ARCHIVED REPORTS_XR0002637 - Laserfiche WebLink

Home Browse Search

A S S 0 C f A T E S I N C 6.0 EVALUATION OF LOW RISK CRITERIA 661 Low Risk Groundwater Case Evaluation Using the criteria listed in CRWQCB's No Further Action (NFA) requirements, as outlined by CRWQCB (Appendix B of the Tri-Regional Recommendations), conditions at the subject site i� were evaluated to determine if NFA as a "low risk, Groundwater Case" is appropriate for this site. i� Criterion 1: Contaminants remaining in the vadose zone must not reverse or threaten to reverse the mass reduction rate of groundwater pollutants. Over the course of the investigation, aquifer levels have been measured at depths of z approximately 22 to 26 feet. One soil sample collected in 1998 contained a detectable concentration of ethyl benzene at 0.3 mg/kg. The same soil sample also contained TPHg rte, and TPHd at concentrations of 300 mg/kg and 2,100 mg/kg, respectively. The soil Id samples collected during the overexcavation of the former tank pit contained low concentrations of benzene and xylenes and detectable concentrations of TPHg, TPHd, and TPHss. The estimated mass of BTEX remaining in soil is approximately 4.6 pounds. Concentrations of BTEX constituents in groundwater well MW3 have generally been decreasing and only benzene has ever detected above its respective maximum contaminant level in December 1999. Concentrations of TPHss, TPHd, and TPHg in groundwater well MW3 have generally been decreasing over time. TPHg has never been ' detected in wells MW1, MW2, and MW4. TPHd and TPHss have not been detected in wells MWi or MW2 since March 1999 and have never been detected in well MW4. Therefore, the likelihood of contaminants in the vadose zone reversing or threatening to ' reverse the mass reduction rate of groundwater pollutants is very low. Criterion 2: Separate phase product has been removed to the extent practicable. ' Free product has never been detected at this site. ' Criterion 3: No existing water supply wells, deeper aquifers, surface waters, or other receptors are threatened by pollutants remaining in the aquifer. ' Well MW1, located 33 feet in the generally downgradient direction, had one detectable concentration of TPHd at 68 ug/L in December 1998 and detectable concentrations of TPHd and TPHss at 52 and 95 ug/L, respectively in March 1999. No other constituents have been detected in this well. TPHd was detected once in well MW2, located crossgradient from the former UST, at 77 ug/L in March 1999. The closest water supply well is located approximately 1,200 feet downgradient from the site and the well is not active. The nearest surface water is located approximately 2,300 feet from the site. For w:\172951reportsklosure.doc 7