Hydrological investigation of a multi-stratified pit lake using radioactive and stable isotopes combined with hydrometric monitoring Javier Sánchez-España a,⇑ , Marta Diez Ercilla a , Fernando Pérez Cerdán a , Iñaki Yusta b , Adrian J. Boyce c a Instituto Geológico y Minero de España (IGME), c/Calera 1, 28760 Tres Cantos, Madrid, Spain b Departamento de Mineralogía y Petrología, Universidad del País Vasco, Apdo. 644, E-48080 Bilbao, Spain c Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, Scotland, UK article info Article history: Received 22 March 2013 Received in revised form 17 October 2013 Accepted 1 February 2014 Available online 10 February 2014 This manuscript was handled by Laurent Charlet, Editor-in-Chief, with the assistance of Tamotsu Kozaki, Associate Editor Keywords: Pit lake Acid mine water Stratification Groundwater dynamics Flooding history Stable isotopes summary 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-iso- topic approach ( 3 H w , d 2 H w , d 18 O w , d 34 S SO4 ) with meteorological, hydrological and hydrochemical monitor- ing 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 pro- files 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 coinci- dent with the dates of mine closure. The oxygen and deuterium isotope composition of the different lay- ers 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 mon- imolimnion. 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 phys- ical 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. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Pit lakes are a very special case of artificial lake formed after the abandonment and later flooding of open-pit mines by the influx of different waters (e.g., groundwater, precipitation, surface runoff, river water, acid mine drainage from adjacent mine galleries; Gel- ler et al., 1998). Due to strong density gradients provoked by the elevated sulfate and metal contents of mine water input at depth, and also to their high depth to surface ratio (which normally ex- ceeds those found in natural lakes) circulation currents cannot reach the bottom and these lakes may become meromictic (Ander- son et al., 1985; Doyle and Runnells, 1997; Boehrer and Schultze, 2008). The form of the lake basin is not the only prerequisite for meromixis and the presence and influx of waters of different chemistry and density appears to be essential to form and sustain meromixis (Schultze and Boehrer, 2009). In meromictic lakes, the bottom-most part of the water mass (so-called monimolimnion), does not mix with the upper, seasonally-mixed layer (known as mixolimnion) for at least one hydrological year (Walker and Likens, 1975; Wetzel, 2001; Boehrer and Schultze, 2008). Because of its isolation from the atmosphere, elevated concentrations of reduced species (NH þ 4 , Fe 2+ ) and gases (H 2 S, CH 4 , CO 2 ,N 2 ,H 2 ) may accumu- late in the monimolimnion as a result of diverse biogeochemical processes. This type of lake offers a good opportunity to study the biogeochemical cycling of nutrients and metals across the red- oxcline (e.g., Boehrer and Schultze, 2008; Wendt-Potthoff et al., 2012), often within a precisely defined timescale. One of the highest concentrations of acidic pit lakes resulting from surface metal mining in Europe is presently observed in the Iberian Pyrite Belt (IPB), SW Spain, where more than 80 different mines have been exploited since Pre-Roman times until present-day http://dx.doi.org/10.1016/j.jhydrol.2014.02.003 0022-1694/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Address: Unidad de Mineralogía e Hidrogeoquímica Ambiental (UMHA), Instituto Geológico y Minero de España (IGME), c/Calera 1, 28760 Tres Cantos, Madrid, Spain. Tel.: +34 91 7287218. E-mail address: j.sanchez@igme.es (J. Sánchez-España). Journal of Hydrology 511 (2014) 494–508 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol