Fracture Occurrence and Connectivity in a Siliclastic Aquifer Near Public Supply Wells in Southern Wisconsin

Historically, most hydrogeologists and water-supply engineers have treated siliclastic “sandstone” aquifers as classic porous media for the purposes of well design and groundwater protection. Recent discovery of human viruses in deep public supply wells in southern Wisconsin suggests that rapid flow pathways occur in the sandstone aquifer system there. Geophysical, flowmeter, and optical logs of multiple boreholes, combined with hydraulic testing, show that the confined aquifer system contains a network of interconnected fractures. Major near-horizontal fractures occur at discrete stratigraphic positions and conduct flows exceeding 100 gallons per minute in the vicinity of pumping wells. In places, these fractures account for most of the transmissivity of the aquifer and could facilitate rapid movement of contaminants. Steeply dipping fractures that could act as vertical pathways also occur but are less frequently observed. Two significant near-horizontal fracture zones have been incorporated into a new regional groundwater flow model, and model results demonstrate their influence on groundwater flow paths and velocities.

Understanding the presence and significance of these fracture networks is important for appropriate analysis of well vulnerability and for wellhead protection planning. Fracture networks help explain the apparent vulnerability of deep wells to viruses and other contaminants and can significantly influence the calculated zones of contribution used for wellhead protection plans.