Transient hydrogeophysical investigation of a gasoline impacted site, Enid, Oklahoma

Scope and Method of Study: The purpose of this study was to examine a gasoline impacted site during remediation. This was done using Multi-channel Analysis of Surface Waves (MASW), Electrical Resistivity Tomography (ERT), and geochemistry of groundwater and soil samples. A second purpose of this study was to determine the effectiveness of ERT cables and electrodes that were left in the boreholes before remediation began. The ERT data collected were then compared to ERT data that were collected pre-remediation to determine the percent change in resistivity of the subsurface, transient ERT. The final methods used were the collection of core and groundwater samples to compare with the results of the transient ERT data. The cores were examined using photoionization detection and samples were sent to a geochemical lab along with the groundwater samples.;Findings and Conclusions: Using MASW the stratigraphy of the subsurface was determined in some locations but raised more questions in other locations. The areas where the data raised more questions were often the location of cores from previous studies conducted at this site. The ERT data collected showed the location of the LNAPL anomalies (higher resistivity). When the data were time lapse processed with data collected pre-remediation there were both positive and negative changes in resistivity that are associated with the LNAPL moving in the subsurface. The core and groundwater sample geochemistry were consistent with the transient ERT; they showed that the site is still contaminated. Specifically, the groundwater geochemistry data showed that the LNAPL is biodegrading as well as volatilizing in different areas of the field site.