Characterizing aquifer geometries in Northern Indiana by profiling the buried bedrock surface with geophysical techniques
A research project funded by the Indiana Water Resources Research Center through the U.S. Geological Survey’s 104B annual base grants (section 104 of the Water Resources Research Act of 1984, as amended).
Start Date: 2019-06-01 End Date: 2020-05-31
Total Federal Funds: $14,994 Total Non-Federal Funding: $31,032
Groundwater resource assessments in glaciated regions require buried bedrock elevation data to provide information related to aquifer geometry and to determine water-resource availability. In northern Indiana such assessments are needed because expanded irrigation is increasing groundwater withdrawals in the region. Furthermore, naturally occurring contaminants have become an emerging issue, based on recent groundwater sampling data. An improved understanding of bedrock valley morphology is also warranted by these concerns because rock-water interactions at the sediment/bedrock interface often control the geochemical characteristics of groundwater, highlighting the need to better constrain the hydrogeologic conditions at the base of glacial aquifers.
This project utilizes newly developed passive seismic geophysical techniques to establish additional bedrock elevation data and support higher-resolution maps of buried valleys that lie below the St. Joseph Aquifer System in St. Joseph and Elkhart Counties.
Figure 1. Tromino set up used to measure vertical and horizontal components of ambient seismic noise. Water well located in the distance.
- Update sediment thickness maps in Northeastern St. Joseph County and Northwestern Elkhart County, Indiana. Horizontal-to-vertical spectral ratios (HVSR) of ambient seismic noise (microtremors caused by ocean waves, wind, rainfall, traffic, industrial activities, etc.) were used to estimate the thickness of sediment over the bedrock. This method has been used successfully in areas of unconsolidated sediments above bedrock, and this project investigated its use where thin glacial sediments occur.
- Update existing database of bedrock topography control points to develop a high-resolution bedrock topography map for the study area. Data acquired during this study underwent a Quality Assurance/Quality Control evaluation in order to merge with an existing Indiana Geological and Water Survey database. This allowed geologists to update the bedrock topography map of the area.
Major Conclusions & Significance
The HVSR geophysical survey conducted for this study produced 233 data points that facilitated mapping the bedrock surface at higher resolutions with better accuracy. The improved bedrock topography map and associated data will:
- Help determine where existing water supply wells are in proximity to the bedrock surface and possibly at a higher risk to naturally occurring contamination, assisting county and state officials in better targeting expensive sampling efforts;
- Improve the resolution of bedrock geologic maps so we can delineate the spatial distribution of rock formations that are high in arsenic and where they coincide with overlying glacial aquifer systems;
- Establish a methodology for improving the resolution of the current statewide bedrock topography map to constrain the lower bounding surfaces of glacial aquifers that lie on the bedrock interface.
What Does This Mean For Indiana?
The results, shown in Figure 2, confirm the existence of a deep north-south trending valley in the northeastern part of the study area. It is possible that this valley trend continues south toward the town of Nappanee (Shawn Naylor, Ohio-Kentucky-Indiana Water Science Center, USGS, personal communication, February 2021). A second north-south trending valley is found in the northwestern part of the study area but is based on limited borehole lithologic data and is constrained mainly with seismic data. Both these valleys provide areas of focus for future transects to determine their extent and depth. The location of these valleys may affect the geometries and boundaries of the St. Joseph, Nappanee, and Hilltop aquifers (Indiana Department of Natural Resources).
Training The Next Generation
One of the missions of the Indiana Water Resources Research Center, and all Water Centers, is to train the next generation of water scientists. This project successfully funded and trained one undergraduate researcher.