oa Long Term ERT Monitoring of Remediation at an LNAPL Site, Enid, Ok

Monitoring of the subsurface changes that take place during contamination remediation is difficult. One method that allows for stable long term monitoring is Electrical Resistivity Tomography (ERT). A contaminated site in Enid, Oklahoma, a saturated sand aquifer with overlying unsaturated clay, was evaluated to determine the distribution and migration of LNAPL. Fifteen direct push boreholes were drilled at ten meter spacing and 27 electrodes were grouted into place at a vertical spacing of 1.5 feet (0.46 meters) giving a total depth of about 40.2 feet (12.26 meters). Data were collected prior to remediation, during remediation and eight years after remediation began. the time lapse data shows the difference in resistivity values from one data set to the next and allows for the determination of background resistivity values and the movement of the contamination across the site. the data indicate that the LNAPL can be quantitatively evaluated and that the degradation over time appears to impact the unsaturated clay layer more strongly than the saturated sand. the clay layer is becoming more conductive with time suggestive of biological impacts in the clay.