HR: 0800h
AN: H31D-0441    [Abstracts]
TI: Spatial variability of ground-water recharge in selected principal aquifers of the eastern United States
AU: * Nolan, B T
EM: btnolan@usgs.gov
AF: U.S. Geological Survey, 413 National Center, Reston, VA 20192 United States
AB: Over 500 vadose-zone sediment cores were collected as part of a regional study of ground water recharge to aquifers comprising glacial deposits and the Floridan, Coastal Lowlands, Piedmont and Blue Ridge, and North Atlantic Coastal Plain aquifer systems. Study objectives were to compare methods for estimating recharge; to compare and contrast recharge estimates for selected principal aquifers in the eastern U.S.; and to identify landscape factors that significantly influence recharge. We evaluated the Darcian, water-budget, water table-fluctuation, and tracer methods for estimating recharge. Sediment cores were analyzed for particle size distribution, moisture content, bulk density, organic matter, and selected anions. Direct measurements of unsaturated hydraulic conductivity (K) were made on a small number of cores by the steady-state centrifuge method. For all cores, K was derived from pedotransfer functions based on texture and bulk density data. Darcian water fluxes were calculated assuming either nonuniform matric potential or a unit gradient (q about equals K). Unit gradient estimates of recharge represent homogeneous sediments and thick layers in heterogeneous systems. Preliminary results indicate that the point estimates of recharge vary considerably within principal aquifers, and that median recharge is highest in glacial deposits in the northeastern U.S. Median recharge estimated by the Darcian method was similar to estimates derived from a base-flow index. Overall, there was good correspondence between unit-gradient and non-unit-gradient estimates of recharge, indicating that matric forces were not dominant in sampled sediment layers. Unit-gradient recharge was strongly related to moisture content and sediment texture. For sands, hydraulic conductivities derived from pedotransfer functions compared favorably with those measured by the centrifuge method. The pedotransfer method, however, overpredicted K for a silty sample with high moisture content. It is possible that the pedotransfer method is biased towards sandy samples because the empirical model on which it is based is calibrated to predominantly coarse-textured soils.
DE: 1800 HYDROLOGY
DE: 1829 Groundwater hydrology
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting