Ground-water hydrology of the Toppenish Creek basin, Yakima Indian Reservation, Washington

A groundwater flow model was constructed for the Toppenish Creek basin aquifer system in eastern Washington. Flow was simulated in three aquifer units: (1) the confined old valley fill and shallow basalt (unit 2); (2) the underlying primary basalt (unit 3); and (3) the deep basalt (unit 4). Water levels in the overlying unconfined alluvial aquifer (unit 1) were held fixed. Calibrated transmissivities ranged from 0.01 to 0.48 foot squared per second. Calibrated storage coefficients were 0.0004 to 0.006. The confining-bed leakance ranged from 2.0 x 10 to the minus 11th power to 2.5 x 10 to the minus 10th power feet per second per foot. Under steady-state conditions (1954) annual natural recharge was about 29,000 acre-ft underflow from adjacent basins. Annual pumpage increased from less than 500 acre-ft in 1954 to an average of 19,600 acre-ft for 1971 and 1972. Pumpage caused simulated declines in unit 3 of up to 95 ft for 1955-72. Projected annual declines from 1973-77 using 1971-72 pumpage were about 0.5-1.5 ft in unit 2 and 0.2-1.5 ft in unit 3. The corresponding declines from 1978-82 were 0.2-1.2 ft and 0.2-0.8 ft, respectively. Using 1971-72 pumpage plus 12 ,400 acre-ft per year from unit 3, the calculated annual declines from 1978-82 were 1-36 ft in unit 2 and 4-20 ft in unit 3. (USGS)