A multiscale assessment of shallow groundwater salinization in Michigan.

Managing non-point-source (NPS) pollution of groundwater systems is a significant challenge because of the heterogeneous nature of the subsurface, high costs of data collection, and the multitude of scales involved. In this study, we assessed a particularly complex NPS groundwater pollution problem in Michigan, namely, the salinization of shallow aquifer systems due to natural upwelling of deep brines. We applied a system-based approach to characterize, across multiple scales, the integrated groundwater quantity-quality dynamics associated with the brine upwelling process, assimilating a variety of modeling tools and data - including statewide water well datasets scarcely used for larger scientific analysis. Specifically, we combined 1) data-driven modeling of massive amounts of groundwater/geologic information across multiple spatial scales with 2) detailed analysis of groundwater salinity dynamics and process-based flow modeling at local scales. Statewide "hot-spots" were delineated and county-level severity rankings were developed based on dissolved chloride (Cl- ) concentration percentiles. Within local hot spots, the relative impact of upwelling was determined to be controlled by: 1) streams - which act as 'natural pumps' that bring deeper (more mineralized) groundwater to the surface; 2) the occurrence of nearly impervious geologic material at the surface - which restricts freshwater dilution of deeper, saline groundwater; and 3) the space-time evolution of water well withdrawals - which slowly induces migration of saline groundwater from its natural course. This multi-scale, data-intensive approach significantly improved our understanding of the brine upwelling processes in Michigan, and has applicability elsewhere given the growing availability of statewide water well databases.