ARCHIVED REPORTS_XR0005356 - Laserfiche WebLink

Home Browse Search

Ms. Margaret Lagorio { Page 3 • CONCLUSIONS I The limits of dissolved MTBE in well MWI have been defined, verified, and are of limited extent. No petroleum hydrocarbons, gasoline oxygenates, EDB or 1,2 DCA are present in well MW2, downgradient of the former USTs at the site. Sample analytical results are consistent with previous results. No evidence has developed that indicates that hydrocarbons associated with the former onsite USTs have significantly impacted groundwater. The lateral and vertical extent of residual soil contamination has been defined. Soil contamination is confined to an area of approximately 700 square feet within close proximity to the former UST location. Soil contamination occurs between the depths of approximately 17.5 and 32.5 feet below grade, and consists of highly weathered gasoline range hydrocarbons depleted in BTEX. The lack of detectable contaminants in water samples from well MW2 and from previous hydropunch boring HP4 indicate that the residual soil contamination has not impacted groundwater. The low levels of MTBE present in water samples from MWI are almost certainly due to encroachment from an off-site source such as the release at 601 East Miner Avenue. We understand that samples were recently collected from boring CPT-I as part of the 601 E. Miner investigation. CPT-1 was located upgradient of Valley Volkswagen well MWI and between MWI and the former dispenser islands at 601 E. Miner (Figure 3). We understand that MTBE was detected at a concentration of 22 ppb in a sample collected at a depth of 61 feet. Note that MW 1 is screened to this depth. Ina hone conversation on November 26 2001 Mr. M Hartzell with R P arty t WQCB requested that we provide estimates of the mass of contamination in soil and in groundwater. Estimates of the mass of residual soil contamination were presented in the Subsurface Investigation Summary Report and Request for Case Closure dated December 20, 1999. It was estimated that approximately 385 yards of soil contain approximately 4,400 pounds of total petroleum hydrocarbons and less than one pound of benzene. With respect to dissolved MTBE in groundwater, an estimate can be made by assuming that the contamination in MWI occupies the same 700 ftz area as the soil contamination, assuming an affected aquifer interval of 30 feet, a soil porosity of 35% and MTBE concentration of 50 ppb. These assumptions result in an estimated 0.023 pounds of MTBE dissolved in groundwater. Of course, it is our opinion that the dissolved MTBE originates to the west-northwest, and consequently occupies a greater area than 700 2 ft . RECOMMENDATIONS Ground Zero recommends case closure. We recommend the destruction of monitoring well MWI. Monitoring well MW2 may be suitable for use in the 601 E. Miner investigation and it may be possible to transfer ownership of the well to the party responsible for that investigation. Otherwise the well should also be destroyed. UGZA_FS1S YS011DatMOROUNDZEIVALLEY V WIQMR12QM2002.doc