Ground-water flow simulation of the Columbia Plateau regional aquifer system, Washington, Oregon, and Idaho

A thick sequence of flood basalts of Miocene age covering 50,600 square miles forms a major aquifer system in eastern Washington, eastern Oregon, and a small part of Idaho. About 4,509 square miles of saturated sedimentary overburden materials overlie the basalts, mainly in the central part of the Columbia Plateau. Together, the sedimentary and basalt materials make up the Columbia Plateau regional aquifer system. The aquifer system, for study purposes, was divided into four aquifer units and three confining units. The aquifer units are, from youngest to oldest, the overburden aquifer, the Saddle Mountains unit, the Wanapum unit, and the Grande Ronde unit. For modeling purposes, the Grande Ronde unit is separated into two layers, the upper and lower Grande Ronde. The base of the aquifer system is composed of rock materials older than the basalts and is called the basement confining unit. The other two confining units are generally equivalent, and are from youngest to oldest, the Saddle Mountains-Wanapum interbed and the Wanapum-Grande Ronde interbed. The Wanapum-Grande Ronde interbed is not mapped fully and is missing in several locations. These two confining units are areally extensive but extremely thin when compared with the thickness of the basalts. The two interbeds, however, are productive aquifers in some areas. A numerical finite-difference ground-water model covering 32,688 square miles was used to simulate the regional flow system. The model consists of 5 layers and 17,899 cells. A time-averaged, steady-state approach was used for model construction. The model parameters initially were estimated for a steady-state simulation representing predevelopment conditions. Subsequently, the model was calibrated to the 1983-85 period. After calibration of the time-averaged (1983-85 period) model, the steady-state, predevelopment flow system was simulated. Recharge to the aquifer system is mainly from precipitation and irrigation. Recharge ranges from zero in low-lying areas that receive precipitation of less than 8 inches annually, to more than 30 inches per year in the mountainous and irrigated areas. Recharge was estimated to be 6,568 cubic feet per second (2.72 inches per year) for predevelopment conditions and 10,208 cubic feet per second (4.24 inches per year) for time-averaged conditions. For cells receiving more than about 3 inches per year of recharge, 25 percent discharges locally; whereas, for cells receiving less than 3 inches per year and where there are no major rivers draining the regional system, less than 5 percent discharges locally. Initial mapping of lateral hydraulic conductivities in the basalts showed little spatial correlation between conductivities. Such randomness initially was assumed to be in areas lacking specific-capacity data, such as in the folded uplands. To achieve calibration, however, small values were estimated for the uplands and large values were estimated for low-lying areas near major discharge features. The model-derived lateral hydraulic conductivity of the overburden aquifer has a median of about 5xlO"4 feet per second and ranges from about 5xlO"7 to 13xlO"3 feet per second. Conductivities of the basalt range from about IxlO"6 to IxlO"4 feet per second and have a median of about 1.7xlO"5 feet per second. Few data are available for vertical conductances. Model-calibrated values for all units range from 4.5xlO~13 to l.OxlO"5 per second. The model-calibrated distribution of vertical conductances generally shows low values in the upland, folded areas and over broad, low-lying areas surrounding the major rivers. Vertical conductances in the two uppermost layers, however, were made anomalously large for model cells beneath the major rivers. One probable explanation for such large conductances is that these rivers cut deeply into the basalts and were backfilled with coarse materials. Recharge is the principal inflow component to the flow system during the predevelopment and the time-averaged periods. Leakage from rivers provides an average of 555 cubic feet per second of inflow for both periods. Discharge from the aquifer system is by leakage to rivers, leakage to drains, and ground-water pumpage. Leakage to rivers and drains during predevelopment conditions was about 2,750 and 3,940 cubic feet per second, respectively, and for the time-averaged conditions was 3,800 and 5,600 cubic feet per second, respectively. Time-averaged conditions have the additional discharge component of ground-water pumpage, 1,135 cubic feet per second (822,000 acre-feet). The root-mean-square head residual for the upper three model layers is about 100 feet (70 percent of the differences between calculated and observed heads is less than 100 feet). The root:mean-square error for the upper Grande Ronde is about 292 feet. A comparison of manually contoured head configurations with model-calculated configurations shows good agreement over much of the modeled area. The use of water associated with water development has increased recharge by 3,640 cubic feet per second, saturated previously unsaturated rock materials, and caused water-level declines. Increase in recharge and volume of saturated materials is due to the infiltration of surface water applied to croplands. Water-level rises have been more prevalent and widespread than water-level declines. This is because a larger area of croplands is irrigated with surface water rather than with ground water. The average surface-water annual application rate is about 4 feet per year. Water-level declines have occurred mainly in the Odessa-Lind area, the Umatilla area, the Horse Heaven Hills area, the Pullman-Moscow area, the Walla Walla River area, and in some of the small tributary valleys to the Yakima River. Water-level changes in the top four layers vary by more than 200 feet. Of 27 basins analyzed, 17 show a decrease in ground-water discharge. The discharge within the remaining basins either stayed the same or increased slightly.