CONCEPTUAL SITE MODEL DEVELOPMENT FOR AN UNLINED SOLID WASTE LANDFILL IN THE VIRGINIA COASTAL PLAIN: EVIDENCE FOR NATURAL ATTENUATION OF CHLORINATED SOLVENTS

The Wilcox Sanitary Landfill is a closed, unlined landfill located in Caroline County, Virginia in the Atlantic Coastal Plain Province. Three unconsolidated Coastal Plain strata can be defined at the site: 1) Pliocene Sand and Gravel Unit 1 (K values=2.66E-7 to 4.18E-5 cm/sec) consists primarily of inorganic silts (ML), silty sands (SM), clays (CL) and clayey sand (SC); 2) Pliocene Sand and Gravel Unit 2 (K values=9.57E-4 to 4.55E-3 cm/sec) consists of silty sand (SM) and medium to coarse sand (SP); and, 3) Miocene Calvert Formation (K values typically less than 1.0E-7 cm/sec) predominantly consists of clay (CH-CL). The water table aquifer is present approximately 20 feet below ground surface within the Pliocene Sand and Gravel Unit. Potentiometric surface maps suggest that a stream west of the landfill is a discharge point for groundwater. Surface water sample results indicate the presence of Volatile Organic Compounds (VOCs) in surface water at concentrations consistent with map projections at the sample locations. Data indicate that groundwater emanating from the landfill discharges to this surface water body.

Groundwater monitoring indicates that VOCs are the most common class of compounds and that a VOC plume is confined to the water-table aquifer. Data indicate 1) natural degradation of the plume (VOC concentrations decreasing since 1997); 2) degradation of 1,1-dichloroethane to chloroethane is the primary degradation pathway at the Site; and, 3) a constant degradation rate (the ratio of mother and daughter compounds of 1,1-dichloroethane and chloroethane remain consistent). Site data can be used to develop a Conceptual Site Model indicating that constituent migration is limited to leaching from waste cells into groundwater that discharges to surface water. Results of application of the Bioattenuation Screening Process indicate adequate to strong evidence supporting Monitored Natural Attenuation as a remedial alternative at the Site.