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ARCADIS Mr. Timothy A. Cazier <br /> August 4, 2014 <br /> Although not as strong, the correlation between DTW and sodium concentration is <br /> also exhibited in monitoring well MW-13 (Figure 3). In the case of MW-13, DTW has <br /> decreased over time and sodium concentrations have also decreased. The reason <br /> for the opposite trends in these two wells is not clear. Resource Geosciences, Inc. <br /> (RGI) speculated that groundwater levels in monitoring well MW-7 were influenced <br /> by the water level in the Arkansas River, owing to the relatively close proximity of this <br /> well to the river (about 250 feet) (RGI, 1999), whereas monitoring well MW-13 is <br /> located nearly 1000 feet from the river and would not be influenced by river level <br /> fluctuations. The geology at both MW-7 and MW-13 consists of the Codell <br /> Sandstone from the bedrock surface down to the water table, as shown in <br /> Attachment 2. Under these unconfined aquifer conditions, the depth to <br /> water/sodium concentration relationship observed is reasonable in that higher <br /> groundwater elevations equate to greater dilution and conversely lower groundwater <br /> elevations result in less water available for dilution. <br /> As such, sodium concentration is not a good indicator of potential impact from the <br /> CKD landfill. As presented below, based on the chemistry of CKD, the use of <br /> potassium to sodium ratio (K:Na) is a much better indicator of groundwater impacts <br /> from CKD. <br /> Literature Data <br /> CKD is comprised of many alkaline compounds (commonly referred to as alkalis), <br /> including potassium and sodium oxides. As such, the leachate from CKD exhibits a <br /> very high pH. Thus elevated pH is the best indicator of CKD impacts in groundwater <br /> or surface water. However, because pH is essentially a measure of the hydrogen ion <br /> concentration, it decreases relatively quickly away from the CKD source, as the <br /> hydrogen ion concentration changes upon encountering more neutral pH values in <br /> the groundwater or surface water environments. On the contrary, both sodium and <br /> potassium are generally considered conservative in the environment in that they are <br /> not readily adsorbed to soil and are not generally reactive under varying water quality <br /> environments (for example changing pH and eH). In addition, because they are both <br /> highly leachable, the concentrations of sodium and potassium in water impacted by <br /> CKD would tend to mimic what is found in the CKD. <br /> Table 2 was developed based on information provided in the Report to Congress on <br /> Cement Kiln Dust(RTC)(USEPA, 1993), for kilns similar to that found at the Portland <br /> plant(dry kiln with pre-heater). Table 2 indicates that the range of concentrations for <br /> potassium is much higher than that of sodium. As a result, the K:Na in the <br /> environment downgradient of a CKD source should be elevated when compared to <br /> background. <br /> Page: <br /> C IUwm1yusko1DocumentstHo.m12014 MW-7 Sodium12014 0804 Proposal to Remove Ne from Groundwater Monitoring-FI NAL.docx 2/4 <br />