2014-05-05_HYDROLOGY - M1977348 - Laserfiche WebLink

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April 2014 22 10381640A levels are only observed for barium. Elevated or upward trending concentrations of other constituents would be expected if CKD leachate was influencing groundwater concentrations in the Site wells. In particular, potassium in the CKD leachate at approximately 500 mg /L is two orders of magnitude greater than the native materials. However, potassium concentrations in groundwater are typically less than 15 mg /L and show no increasing concentrations. These groundwater chemistry observations suggest that the elevated concentrations of barium and TDS are attributed to local geologic conditions and not CKD leachate. This conclusion is consistent with the observation that naturally occurring barium and TDS background concentrations are regionally elevated in the Niobrara aquifer groundwater. 5.1.2 Hydrogeologic Considerations The CKD was placed in disposal area A2 beginning in the fall of 1999. Approximately 15 years have passed since CKD material was first placed in the disposal area. The most conservative groundwater velocity (i.e., fastest) was calculated to be 7.9 x 10 -3 ft/day. Based on conservative assumptions, it is estimated that groundwater has traveled approximately 40 feet through the bedrock beneath the Site since CKD was first placed in the disposal area. Due to the distances of the wells from the CKD disposal area, groundwater influenced by CKD from area A2 is possibly being monitored in wells MW -3 and MW -2. However, it is unlikely that groundwater that has come into contact with the CKD from area A2 is currently being monitored in the other Site wells. Furthermore, Secor performed sequential batch testing as part of their 1998 hydrogeologic and geochemical assessment ( Secor 1998). The results of this study indicate that barium at concentrations reported in the SPLP leachate sorb to the native materials. This suggests that the particle velocities of barium traveling through the native materials are slower than groundwater velocities. Finally, based on the observed groundwater elevations in the Site wells, the groundwater flow direction is generally west to east across the Site. Monitoring wells MW -4 and MW -7 are located more than 500 ft south of CKD disposal area A2 and are not directly downgradient of the disposal area. It is therefore highly unlikely that CKD leachate from disposal area A2 would influence the water chemistry in these wells. Based on these observations, groundwater chemistry observed in wells MW -4, MW -6 and MW -7 are most likely not influenced by leachate from CKD in area A2. 5.1.3 Responses to Comments The June 25, 2012 DRMS letter (DRMS 2012) included the following questions: Golder i \10 \81640a\ 0400 \gw han llinslallfieldrep _fnl- 30aprl4 \10381640a gwcharrpt _holcim- boettcher_fnl- 28aprl4- with holcm edits.dou Associates