2011-07-22_HYDROLOGY - M1977344 (4)

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1 ' ARCAD1S monitoring well MW -7, Holcim is required to modify the sampling frequency from annual to semi - annual in this well. The measured sodium concentration of 232 mg /L was three percent greater than the compliance value of 226 mg /L. The concentration of iron in the duplicate sample collected from MW -7 was 5.13 mg /L, or 14 percent above the compliance value of 4.5 mg /L. It should be noted that the concentration of ' iron in the non - duplicate sample collected from MW -7 was below the compliance value. Though two parameters exceeded their compliance standards, only one was by more than 10 percent. Therefore, the sampling frequency for this well can remain annual. ' In addition to MW -7, groundwater monitoring wells MW -12 and MW -13 were sampled as part of the annual groundwater monitoring program. Analytical results are located in Tables 2 and 3. In the proposed GMP dated July 7, 2010 and provided in Appendix B, Holcim provided proposed compliance standards for monitoring well MW -13. The March 2011 groundwater sampling results for MW -13 (compliance well) do not exceed these proposed compliance standards. To date, Holcim has not received notice from DRMS accepting the proposed compliance parameters and standards for MW -13 ' (compliance well). Holcim anticipates that the DRMS will set compliance parameters and standards for ' MW -13 (compliance well). Time series graphs of TDS, sulfate and manganese for MW -7, MW -12 and MW -13 ' were prepared (Figures 3, 4 and 5) to evaluate potential impacts of CKD disposal to the groundwater. In addition, the potassium sodium ratio (K:Na) was plotted for these same wells (Figure 6). The K:Na is a useful diagnostic tool in determining CKD impact. Generally a K:Na value of greater than 0.5 is indicative of impact from CKD. The graphs indicate that TDS and sulfate concentrations in MW -7 were decreasing ' through time but are higher than observed during the 2010 annual groundwater sampling event. TDS and sulfate concentrations in MW -12 have remained about the same when compared to historical concentrations. The graphs also indicate that TDS 1 and sulfate concentrations at MW -13 appear to be stable or decreasing. In general, manganese concentrations in MW -12 and MW -13 have continued to decrease since over time; concentrations at MW -7 were decreasing through time but are higher than observed during the 2010 annual groundwater sampling event. The concentrations found in monitoring well MW -12, which is considered the background well, from the March 2011 sampling event (Table 2 and Figures 3, 4 and 5) are as high as or higher than the wells installed downgradient of the CKD with the exception of manganese in 1 1 2011 Groundwater Monitoring Report Florence, Colorado 8