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March 2014 Mill MLGW -7 Groundwater Quality Assessment ♦ 6 <br />In 2013, the lowest pH measurements occurred during July and August. A higher <br />pH was observed during the September 2013 monitoring event but pH has <br />fluctuated around 6.5 since then. <br />• As noted above in 2013, the lowest pH values reported at MLGW -7 occurred during <br />the summer period following spring runoff, suggesting a potential relationship <br />between aquifer recharge and groundwater chemistry. Figure 9 is a graph of pH <br />versus water level in MLGW -7 since February 2011. The graph shows some recent <br />correlation between shallow water levels, indicating recharge conditions, and low pH. <br />However, historical low pH values also have been recorded during low flow /winter <br />conditions. <br />Analytical results for other indicator parameters established in the GWMP as well as <br />parameters added by Henderson to further evaluate conditions at MLGW -7 are presented in <br />Table 2. Analytical results show that all other indicator parameters were well below <br />respective NPLs defined in the GWMP. This includes all samples collected in 2013, <br />including samples corresponding to low pH measurements in MLGW -7. A summary of <br />findings is presented below. <br />• Preliminary trend analyses of key indicator parameters appear to show that low pH <br />values do not correlate with higher total dissolved solids (TDS), sulfate, and metals <br />concentrations at MLGW -7. For example, Figure 10 presents graphs of pH and <br />total dissolved solids, for the 2007 to 2014 period. The graphs show that increases in <br />TDS do not always correspond to decreases in pH and there is no correlation <br />between pH values and TDS. <br />• The sulfate concentrations show a similar pattern as TDS (Figure 11). <br />• Recent manganese data show that increases in manganese concentrations do not <br />correlate with low pH values (Figure 12). <br />AJAX Ltd ♦ 28828 Cedar Circle ♦ Evergreen, CO 80439 ♦ (303) 241 -2057 <br />Clear Creek Associates♦ 6155 E. Indian School Rd. ♦ Scottsdale, AZ 85251 ♦ (480) 659 -7131 <br />