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2007 Ground Water Summit |
Wednesday, May 2, 2007 : 9:20 a.m.
Regional-Scale Influence of Large Incised-Valley Fill Deposits on Ground Water Flow
Simulation results using a regional-scale ground-water model indicate that large incised-valley fill deposits (IVFD) within fluvial fans of the eastern San Joaquin Valley significantly influence ground-water flow patterns by: (1) creating a large zone of flow convergence near the fan apex; (2) creating a regional-scale preferential pathway for ground-water flow; (3) increasing overall dispersion; and (4) reducing average ground- water residence times. These coarse-grained deposits, measuring approximately 1.5-2 km wide, 30 m deep, and extending the length of the fans (approximately 40-60 km), act as preferential pathways for moving ground water from the surface to deeper zones in the heterogeneous open-fan deposits. To investigate the effect of IVFD on ground-water flow, a regional-scale ground-water model developed by the US Geological Survey (USGS) was modified for the area of the Tuolumne River fluvial fan, beneath and around Modesto, California , to explicitly include defined IVFD stratigraphy and geometries (the USGS model implicitly included the IVFD through geostatistical interpolation of sediment texture). The IVFD geometries were delineated through interpretation of driller’s logs and sparse geophysical well logs and from a conceptual model of IVFD geometries based on the Kings River IVFD described by Weissmann et al. (2004). Simulation results indicate that the coarse-grained IVFD potentially enhance the amount of water that can be recharged near the fan apex and potentially facilitate flow deep into the aquifer system. The regional influence of these coarse-grained IVFD may be significant for developing artificial recharge projects in the area, as well as understanding contaminant fate and transport from regional non-point sources. Future work is needed to (1) verify the geometry of IVFD in the Tuolumne River fan, (2) provide physical evidence to show the influence of the IVFD on ground-water flow, and (3) evaluate the potential for using the IVFD as part of artificial recharge projects.