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220 --__ -__. - ---------- -- -------- ------ --._. _.-__._... <br /> 170 _--._----- - -- ---___ ----_ <br /> 120 <br /> 70 -----._._ __----- <br /> ti <br /> 20 <br /> ■ <br /> .. .. .. :. :. .. .. .i .. .. .. .. .. .. .. .. ... ..1 .. .. <br /> -GVMW 22B-30 -■-GVMW 22A-70 --GVMW SA-250 -GVMW 88-50 <br /> Figure 19: Sulfate data from Grassy Valley monitoring wells. <br /> Manganese data are shown in Figure 20. Wells GVMW 2A-70 and GVMW 813-50 have geometric mean <br /> manganese concentrations below the domestic well standard, GVMW 226-30 is slightly above the <br /> standard, and GVMW 8A-250 has a geometric mean concentration significantly higher than the standard. <br /> However, manganese concentration in GVMW 8A-250 has been declining since the well was installed, <br /> although manganese concentration increased in 2015 and 2016. The most recent analytical results <br /> available for GVMW 8A-250 (28 November 2016 and 25 January 2017) indicate that manganese has <br /> declined to less than 0.07 mg/L. <br /> It is unclear why manganese has been elevated in GVMW 8A-250, or why the concentration has for the <br /> most part been declining to levels similar to those measured in the other wells. It may be that the well <br /> bore intersected fractures that contained manganese oxide minerals, and over time these have been <br /> dissolved and flushed down-gradient. Continued monitoring will demonstrate whether groundwater in the <br /> well remains at low manganese concentrations. <br /> Zinc has never been present at quantifiable concentrations in any of the Grassy Valley wells. <br />