HR: 0800h
AN: H21E-1057    [Abstracts]
TI: Influence of Coarse-Grained Incised Valley Fill on Ground-Water Flow in Fluvial Fan Deposits, Stanislaus County, California, USA
AU: * Lansdale, A L
EM: lansdal1@msu.edu
AF: Department of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, MI 48824-1115 United States
AU: Weissmann, G S
EM: weissman@msu.edu
AF: Department of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, MI 48824-1115 United States
AU: Burow, K R
EM: krburow@usgs.gov
AF: U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819-6129 United States
AB: A relatively coarse-grained incised valley-fill deposit was identified within the Tuolumne River fluvial fan beneath the city of Modesto, Stanislaus County, California, USA. Incised valley fill deposits in the eastern San Joaquin Valley resulted from rapid decreases and increases in sediment supply during interglacial and glacial periods. Ground-water flow and contaminant transport modeling of similar deposits on the Kings River fan near Fresno, California, indicate the potential for (1) relatively high ground-water production rates, (2) rapid contaminant transport within incised valley-fill sediments, and (3) rapid contaminant movement through the incised valley-fill sediments into adjacent aquifer sediments. If these characteristics also apply to the incised valley fill beneath Modesto, then it may have implications for ground-water quality and potential artificial recharge to city supply wells and wells in the surrounding region. We identified the location of the incised valley fill and surrounding deposits in the subsurface through the use of drillers' well logs, geophysical well logs, and core samples, and compared the spatial dimensions of the subsurface incised valley fill to the modern Tuolumne River incised valley. Because the interpretation of drillers' well logs leaves uncertainty regarding the geometry of the subsurface incised valley fill, we assumed that the three-dimensional geometry of the subsurface valley fill is similar to the modern Tuolumne River valley. From this analysis, we estimate the incised valley fill (1) is 0.7 to 1.6 kilometers wide, (2) ranges from approximately 30 meters thick at the apex of the fan to approximately 0.3 meters thick at the toe of the fan, and (3) has a 5 to 9-meter-thick gravel base near the fan apex that thins downfan. Our regional geologic model uses digital elevation model (DEM) and subsurface lithologic data to describe the approximate geometry, subsurface location, and permeability of the incised valley fill. The geologic model will be used to provide a framework for a ground-water flow model that will test whether the incised valley fill enhances flow for prospective artificial recharge and increases potential for contaminant transport to drinking-water wells in the region.
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting