Influence of regional hydrogeological systems at a local scale: Analyzing the coupled effects of hydrochemistry and biological activity in a Fe and CO₂ rich spring

A multidisciplinary approach was used in this study to determine the origin of a Fe and CO₂ rich spring, an extreme freshwater system and to evaluate the coupled effects of hydrochemistry and biological activity at a local scale. An electrical resistivity tomography survey was conducted to delineate the geological context in which water emerges from a Plio-Quaternary supracrustal fault zone, and a bulk resistivity decrease was detected when CO₂ rich groundwater occurred. Nine water samples, together with eight biofilm samples, and three sediment samples were taken along the spring canal for their analysis. Major ions, nutrients, and metals were analysed in water samples. Sediment analyses determined the main solid phases precipitated (mainly as CaCO₃ and Fe(OH)₃(a)). Biofilm analyses permitted to obtain biovolume per cell measures, total biovolume values, diatom density, chlorophyll a concentrations, and the Margalef Index values. Inverse modeling and batch reaction models were used to determine the physicochemical processes affecting the spring water, obtaining the total amount of CaCO₃/L formed; the Fe and Mn compounds, which mainly precipitated as Fe(OH)₃(a) and Mn(OH)₂; as well as the total CO₂ released to the atmosphere. Analyzing these results together with the patterns of variation of hydrochemical and biological parameters, different interactions were observed: a) the effects of Fe inhibition in travertine formation, even though when the highest CO₂ release was occurring; b) the fate and effects of chemicals limiting and/or inhibiting algal growth (mainly Fe, As and phosphate); c) the lack of coincidence between algal growth and tufa limestone precipitation; d) the relationship between some divalent metals (Mn and Co) and biotic activity. - Highlights: • Electrical resistivity tomography detected CO₂ rich groundwater. • High Fe concentrations inhibits both algal growth and CaCO₃ precipitation. • Tufa precipitation doesn't coincide with the highest growth of algae. • Physicochemical and biological processes may improve groundwater quality from regional systems.