SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

ErniQ's General Store - 2 - 22 November 2011 4407 E. Waterloo Rd., Stock 40 San Joaquin County In September 2001, two grab groundwater samples from borings B2 and B3 were analyzed, and both samples were reportedly impacted by petroleum hydrocarbons. In July 2003, four groundwater monitoring wells (MW-1 through MW-4) were installed, followed by MW-5 in December 2006. Groundwater monitoring was conducted quarterly from August 2003 until July 2008, when semi-annual sampling was approved by the SJCEHD. Groundwater monitoring ceased in May 2010. A history of groundwater monitoring results can be seen on attached Table 3. While groundwater monitoring has shown a trend of increases and decreases since 2005, the maximum peak concentrations have declined to less than 33% of their 2005 maximum concentration since 2008 (Graph 1). The minimum concentrations from 2008-2010 have declined approximately an order of magnitude lower than the prior minimum concentrations in 2006-2007, indicating that mass reductions are occurring at a slow rate through natural attenuation. Groundwater monitoring has shown that the groundwater plume is stable and slowly declining. While water levels have risen in recent years, there is no clear correlation between rising water levels and groundwater concentrations (Graph 1). A November 2007 cone penetrometer testing (CPT) grab groundwater sampling (CPT1 and CPT2) did not report groundwater contamination from 88' bgs to 115' bgs. Maximum grab groundwater concentrations in 2001, and maximum August 2005 and final May 2010 groundwater monitoring concentrations (MW-4) are listed below in Table 2. Table 2 Maximum Groundwater Sample Concentrations' Date TPHg benzene toluene eth (benzene xylenes MTBE TBA DCA 9/4/2001 8,300 8.5 150 120 1,300 29 <13 6.2 grab sample 8/17/2005 15,000 <0.5 <0.5 <0.5 <0.5 9,900 3,700 29 monitoring 5/7//2010 <50 NA NA NA NA 36 <10 <0.5 monitoring 1 =concentrations given in micrograms per Liter(ug/L) 2=sample taken 5/11/2005 In December 2006, six soil vapor extraction (SVE) wells (SV-1 through SV-6)were installed as an interim remedial measure, and an SVE pilot study was conducted. The shallow SVE wells pilot study was reported as successful, while the deep SVE wells failed to remove an adequate volume of soil vapor to constitute a successful test. Due to declining peak concentrations in monitoring wells with the stable MTBE plume near the USTs, Regional Board staff met with the consultant in April 2010 and agreed to postpone implementation of SVE, pending additional groundwater sampling and completion of a soil vapor survey. In September 2010, a soil vapor intrusion study was conducted by advancing and sampling seven shallow borings (V1 through V7). Maximum soil vapor concentrations for TPHg, 3,600 micrograms per cubic meter (ug/m3), benzene, 5.4 ug/m3, toluene 7.6 ug/m3, and xylenes, 10 ug/m passed the residential and commercial San Francisco Bay Environmental Screening Levels (ESLs) and commercial California Human Health Screening Levels (CHHSLs). Risk Assessment As stated above, a soil vapor investigation was conducted in September 2010, which passed the ESLs for residential and commercial use, and CHHSLs for commercial use. Soil concentrations failed the ESLs for gross contamination and direct exposure; however, the consultant states that the most contaminated soil is located at least 50' from any building