TOWARDS UNDERSTANDING PERIOD DEPENDENCE IN FLOW PROPERTIES OF A FRACTURED BEDROCK AQUIFER

Secondary porosity features (i.e., fractures) act as conduits of enhanced flow and transport in bedrock aquifers, which are found extensively throughout the bedrock of Upper Midwestern region of the United States. As such, understanding flow and transport through bedrock aquifers along these fractures is of critical importance for understanding regional groundwater resources. Recent advances in aquifer characterization techniques has seen the development and implementation of periodic pumping tests to estimate bulk fracture flow properties, with multiple studies observing that these estimated flow properties are dependent upon the period of the input signal.

We conducted a series of cross-well periodic pumping tests at a fractured sedimentary bedrock aquifer near Madison, WI. Employing zonal isolation techniques, we isolated and tested a single fracture across a range of periods from 4 to 360 seconds and inter-well spacings from 5 to 15 meters. Using developed analytical solutions, we estimate the hydraulic diffusivity of the fracture, and find that estimated diffusivity decreases with increasing input period, which is consistent with previous studies.

As a first step towards understanding the observed period dependence, we develop a variety of numerical models with varying levels of heterogeneity complexity. We start with simple idealized heterogeneity patterns and move to more complex and realistic heterogeneity patterns that more closely resemble natural fractures surfaces. Our numerical modeling efforts indicate that fracture surface heterogeneity, and thus fracture transmissivity, is one potential mechanism that reproduces the observed period dependence in bulk fracture flow parameter estimates.