T58 Contaminant Hydrogeology of Karst—Characterization, Modeling, and Remediation

Sunday, 31 October 2010
1:30 PM - 5:30 PM
Room 402

MNA AS AN APPROVED GROUNDWATER REMEDY FOR A CARBONATE BEDROCK AQUIFER CONTAMINATED WITH PAHs

Paper number: T58-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

Monitored Natural Attenuation (MNA) is approved as the main component of groundwater remediation at a Site contaminated with free phase visible coal tar and dissolved phase SVOCs with the constituents of interest consisting of the 16 PAHs recognized by USEPA as priority pollutants, as well as 1-methylnaphthalene, 2‑methylnaphthalene, dibenzofuran, various coal-tar-related phenols, and carbazole. Assessment activities have identified the presence of DNAPL coal tar material in surface and subsurface soil and the underlying carbonate bedrock aquifer. Hydrogeologic characterization included downhole electromagnetic borehole flowmeter (EBF) testing of the uncased portion of coreholes, hydraulic analyses of aquifer response to precipitation events, and hydrogeologic mapping. Investigations identified the presence of horizontal or sub‑horizontal voids indicative of karst development, mud seams, fractures, and fracture zones in the bulk rock mass of the Ketona dolomite which behaves as a heterogeneous aquifer with varying degrees of weathering, fracturing and groundwater transmission.

Evaluation of natural attenuation as a remedial alternative included assessment of multiple lines of evidence: plume stability (i.e., determining whether a plume is shrinking, static, or expanding), the predominant in situ biogeochemical condition (oxidizing or reducing) to determine whether natural attenuation conditions are present, and laboratory microbial (microcosm) studies using crushed bedrock and groundwater from the Site. Results of the microcosm study indicate that indigenous facultative microorganisms are capable of biodegrading naphthalene effectively under aerobic and anaerobic conditions, with the aerobic conditions being the most favorable.

The selected remedial alternative of source removal in the soils coupled with natural attenuation of groundwater, along with maintaining existing land use and administrative controls, meets the three corrective action objectives for groundwater: 1) Protect potential future receptors, 2) Reduce to the extent practicable future groundwater plume migration from the current impacted areas, and 3) Reduce concentrations of chemicals of concern in groundwater to acceptable levels over a reasonable period.