HYDRO-GEOPHYSICAL COMPARISON OF OBSERVATION WELL #2 AT THE DICKINSON COLLEGE FARM, SOUTH MIDDLETON TOWNSHIP, PA

Here we present the results of hydrogeophysical characterization of a well field within an agricultural site in south-central Pennsylvania. The well field, located on the Dickinson College Farm, contains four 60 m deep observation wells less than 22 meters from a central pumping well. The boreholes are within the Cambrian Elbrook formation, an interbedded silt-shale limestone commonly associated with dissolution features such as sinkholes, springs, and karstic topography at the surface. The goal of this study is to better understand how inherited bedrock features (i.e., fractures, bedding planes, and karstic features) influence subsurface groundwater flow. We characterize these features using multiple downhole measurements including: optical and acoustic televiewing, sonic velocity, flow meter, temperature, resistivity, natural gamma, and caliper logs. WellCAD analysis of optical and acoustic televiewer data show fractures (54 measured) and bedding planes (152 measured) with similar strike (S055^oW and S050^oW, respectively) and dip (W40^oN and W33^oN, respectively). These measurements compare to measurements at the surface quite closely however the orientation of measurements are switched. Bedding strike was measured ( N080^oE and N045^oE) and dip ( 55^oNW and 25^oSE, respectively). We also compare flow meter data with caliper and sonic velocities and see depth interval correlation between larger caliper size, higher flow velocity, and slower P-wave velocity. Finally, seismic refraction surveys at the surface adjacent to the well field show a gradual increase in p-wave velocity from weathered material and clay residuum near the surface toward bedrock velocities similar to those observed from sonic velocity measurements down hole. These independent measurements of seismic velocity show good agreement despite the differences resolution. Our surveys underscore the importance of understanding both inherited bedrock features and dissolution features in characterizing groundwater flow.