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♦W -.J
<br /> Groundwater Monitoring and Remediation Page 2 of 3
<br /> Canepa's Car Wash December 16, 2014
<br /> 248 E. Park St.
<br /> Stockton, CA
<br /> (88,000 pg/L), MW-11 (46,000 pg/L), and MW-12 (25,000 pg/L). Wells inferred to be obliquely
<br /> cross-gradient (and approximately 140 feet) north-northeast of the former UST area are MW-13
<br /> (less than 50 pg/L) and MW-14 (100,000 pg/L). Wells inferred to be downgradient (and
<br /> approximately 380 feet) north-northeast of the former UST area are MW-15 (less than 50 pg/L),
<br /> MW-16 (160 pg/L), and MW-17 (3,400 pg/L).
<br /> Wells used for groundwater monitoring purposes which were not sampled in the 1St Quarter
<br /> 2014 are MW-1 through MW-4, and MW-6 through MW-8. Several historical sarnpling events
<br /> have yielded non-detect results for chemicals of concern (COC) in MW-1, MW-3, and MW-4.
<br /> MW-2, located in the plume of contaminants, has not been sampled since May 7, 2012, at which
<br /> time TPH-g concentrations were detected at 12,000 pg/L. MW-6 and MW-7 have historically
<br /> been non-detect for all COCs, except for low concentrations of methyl tertiary-butyl ether
<br /> (MTBE) detected in MW-6 up to January 2008, but have not been detected in subsequent
<br /> sampling events. MW-8, located in the plume of contaminants, has not been sampled since
<br /> April 7, 2011, when TPH-g concentrations were detected at 600 pg/L.
<br /> Based on these findings, the EHD directs the immediate implementation of the following
<br /> groundwater sampling schedule:
<br /> • Biennially (approximately two-year intervals): MW-1, MW-3, MW-4, MW-6, and MW-7
<br /> (sample the wells in rotation, one each semi-annual event);
<br /> • Annually (approximately one-year intervals): MW-8, MW-13, MW-15, and MW-16.
<br /> • Semi-annually (approximately six-month intervals): MW-2, MW-5, MW-9, MW-10,
<br /> MW-11, MW-12, MW-14, and MW-17.
<br /> At minimum, please have the groundwater samples analyzed the following chemicals: TPH-g,
<br /> benzene, toluene, ethylbenzene, and total xylenes (BTEX), tert-Amyl methyl ether (TAME), 1,2-
<br /> dichloroethane (1,2-DCA), and naphthalene.
<br /> Further investigation is needed in various directions in the main depth zones. The First
<br /> Intermediate Zone requires further delineation northwest, northeast, and east of MW-5. The
<br /> Second Intermediate Zone requires further delineation in all directions except toward the north-
<br /> northeast, where it is delineated by MW-16. In addition, an evaluation of the need for
<br /> remediation of the Second Intermediate Zone should be conducted, as the DPE remediation
<br /> system may not be effective in this interval. The Deep Zone requires further investigation in all
<br /> directions, and the vertical extent of groundwater contamination has yet to be determined.
<br /> The EHD directs that a work plan to complete the assessment of impacted groundwater and an
<br /> evaluation pertaining to the remediation of the Second Intermediate Zone be submitted to the
<br /> EHD by February 16, 2015.
<br /> Based on the EHD review of the three dual phase extraction reports, the EHD directs the
<br /> continuation of the dual phase remediation system operation. Monthly monitoring should be
<br /> conducted to determine the contaminant mass removal rate and total mass removed. Operation
<br /> and maintenance logs for the remediation system should be included in the quarterly
<br /> remediation reuorts. In addition, the aforementioned remediation reports have been denied on
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