Hydrological investigation of a multi-stratified pit lake using radioactive and stable isotopes combined with hydrometric monitoring

Abstract

The internal configuration and hydrological dynamics of meromictic pit lakes is often complex and needs to be studied by different tools including stable and radiogenic isotopes. This study combines a multi-isotopic approach (3Hw, δ2Hw, δ18Ow, δ34SSO4) with meteorological, hydrological and hydrochemical monitoring to deduce the flooding history and hydrological dynamics of a meromictic and deeply stratified pit lake (Cueva de la Mora mine, SW Spain). The mine system is complex and includes horizontal galleries, shafts and large rooms physically connected to the mine pit. Specific conductance and temperature profiles obtained in the pit lake draw a physical structure with four monimolimnetic sub-layers of increasing density with depth. This characteristic stratification with m-scale layers separated by sharp transitional zones is rather unusual in other pit lakes and in most natural lakes. Tritium abundance in the different layers indicates that the deep lake water entered the pit basin between 1971 and 1972 which is coincident with the dates of mine closure. The oxygen and deuterium isotope composition of the different layers describes a marked and stable stratification, with an increasing evaporative influence towards the lake surface and a minimal influence of groundwater flow on the structure and composition of the monimolimnion. This study reveals that the initial stages of flooding (via influx of metal- and sulfate-loaded mine drainage from underlying galleries at different depths) may be essential to imprint a layered physical structure to pit lakes which would be very difficult to explain merely by physical processes. After reaching its present water level and morphology, the monimolimnion of this pit lake seems to have remained essentially isolated and chemically unmodified during decades.