DETERMINATION OF HYDRAULIC PROPERTIES IN AN AQUIFER-AQUITARD SYSTEM HYDROLOGICALLY CONNECTED TO A RIVER

Field and laboratory methods are used to determine the hydraulic properties in a multiple-layer aquifer-aquitard system which is hydrologically connected to a river. First, hypothetical pumping tests in aquifer-aquitard systems are performed to evaluate the feasibility of MODFLOW-PEST in determining these parameters. Sensitivity analyses show that the horizontal hydraulic conductivity in the aquifer has the highest composite sensitivity; the vertical hydraulic conductivity has higher composite sensitivity than the horizontal hydraulic conductivity in the aquitard; a partial penetration pumping well in an aquifer layer can improve the quality of the estimated parameters. A pumping-recovery test is conducted near the Platte River in southeastern Nebraska. Observation wells are constructed in aquitard and aquifer layers for the pumping-recovery test. The thickness of aquitard layer is determined based on an electric conductance log. This inverse approach is then used to calculate the hydraulic conductivities, storage coefficient and specific yield for the four-layer aquifer-aquitard system. The direct-push technique is used to generate sediment columns; permeameter tests on these sediment columns produce the vertical hydraulic conductivities which are compatible to that obtained from the pumping-recovery test. Thus, the combination of direct-push technique with permeameter tests provides a new method for estimation of vertical hydraulic conductivity. The hydraulic conductivity, determined from grain-size analysis, are smaller than the horizontal one but larger than the vertical one determined by the pumping-recovery test.