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Abstract: Use of Aquifer Stratigraphy for Building Numerical Models of Groundwater Flow: Case Study of the Heterogeneous Gulf Coast Aquifer in Matagorda and Wharton Counties, Texas

Alan R. Dutton

Deterministic models of groundwater flow require assigned values of hydrological properties. Assigning a uniform distribution of properties, for example, mean values, to all blocks or nodes of the computer model is generally unacceptable because of heterogeneity within regional aquifers. Subdividing the model area around aquifer test locations results in unnatural discontinuities with sometimes spurious results, for example, in particle tracking. Assigning hydraulic properties on the basis of spatial stratigraphic variables such as sand-bed thickness allows hydrologic properties to be continuously distributed for simulation of aquifer heterogeneity.

Aquifer stratigraphy, hydrologic properties, and groundwater availability are complexly interrelated in the heterogeneous Gulf Coast aquifer. The aquifer is made of a coastward-thickening wedge of Pliocene and Pleistocene formations deposited in fluvial meander-belt, fluvio-deltaic, and wave-dominated delta systems. To evaluate groundwater resources in the aquifer underlying Matagorda and Wharton counties, Texas, a quasi-three-dimensional numerical model that accounts for aquifer heterogeneity was based on detailed maps and cross sections of sand-bed distribution, hydraulic head, and hydrochemical facies. Transmissivity, storativity, and compaction potential were initially assigned to model blocks as functions of sand percentage mapped for each aquifer unit. Transmissivity, vertical c nductance, river leakage rates, and head-dependent recharge and discharge rates were then adjusted to match simulated and "observed prepumping" hydraulic heads. Storativities were adjusted to match simulated and "historic" hydraulic heads. With future pumping rates assumed to be as much as 639,000 ac-ft/yr through 2030, average drawdown rate might reach 1.2 to 2 ft/yr by 2000 but decrease to about 0.5 ft/yr by 2020. Hydraulic head might decline in some areas by another 110 to 240 ft, falling to elevations below the tops of confined aquifers. Land-surface subsidence might increase locally to as much as 2.5 ft by 2030. The model indicates that 18% of the decrease in stored water is nonrecoverable because of clay-bed compaction.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994