SITE INFORMATION AND CORRESPONDENCE_1

Home Browse Search

Del Monte/Parmaceast Page 2 i extraction may not be necessary. The report states that the Cal Water and Del Monte wells are closed when in actuality they are not currently in use and until the wells are destroyed there is a potential that they could be put back into use . In the Executive Summary of the report it states that groundwater modeling indicates that the shallow groundwater at the site was not likely influenced by the Del Monte production wells . At a meeting on October 17, 1995, CH2M Hill provided gradient data illustrating the influence of the production wells . On page 3-15 of the report it states that the change in groundwater flow may have been caused by the discontinuation of the production well pumping. The pump tests were conducted on monitoring wells at low pumping rates (1-5 gallons per minute) for a limited time, whereas the production wells extract groundwater at 375-1000 gallons per minute over a long period of time so PHS/EHD does not agree that the pump test indicate the Del Monte wells did not influence the shallow groundwater at the site. The necessity of groundwater extraction will be evaluated as remediation progresses . PHS/EHD also does not agree that gasoline constituent concentrations have shown a favorable declining trend from 1991- 1995 . The groundwater samples collected from monitoring wells 1 and 2 were diluted by the laboratory prior to analysis from June i 1991 to July 1994 so the results are not comparable . Since the underground storage tanks were reportedly last used in 1976, PHS/EHD does not feel that intrinsic remediation has biodegraded the gasoline contamination significantly. The report refers to the I criteria for the initial screening of natural attenuation effectiveness contained in the EPA' s "Guide For Corrective Action Plan Reviewers" . This document also states that "rates of natural processes are typically slow; contaminant levels may not be reduced to acceptable regulatory levels for years" . The report shows that in the mid 1970' s groundwater was 100 feet below grade (fbg) when the UST' s last samples from SB3 and MWDIS-2 evidenced soil econtamination at 851fbg. In addition, SB1 and SB3 evidenced soil contamination at 21 and 36 fbg. Remediation of all contaminated soil as well as groundwater must be addressed. The vapor extraction wells are screened from 59 to 69 fbg. Piezometer NP2-45 is screened from 43-45 fbg and showed less influence from the vapor extraction system than piezometer NP2-65 which is screened from 63-65 fbg. The determination of the radius of influence of the vapor extraction wells was limited to observations of the nested piezometers in NP1 and NP2 . The report does not discuss whether the vapor extraction tests were conducted while the air sparge wells were operating. Further tests need to be conducted in order to evaluate the number of air sparge or vapor extraction wells required to remediate all of the soil and I I