ARCHIVED REPORTS_XR0010993 - Laserfiche WebLink

Home Browse Search

i SECTION 5 Conclusions and Recommendations 5.1 Conclusions The purpose of this investigation was to provide additional site characterization of petroleum constituents in soil and groundwater beneath and downgradient of the Disco property to reduce uncertainty regarding the migration of contaminants from the site. The investigation was successful in assessing the horizontal and vertical extent of petroleum hydrocarbons,EDB,and 1,2-DCA in soil and groundwater in the areas of concern. The CPT boring logs used in conjunction with cross-sections of the Disco property identify an apparently continuous sand unit approximately 5 to 10 feet thick,and located between approximately 90 to 110 feet below the former tank and pump area. The unit is thickest in the vicinity of MW-DIS-8S/SD and thins toward the south and east(see Figs. 4-3,4-5,4-6, j and 4-7). The unit is not present in borings CPT-9,and-13. The geometry and extent of the sand unit in conjunction with the current groundwater flow direction provide insight to the migration characteristics of the constituents of concem. A fairly continuous fine- grained/clay unit underlies the permeable zone in all CPT borings,and appears to be an effective barrier to vertical migration of compounds. Analytical results of soil and groundwater samples from this investigation indicate petroleum hydrocarbons,TBA,EDB,and 1,2-DCA are the primary constituents of concern for the Disco Site. These compounds area primarily located within the 5-to 10-foot thick sandy unit located about 90-to 100-feet below the site.The geometry,orientation,and extent of the dissolved hydrocarbon plume coincides with this permeable unit,and is consistent with the groundwater flow direction.Also,this depth coincides with the depth to groundwater in the Stockton area between 1972 and 1977(See Figure 2-2).The Disco igasoline station operated from the mid-1960's to 1976, therefore,any leaks.Qr spills that occurred during this time would have migrated downward to the top of the water table.The dissolve hydrocarbon plume appears to be confined to the sandy unit between 90 and 100 ft bgs, as supported by lithologic logs,and the shallow CPT groundwater samples. The horizontal and vertical extent of TPH-gasoline and BTEX compounds in groundwater is graphically shown on Figures 4-8 and 4-10. The horizontal and vertical extent of 1,2-DCA is shown on Figures 4-9 and 4-11. The maximum horizontal extent of TPH-gas and BTEX compounds is estimated at approximately 250 feet downgradient. The 1,2-DCA and TBA is found at least 350 feet downgradient,and the maximum horizontal extent has not been completely defined. EDB is not found in any downgradient groundwater samples. The vertical extent of the contamination is predominantly confined to the sandy zone between 90 and 110 feet below ground surface. Hydrocarbon Mass Calculation The total mass of TPH-gas present within the hydrocarbon plume at the Disco site was l estimated. TPH-gas isoconcentration plume maps in plan view,and longitudinal and lateral cross-sectional views were drafted in intervals of 200, 150, 100,50,and 20 mg/L,see Figures ti • 5FOICPT REPORT MOV003670036 rye