2012-11-14_HYDROLOGY - M2002004 - Laserfiche WebLink

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RESULTS AND DISCUSSION Results of the September 26, 2012 groundwater sampling are provided in Table 1. Historical results for the compliance suite for all wells are shown in Table 2. Table 2 also presents the average and standard deviation statistics for compliance suite constituents, based on the monitoring history for each well. Figure 2 presents time versus concentration plot for TDS. Analytical results provided by the contract laboratories are presented in Appendix A. General Chemistry (TDS) General chemistry parameters do not have primary health based standards. TDS has a secondary drinking water standard of 500 mg/L and a Colorado State Numeric Protection Level (NPL) of 2,630 mg/L. As shown in Table 2, TDS in concentrations have exceeded the 500 mg/L secondary domestic drinking water standard in each of the sampling events since 2003. High TDS waters are typically hard and have poor quality for drinking. TDS in well MW -03 exceeded the NPL in three consecutive events beginning in October 2011. TDS concentrations at MW -04 exceeded the NPL in two of the three previous events. Therefore, quarterly sampling for TDS has been re- initiated at all wells The DRMS sent a letter to Mr. Brian McGill of GCC dated December 8, 2009. The letter presented NPLs for groundwater monitoring for four parameters (TDS, Sulfate, Radium 226, and Radium 228). The letter presents an approach for evaluation of potential of exceedences, and subsequent actions, based on results from the groundwater monitoring program. The following approach was presented. "Sampling for each well will be conducted semi annually and results to be submitted annually. If an upward trend above the set limits is noted, the frequency of sampling will increase to quarterly. If the upward trend still continues, the frequency of sampling will be conducted monthly. If two consecutive monthly samplings still show an upward trend, GCC will present a remedial action to the Division, which will indentify the source of the upward trend and submit the actions GCC proposes to take a receive written approval from the Division." TDS concentrations at well MW -03 have defined an upward trend, with three consecutive data points exceeding the NPLs. No data was obtained for TDS in the current sampling event at MW -03. TDS concentrations at well MW -04 have fluctuated around the NPL, though no clear trend is apparent. No trend is defined above NPLs for sulfate in well MW -03, as only one exceedence has been reported. Prior to April 2011, TDS and sulfate results were characterized by alternating highs and lows that were likely associated with seasonal groundwater fluctuations. Recent TDS highs have been recorded during October sampling events that 3