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May 2014 Groundwater POC Update Memorandum♦7 <br /> ' Concentrations of sulfate ranged from 31.9 mg/L to 41.9 mg/L in MLGW-17 and <br /> 24.3 mg/L to 75.6 mg/L in MLGW-20. Concentrations of manganese ranged from <br /> 0.002 mg/L to 0.005 mg/L in MLGW-17 and 0.004 mg/L to 0.011 mg/L in <br /> MLGW-20. Field pH measurements ranged from 6.4 to 7.2 in MLGW-17 and 5.5 to <br /> ' 6.0 in MLGW-20. <br /> • Concentrations of seepage indicator parameters are within the range of interpreted <br /> ' background (natural) conditions for the Qd aquifer at the Henderson Mill site and <br /> indicate groundwater at MLGW-17 and MLGW-20 is not currently impacted by <br /> seepage from 3-Dam. Internal monitoring at Henderson has shown that seepage <br /> timpacted groundwater typically has the following parameter concentrations: sulfate <br /> > 1000 mg/L, pH < 4, dissolved manganese > 50 mg/L. As noted above, <br /> ' concentrations of these three indicator parameters in MLGW-17 and MLGW-20 are <br /> well below the levels indicative of seepage impacts. <br /> ' Results suggest natural (background) groundwater at these locations is slightly acidic, <br /> which is interpreted to reflect the influence of infiltrating surface water, which has a <br /> pH usually below 5.5 (Langmuir, 1997). The meteoric response would be more <br /> apparent in shallow wells (e.g., MLGW-20) resulting in lower pH values and <br /> ' potentially higher natural concentrations of sulfate and total dissolved solids. <br /> ' 4.4 3-DAM POC FINDINGS AND RECOMMENDATIONS <br /> The results of quarterly groundwater monitoring at 3-Dam support the original <br /> ' interpretation that a laterally extensive aquifer system does not exist at 3-Dam. Of the five <br /> monitor wells installed, only one (MLGW-17) has had measureable groundwater during all <br /> ' monitoring events. All other wells have been observed to go dry. Hydrographs and water <br /> level elevation data suggest water infiltrates the glacial sediments during late Spring/early <br /> ' Summer snowmelt and following storm events. This transient water is interpreted to flow <br /> vertically until it reaches the bedrock contact (water in MGLW-20 migrates downward to <br /> ' MLGW-17). At that point, the water is interpreted to migrate along the bedrock contact <br /> following the slope of the contact northeastward toward the center of the Williams Fork <br /> ' River valley. More continuous saturation of the glacial deposits occurs where the sediments <br /> are deepest, which corresponds to the area of MLGW-17. The ultimate fate of water in <br />