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West Clay Property -2 - 26 July 2016
<br /> 639 West Clay Street
<br /> Stockton, San Joaquin County
<br /> 1,2-DCA. Therefore, by 30 August 2016, submit a work plan to complete vertical
<br /> delineation, below HU2, in the vicinity and down-gradient of MW-29.
<br /> 2. In July 2015, two grab groundwater samples were collected to the east of the Site, along
<br /> Harrison Street, approximately 175 feet east of well MW-29. Grab groundwater samples
<br /> were collected at 54 ft bgs in B16 and at 46 ft bgs in B18, B17 was advanced to 50 ft bgs
<br /> and was dry. Grab groundwater samples were analyzed for TPH-G, BTEX, MTBE, DIPE,
<br /> ETBE, TAME, TBA, ethylene dibromide (EDB), and 1,2-DCA. Petroleum hydrocarbons
<br /> were not detected in either of the grab groundwater samples.
<br /> Staff are concerned that.these grab sample locations may have been located too far south
<br /> to intersect the groundwater pollution plume. The groundwater gradient is predominantly
<br /> towards the northeast, and grab samples B16 and B18 are located due east of MW-29.
<br /> Therefore, in the Work Plan due 30 August 2016, include a scope of work to collect grab
<br /> groundwater samples down-gradient, northeast, of well MW-29. A grab groundwater
<br /> sample should be collected from the same approximate depth as the screen interval of MW-
<br /> 29 (40 to 60 ft bgs), and a second, deeper groundwater sample should also be collected to
<br /> define the vertical extent of contamination down-gradient of well MW-29.
<br /> 3. Groundwater monitoring has not been performed since first quarter 2015, and many of the
<br /> Site's wells have not been sampled since before shut-down of the ozone injection system.
<br /> Regular groundwater monitoring is needed to establish trends and plume stability, and to
<br /> evaluate risk associated with the release. Groundwater monitoring must resume and
<br /> continue until directed to stop by Central Valley Water.Board staff. Please resume
<br /> groundwater monitoring on a semi-annual basis, during the first and third quarters. The
<br /> Third Quarter 2016 Groundwater Monitoring Report is due 15 November 2016. All Site
<br /> monitoring wells are to be gauging and sampled. Groundwater samples are to be analyzed
<br /> for the following parameters: TPH-G, benzene, toluene, ethylbenzene, total xylenes
<br /> (collectively BTEX), MTBE, di-isopropyl ether (DIPE), ethyl tert butyl ether (ETBE), tert amyl
<br /> methyl ether (TAME), tert butyl alcohol (TBA), 1,2-DCA, and naphthalene.
<br /> 4. There is a discrepancy in the reported dates of operation of the ozone injection system.
<br /> Ozone injection was reportedly performed at the Site from 17 November 2006 until fourth
<br /> quarter 2010. The Quarterly Report— First Quarter 2010 (1Q2010 Report) dated 15 June
<br /> 2010 submitted by AGE, states that ozone injection occurred through fourth quarter 2009,
<br /> and stopped in January 2010 due to ozone generation unit and compressor malfunction.
<br /> Table 5 in the 1 Q2010 Report, lists 28 September 2009 as the last date of ozone
<br /> remediation operation. Therefore, in the Third Quarter 2016 Groundwater Monitoring
<br /> Report, due 15 November 2016, please clarify the last known date of ozone injection.
<br /> 5. Formation of hexavalent chromium can occur due to ozone sparging. Therefore, during the
<br /> third quarter 2016 groundwater monitoring event, please add analysis for hexavalent
<br /> chromium for all Site wells.
<br /> 6. As Site wells monitor 4 hydrogeologic units, groundwater flow direction and gradient and
<br /> groundwater data should be presented on individual figures for each hydrogeologic unit. In
<br /> the 1 Q2015 Report, a groundwater gradient map and an iso-concentration contour map for
<br /> HU2 was included. Gradient maps for the other 3 monitored hydrogeologic units were not
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