Hydrogeochemical processes controlling water and dissolved gas chemistry at the Accesa sinkhole (southern Tuscany, central Italy)

Submitted: 4 February 2014
Accepted: 16 May 2014
Published: 26 May 2014
Abstract Views: 3987
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The 38.5 m deep Lake Accesa is a sinkhole located in southern Tuscany (Italy) that shows a peculiar water composition, being characterized by relatively high total dissolved solids (TDS) values (2 g L-1) and a Ca(Mg)-SO4 geochemical facies. The presence of significant amounts of extra-atmospheric gases (CO2 and CH4), which increase their concentrations with depth, is also recognized. These chemical features, mimicking those commonly shown by volcanic lakes fed by hydrothermal-magmatic reservoirs, are consistent with those of mineral springs emerging in the study area whose chemistry is produced by the interaction of meteoric-derived waters with Mesozoic carbonates and Triassic evaporites. Although the lake has a pronounced thermocline, water chemistry does not show significant changes along the vertical profile. Lake water balance calculations demonstrate that Lake Accesa has >90% of its water supply from sublacustrine springs whose subterranean pathways are controlled by the local structural assessment that likely determined the sinking event, the resulting funnel-shape being then filled by the Accesa waters. Such a huge water inflow from the lake bottom (~9·106 m3 yr-1) feeds the lake effluent (Bruna River) and promotes the formation of water currents, which are able to prevent the establishment of a vertical density gradient. Consequently, a continuous mixing along the whole vertical water column is established. Changes of the drainage system by the deep-originated waters in the nearby former mining district have strongly affected the outflow rates of the local mineral springs; thus, future intervention associated with the ongoing remediation activities should carefully be evaluated to preserve the peculiar chemical features of Lake Accesa.

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Supporting Agencies

The work was financially supported by the Laboratories of Fluid and Rock Geochemistry (Resp. F. Tassi) and Stable Isotopes (Resp. O. Vaselli).
Franco Tassi, University of Florence
Department of Earth Sciences

How to Cite

Tassi, Franco, Gabriele Bicocchi, Jacopo Cabassi, Francesco Capecchiacci, Orlando Vaselli, Enrico Capezzuoli, and Andrea Brogi. 2014. “Hydrogeochemical Processes Controlling Water and Dissolved Gas Chemistry at the Accesa Sinkhole (southern Tuscany, Central Italy)”. Journal of Limnology 73 (3). https://doi.org/10.4081/jlimnol.2014.961.

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