Effects of drain wells on the ground-water quality of the western Snake Plain Aquifer, Idaho

Abstract

Approximately 3,100 drain wells are used to inject irrigation waste water, urban runoff, septic-tank effluent, and industrial waste water in Minidoka, Gooding, Jerome, and Lincoln Counties. About 29,000 acre-feet(3.6xl0 7 m 3 ) of irrigation waste water, 100 acre-feet (1.2xl0 5 m 3 ) of urban runoff, 400 acre-feet (4.9xl0 5 m 3 ) of septic-tank effluent, and 1,000 acre-feet (1.2xl0 6 m 3 ) of industrial waste water are injected into the Snake Plain aquifer annually in the study area. Irrigation waste water contains significant concentrations of nutrients and bacteria; urban runoff may contain high concentrations of sodium, chloride, and bacteria; and septic-tank effluent contains high concentrations of nutrients, chloride, and bacteria. The only sample of industrial waste water collected was chemically unchanged from its source, but the temperature was significantly increased. No regional effects of drain-well inflow could be identified within the study area. Several examples of local effects were noted, however. In the observations of local effects, water samples collected from the aquifer before, during, and after injection of waste water through nearby drain wells indicated changes in quality toward the drainwell inflow. In some wells, significant increases in bacterial concentrations were observed. The data clearly suggest that disposal of toxic wastes or other substances which might have a detrimental effect on water use would be hazardous. One must assume that any water injected into the aquifer through drain wells may, at some future time, affect a downgradient supply well.