Jose Luis Antinao,
||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.
||To improve the density of data related to bedrock depth below the St. Joseph Aquifer System in northeastern St. Joseph and northwestern Elkhart County, we will employ newly developed passive seismic methods. The data will facilitate mapping at an improved resolution the base of the aquifer, where it coincides with the bedrock surface. The methods developed can be applied to valley-fill aquifers in the rest of the state. The enhanced bedrock topography data will also provide hydrologists and health department officials with better tools to investigate the geologic constraints on naturally occurring arsenic in groundwater in deep wells.
||A series of horizontal-to-vertical spectral ratio (HVSR) seismic transects will be collected to develop bedrock profiles and systematically increase data density in the study area. An initial calibration period will entail collecting HVSR data at sites where bedrock elevation is known such that a power law relationship can be established between peak frequency response and bedrock depth. Following the calibration phase, geophysical data transects will be collected along county roads using the non-invasive approach.
||• A published map (1:100,000 scale) showing bedrock topography in NE St. Joseph County and NW Elkhart County. The map and corresponding GIS files (digital elevation model, digital contours, control points) will be provided to John Carlson (St. Joseph Co. GIS Director). A summary of the work, including relationships of overlying aquifer sequences to the bedrock surface and how they relate to human health concerns, will be presented to Mark Espich (St. Joseph Co. Env. Health Division) and Joe Foy (Elkhart Public Works and Utilities Dept.).
• A digital data compilation including processed HVSR data, a digital elevation model of the interpolated bedrock surface, digital elevation contours of the bedrock surface, HVSR transect locations, and well log data related to bedrock control points and lithologic characteristics of glacial sediments.
• A new database structure that demonstrates how to process passive seismic data and merge it into an existing database for bedrock topography in Indiana. This information will be available to hydrologic consultants in Indiana who are collecting HVSR data and geologists in adjacent states who will be able to share data and improve data density along state borders.
||The HVSR geophysical surveys will produce 100 to 200 data points that will facilitate mapping the bedrock surface at higher resolutions with better accuracy. The improved bedrock topography map and associated data will:
a) help us 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;
b) improve the resolution of bedrock geologic maps so we can delineate the spatial distribution of lithologies that are high in arsenic and where they coincide with overlying glacial aquifer systems;
c) 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.
A database of existing lithologic logs will also be compiled in preparation for building a fully three-dimensional geologic model of the aquifer system.