Resolving spatial heterogeneity in the Kirkwood-Cohansey aquifer using electrical resistivitytomography and nuclear magnetic resonance

Abstract:

The Kirkwood-Cohansey aquifer system underlying the New Jersey Pinelands is an important source of freshwater for residents of the southern New Jersey Coastal Plain. The aquifer system consists primarily of unconsolidated sand and gravel separated by silt and clay confining layers. In 2004, a USGS study was commissioned to assess the hydrogeology of and examine the impact of human development on three primary drainage basins. To study the regional groundwater dynamics, 471 wells were drilled across over 216 square kilometers, however, only a small number were drilled deeper than a few meters below the subsurface limiting the resolution of the resulting groundwater model. We conducted an array of geophysical methods over a four kilometer section between two USGS wells in order to better resolve lateral variations in the hydrostratigraphy of the Kirkwood-Cohansey aquifer system. Electrical resistivity tomography was used to image variations in resistivity along the transect; ten surface nuclear magnetic resonance soundings were used to obtain 1D profiles of water content and pore size; and borehole nuclear magnetic resonance measurements were collected at each well and used to correlate nuclear magnetic resonance measurements with stratigraphic data from driller’s logs. The results show that the geophysical data resolve spatial heterogeneities that would otherwise have gone undetected. Our study demonstrates the potential value of geophysical measurements for characterizing the hydrostratigraphy underlying the New Jersey Pinelands.