HR: 0830h
AN: H21E-0897    [PDF]
TI: The Effect of Heterogeneity on Salinization in the Western San Joaquin Valley
AU: * Zhang, H
EM: hhzhang@ucdavis.edu
AF: 1. Department of Land, Air and Water Resources, University of California, Davis, One Shields Ave., Davis, CA 95616 United States
AU: Harter, T
EM: thharter@ucdavis.edu
AF: 1. Department of Land, Air and Water Resources, University of California, Davis, One Shields Ave., Davis, CA 95616 United States
AU: Weissmann, G
EM: weissman@msu.edu
AF: Department of Geological Sciences, Michigan State University, Department of Geological Sciences, Michigan State University, East Lansing, MI 48824 United States
AB: Salinity in the upper, semi-confined aquifer of the Western San Joaquin Valley, California, is locally high due to downward transport from irrigated shallow saline soils. We are using a stochastic approach for implementing a risk analysis of future aquifer salinization. A key issue for the analysis is an appropriate geostatistical characterization of the alluvial aquifer stratigraphy. Due to a lack of aquifer tests and other hydraulic data, we are using a hydrostratigraphically based transition probability/Markov chain approach to build our geostatistical model. Commonly, geostatistical models are used to model local variability in hydrogeologic properties, but with little regard to regional variability. However, our study area consists of four distinct geologic units: upper fan, distal fan, inter-fan and Sierran sand. Each of these units represents a different depositional environment with potentially distinct facies assemblages. The geostatistical analysis, which is based primarily on drillers logs and soil maps, shows that hydrostratigraphic facies categorization, facies mean lengths, facies proportions, and juxtapositional preferences among the hydrostratigraphic facies are significantly different between the four units. The proportion of coarse material in the upper fan is about 10 percent higher than that of the distal fan and inter-fan unit. The lateral mean lengths of fine material in distal fan unit are around two times those in upper fan unit. The juxtapostional preferences also vary considerably between these three units. The Sierran sand unit consists of three hydrostratigraphic facies, while four facies can be distinguished in the other three units of the Coast Ranges alluvium. Sensitivity analysis indicates that such differences significantly affect the groundwater flow and salt transport. Therefore, the regional spatial variability model of aquifer properties must distinguish the four sub-regional hydrogeologic units. We developed a composite geostatistical aquifer simulator to account for these sub-regional differences. In the hydrostratigraphic simulation, continuity between adjacent units with different geostatistical properties is preserved through step-wise simulation of individual units using sequential conditional simulation. Soil maps and well drilling logs are also used as conditional data. Probability of early arrival of highly saline water in the extensive interconnected coarse-textured materials is computed from Monte Carlo simulations of groundwater flow and transport through these synthetic hydrostratigraphic facies assemblages.
DE: 1832 Groundwater transport
DE: 3210 Modeling
DE: 6309 Decision making under uncertainty
SC: Hydrology [H]
MN: 2003 Fall Meeting