Hydrofacies distribution and correlation in the Miami Valley Aquifer System

This study combines geostatistical analyses with geologic interpretations to further the quantitative understanding of physical heterogeneity within glaciofluvial aquifers. The stochastic simulation of aquifers requires quantitative measures of heterogeneity, including both cumulative distribution and spatial correlation functions. The heterogeneity in glaciofluvial aquifers is typified by low-permeability facies (e.g., till or lacustrine clay) juxtaposed with high-permeability facies (e.g., sand and gravel outwash). The Miami Valley aquifer system was examined at multiple sites for the spatial distribution and correlation of these two hydrofacies. Binary indicator geostatistics were used to quantitatively determine, at each site, the relative volume of each hydrofacies, their spatial distribution, the major principal direction of their spatial correlation, the minor principal direction, and the correlation range in these directions. The percent by volume of the system that is aquitard material decreases down the valley, from 31% to 12%. Each site has an elevation zone with more aquitard material relative to other elevations at that site. The percent aquitard material in these zones decreases down the valley from 45% to 22%. The maximum principal direction of spatial correlation in the aquitard zones generally is NE-SW, subparallel to the trend of the bedrock valley, with a range of the order of 0.75 km and minimum/maximum anisotropy ratio of 0.4. The locations exceeding 0.5 probability of aquitard occurrence generally occur on the valley margins. Thus, among the sites investigated, there is a trend down the valley in the ratio of aquitard volume to aquifer volume, and the spatial correlation and distribution of aquitard material are similar within the aquitard zones. Furthermore, these findings are consistent with aspects of the subglacial and proglacial depositional environments responsible for the facies assemblage and thus are likely to be applicable to other parts of the aquifer system and other aquifer systems where similar geologic processes are inferred to have existed.