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Recent trends of assimilating water well records into statewide databases provide a new opportunity for evaluating spatial dynamics of groundwater quality and quantity. However, these datasets are scarcely rigorously analyzed to address larger scientific problems because they are of lower quality and massive. We develop an approach for utilizing well databases to analyze physical and geochemical aspects of groundwater systems, and apply it to a multiscale investigation of the sources and dynamics of chloride (Cl ) in the near-surface groundwater of the Lower Peninsula of Michigan. Nearly 500,000 static water levels (SWLs) were critically evaluated, extracted, and analyzed to delineate long-term, average groundwater flow patterns using a nonstationary kriging technique at the basin-scale (i.e., across the entire peninsula). Two regions identified as major basin-scale discharge zones-the Michigan and Saginaw Lowlands-were further analyzed with regional- and local-scale SWL models. Groundwater valleys ("discharge" zones) and mounds ("recharge" zones) were identified for all models, and the proportions of wells with elevated Cl concentrations in each zone were calculated, visualized, and compared. Concentrations in discharge zones, where groundwater is expected to flow primarily upwards, are consistently and significantly higher than those in recharge zones. A synoptic sampling campaign in the Michigan Lowlands revealed concentrations generally increase with depth, a trend noted in previous studies of the Saginaw Lowlands. These strong, consistent SWL and Cl distribution patterns across multiple scales suggest that a deep source (i.e., Michigan brines) is the primary cause for the elevated chloride concentrations observed in discharge areas across the peninsula.

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[1] We present in this paper a critical review of recent research on nonuniform mean flows in heterogeneous porous media, examine why existing stochastic methods are computationally so difficult to implement, and introduce a new and efficient alternative. Specifically, we reformulate the nonstationary spectral method of McLaughlin (1991, 1995) and present a new way for its numerical implementation, combining the best advantages of efficient analytical solutions and flexible numerical techniques. The result is a substantially improved stochastic technique that allows modeling efficiently the nonlinear scale effects for moderately heterogeneous media in the presence of general nonstationarity. In particular, the reformulated approach allows computing the nonlocal and nonstationary mean ''closure'' flux using a coarse grid without having to resolve numerically the small-scale heterogeneous dynamics. The methodological innovation significantly increases the size and expands the range of groundwater problems that can be analyzed with stochastic methods. The effectiveness of the new spectral approach is illustrated with two concrete examples and a systematic comparison with existing stochastic methods.

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