Extraction and development of inset models in support of groundwater age calculations for glacial aquifers

Daniel T. Feinstein; Leon J. Kauffman; Megan J. Haserodt; Brian R. Clark; Paul F. Juckem

doi:10.3133/sir20185038

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Extraction and development of inset models in support of groundwater age calculations for glacial aquifers

Daniel T. Feinstein·Leon J. Kauffman·Megan J. Haserodt·Brian R. Clark·Paul F. Juckem

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Abstract

The U.S. Geological Survey developed a regional model of Lake Michigan Basin (LMB). This report describes the construction of five MODFLOW inset models extracted from the LMB regional model and their application using the particle-tracking code MODPATH to simulate the groundwater age distribution of discharge to wells pumping from glacial deposits. The five study areas of the inset model correspond to 8-digit hydrologic unit code (HUC8) basins. Two of the basins are tributary to Lake Michigan from the east, two are tributary to the lake from the west, and one is just west of the western boundary of the Lake Michigan topographic basin. The inset models inherited many of the inputs to the parent LMB model, including the hydrostratigraphy and layering scheme, the hydraulic conductivity assigned to bedrock layers, recharge distribution, and water use in the form of pumping rates from glacial and bedrock wells. The construction of the inset models entailed modifying some inputs, most notably the grid spacing (reduced from cells 5,000 feet on a side in the parent LMB model to 500 feet on a side in the inset models). The refined grid spacing allowed for more precise location of pumped wells and more detailed simulation of groundwater/surface-water interactions. The glacial hydraulic conductivity values, the top bedrock surface elevation, and the surface-water network input to the inset models also were modified. The inset models are solved using the MODFLOW–NWT code, which allows for more robust handling of conditions in unconfined aquifers than previous versions of MODFLOW. Comparison of the MODFLOW inset models reveals that they incorporate a range of hydrogeologic conditions relative to the glacial part of the flow system, demonstrated by visualization and analysis of model inputs and outputs and reflected in the range of ages generated by MODPATH for existing and hypothetical glacial wells. Certain inputs and outputs are judged to be candidate predictors that, if treated statistically, may be capable of explaining much of the variance in the simulated age metrics. One example of a predictor that model results indicate strongly affects simulated age is the depth of the well open interval below the simulated water table. The strength of this example variable as an overall predictor of groundwater age and its relation to other predictors can be statistically tested through the metamodeling process. In this way the inset models are designed to serve as a training area for metamodels that estimate groundwater age in glacial wells, which in turn will contribute to ongoing studies, under the direction of the U.S. Geological Survey National Water Quality Assessment, of contaminant susceptibility of shallow groundwater across the glacial aquifer system.

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Source-reported bounding extent: 40.413496049701955° to 46.830133640447386° latitude; -90.615234375° to -81.5185546875° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Daniel T. Feinstein, Leon J. Kauffman, Megan J. Haserodt, Brian R. Clark, Paul F. Juckem. 2018-06-22. Extraction and development of inset models in support of groundwater age calculations for glacial aquifers. https://doi.org/10.3133/sir20185038

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