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mt By:. RWOCB; 2553015; Mar-7-do2:15PM; Page 2 wm �- .� California Regional Water Quality Control Board Central Valley Region u s .. inion H.Hlekoc Steven T.Butler,Chair . CvraY Davis Secret ,N fur S2cramento Main office Guvamo fn mwfi+ua1 T�a+`cJ�an Intemet Address: http://..swrcb.ca.pv/—Rvgcb5 3443 Routier Rome,Suite A.Sacramento,C•aliromia 95827-3003 Phmc(916)255-3000•FAX(916)55-301.5 28 February 2000 Durhim -Ali — Nagi Saleh-Ali Mohamed &Ruda Gubary 667 O'Farrell Street #10 PO Box 1322 25 Poncetta Drive#201 San Francisco, CA 94109 Riverbank, 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 (bg6), 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(µg/1) of total petroleum hydrocarbons as gasoline (TPHg), 840µg/l of methyl tertiary butyl ether (MtBE), 5,120 µg/1 of benzene, 7,390 µg/1 of toluene, 696 µg/1 of ethyl benzene, and 5,020 µg/1 of total xylenes- The wells were not sampled for a y=_ On28 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 filft. 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. TPHg MtBE Benzene Toluene Ethyl benzene Xyl es 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-1 <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 <0.3 DW-4 <50_ < 5 <0.3 < 0.3 < 0.3 c 0.3 California Environmental Protection Agency