Depth influence on the distribution of chemical elements and saturation index of mineral phases in twins maar lakes: The case of the Monticchio lakes (southern Italy)

- A systematic geochemical study of the twins Monticchio maar lakes was performed.
- The two lakes are separated by less than 200 m and they exhibit different behaviour.
- The “Lago Grande” water are governed by groundwater interactions with host rocks.
- The “Lago Piccolo” hypolimnion water are anoxic with a chemical stratification.
- The cycling of redox-sensitive elements promotes mineral precipitation with depth.

In this paper we provide a systematic geochemical study of the Monticchio maar lakes, on the Mt. Vulture volcano, southern Italy, to understand the processes affecting the distribution of chemical elements. A high-resolution conductivity-temperature-depth profile was obtained, and water samples were collected at various depths for analyses of major and trace elements. Although the two lakes are separated by less than 200 m, they exhibit different behaviour. The Ca-Na-HCO3 composition of “Lago Grande” water suggests that low-temperature fluids are leached from the host volcanic rocks. Na-Ca-HCO3 in near-surface “Lago Piccolo” water is derived from the dissolution of local volcanic rocks, while the deepest water samples are bicarbonate alkaline-earth in composition and generally show an enrichment in solutes with respect to the epilimnion layer (except for SO42 − content). The hypolimnion water is principally governed by both the input of saline groundwater from the lake bottom and authigenic processes within bottom sediments. In addition, the occurrence of anoxic conditions and microbial activity is responsible for the transformation of sulphates to hydrogen sulphide, and for the precipitation of Fe sulphide phases. Both processes cause a change in the chemical composition of saline groundwater as it moves upwards. Overall, the water chemistry of the Monticchio lakes is principally affected by: i) the input of CO2-rich volcanic gas; ii) evaporative processes that deplete near-surface water in some elements; and iii) the occurrence of chemical, physical and biological conditions causing the transformation of molecular complexes and consequently promoting mineral precipitation.