Aquitard Distribution in a Northern Reach of the Miami Valley Aquifer, Ohio, USA: Part 1. Three-Dimensional Geostatistical Evaluation of Physical Heterogeneity

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In the glacially-filled valleys of the western and central glaciated plains of North America, assemblages of sand-and-gravel outwash form large, productive regional aquifer systems. In these buried-valley aquifers, low-permeability facies, including till or lacustrine clay, are commonly juxtaposed with the permeable outwash. The geometry of the boundary between these facies must be defined in order to address contaminant problems. Although the exact geometry of the boundary between high- and low-permeability facies can only be determined by a prohibitive amount of subsurface information, indicator geostatistics can be used to evaluate the uncertainty at points between actual observations. As an example, the heterogeneity in the Miami Valley aquifer system, which extends 150 km through southwestern Ohio, USA, was examined. In a northern reach, a three-dimensional lithological data base with 30,094 observations was developed by coding all available logs with a binary indicator system, indicating the presence of either high-permeability facies (aquifer) or low-permeability facies (aquitard). This reach of the aquifer system has 36 percent by volume aquitard material, based upon the declustered global mean of the data. The aquitard material is stratified, and the elevation between 204-213 m has the most aquitard material. An interval 6.1 m thick within this zone was studied in more detail to determine the spatial correlation and areal distribution of the aquitard material. Within this interval, the aquitard facies occupies 45 percent of the volume. Based upon variography, the spatial correlation of the facies is greatest in the N50°E direction with a range of about 700 m. The min/max anisotropy ratio is 0.4. The probability of aquitard material existing at any given location was examined with indicator point kriging. Locations in the subsurface having a higher probability for the existence of aquitard material occur more along the valley margins, and locations with a lower probability occur more in the valley interior.