Opencast mining of sulfide ore deposits may lead to the formation of anthropogenic acidic lakes with highly polluted waters. In these systems, it is crucial to understand the hydrological connections between surface and groundwater and their contribution to the pollutant load delivered to the downgradient streams. This study characterizes the interactions between surface and groundwater in an acidic pit lake using different geochemical tracers (i.e., REE and other trace metals). The San Telmo pit lake, located in one of the most pollutant sources of the Iberian Pyrite Belt (IPB), can be considered as a flow-through pit lake except during dry periods, when it behaves as a terminal lake due to lower inputs by surface waters and higher outputs by evaporation. Results based on geochemical tracers indicate that the main inputs to the pit lake come from surface waters, with minor groundwater inputs rich in As, Cr, Cu, Fe and Pb. The contaminant load released from the mining area is very high (e.g., median values of 520 kg/day of Fe and 38 kg/day of Zn), causing the degradation of the fluvial network downstream. Most of released pollutants come from waste dumps located at the W of the mining zone (~50-70% of Al, Cd, Mg, Mn, Ni, SO