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ent By: RWOCB; 2553015; Mar-7-0 :15PM; Page 2 p / r t / LY.. L... .� California Regional Water Quality Control Board vCentral Valley Region ioston H.MCkox Steven T.Butter,Chair (.rav Davis $rcrrrmvlur Sacramento Main tutee GOvenipr Emwnw+renml Internet Addrm2 hrT;1/w .sw b.=._-oW-twgch5 Prorstuun 3443 Rosner Rout,Suite A.Sacramento,QUamia 95827-3003 Phone(916)255-3000•FAX(916)55-3015 28 February 2000 (a FDurhim3 -Ali — agi Saleh-Ali Mohamed &Huda Gubary /ot5�"Jo rrell Street #10 PO Box 1322cisco, CA 94109 Rive k CA 95367 Daly City, CA 94015 REQUEST FOR WORKPLAN TO ASSESS GROUNDWATER CONTAMINATION, RANCH MARKET, 23569 SOUTH SANTA FE ROAD, RIVERBANK, SAN JOAQUIN COUNTY We received your Phase II Environmental Site Investigation on 7 December 1999, and the Quarterly Monitoring;Report on 31 January 2000 both prepared by Arrowhead Consultants (Arrowhead). In addition, you provided a plan and schedule to supply potable water to the residence at 23565 South Santa Fe Road on 31 December 1999. A water line was installed from the Ranch Market well to the holding tank at the residence on 6 January 2000. With this work completed, as requested in our letter dated 8 December 1999, you are considered to be in compliance with our directives. On 24 November 1998, Arrowhead installed four monitoring wells to 40 feet below ground surface (bgs), as reported in the Phase II Environmental Site Investigation, Groundwater samples collected on 9 December 1998 were found to contain up to 65,900 micrograms per liter(µg11) of total petroleum hydrocarbons as gasoline(TPHg), 840µg11 of methyl tertiary butyl ether (MtBE), 5,120 µg/1 of benzene, 7,390µg/1 of toluene, 696µg/1 of ethyl benzene, and 5,020 µgll of total xylenes- The wells were nor sampled for a year- On 28 December 1999, Arrowhead sampled the four on-site monitoring wells and four downgradient and cross gradient domestic water supply wells. The results of this sampling event are presented in the table below. The average depth to water was approximately 26 feet bgs, and the groundwater flow was to the southwest at a reported gradient of 0.0015 ft/ft. All four monitoring wells showed high concentrations of TPHg,MtBE, and BTEX constituents, with the highest concentrations of TPHg and BTEX at MW-1. The highest concentration of MtBE was at MW-2 upgradient of the former USTs. The attached Figure 2 is a site map from Arrowheads Quarterly Monitoring Report. Results of 28 December 1999 Groundwater Sam lin in at Ranch Market. TPHE MtBE Benzene Toluene Eth 1 benzene Xylenes MW-1 72900 210 8150 10700 1020 4750 MW-2 19800 300 3840 2930 230 1210 MW-3 10300 < 5 1750 1170 184 698. MW-4 7120 110 1060 714 207 768 DW-I < 50 < 5 <0.3 <0.3 < 0.3 <0.3 DW-2 < 50 11 <0.3 < 0.3 < 0.3 <0.3 DW-3 < 50 < 5 <0.3 <0.3 < 0-3 <03 DW-4 < 50 < 5 <0.3 < 0.3 < 0.3 California Environmental Protection Agency