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TABLE 6 <br /> HISTORIC GROUNDWATER ANALYTICAL RESULTS AND PROPOSED COMPLIANCE WELL BACKGROUND CONCENTRATION LIMITS <br /> FULL SCALE IN SITU HEXAVALENT CHROMIUM REDUCTION TREATMENT <br /> FORMER MARLEY COOLING TOWER SITE <br /> STOCKTON, CALIFORNIA <br /> Laboratory Analyses <br /> Hexavalent <br /> Nitrate as N Chromium Chromium Manganese <br /> Well Sample Date Event/Purpose m L * Sulfate (mg/L Calcium (mg/L Arsenic(pg/L) (gg/L L Iron L /L TDS m L <br /> Water Quality Objective 10 250 -- 10 50 50** 300 50 450 <br /> Background/Compliance Wells <br /> MW-325(Supplemental) 11/3/1986 1985-1987 Common Ion 17.5 18 -- 21 -- -- -- 360 <br /> 1/28/1987 1985-1987 Common Ion 51.0 22 -- -- -- -- -- 495 <br /> 1/1/1988 Hist. Sum.GW Quality -- -- -- 10 -- -- -- -- <br /> 10/1/2000 Hist. Sum.GW Quality -- -- -- 8.4 -- -- -- -- <br /> 1/1/2001 Hist. Sum.GW Quality -- -- -- 9 -- -- -- -- <br /> 1/1/2002 Hist. Sum.GW Quality -- -- 8 -- -- -- -- -- <br /> 4/1/2006 Dudek 2007 Annual Rpt -- -- -- -- <5 -- -- -- -- <br /> 4/1/2007 Dudek 2007 Annual Rpt -- -- -- -- 7 -- -- -- -- <br /> 1/21/2008 Background 5.0 17.3 NA 9.0 <10 <10 157 <10 330 <br /> Historic Concentration Range 5.0-51.0 17.3 -22 <5 -21 <10 157 <10 330-495 <br /> WDR/MRP Suggested Upper Limit Background Concentration 6.5 22.5 13 13 204 13 429 <br /> Proposed Upper Limit Background Concentration A I 10.0'M 22.5 11. 25 M 300 50 <br /> 4/1/2008 Dudek 2008 Annual Rpt -- -- -- -- 12 -- -- -- <br /> 4/1/2008 2nd Qtr./Compliance 5.2 15.4 NA 9.0 <10 <10 140 <10 360 <br /> 7/10/2008 3rd Qtr./Compliance 5.5 16.3 NA 10.0 <10 <10 382 <10 394 <br /> 10/21/2008 4th Qtr./Compliance 12.7 31.4 NA 5.0 31 <10 <100 <10 638 <br /> 1/12/2009 1st Qtr./Compliance 6.5 17.0 NA 10.0 15 <10 <100 <10 382 <br /> 4/7/2009 2nd Qtr./Compliance 6.4 17.8 NA 10.0 <10 <10 264 <10 403 <br /> 7/14/2009 3rd Qtr./Compliance NA 29.0 <5 <10 <10 528 <10 646 <br /> 10/12/2009 4th Qtr. NA 28.1 5.0 20 <10 498 <10 660 <br /> 2/1/2010 1st Qtr. NA 30.0 5.0 26 <10 623 <10 636 <br /> 4/13/2010 2nd Qtr. NA 22.4 6.0 14 <l0UJ 319 <10 514 <br /> 7/7/2010 3rd Qtr. NA 21.6 6.0 12 <10 359 <10 492 <br /> 10/12/2010 4th Qtr. NA 23.9 5.0 14 <10 350 <10 503 <br /> 1/20/2011 1st Qtr. NA 30.3 NA 5.0 12 6.13 476 <10 636 <br /> 4/13/2011 2nd Qtr. NA 15.9 NA 8.0 81 3.55 240 <10 395 <br /> 7/19/2011 3rd Qtr. NA 5.6 NA 19.0 <10 <10 42J 70 162 <br /> 10/12/2011 4th Qtr. NA 13.6 NA 4J <10 <0.5 171 20 315 <br /> (Page 8 of 9) <br />