2017-06-26_HYDROLOGY - M1980244

• Permit No: M1980244
• IBM Index Class Name: Hydrology
• Doc Date: 6/26/2017
• Doc Name: Demonstration of Compliance with WQCC Regulation No 41
• From: Newmont
• To: DRMS
• Email Name: TC1
• Media Type: D
• Archive: No

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