Assessing the spatial variability of constraints on groundwater abstractions due to potential adverse resource impacts on surface water ecosystems - a GIS based approach

Groundwater contributions to streams, particularly in periods of low flow, can be critical to sustaining aquatic ecosystems. Groundwater abstractions in areas where the groundwater is in hydraulic connection with the surface water can deplete these flows potentially causing adverse resource impacts. In particular, the passage of the Great Lakes—St. Lawrence Basin Water Resources Compact in 2008 has brought increasing awareness to this issue in the Great Lakes Basin. As a requirement of this legislation, each of the Great Lakes States must take steps to limit water withdrawals that may potentially impact water-dependent natural resources. The State of Michigan has developed an automated “Water Withdrawal Assessment Tool” to assist in this process. By using the methodology as developed for the Michigan Water Withdrawal Assessment Tool, this study examines spatial variations in maximum allowable pumping rates under these constraints. The pumping rates are constrained either by the local hydrogeology or concerns related to adverse impacts to the surface water ecosystems. A simple analytical model is used to calculate streamflow depletion as a function of hypothetical groundwater abstraction rates and positions. The inputs to this model are obtained from a GIS database including such spatially relevant information as aquifer characteristics, streamflows, and a stream network. The maximum pumping rates are averaged over the HUC-8 watershed scale. We explore the characteristics that play the largest role in the variability of maximum pumping rates, such as hydrogeologic parameters, stream density, and stream flows. We also discuss limitations of the analytical approach to assessing water availability. Understanding how these restrictions on adverse resource impacts constrain groundwater usage and which hydrogeologic characteristics and spatial variables have the most influence on potential streamflow depletions have important water resources policy and management implications.