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ARCADIS <br />to historical concentrations. The graphs show that sulfate concentrations in MW -12 and <br />MW -13 appear to be generally stable with a slight increase. The concentrations found <br />in monitoring well MW -12, which is considered the background well, from the March <br />2013 sampling event (Table 2 and Figures 4, 5, and 6) are higher than the wells <br />installed downgradient of the CKD. <br />Sodium concentrations in downgradient monitoring well MW -7 continue to exceed the <br />NPL for this parameter and are generally increasing over time (see historical <br />groundwater data tables in Appendix E). The cause of the increasing sodium <br />concentration is not known, but this trend is not caused by the CKD landfill. As stated <br />previously, the potassium to sodium ratio (K:Na) is a more useful diagnostic tool in <br />determining CKD impact than sodium concentrations. The K:Na in all monitoring wells <br />continues to be generally less than 0.10, indicating that there is no discernible impact <br />from CKD. <br />Based on March 2013 sampling results that indicate there is no discernible impact from <br />CKD, and the fact that the concentrations of only one parameter (sodium) exceeds its <br />NPL, ARCADIS recommends that annual groundwater monitoring resume unless <br />future results require an increased monitoring frequency. <br />2013 Groundwater <br />Monitoring Report <br />Florence, Colorado <br />10 <br />