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reports to the PHS/EHD and RWQCB Groundwater samples would be analyzed for TPH/D and BTEX All monitoring activities would be discontinued five years after the system is decommissioned This is assumed to occur in 17 years 5 3 5 In-Situ Bioremediation - Option 5 Option 5 also consists of in-situ biological treatment to reduce the concentration of hydrocarbons in soil However, this option would add moisture and nutrients to improve naturally-occurring microbial growth A groundwater monitoring program identical to that developed for Option 2 would also be undertaken until the biological treatment is completed A percolation field consisting of perforated pipes or soaker hoses placed in gravel Iined trenches beneath the existing pavement would be used to deliver an aqueous solution of nutrients (probably calcium nitrate) to areas of remaining soil impacts The water and nutrient delivery rate would depend on the soil type, thickness of the impacted soil column, and areal extent of impacts With the addition of water to the impacted soil system, increased infiltration and consequent migration of hydrocarbon constituents and/or nitrate to the groundwater table is likely at some time during the treatment process This option therefore also includes groundwater plume capture and reinfection through the percolation field to contain and treat groundwater impacts resulting from the in-situ bioremediation process Although Option 5 can be expected to perform better than Option 4 in terms of hydrocarbon degradation and speed of remediation, there are still practical limits on the degree of contaminant mass reduction that could realistically be achieved with this technology It is estimated that in-situ bioremediation could ultimately achieve a cleanup level of about 500 to 1,000 mg/kg, beyond which little further reduction in hydrocarbon concentrations would be expected O&M of the system could be expected to continue for a minimum of approximately eight years As with Option 4, monitoring of system effectiveness would consist of collection and analytical testing of representative soil samples from both of the primary areas of soil impact Because of the relatively slow response expected, soil sampling and testing would be performed every two years Option 5 also includes quarterly monitoring of groundwater elevations in on-site monitoring wells (Dames & Moore, 1994b), annual groundwater sampling and analysis of the five wells, and preparation and submittal of annual monitoring reports to the PHS/EHD and sAc127 io 16 DAMES & MOORE