A week of automated continuous measurements of temperature, pH and specific conductance in a mining-polluted river in Sardinia (Italy): The effect of rainfall as a case of serendipity

This study presents a week of high-frequency automated measurements of temperature, pH and specific conductance collected every 15 min at two locations along the Rio Irvi (SW Sardinia, Italy), a river heavily affected by past mining activity. The original objective of the monitoring campaign was to investigate short-term hydrogeochemical variability under relatively stable weather conditions. However, several rainfall events occurred during the monitoring period, producing an unexpected opportunity to observe the hydrogeochemical response of the system to dilution processes. The results show disturbances in diel cycles of temperature and pH, and significant decreases in specific conductance during rainfall events due to dilution with rainwater. Geochemical modelling performed using PHREEQC indicates that dilution can shift the saturation state of sulphate green rust (an interesting, rare solid phase that precipitates in a stretch of the Rio Irvi) from oversaturated to undersaturated conditions, potentially promoting dissolution of previously precipitated phases. These observations highlight how short-term hydrological perturbations can influence hydrogeochemical processes in mining-impacted rivers.