2018-10-16_PERMIT FILE - M2018039 ‎(2)‎

• Permit No: M2018039
• IBM Index Class Name: PERMIT FILE
• Doc Date: 10/16/2018
• Doc Name: Adequacy Review Response
• From: Blue Earth Solutions
• To: DRMS
• Email Name: JLE
• Media Type: D
• Archive: No

3.0 POTENTIAL DEWATERING IMPACTS
To estimate potential impacts from mine dewatering operations, the steady-state two-
dimensional analytical solution of Marinelli and Niccoli (2000) was used for each phase of the
mine plan. The analytical solution estimates radial groundwater flow toward a circular pit and
assumes that drawdown occurs within a homogeneous, isotropic aquifer of infinite extent. Other
important assumptions include (1) groundwater flow is steady state, unconfined, horizontal, and
radial, (2) recharge is uniformly distributed and captured within the radius of influence, (3) the
aquifer extends significantly beyond the radius of influence, and (4) the base of the pit is
coincident with the base of the aquifer and there is no flow through the pit bottom. Review of
the site hydrogeologic characterization (Section 2) suggests that these assumptions are
reasonably satisfied. Deviations from these assumptions are described in more detail below.
The analytical solution for groundwater saturated thickness in the aquifer adjacent to a mine
dewatering operation is given as:
-
h =FW+ ra r
in - r —rp
rp 2
where
h is saturated thickness above the pit base at r(radial
distance from pit center) [L],
hp is saturated thickness above the pit base at rp(at
the mine wall) [L],
W is distributed recharge flux JUT],
KI, is horizontal hydraulic conductivity of
surrounding geologic materials [L/TJ.
r; is radius of influence (maximum extent of the
cone of depression) [L],
r is radial distance from pit center IL1,
rp is effective pit radius [L].
Given input values of hp, W, Kh, rp, and initial (pre-mining) saturated thickness above the pit
base (h at ri), the radius of influence (ri) can be determined through iteration by setting r equal
to ri. Once ri is determined, h can be calculated for any radial distance from the pit, and
drawdown can be calculated as pre-mining h minus calculated h at r.
Key parameters in the analysis include the alluvial aquifer saturated thickness at the pit wall
prior to dewatering impacts, hydraulic conductivity (Kh), and recharge (W). Pre-mining aquifer
characteristics were discussed previously in Section 2, including an alluvial groundwater
saturated thickness of 15 feet (h at ri) and hydraulic conductivity of 30 feet/day (Kh).
Additionally, it was assumed that the alluvial groundwater saturated thickness is drawn down
to 2 feet at the pit wall (hp at rp) during dewatering.
(blue earth Page 7 of 12
SOLUTIONS