Fractionary study of Hg in the water column of El Entredicho open-pit (Almaden, Spain)

El Entredicho is a cinnabar open-pit mine located in the Almad\ue9n mining district (Ciudad Real, Spain). It is the second most important mine within the district, since it has produced 290,000 flasks of mercury (Hg) during its short period of activity (1978-1997). Mining operations created a large hole reaching a maximum of 92 m depth, which was purposely flooded with waters from the Valdeazogues River in 2004. Today, this open-pit lake is only fed by runoff waters and as a consequence, the water column has remained isolated during the last ten years, thus creating an unique environmental laboratory for studying the effects of interactions between freshwaters and permanently flooded cinnabar ore rocks. This research focuses on a preliminary characterization of Hg abundance and partitioning in the water column of El Entredicho open-pit to depict possible effects of physical and biogeochemical factors on element distribution and speciation. The main physico-chemical parameters of freshwaters were recorded using an Ysi CTD 6920V2 in five selected points. Water samples were collected at the deepest point by means of a Niskin bottle and were analysed using ICP-MS technique. In the water column, partitioning of Hg varies with depth, being most important the dissolved fraction (DHg) (0.45\ub5m) represents the main fraction. PHg concentration increases with depth (0.059-0.137 \ub5g l-1), peaking at 55 m depth (0.174 \ub5g l-1). Vertical variability for DHg and PHg appears to be similar down to 55 m depth. Below 55 m, DHg quickly decreases to concentrations lower than those detected at the surface (0.175 vs 0.125 \ub5g l-1) suggesting the existence of different mechanism governing Hg partitioning at this stage of the water column. During field works, physicochemical parameters showed a sharp chemocline starting at 46 m depth. In the chemocline, temperature (T\uba) quickly increases from 8.57 to 10.17 \ubaC as conductivity (Cond.) from 854 to 2684 \ub5S cm-1 whereas dissolved oxygen (DO) decreases from 9.81 to 9.20 mg l-1 in only one meter. Then, T\uba, Cond. and pH remain constant, whereas DO and redox potential decrease with depth (0.61 mg l-1 and 56.5 mV, respectively). The bottom layer seems to behave like a \u201cbiochemical reactor\u201d playing an important role in the Hg partitioning between water and cinnabar rocks. Future research should be addressed to this compartment