2011-01-10_HYDROLOGY - M1977348 - Laserfiche WebLink

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Colorado Division of Reclamation Mining and Safety January 6, 2011 Mr. Eric Scott 6 103-81640 comparison are presented in the following section on a well by well basis. Benchmark levels were not established for Gross Beta for the following reasons. ¦ The only regulatory standard for Gross Beta is provided in units of exposure, millirems per year (mrems/yr), which would require the measurement of specific nuclides (Tritium and Strontium) with known energy levels. Specific nuclides were not part of the abbreviated constituent list. ¦ Statistical methods could not be confidently applied to the data because: • Historical concentrations vary widely within each well and between wells. • There are high errors or calculated sample specific uncertainty associated with the reported concentrations. • Reported concentrations for the September 2010 samples are below the Lower Limits of Detection (LLD). 1 MW-1 Monitoring well MW-1 is located upgradient of CKD disposal area A2 based on the groundwater. flow direction presented on Figure 3. Per the SAP (Golder 2010a) benchmark levels were not determined for upgradient well MW-1; however, historical and current constituent concentrations from MW-1 were used to help evaluate downgradient concentrations. MW-2 Barium is the only constituent with a September 2010 concentration slightly above the benchmark level in the sample from MW-2. The concentration of barium is 2.09 milligrams per liter (mg/L), whereas the benchmark level is 2.0 mg/L. The barium benchmark is the Domestic Water Supply-Human Health Standard from Table 1 of the Regulations. It is also worth noting that the gross alpha concentration of the MW-2 sample is 15 picoCuries per liter (pCi/L), which is equivalent to but not above the benchmark (i.e., the Domestic Water Supply -Human Health Standard from Table 1 of the Regulations). MW-3 September 2010 concentrations of barium and boron are slightly above benchmark levels in the sample from MW-3. The concentration of barium is 2.4 mg/L and the benchmark level is 2.0 mg/L. The barium benchmark is based on the Domestic Water Supply-Human Health Standard (Table 1 of the Regulations). The concentration of boron is 0.77 mg/L, whereas the benchmark level is 0.75 mg/L. The boron benchmark is based on the Agricultural Water Quality Standard (Table 3, of the Regulations). MW-4 In the sample from MW-4, the September 2010 concentrations of barium and total dissolved solids (TDS) are above benchmark levels. The barium concentration is 8.69 mg/L compared to a regulatory benchmark level of 2.0 mg/L from the Domestic Water Supply-Human Health Standard (Table 1 of the Regulations). The concentration of TDS is 11,000 mg/L compared to the benchmark level of 10,212 mg/L calculated from Table 4 of the Regulations. 5.0 SUMMARY This report presents results of the groundwater sampling and comparison to benchmark levels for the September 2010 monitoring event at the Site. The constituent concentrations above benchmark levels are as follows: ¦ Barium was present in the September 2010 groundwater samples from all three downgradient wells (MW-2, MW-3, and MW-4) at concentrations above the benchmark level of 2.0 mg/L. The concentrations in samples from MW-2 and MW-3 (2.09 and 2.4 mg/L, respectively) are just slightly above the benchmark level. I is\10\81640\0400\gwrep_fnl_06jan11\10381640holcimgwrep_fn1-Ilr06janll.docx Golder ?L7Assodates