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TABLE 6 HISTORIC GROUNDWATER ANALYTICAL RESULTS AND PROPOSED COMPLIANCE WELL BACKGROUND CONCENTRATION LIMITS FULL SCALE IN SITU HEXAVALENT CHROMIUM REDUCTION TREATMENT FORMER MARLEY COOLING TOWER SITE STOCKTON, CALIFORNIA Laboratory Analyses Hexavalent Nitrate as N Chromium Chromium Manganese 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 Water Quality Objective 10 250 -- 10 50 50** 300 50 450 Background/Compliance Wells MW-401 7/1/2000 Hist. Sum.GW Quality -- -- -- 3 -- -- -- -- 1/1/2001 Hist. Sum.GW Quality -- -- -- 3 -- -- -- -- 4/1/2007 Dudek 2007 Annual Rpt -- -- -- -- 4 -- -- --1/9/2008 Background 3.7 16.9 NA <5 <10 <10 93 <10 284 Historic Concentration Range 16.9 <5 3 -<10 93 <10 WDR/MRP Suggested Upper Limit Background Concentration 22 6.5 13 121 6 Proposed Upper Limit Background Concentration 22 10 4/1/2008 Dudek 2008 Semiannual Rpt -- -- -- -- 5 -- -- -- -- 7/10/2008 3rd Qtr./Compliance 2.3 16.2 NA <5 <10 <10 157 <10 252 10/22/2008 4th Qtr./Compliance 2.4 14.3J NA 5.0 <10 <10 <100 <10 229J 1/8/2009 1st Qtr./Compliance 1.8 20.3 NA 6.0 11 <10 <100 <10 236 4/7/2009 2nd Qtr./Compliance 2.7 17.4 NA 5.0 <10 <10 130 <10 253 7/14/2009 3rd Qtr./Compliance NA 16.2 <5 <10 <10 115 <10 230 10/12/2009 4th Qtr. NA 15.9 5.0 5 <10 119 <10 247 2/1/2010 1st Qtr. NA 17.7 4.0 6 <10 152 <10 266 4/13/2010 2nd Qtr. NA 16.2 5.0 7 <lOUJ <100 <10 283 7/7/2010 3rd Qtr. NA 14.1 5.0 4 <10 124 <10 235 10/12/2010 4th Qtr. NA 14.2 4.OJ 5J <10 69 <10 219 1/20/2011 1st Qtr. NA 14.OJ NA 6.0 <10 0.98 39 <10 257 4/13/2011 2nd Qtr. NA 13.4 NA 5.0 3J 0.74 120 <10 218 7/19/2011 3rd Qtr. NA 14.0 NA 5.0 <10 <10 57 <10 225 10/12/2011 4th Qtr. NA 19.4 NA 4J <10 <0.5 99 20 237 Overall Concentration Range <1.0-69.6 5.6-74 2- 19 1 -480 <0.5 -20 39 - 1,340 6J-70 162- 1430 Notes: BOLD values exceed Water Quality Objective(WQO). Blue highlight values represent proposed upper limit background concentrations. Yellow highlight values exceed the proposed upper limit background concentration,but not WQO. BOLD BOLD and yellow highlight values exceed of the WQOs and proposed upper limit background concentrations. Background sampling represents the first MWH collected samples associated with the Site-wide in situ remedial program to monitor outside the primary chromium plume boundary, and areas of anticipated significant treatment. Quarterly/Compliance sampling was established for the program to track treatment performance by quarter(typically sampled in January,April,July,October). * Nitrate concentrations were reported as NO3 and converted to N by multiplying NO3 concentrations by 0.226(ratio of molecular weight of nitrogen[14]to nitrate[62]). **The Water Quality Objective for the Hexavalent Chromium will use the same guidelines as the Chromium column for reference purposes only. NA-Not Analyzed. -- Data not available. J-Result is estimated mg/L-milligrams per liter MRP-Monitoring and Reporting Program TDS-total dissolved solids µg/l,-micrograms per liter WDR-Waste Discharge Requirements MW-211(Supplemental)-Wells MW-211,MW-310,and MW-325 have been sampled to supplement the background wells for the program,but are not required per the WDR/MRP. (Page 9 of 9)