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General Chemistry (Sulfate and TDS) <br />General chemistry parameters do not have primary health based standards. TDS has a <br />secondary drinking water standard of 500 mg/L. Sulfate has a secondary standard of 250 <br />mg/L. As shown in Table 2, TDS and sulfate concentrations in all three downgradient <br />alluvial compliance wells have exceeded the 500 mg/L and 250 mg/L secondary domestic <br />drinking water standard in each of the sampling events since 2003. Excess sulfate in <br />drinking water may have negative aesthetic impacts such as laxative effects. High TDS <br />waters are typically hard and have poor quality for drinking. Sulfate exceeded GCC site <br />State NPL of 1,950 mg/L at well MW -03 during the June 2012 sampling event. This is the <br />first sulfate NPL exceedence that has ever been recorded during groundwater sampling for <br />the GCC project. Though sulfate concentrations had been rising in all three wells, based <br />on the previous two sampling events, this trend was reversed in wells MW -02 and MW -04, <br />as sulfate concentrations decreased. <br />TDS concentrations from the June 21, 2012 event ranged from 1,480 mg/L to 4,700 mg/L. <br />TDS concentration exceeded the State NPL of 2,630 mg/L in well MW -03 (4,700 mg/L) for <br />the third consecutive monitoring event. TDS concentrations at wells MW -02 and MW -04 <br />decreased from the previous sampling event and (similar to sulfate) ended the trends of <br />increasing TDS previously defined at these wells. Figure 2 shows the results of the TDS <br />analyses for all three wells since TDS sampling started in March 2008. <br />The DRMS sent a letter to Mr. Brian McGill of GCC dated December 8, 2009. The letter <br />presented NPLs for groundwater monitoring for four parameters (TDS, Sulfate, Radium <br />226, and Radium 228). DRMS generally selected the historic maximum concentration prior <br />to May 2009 as the numeric standard for each of the four parameters. Tables 1 and 2 each <br />list the DRMS NPLs for the four constituents of concern. The letter also described an <br />approach for evaluation of potential of exceedences, and subsequent actions, based on <br />results from the groundwater monitoring program. The following approach was presented. <br />"Sampling for each well will be conducted semi annually and results to be submitted <br />annually. If an upward trend above the set limits is noted the frequency of sampling will <br />increase to quarterly. If the upward trend still continues, the frequency of sampling will be <br />conducted monthly. If two consecutive monthly samplings still show an upward trend GCC <br />will present a remedial action to the Division, which will indentify the source of the upward <br />trend and submit the actions GCC proposes to take a receive written approval from the <br />Division." <br />Currently, TDS concentrations at well MW -03 define an upward trend, with three <br />consecutive data points exceeding the NPL. No trend is defined above NPLs for sulfate in <br />well MW -03, as only one exceedence has been reported. Prior to April 2011, TDS and <br />sulfate results were characterized by alternating highs and lows that were likely associated <br />with seasonal groundwater fluctuations. Recent TDS highs have been recorded during <br />October sampling events that have followed dry summers with little fall precipitation. Low <br />groundwater levels may tend to concentrate dissolved chemical species, raising the level of <br />TDS. GCC expected to see TDS concentrations decrease in the current spring and early <br />4 <br />