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1 <br /> 3.0 CONCLUSIONS AND RECOMMENDATIONS <br /> ' Since May 1992, the groundwater surface has risen approximately 20 feet This extreme rise in <br /> the groundwater table has resulted in a groundwater elevation significantly above the screened <br /> ' interval of the five groundwater monitoring wells This situation has probably resulted in <br /> inconsistent groundwater flow direction and gradient interpretations and groundwater sample <br /> ' collection and analytical results that are unrepresentative of groundwater table conditions To <br /> obtain data representative of groundwater table conditions, groundwater samples were only <br /> collected for analysis from two deep sod vapor extraction wells with screen intervals coinciding <br /> with the current groundwater table <br /> Groundwater samples collected this quarter from soil vapor extraction wells VW-1 and VW-2 <br /> indicate a general decrease in gasoline hydrocarbon concentration since the last sampling was <br /> Performed on May 15, 1997 In VW-1, the concentration of TPH-as-gasoline declined from <br /> 12,000 ppb in May 15, 1997 to 2,400 ppb this quarter Benzene concentrations also declined in <br /> VW-2 from 1,000 ppb to 110 ppb In VW-1, concentrations of TPH-as-gasoline and benzene <br /> ' were slightly higher this quarter than on May 15, 1997 To comply with recent Regional Water <br /> Quality Control Board guidelines, the gasoline oxygenate compound MTBE was analyzed <br /> Groundwater sampled from VW-2 contained 1,800 ppb of MTBE <br /> Based on the elevated concentrations of gasoline hydrocarbons in VW-1 and VW-2 Ramage <br /> 9 Y � 9 <br /> ' Environmental recommends the following environmental corrective actions <br /> 1 To enhance natural biodegradation rates and cost-effectively begin to remediate impacted <br /> groundwater, install Oxygen Release Compound (ORC) socks into three of the groundwater <br /> monitoring wells (MW-1, MW-2 and MW-3) These three wells geometncally triangulate <br /> ' around VW-2, which contains the highest concentration of dissolved gasoline hydrocarbons <br /> Dissolved oxygen and gasoline hydrocarbon concentrations should be monitored to evaluate <br /> the effectiveness of the ORC and its remaining oxygen release potential In most cases, the <br /> oxygen release potential of the ORC is depleted after six months If the use of ORC proves <br /> effective, additional socks may be installed <br /> 2 Continue quarterly groundwater monitoring and sampling of VW-1 and VW-2 Groundwater <br /> samples should continue to be analyzed for TPH-as-gasoline and BTEX To confirm the <br />-' presence of MTBE and other oxygenate additives, the samples collected next quarter should <br /> also be analyzed for volatile organic compounds by EPA Method 8260 <br /> 4 <br />