2014-05-29_HYDROLOGY - M1977342 - Laserfiche WebLink

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' CMC—Henderson Operations 5-Quarter Water Quality Data and Baseline Parameters Report May 2014 1 performed at MLGW-ACR over a period of five (5) calendar quarters from the 4th quarter of 2012 through the 4th quarter of 2013 for the parameters listed in GWMP Tables 4-1 and 4-3. ' 3.1.2 Baseline Parameters Data Assessment MLGW-ACR baseline monitoring data is summarized in Table A — MLGW-ACR Monitoring Data. Results were compared against the domestic water supply standards specified in Table 1 and Table 2 (refer to Appendix B) of the Colorado Water Quality Control Commission Basic Standards for Groundwater (CBSG). All results were below the standards with the exception of iron and manganese, ' both secondary aesthetic standards.A graph summarizing iron and manganese concentrations is presented in Figure 1. Iron exceeded the CBSG Table 2 standard of 300 ug/L in two out of the five monitoring events on 12/20/2012 and 2/5/2013 with measured concentrations of 428 ug/L and 340 ug/L,respectively. Manganese exceeded the CBSG Table 2 standard of 50 ug/L in two out of the five monitoring events on 1 12/20/2012 and 2/5/2013 with measured concentrations of 225 ug/L and 72.5 ug/L respectively. These elevated iron and manganese concentrations do not appear to be in any way related to mining activities, rather, they may be due to the condition of the steel casing in the well and potential presence of iron reducing bacteria. Henderson intends to conduct further research to explore this possibility. As mentioned,no other baseline monitoring parameter results exceeded applicable CBSG standards nor were tthere any other apparent trends that were a cause for concern. The baseline water quality assessment did not result in the identification of any additional parameters that warranted consideration for inclusion in the established indicator parameter list for future monitoring at MLGW-ACR. The original set of indicator parameters summarized in Section 4.1 of the GWMP, including consideration for trace elemental cations, anionic species, oxyanions and field data, appears to continue to be an appropriate approach for this well. As stated in Section 4.2 of the GWMP, upon ' completion of baseline monitoring at MLGW-ACR, only those indicator parameters that also appear in CBSG Tables 1 and 2 (Domestic Water Supply)will be monitored on an ongoing basis: iron, manganese, selenium,zinc,pH and sulfate. 3.1.3 NPL Assessment ' Consistent with Section 5.1 of the GWMP, Henderson proposes that NPLs for MLGW-ACR be established using the most stringent domestic water supply use standards specified in CBSG Tables 1 and ' 2 (refer to Appendix B) for dissolved selenium, zinc and sulfate. The NPL range for pH was developed using ambient data as discussed in Section 3.4 below. Consistent with the Technical Consulting Report Establishing Background Threshold Values (BTVs) - Henderson Mill (Gateway Enterprises, 2014), ' included as Appendix C, Henderson proposes that NPLs for dissolved iron and dissolved manganese be developed after an investigation to determine the source of elevated Fe and Mn in the well, including the ' Page 3 of 9