Hydrogeochemical overview and natural arsenic occurrence in groundwater from alpine springs (upper Valtellina, Northern Italy)

•We studied the hydrogeochemical complexity of recent alpine aquifers.•We characterize the natural arsenic presence in alpine aquifers during 13 years.•We studied speciation importance of arsenic for characterization of alpine aquifers.•Arsenic (V) and (III) have been found in cold alpine springs and thermal springs.•The natural arsenic concentration varies seasonally and depends on pH and redox conditions.

SummaryHigh arsenic (As) concentrations (up to 230 μg/L) have been historically observed (since 1999) in the upper Valtellina valley groundwater (UVV, central Italian Alps), and measured in samples collected during four campaigns of one full hydrological year (summer 2012-summer 2013). During these campaigns, water has been collected from both cold springs and thermal springs. The hydrogeochemistry of aquifers and superficial waters through the hydrologic year, and the long-term regional As distribution and time variability were analyzed.Although the studied springs belong to different catchments with different hydrochemical and lithological conditions, they present some typical characteristics: (1) the water types are dominated by Ca-Mg and SO4-HCO3 main ions, with seasonal variations for the second end members; (2) the Cl concentration is always very low, and poorly correlated with other ions; (3) the circulation time obtained from isotopic data ranges between 5 and 10 years for thermal springs and it is lower than 2 years for cold springs; (4) dominant oxidizing conditions have been observed for most of the cold and for the thermal springs; (5) anthropogenic contamination is absent, while natural contamination of arsenic affects most of the springs, with a natural background level for the entire UVV of 33 μg/L; (6) both As (V) and As (III) are present in all the springs analyzed, with a marked prevalence of As (V) among the cold ones. These conditions suggest that the latter belong to recent hydrochemical immature aquifers, where the presence of arsenic is mostly related to alkali desorption and sulfide oxidation, while the thermal springs derive from the rapid uprising of deep-circulation water, with a high concentration of geothermal arsenic.