Simulation of Flow in a Complex Aquifer System Subjected to Long-Term Well Network Growth.

In west-central Lower Peninsula of Michigan, population growth and expanded agricultural activities over recent decades have resulted in significant increases in distributed groundwater withdrawals. The growth of the extensive well network and anecdotes of water shortages (dry wells) have raised concerns over the region's groundwater sustainability. We developed an unsteady, three-dimensional (3D) groundwater flow model to describe system dynamics over the last 50 years and evaluate long-term impacts of groundwater use. Simulating this large aquifer system was challenging; the site is characterized by strong, spatially distributed, and statistically nonstationary heterogeneity, making it difficult to avoid over-parameterization using traditional approaches for conceptualizing and calibrating a flow model. Moreover, traditional pumping and water level data were lacking and prohibitively expensive to collect given the large-scale and long-term nature of this study. An integrated, stochastic-deterministic approach was developed to characterize the system and calibrate the flow model through innovative use of high-density water well datasets. This approached allowed (1) implementation of a "zone-based," nonstationary stochastic approach to conceptualize complex spatial variability using a small set of geologic material types; (2) modeling the spatiotemporal evolution of many water well withdrawals across several decades using sector-based parameterization; and (3) critical analysis of long-term water level changes at different locations in the aquifer system for characterizing the system dynamics and calibrating the model. Results show the approach is reasonably successful in calibrating a complex model for a highly complex site in a way that honors complex distributed heterogeneity and stress configurations.

A Multiscale Assessment of Shallow Groundwater Salinization in Michigan.

Managing nonpoint-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 "hotspots" were delineated and county-level severity rankings were developed based on dissolved chloride (Cl- ) concentration percentiles. Within local hotspots, 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 fresh water dilution of deeper, saline groundwater; and (3) the space-time evolution of water well withdrawals-which induces slow migration of saline groundwater from its natural course. This multiscale, 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.

Data-Driven Approach for Analyzing Hydrogeology and Groundwater Quality Across Multiple Scales.

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.