Impacts of groundwater withdrawals on lake hydrology and ecology in a water-rich region: what matters most?

Groundwater supports over 300 lakes, 800 miles of trout streams, and a $500 million irrigated agricultural industry in the Wisconsin Central Sands region. In this water-rich region, groundwater availability is affected by climate, land use, and groundwater pumping and the relative importance of each may vary with time and location. Uncertainty in how these factors interact to cause reduced lake levels has led to conflict, with environmental and lakeshore resident groups pointing to high-capacity wells as the main reason for periods of low water levels, while many in the agricultural industry perceive precipitation variability and forest evapotranspiration as the bigger drivers. In response to this ongoing debate, the Wisconsin State Legislature directed the Wisconsin Department of Natural Resources (WDNR) to evaluate whether existing and/or future groundwater withdrawals are causing or could cause "significant impacts" to three seepage lakes in the region. To tackle this question, WDNR brought together over 30 scientists from state, federal, and academic institutions with a wide range of expertise and took a multi-pronged approach that included field surveys, social surveys, compilation of historic data and reports, literature reviews, and groundwater-flow modeling. We first defined the historical hydrologic regime of these lakes, drawing from the rich literature on hydrologic regimes of rivers and streams to define appropriate metrics that capture the variability of water levels in lakes. We then used groundwater flow models to untangle the extent to which current groundwater withdrawals, land use, and climate affect the hydrologic regimes of the study lakes. Finally, we examined how changes in hydrologic regimes due to current groundwater withdrawals could impact lake ecosystems in terms of water chemistry, aquatic life (plants and fish), and recreation. We show that characterization of hydrologic change can be generalized, but ecosystem vulnerabilities are unique to each lake.