Laserfiche WebLink
mine plan. The analytical solution estimates radial groundwater flow toward a circular pit and <br />assumes that drawdown occurs within a homogeneous, isotropic aquifer of infinite extent. Other <br />important assumptions include (1) groundwater flow is steady state, unconfined, horizontal, and <br />radial, (2) recharge is uniformly distributed and captured within the radius of influence, (3) the <br />aquifer extends significantly beyond the radius of influence, and (4) the base of the pit is <br />coincident with the base of the aquifer and there is no flow through the pit bottom. Review of <br />the site hydrogeologic characterization (Section 2) suggests that these assumptions are <br />reasonably satisfied. Deviations from these assumptions are described in more detail below. <br />The analytical solution for groundwater saturated thickness in the aquifer adjacent to a mine <br />dewatering operation is given as: <br />h = F2+w - In 1, rp 2 <br />where <br />h is saturated thickness above the pit base at r (radial <br />distance from pit center) [L], <br />hr is saturated thickness above the pit base at rp (at <br />the mine wall) [L], <br />W is distributed recharge flux [UT], <br />K1r is horizontal hydraulic conductivity of <br />surrounding geologic materials [L/T], <br />r; is radius of influence (maximum extent of the <br />cone of depression) [L], <br />r is radial distance from pit center [L], <br />rp is effective pit radius [L] <br />Given input values of hp, W, Kh, rp, and initial (pre- mining) saturated thickness above the pit <br />base (h at ri), the radius of influence (r) can be determined through iteration by setting r equal <br />to r;. Once r; is determined, h can be calculated for any radial distance from the pit, and <br />drawdown can be calculated as pre- mining h minus calculated h at r. <br />Key parameters in the analysis include the alluvial aquifer saturated thickness at the pit wall <br />prior to dewatering impacts, hydraulic conductivity (Kh), and recharge (W). Pre - mining aquifer <br />characteristics were discussed previously in Section 2, including an alluvial groundwater <br />saturated thickness of 23 feet (h at ri) and hydraulic conductivity of 480 feet/day (Kh). <br />Additionally, it was assumed that the alluvial groundwater saturated thickness is drawn down to <br />2 feet at the pit wall (hp at rp) during dewatering. <br />Recharge flux within the radius of influence (W) was estimated based on annual precipitation <br />(WRCC 2010) and inputs from irrigated agriculture (CSU 2009). Annual precipitation in the <br />region averages 12 inches/year (WRCC 2010). With the majority of precipitation occurring <br />Cbll-..je earth Page 6 of 11 <br />SOLUTIONS <br />