HIGH FREQUENCY MONITORING OF DETAILED AND DEPTH-DISCRETE HEAD PROFILES TO CHARACTERIZE VARIABILITY IN HYDROGEOLOGIC PROPERTIES IN THE SILURIAN-DEVONIAN AQUIFER IN IOWA

Delineation of hydrogeologic units (HGUs) within bedrock aquifer systems can be challenging, particularly if blended data from long screened wells is all that is available. However, recent research has shown that sharp inflections in high-resolution head profiles associated with changes in bulk vertical hydraulic conductivity (K_v) can be useful for delineating the boundaries of HGUs. The Silurian-Devonian Aquifer (SDA) in Iowa is an important aquifer for the region and is often treated as a single HGU despite consisting of over 100 meters of lithologically variable strata. We hypothesize that hydraulic contrasts, some rising to the level of distinct HGUs, exist in the SDA that are important to understanding and simulating groundwater flow and contaminant transport. To test this hypothesis, we used 2 existing bedrock boreholes (Ely and Bertram) drilled as part of a 1970s USGS/IGS study for high frequency monitoring of depth-discrete, detailed vertical hydraulic head profiles. The monitoring was accomplished by installing a string of 35 RBR pressure sensors, each surrounded by a mesh screen and collecting data every 5 seconds, into the 85-115 m open portions of the boreholes. The entire hole, with the exception of the mesh intervals, was then sealed with a FLUTe liner to prevent cross-connective flow. The depth of each sensor was chosen based on geologic data collected during drilling and geophysical logs collected in 2019. The Ely head profiles showed little vertical gradient between monitoring intervals indicating similar K_v throughout the borehole. However, the maximum head difference within the profile was small, <0.7m, so the ability of the head profiles to resolve changes in K_v was limited at this point in the flow system. Head profiles from Bertram are still being processed, but preliminary inspection indicates the possibility of two distinct K_v zones. In addition to utilizing the head profiles to assess potential HGU boundaries, transient responses throughout the profile to barometric pressure fluctuations, earth tides, and seismic events are being used to further inform hydrogeologic heterogeneity in the SDA. The method utilized in this research demonstrates the potential for using existing boreholes to obtain novel data sets useful for improving hydrogeologic characterization in bedrock aquifers.