Layered Radial Aquifer Model for Open-Pit Mine Closure with Managed Aquifer Recharge.

Abstract

Open-pit mine dewatering can delay post-closure pit-lake recovery and is increasingly managed using managed aquifer recharge (MAR). We develop a radially symmetric two-aquifer model that couples an unconfined aquifer, a leaky aquitard, and an underlying confined aquifer to a pit water-balance equation, extending Cook et al. (2022) to layered systems. MAR is represented by reinjecting prescribed fractions of the time-varying pumping rate at a ring radius r₁, with flexible allocation between unconfined and confined injection. Nondimensionalization yields transferable type curves and a design workflow controlled by K_u/S_y, Kc/Sc, Λ/S_c, r₁/r₀, and the far-field head ratio α. We evaluate 15 hydrogeologic scenarios (H0-H14) under three MAR allocations (no MAR, MAR_u80, MAR_split). Across scenarios, reinjecting 80% of pumped water to the unconfined aquifer (MAR_u80) reduces the time for the pit water level to return within 5 m of the pre-mining unconfined head from 21-48 year (no MAR) to 1.8-28 year, with a median recovery-time ratio of 0.17, while increasing mean pumping by a median factor of 1.31. Splitting the same reinjection fraction between unconfined and confined aquifers can further shorten recovery when vertical coupling is moderate to high (Λ/S_c ≳1), but increases mounding and recycling. Layering matters: for the same MAR policy, the layered base case predicts recovery times a factor of ~2-3 shorter than the homogeneous limit. The results provide screening guidance on when confined-aquifer injection can accelerate recovery without unacceptable unconfined mounding.