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5.2 Arequa Gulch <br /> Compared with the other five drainages, Arequa Gulch has the second highest concentration of sulfate, <br /> third highest manganese and fourth highest zinc, based on samples from two monitoring wells (CRMW <br /> 3A-35 and CRMW 313-63). Sulfate and total precipitation between sampling events, as measured at the <br /> Rigi meteorological station, are shown in Figure 8. <br /> Sulfate concentrations in both of the Arequa Gulch monitoring wells are similar, but the data for <br /> CRMW 313-63 show less variability. Overall the sulfate data spread over a range of about 800 mg/L. <br /> Sulfate concentration increased from early 2000 until late 2011 and declined until mid-2015, when it <br /> began to increase. The data in Figure 8 suggest an inverse relationship between sulfate and precipitation <br /> in Arequa Gulch groundwater. It is probable that the sulfate load is relatively constant in the groundwater, <br /> but when there is an increase in recharge, the sulfate concentration is diluted. <br /> There is a noticeable periodicity in the sulfate data, with approximately four years from low to low, until <br /> 2007, when the periodicity becomes annual. <br /> 1400 - --- _... ----- ---_-__. __..- -- 20 <br /> 1200 <br /> i <br /> I 18 <br /> 16 <br /> I <br /> I <br /> 1000 -... _-. -------� <br /> 14 <br /> Boo 12 E <br /> 600 - - — -- --- --- a <br /> 8 L¢3 <br /> 400 <br /> >P <br /> 6 <br /> • 6 <br /> f <br /> 4 <br /> 200 <br /> 0 y y 0 <br /> 1% % <br /> -+-CRMW 3A-35 -}CRMW 30-63 - Precip Poly.(CRMW 38-63) Poly.(Precip) ~ <br /> Figure 8: Sulfate and total precipitation between sampling events data for Arequa Gulch. Third order <br /> polynomial trend lines have been added to the data from CRMW 38-63 and the precipitation data to illustrate <br /> the relationships between these two parameters. <br /> Data for manganese and zinc are shown in Figure 9. Manganese and zinc co-vary in the two wells, and <br /> both constituents exhibit increasing trends in CRMW 36-63 until late 2011, when concentrations began to <br /> decline. Since late 2013 zinc and manganese concentrations have been relatively stable. The very high <br /> zinc concentration in CRMW 313-63 (0.4 mg/L) in September 2014 must be considered as a spurious <br /> measurement. Manganese in CRMW 3A-35 was elevated until late 1999, when it declined to less than <br /> 0.1 mg/L. However, beginning in early 2002 very few samples returned results for manganese above <br /> detection limits. Zinc in CRMW 3A-35 was more or less stable at around 0.6 mg/L until late 2003, after <br /> which very few samples returned results above detection limits. The reason for the change to non-detect <br /> results for manganese and zinc in CRMW 3A-35 is not clear. <br />