EVALUATING VERTICAL GROUNDWATER FLOW THROUGH FRACTURED BEDROCK NEAR A MUNICIPAL SUPPLY WELL: PLANNED FIELD EXPERIMENTS INFORMED BY PRELIMINARY MODELING

Groundwater flow through high angle fracture networks will be examined as an explanation for the fast vertical transport of human enteric viruses from near surface sources to municipal supply wells screened in a deep, confined, siliciclastic aquifer in south-central Wisconsin. Preliminary modeling of site characteristics is used to inform field experiment design.

Multi-frequency Oscillatory Flow Interference (OFI) testing is the primary field experiment scheduled for the summer of 2015. This is a method that uses the propagation of a pressure signal to observation points to characterize flow within an aquifer. This project will employ a pneumatic pump to pressurize the air column in a sealed off test well, and then vary the water level in a sinusoidal manner in an isolated interval of the test well. Pressure transducers at locations in adjacent isolated intervals of observation wells will record any signal detections. Aquifer parameters such as transmissivity, and pressure signal propagation pathways can be extracted from observation well data through analysis of signal propagation.

The results of the OFI tests will be validated with several other aquifer test techniques. Fracture connectivity at the site will be confirmed with an induced gradient tracer test. Aquifer parameters, such as transmissivity obtained from OFI testing, will be compared with results of fixed-rate slug and pump tests conducted on the same intervals. Validation tests will serve to further support results produced by OFI testing.

Aquifer parameters estimates, optimized OFI testing frequencies, and minimum required pumping rates are determined with preliminary modeling using existing site data. These estimates are employed in the selection of field equipment and design of OFI, tracer, and pump test experiments.