BIOREMEDIATION OF PAHs IN A DEEP AQUIFER AT A FORMER MANUFACTURED GAS PLANT SITE
David Falatko, P.E. (d.falatko@iesionline.com) (Innovative Engineering Solutions, Inc.- Portland, Maine)
Joe Higgins, P.E., L.S.P.,
Sami A. Fam, Ph.D., P.E. L.S.P. (Innovative Engineering Solutions, Inc. - Walpole, Massachusetts)
George Pon, Ph.D. (Bioremediation & Treatability Center-
Walpole, Massachusetts)
Elizabeth Greene, Brian Quillia, (National Grid - Westborough, Massachusetts)
Abstract
A former manufactured gas plant (MGP) site released coal tar which contaminated groundwater with polycyclic aromatic hydrocarbons (PAH), primarily naphthalene and 2-methylnaphthalene, as well as benzene, toluene, ethylbenzene and xylene (BTEX) compounds. The coal tar and resultant dissolved PAH and BTEX compounds in groundwater migrated from the former MGP area to a downgradient sand and gravel drinking water aquifer. The impacted aquifer is approximately 100-feet thick with a 90-foot saturated thickness. The highest contaminant levels are present in the deepest portions of the aquifer just above bedrock; this is consistent with localized evidence of separate phase coal tar in these areas. Site data indicated that biological degradation of the contaminants was occurring naturally but was limited by the lack of oxygen in the aquifer. Delivering oxygen to the deepest impacted zones with a typical air sparging system was not considered practical as vapors generated from the sparged air could not be efficiently recovered with a vapor extraction system and low residual dissolved oxygen levels from sparging would require numerous sparge points. An oxygen sparging system was selected for higher oxygen transfer efficiency and to provide higher residual levels of dissolved oxygen. The results show significantly decreased contaminant concentrations around the sparge well transect concurrent with increases in dissolved oxygen. Dissolved oxygen concentrations have increased significantly with levels as high as 52 milligrams per liter (mg/l) observed. Contaminant levels have decreased 95-100% within approximately 100-feet of the sparge system. The effective treatment zone around the oxygen sparge transect increased with time of operation as contaminants were degraded. The primary factor limiting continued expansion of the treatment zone appears to be the localized presence of adsorbed phase coal tar at the base of the sandy aquifer within the targeted treatment zone.
