The Spokane aquifer, Washington: its geologic origin and water-bearing and water-quality characteristics
The Spokane aquifer is an unconfined aquifer consisting of coarse sand, gravel, cobbles, and boulders deposited during several catastrophic glacial outburst floods--known as the Spokane Floods---of Pleistocene time. The aquifer is one of the most productive in the United States, and, as the only significant source of good-quality water supply in the Spokane Valley, it has been designated as a 'Sole Source Aquifer' by the U.S. Environmental Protection Agency. The Spokane aquifer underlies an area of about 135 square miles in the Spokane Valley and varies in saturated thickness from a few feet to 500 feet or more. The aquifer is recharged by ground-water underflow from the Rathdrum Prairie aquifer in Idaho on the east, by ground-water underflow and surface-water seepage from small drainage areas along the Spokane Valley margins, and by percolation from various sources--from rainfall and snowmelt, from some reaches of the Spokane and Little Spokane Rivers, and from septic-tank drain fields, cesspools, and irrigation water. Discharge from the aquifer occurs by ground-water underflow from the lowermost end of the valley, by leakage to the Spokane and the Little Spokane Rivers, by evapotranspiration, and by ground-water withdrawal by pumping. The transmissivity of the aquifer ranges from less than 0.05 to 70 feet squared per second, and its specific yield ranges from less than 5 to 20 percent of the aquifer volume. Seasonal water-level fluctuations in wells tapping the aquifer are generally less than 10 feet. The annual pumpage from the aquifer in 1977 was about 164,000 acre-feet, of which about 70 percent was for municipal supplies, which included some industrial and commercial supplies. Land use over the aquifer includes predominantly agricultural activities in the eastern one-third of the valley and urban and residential developments in most of the remaining area. Potential sources of contamination of the aquifer include percolation from cesspools, septic-tank drain fields, and municipal and industrial waste-disposal sites. In general, the high rate of ground-water movement through the highly permeable aquifer materials has resulted in the ground-water quality being little affected by the overlying land use activities. Some local degradation of water quality has occurred due to industrial waste-disposal practices, however. During the water-quality study period of May 1977 to May 1978, average specific conductance of the ground water ranged from less than 100 to about 500 micromhos per centimeter at 25 degrees Celsius, average chloride concentration ranged from less than 2 to about 12 milligrams per liter (equivalent to parts per million}, and average nitrate nitrogen concentrations ranged from less than 1 to about 8 milligrams per liter. The streamflow and water quality of the Spokane River, which are related to the flow and quality of water in the Spokane aquifer, indicate that, during the period 1913 to 1978 inclusive, the river at Post Falls, Idaho, had an average annual discharge of 6,307 cubic feet per second, a maximum discharge of 50,100 cubic feet per second, and a minimum discharge of 65 cubic feet per second. The quality of the river water along its course through the study area is affected to some extent by inflows of industrial wastewater and treated municipal sewered water. In the 30-mile reach between the State line and Riverside State Park, during the 1975 to 1978 water years inclusive, concentrations of nearly all the constituents analyzed increased, and concentrations of dissolved oxygen correspondingly decreased from 1968 to 1977 inclusive; coliform bacteria also showed notable increases in the downstream direction.