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Cieologual Tecluucslnc Page 5 ' In Situ Remediation Feasibility Study Project No 507 2 September 20,2002 1 3.0 FEASIBILITY STUDY ' GTI will discuss several remediation alternatives and evaluate their applicability to the geologic,hydrogeologic and contaminant distribution factors discussed above ' 31 Monitored Natural Attenuation 3.1.1 Technology Review Monitored natural attenuation (NWA) consists of long term monitoring of groundwater ' conditions Attenuation of a groundwater plume can occur through physical processes such as advection and dispersion, or through biological processes In theory, biological organisms that can degrade hydrocarbons are ubiquitous in the subsurface These organisms metabolize the hydrocarbons in oxidation-reduction processes Under MNA, the laboratory analyses include chemical parameters that indicate whether oxidation-reduction reactions are taking place and natural biological mechanisms are decreasing the mass of ' residual contamination at a site The chemical parameters may include dissolved oxygen, sulfate, nitrate, oxidation-reduction potential, conductivity and pH A decrease in residual mass can be monitored by measuring concentrations of these parameters, along with a demonstrated decrease in contaminant concentrations in the sites' wells 3.1.2 Concerns & Limitations Table 3 includes the wells' historical groundwater concentrations The concentrations of ' TPH-G measured in well MW-9 have fluctuated during the course of the investigation but do not display a decreasing trend This suggests that the natural capacity of the aquifer to mitigate the plume is not sufficient to reduce the concentrations of the residual mass The ' former USTs were removed in the early 1980's and after almost twenty years there is no evidence that the natural processes can effectively remediate the plume Although it is noted that the plume does not appear to be migrating to any significant degree, the groundwater continues to be impacted by the gasoline release 3.2 Soil Vapor Extraction — 3.2.1 Technology Review Vapor extraction is the process of drawing a vacuum on the vadose zone (the unsaturated zone of the soil profile) to produce airflow through the subsurface The airflow removes volatile compounds from the soil and soil pore spaces through evaporation and ' concentration gradient diffusion Typically the extracted air is treated using thermal destruction (a thermal oxidizer or internal combustion engine) or granular activated carbon I