Tuesday, May 3, 2011: 11:45 a.m.
Columbia/Frederick (Hyatt Regency Baltimore on the Inner Harbor)
Paul C. Miller, P.E., Environmental Alliance Inc.;
Jason S. Early, P.G., Environmental Alliance Inc.;
Paul Yaniga, W I S E Ltd;
An evaluation of monitored natural attenuation (MNA) was performed for an industrial site with a low-concentration (<200 ug/L) groundwater TCE plume. Previous active remediation at the site included removal of a former waste solvent UST, removal of impacted source-area soils, and operation of a pump-and-treat system for over 24 years. Capture zones for the pump and treat system were drawn using flownets constructed from field-measured hydraulic heads corresponding to varying system operating conditions, including a single extraction well in the former UST area operating for 24 years and two additional extraction wells downgradient from the former source area operating for four years. A trend analysis was then performed to compare the TCE attenuation rate within the capture zones of the pumping wells to the rate of TCE attenuation outside the capture zone of the pumping wells.
A key monitoring well within the capture zone of the pumping well at the former UST area indicated an approximate TCE attenuation rate of -0.0055 ug/L/day. In contrast, a key monitoring well outside the capture area of this pumping well showed nearly twice the rate of TCE attenuation, at approximately -0.0095 ug/L/day. These data indicate that natural attenuation of TCE is occurring at a rate faster than the rate realized by continued operation of the pump-and-treat system. Further, the study suggests that operation of the pump-and-treat system may be physically and/or biochemically inhibiting natural attenuation.
Based on the results of this study, the pump-and-treat system has been deactivated and a long-term MNA pilot study has been initiated. This study illustrates the importance of reviewing long-term historical contaminant concentration data when evaluating performance of an ongoing site remedy.
