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TABLE 9 CHANGE IN GROUNDWATER CHROMIUM PLUME SIZE OVER TIME FORMER MARLEY COOLING TOWER SITE STOCKTON, CALIFORNIA Shallow Zone Intermediate Zone Area with Change Area with Change Area with Change Area with Change Area with Change Cr above Relative to Cr above Relative to Cr above Relative to Cr above Relative to Cr above Relative to 50µg/L Previous 500µg/L Previous 5,000 Itg/L Previous 50 gg/L Previous 500 Itg/L Previous Date (acres) Period (acres) Period (acres) Period (acres) Period (acres) Period April 1989 34 NA 23 NA 4 NA NA NA NA NA January 1992 NA NA NA NA NA NA 169 NA 20 NA January 1997 46 35% 26 11% 3 -19% 76 -55% 16 -20% April 1998 37 -20% 11 -58% 3 -26% 61 -20% 18 13% January 1999 39 6% 12 6% 2 -16% 38 -38% 38 111% January 2002 27 -31% 15 24% 4 95% 38 0% 26 -32% January 2004 40 48% 18 24% NA NA 34 -12% NA NA January 2005 47 19% 19 6% 3 -27% 34 0% 9 -65% April 2006 37 -23% 13 -32% 1 -67% 30 -10% 10 11% April 2007 34 -8% 12 -8% 0.1 -90% 36 20% 14 40% April 2008 32 -5% 8 -33% NA NA 31 -14% 17 21% April 2009 26 -18% 3 -66% NA NA 34 10% 15 -12% April 2010 17 -35% 3 11% NA NA 32 -6% 9 -43% April 2011 6 -65% 2 -33% 0.2 100% 16 -50% 7 -14% July 2011a 4 -33% 0.1 -95% NA NA 12 -25% 1 -86% October 2011a 4 0% 0.1 0% NA NA 12 0% 5 400% From 1989/1992 -30 -88% -23 -100% -4 -95% -157 -93% -15 -75% to April 2011 From April 2007 to April 2011b -30 -88% -12 -99% 0.1 100% -24 -67% -9 -64% Notes: Addition of monitoring points(new wells)over time provides greater accuracy for Cr concentration contouring;an apparent increase in plume size or concentration over time may represent an improved understanding of site conditions,not expansion of the plume. Plume surface area data provided by Dudek and Associates,Inc. a=Quantile Kriging used to estimate plume extents. Previous estimates used logarithmic interpolation. b=Values reflect results from the full-scale in situ chromium reduction effort initiated in January 2009 µg/L=microgram per liter Cr=chromium(total dissolved) NA=not available (Pagel of 1)