BEDROCK AND POTENTIOMETRIC SURFACE MAPPING BY GIS ANALYSIS OF DIGITAL WELL RECORDS, WAYNE COUNTY, INDIANA

Ground water well records have been collected for many decades as the result of the recognized need to regulate ground water resources. Well records are useful but in the most common form (paper sheets filled out by the well driller) are often difficult to acquire, may be imprecise and are unwieldy for large dataset projects. This is changing as well records become digitized and well locations are recorded via GPS. A total of 2350 digitized well records are now available from the Wayne County GIS Program. The digitized data include many useful fields: ownership; driller; dates; elevation of land surface and static water level; depth to bedrock; development and pumping yields; screen, casing and grouting construction details; pump configuration and UTM location coordinates (with uncertainty estimates).

This study is an undergraduate independent study project applying a Geographic Information System (GIS) to ground water resource characterization. Ground water in Wayne County is likely dominated by flow in discontinuous glacial till and outwash and in bedrock fractures; yet the subsurface geometry of bedrock and overburden has only been poorly known. To improve our understanding, GIS analysis of the well data was aimed at answering specific questions: 1.) What is the 3-dimensional configuration of the bedrock surface? 2.) What is the spatial variability of overburden thickness? 3.) What is the configuration of the potentiometric surface?

Well data were converted to DbaseIV (dbf), were imported into ArcGIS 8.1.3, were spatially rectified by UTM coordinates and were converted from vector to raster (grid) format using Spatial Analyst 8. 3-D maps of the bedrock and potentiometric surfaces were created by subtracting depth of bedrock or static water level from land surface elevations using Spatial Analyst. An isopach map was created by subtracting the derived bedrock elevation from the land surface elevation. Artistic 3-D maps of the bedrock and potentiometric surfaces and the overburden thicknesses were created using 3D Analyst with a vertical exaggeration of 5x and color coding elevations by contour range.

The data reveal a bedrock surface that slopes toward the SSW with several large NNE-SSW trending buried valleys reflecting meltwater incision during Late Pleistocene deglaciation.