Previous Modeling

Previous models of transport (WHPA's) in the Hays aquifers have incorporated successively increasing levels of geologic detail. Initial estimates were prepared using the Calculated-Fixed-Radius (CFR) approach, which considers only aquifer porosity, saturated thickness, and well pumping rate. Kraemer and Burden (1994) calculated a mean WHPA radius of 683 m for Hays wells pumped for 5 years, and 966 m over 10 years. An improved estimate was made using the Analytic Element Method [Haitjema, 1995] based on a surface contour map of the bedrock shale (Fig. 9) to estimate the transmissivity distribution in the area [Kraemer and Burden, 1994]. The presence of a deep channel cut into the shale beneath Hays was predicted to greatly extend city-well capture zones in a northeasterly direction. For example, at 5 years, capture zones were predicted to extend 1-2 km to the NNE from the southeastern city wells. Moderate correlation between the observed water table and channel geometry in the shale (compare Figs. 9 and 10) supports this hypothesis. Additional effects related to the detailed connectivity between observed bodies or ``stringers'' of thick Qal material were not considered in these models.

Figure 9: Surface contour (elevation in meters) of the top of the Cretaceous Shale, interpolated from well data, wells shown in Fig. 7. Note appearance of arcuate scour features, and deep channel beneath SE Hays. Also shown: City limits (1987), Big Creek, city wells (filled circles) and outline of 100ppb PCE concentration (irregular outline within city limits, 1997 data). Select figure to view full-sized version.

Figure 10: Water table elevation (in meters). Note approximate correlation between water table and shale top elevation (Fig. 9). Select figure to view full-sized version.