T144 Hydrology of Urban Groundwater, Streams, and Watersheds

Tuesday, 6 November 2012
8:00 AM - 12:00 PM
213A

REVERSE WATER-LEVEL FLUCTUATIONS AS INDICATORS OF PUBLIC SUPPLY WELL VULNERABILITY IN FRACTURED SILICICLASTIC AQUIFER SYSTEMS

Paper number: T144-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Public supply wells in many urban settings are vulnerable to contamination from near-surface sources such as leaking sanitary sewers. The cyclic pumping common to these wells can cause widespread responses in the surrounding aquifer system. In some cases, these responses can provide insight into the existence and connectivity of preferential flow pathways that might be responsible for rapid transport of these contaminants. In a study conducted in Madison, Wisconsin, data from pressure transducers located in shallow wells at varying depths and radial distances from a deep public supply well recorded cyclic reverse water-level fluctuations (RWFs) in a fractured siliciclastic multi-aquifer system. RWFs are a phenomenon in which water levels rise briefly in response to pumping, or conversely, decline at the onset of recovery. The shallow wells were completed above the regional aquitard and at lateral distances up to 350 meters from the pumping well.

The magnitude and timing of RWFs provide important information that can help interpret aquifer hydraulics and identify fracture networks near pumping wells. A RWF in a well is normally attributed to poroelastic coupling between the solid and fluid components in an aquifer system. In this study the RWFs are inferred to be generated initially by poroelastic effects in an aquitard and then propagated rapidly through fractures in the overlying siliciclastic aquifer. The rate and cycling frequency of public supply well pumping affected the magnitude of RWFs. The pattern of RWF propagation was used to better define fracture connectivity in this aquifer system. An implication of the rapid, cyclic head changes due to RWFs is they might serve as a mechanism to enhance rates of contaminant transport through fractures.