LOCATING HIGH YIELDING WELLS IN FRACTURED BEDROCK ENVIRONMENTS

Well yields in the fractured bedrock aquifers of the Piedmont and Mountain regions of the southeast and mid-Atlantic states are highly variable. Identifying where to explore for high yielding groundwater supplies can be challenging especially in the subdued terranes of the Piedmont. Traditional geologic techniques including field mapping, lineament analysis, remote sensing, structural analysis and geophysical methods have been applied towards locating high yielding water supply wells with great success at locations in VA and NC. While published geologic maps are available for most of the locations at varying scales, many do not provide the detailed information required to be successful in locating the higher yielding wells in an area. Field mapping, involving rock identification and verification of mapped geologic features like contacts and fault traces in addition to measurements of planar features such as bedding or foliation and fractures, is essential for validating the published data prior to making inferences for the potential of a given site to produce an adequate water supply. The use of remote sensing data is an essential part of the geologic analysis. Utilization of remote sensing data like LIDAR base maps combined with analysis of traditional stereographically paired images is an optimum method for collecting lineament data. These data can then be analyzed and their orientations compared with actual field measurements for an area, allowing for a better understanding of which orientations are correlative at both the regional and local scale. Utilization of GIS allows the traditionally collected data to be spatially modeled resulting in a weighted raster map that indicates the most probable locations for successful groundwater exploration. These locations can be ranked based on the significance of the geologic data; the best ranked locations are ideal for further groundwater exploration. Following the prioritization of the most favorable groundwater exploration sites, detailed surface geophysical investigations (ERI) are carried out to identify subsurface bedrock fracture zones to target for drilling. Examples of this methodology will be discussed through two case studies.