Characterization of low-enthalpy geothermal resources and evaluation of potential contaminants

Abstract

The use of renewable resources alternative to fossil fuels, thus contributing to the reduction of CO2 emissions, requires the assessment of eventual negative impacts on the environment. This study was devoted to the characterization of low-enthalpy geothermal resources and the potential contamination of geothermal effluents into the aquatic system. Thirty-five groundwater samples were collected in the Campidano (southern Sardinia, Italy), an area showing heat flow anomalies and thermal occurrences. Hydrogeological features inferred by literature were implemented by data acquired at each sampling site. Physical–chemical parameters, major, minor and trace components in groundwater were determined, together with the isotopic composition of the water. Six hydrogeological units with variable permeability were identified. According to geological and hydrogeological modeling, four of the six units appeared hydraulically connected, although not everywhere. The predominant groundwater flow was seen from north-east to south-west. The water temperature was in the range 17–42 °C, pH ranged from 6.7 to 8.6, dissolved oxygen varied from 1 km, would rise up along faults or fractured zones in the granitic–metamorphic Paleozoic basement. Maximum temperatures of 90 °C in the thermal reservoir were estimated by silica and Na–K–Ca geothermometers. The δ18O enrichment shift occurring at high temperature was not observed. Due to high concentrations of some contaminants (e.g. Mo, W, B, F−), geothermal effluents derived from exploitation should be either re-injected or treated before discharge for avoiding the contamination of aquatic systems.