SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Remo Canepa Page 2 of 5 ! 6230 Pacific Avenue May 18, 2011 Stockton, CA from you or your consultant for each well justified by the data, with a demonstration that the proposed schedule will achieve all monitoring objectives . In SVIR your consultant recommended additional soil gas sampling ; the EHD does not consider additional soil gas sampling in areas underlain by pea gravel to be necessary at this time. If all the soil gas samples collected during the soil gas survey were collected from native soil, then the results are adequate for the purpose of evaluating the risk from vapor intrusion into your on- site structure, as presumably, the building on the site was constructed over native soil The EHD is of the opinion that the recent soil gas investigation demonstrated that there is minimal risk of vapor intrusion into your on-site structure. The EHD does not approve additional soil gas sampling at this time. ;I a Citing the increased contaminant concentrations in intermediate-depth monitoring well your consultant recommended that the EHD issue an unauthorized release (UAR) for the site. The EHD notes that the increased contaminant concentrations in some wells was the basis for investigating the possibility that a new release may have occurred , but to date, no other line of evidence has been provided that strongly supports the hypothesis that a new release from the current underground storage tank (UST) system has occurred. There may be other causes for the contaminant concentrations trends that have been observed , some of which the EHD identified in the EHD letter dated 17 April 2009. None of the potential causes of the concentration trends listed in that letter has been shown to be invalid with only a new release left as the probable cause . The EHD has reexamined the contaminant concentration trends in monitoring wells MW-1 , MW-3 and SV-4 with depth-to-water and screen intervals considered in relationship to the observed trends. The following graph shows the concentration trends of total petroleum hydrocarbons quantified as gasoline (TPHg ) in MW-1 , MW-3 and SV-4: DTW vs TPHg 1000000 0 100000 10 10000 „ 20 y- MW-1 TPHg MW-3 TPHg 1000 —K— SV-4TPHg � 6230 Pacific DTW u 100 e o 0 40 10 50 1 6D co S=ping Oates Notice that as the depth to water (DTV) decreases (moves upward on the graph, DTW scale on the right side), the TPHg concentrations in MW-1 also decrease (goes down) and vise versa. In LIAR Request Response and Directive Letter 0511 `