Comparison of Analytical and Numerical Flow Models Used to Delineate Capture Zones of Wells at the South Well Field, Columbus, Ohio
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Date
1991-08
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The Ohio State University
Abstract
An analytical flow model of the ground-water flow system in the Southern Franklin County area was used with a two-dimensional particle-tracking program to delineate traveltime-related capture zones of a municipal well field completed in an unconfined glacial-drift aquifer. The results were compared to the flow model and capture zones already constructed for the area using a numerical model coupled with a three-dimensional particle-tracking program. The analytical flow model uses the Theis equation describing the two-dimensional distribution of drawdown surrounding a well completed in an unconfined aquifer and superposes the results on a generalized historical head distribution that reflects pre-1967 non-pumping conditions. In contrast, the numerical flow model uses a finite-difference solution and incorporates two model layers, specified flux-boundaries conditions, leakage from riverbeds, and spatially variable hydraulic parameters. Additional simulations using the analytical flow model were performed reflecting 1990 pumping rates.
The comparisons show that the inability of the analytical flow model and the two-dimensional particle tracking program to account for spatial variations in hydraulic conductivity and aquifer thickness cause the size of the capture zones to be overestimated and their shapes to incorrectly simulate the geologic and hydrologic conditions at the study area. However, the head distribution between the analytical and numerical flow models are reasonably similar due to similar simplifying assumptions and the relatively low discharge rates at the well field.
These results suggest that for the optimum location of sentinel wells used in a wellhead-protection program, the numerical flow model coupled with the three-dimensional particle-tracking program is the most appropriate methodology to use in this type of complex hydrogeologic setting. However, in terms of cost and accuracy, the analytical flow model coupled with the two-dimensional particle-tracking program may be more appropriate for communities in less complex hydrogeologic settings.