up. The concentration of nitrate in Crab Creek Lateral during base flow decreased from about 8 milligrams per liter in 1966 to about 6 milligrams per liter in 1991. This decrease is likely the result of an increase in the percentage of orchards planted, requiring less nitrogen than many row crops, and an increase in the use of sprinkler irrigation systems.

CITATION: Ebbert, J.C., and Jones, J.L., 1997, Sampling irrigation wasteways, surface and subsurface drains to monitor ground-water quality in the Quincy and Pasco Basins, Washington, in Symposium on the Hydrogeology of Washington State, 2nd, Aug. 25-27, 1997.