Sustainable groundwater use is a growing and relatively ubiquitous concern. Yet, managing groundwater sustainably remains a significant societal challenge, largely because management must address a variety of issues across a multitude of scales. With rare exceptions, groundwater investigations have allocated limited resources to a narrow range of scales, in many cases ignoring important process interactions. Local-scale management is often done without proper understanding of the larger system controls, while system-based studies may not be sufficiently supplemented by subscale data collection and analysis, making it difficult to provide concrete, site-specific recommendations for management. This dissertation addresses a concrete example of a complex multiscale sustainability problem, namely, widespread salinization of shallow groundwater in southern Michigan, U.S.A, which appears to be due to natural basin-scale upwelling of deep brines, but is complicated by regional and local-scale human activities (e.g., pumping, road salting and land development). An integrated, end-to-end approach is developed and applied to investigate the complex interplay of upwelling brines, human activity, and climate variability at and across vastly disparate scales. Included are perspectives of basin-scale contamination, regional groundwater-surface water connections, local human-environment interactions, and well-scale analysis of water availability (quantity and quality). Data of different types, qualities, and resolutions/coverages (most of which are pre-existing and ‘big’) are integrated and critically evaluated with various modeling tools (data-driven, process-based and analytical solutions) to understand complex system dynamics and provide a basis for strategic regional planning, local operational management, and site-specific problem-solving. Data-driven approaches are used to characterize spatial patterns and temporal trends and provide diagnostic screening of different groundwater environments across multiple scales. Regional and local-scale process-based flow simulations are used to describe and quantify the relative importance of different processes controlling groundwater sustainability, and to estimate future groundwater conditions by integrating detailed county-and township-level projections of water use and land use into model development. Through this integrated, multi-scale and multi-perspective analysis, several key scientific conclusions emerge: 1) slow, natural upwelling of brines is the dominant source of shallow saline groundwater in low-lying discharge areas across southern Michigan; 2) locally, impacts of brines are most severe where relatively continuous confining materials occur near or at the surface (precluding freshwater recharge as a freshwater flushing mechanism) and where groundwater levels are low; 3) cumulative impacts of gradual (multi-decadal) but significant increases in distributed water well networks have caused large declines (>15 m) in groundwater levels across broad subregions, which in turn is inducing movement of brine-influenced groundwater into areas historically less/not affected and worsening the severity and extent of shallow groundwater salinization; and 4) impacted aquifer systems will begin to stabilize as the rate of increase in pumping slows down, but local groundwater availability will still be limited by acceptable water quality of the user and/or required yield. The multi-scale, big data-enabled approach applied here provides a generalized framework for developing holistic understanding of groundwater systems important to sustainability, and for prioritizing limited resources needed for analysis in key areas to inform subregional and local-scale resource management.
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