Hydrogeological Characterization of Leakage Water Flows in the Subsoil of Ouizert Dam, Algeria

Water leakage through dam foundations and abutments compromises both dam stability and useful storage capacity. The Ouizert Dam, located in north-western Algeria (Mascara Province), is a 60 m high earth-fill structure built on a heterogeneous geological basement of marls, sandstones, conglomerates and limestones, with a documented mean leakage of 10.42 hm³ yr-¹ through its left bank. The present study examined the origin and pathways of this leakage through a hydrogeological approach based on the correlation between the reservoir water level and that recorded in 14 piezometers installed on the left bank between 2009 and 2026. The analysis revealed a marked spatial differentiation of the hydraulic behaviour across the left bank: seven piezometers (P19, P20, P5, P12, P6, P10, P2) showed a very strong correlation with the reservoir level (R² ≥ 0.913) and regression slopes close to or slightly exceeding unity, indicating direct hydraulic connection through the fractured rock mass; four piezometers (P14, P21, P3, P13) showed a good correlation (R² = 0.653-0.797); and three (P16, P1, P7) showed a weak or no correlation (R² ≤ 0.472), suggesting hydraulically disconnected zones. The existence of two distinct flow regimes was confirmed statistically by segmented regression and a Chow test on monthly mean discharge (F = 204.1, p < 0.001), with the optimal breakpoint at 437.9 m NGA: a moderate Darcian regime below this level (slope = 25.9 L s-¹ m-¹) and a steeper enhanced-seepage regime above it (slope = 151.9 L s-¹ m-¹, about six times larger). The verified maximum leakage discharge of 1,967 L s-¹, recorded during the high-reservoir period of mid-2013, exceeds the peaks previously reported at the site and is consistent with an ongoing geomechanical evolution of the fractured rock mass. Long-term piezometric monitoring remains the most reliable tool for tracking leakage development and for guiding remedial action.