URBAN HYDROGEOLOGY OF THE UPPER MIDWESTERN CAMBRIAN-ORDOVICIAN SANDSTONE AQUIFER (Invited Presentation)

The effects of urbanization on groundwater resources have been a long-term focus of Jack Sharp’s research, with a particular emphasis on enhanced recharge and baseflow of urban streams that can be induced by land use changes and urban infrastructure. In cities that rely on groundwater for municipal supplies, long term pumping can also cause significant changes to water levels, discharge to streams and water quality. Impacts of urbanization are becoming even more critical as urban areas continue to expand, evolving in many places to “megacities” with populations that place unprecedented stresses on water resources. While urban expansion can occur rapidly, some of the resulting changes in groundwater quantity and quality are more gradual and may take decades or longer to become apparent. A number of urban and suburban areas of Illinois, Iowa, Minnesota and Wisconsin have used the Cambrian-Ordovician sandstone aquifer system extensively for water supply. Research on this aquifer system over the last two decades in Madison, Wisconsin and nearby areas has revealed a number of impacts of well construction and pumping. These include decreases in discharge to springs and streams that are supplied by groundwater from the shallow bedrock, development of steep vertical gradients across a confining unit that separates the upper and lower bedrock aquifer, vulnerability of the lower confined aquifer to contamination from shallow sources such as sewers, and changes in redox conditions and trace metal mobility resulting from pumping induced mixing of shallow and deeper groundwater. Explaining these impacts and developing strategies to mitigate them has required refining the initial conceptual model of a relatively homogeneous siliciclastic aquifer to one that accounts for stratigraphic variations in hydraulic conductivity and a prevalence of preferential flow features that can be inferred through downhole logging, in-well heat tracer tests, analysis of poroelastic effects, and geochemical and virus sampling.