WORK PLANS_FILE 1 - Laserfiche WebLink

Home Browse Search

r Remedial Action Plan-Tosco 76 Branded Facility No. 11195 Tune 21 1999 A hydraulic conductivity pump test, performed by Alisto, reported conductivity values in monitoring wells at the site ranging from 2.49 x 10-6 to 2.35 x 10' feet/second, which correspond to linear groundwater velocities ranging between 0.13 to 12.72 feet/year. The range of groundwater velocities were calculated using a hydraulic gradient of 0.0006 measured on September 3, 1998, which is typical for the subject site (Alisto, 1999). After 30 minutes of pumping during the performance test performed by GR, the hydraulic gradient from MW-6 toward the conductor casing was 0.016, more than one order of magnitude greater than the typical hydraulic gradient observed at the site. Drawdown data from MW-6, the furthest on-site well from the conductor casing, indicate the radius of influence from pumping the conductor casing extends beyond this well. The shallow cone of depression produced should have some degree of influence throughout a wide area of the site, given the small groundwater velocities and hydraulic gradients observed at the site. Approximately 107,760 gallons of groundwater have been removed from the existing and former tank pits to date. This groundwater removal resulted in a dramatic decrease in petroleum hydrocarbons concentrations in the groundwater around the former/present UST complex backfill area. As a result, petroleum hydrocarbon concentrations in the former UST pit/conductor casing have decreased from 60,000 ppb to not detected for TPHg, 1,500 ppb to 5.3 ppb of benzene, and 130,000 ppb to 6,300 ppb of MtBE. In addition, MTBE concentrations in wells MW-3, MW-4 and MW-5 have decreased from 77,000 ppb, 36,000 ppb, and 160,000 ppb in December 1998 to 12,000 ppb, 4,400 ppb, and 95,000 ppb, respectively, in March 1999. PROPOSED REMEDIAL ACTION PLAN GR is proposing to resume groundwater removal from the former UST pit as a remedial measure. In addition, groundwater removal from the conductor casing uses existing facilities and requires no construction. Groundwater removal and disposal can begin immediately following written regulatory concurrence. This remedial options uses existing Tosco facilities for treatment and disposal of effluent groundwater, thereby eliminating exposure risks and other negative impacts of on-site treatment of hydrocarbon impacted groundwater. Groundwater removal from the conductor casing is cost effective and has a minimal impact on the existing service station operation and its patrons. Data collected to date indicate that groundwater removal can be an effective remedial option and cost effective over time. The performance test conducted by GR indicates that removal of groundwater from the UST backfill (conductor casing) produces a shallow cone of depression influencing groundwater flow over a wide area of the site. Groundwater removal has reduced dissolved hydrocarbons in the former UST pit from 60,000 ppb to less than 10 ppb of TPHg, 1,500 ppb to 5.3 ppb of benzene, and 130,000 ppb to 6,300 ppb of MtBE. Reductions in dissolved hydrocarbon concentrations in some on-site monitoring wells have also been observed since groundwater removal began. 140193.07-1 5