USGS Publications Warehouse

Abstract

Large chloride concentrations in Arkansas River water may degrade water quality in the adjacent Equus beds aquifer. A ground-water flow-model program (MODFLOW) was used to simulate hydrologic interaction of the Arkansas River and the Equus beds aquifer. A particle-tracking program (MODPATH) was used to simulate the movement of chloride from the river through the aquifer. Model-simulation results indicate that declining water levels in the Equus beds aquifer have caused net base-flow gains in the Arkansas and Little Arkansas Rivers to decrease from about 21 and 67 ft3/s (cubic feet per second), respectively, in 1940 to about -52 and 27 ft3/s, respectively, by the end of 1989. In hypothetical simulations (1990-2019) where only pumpage varied, net base-flow loss from the Arkansas River ranged from about 59 to 117 ft3/s for no increase in pumpage and a 3-percent per year increase in pumpage since 1989, respectively. Estimated chloride discharge from the Arkansas River Iassuming a chloride concentration of 630 milligrams per liter) to the aquifer increased from about 21 ton/d (tons per day) in 1940 to about 100 ton/d by the end of 1989 and was estimated to range from about 110 to 200 ton/d by 2019, depending on pumpage and climate conditions. Particle-tracking simulations show that the distribution of particles representing chloride from the Arkansas River expanded from relatively narrow bands near the river to a wider distribution within the aquifer, and may have reached the edge of the Wichita well field by 1963.