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 < 0.2 to 7.8 mg L⁻¹ and electrical conductivity from 0.8 to 10 mS cm⁻¹. Predominant cations were Na⁺ and Ca²⁺, predominant anions were either Cl⁻ or HCO₃⁻. The more saline waters showed anyhow a marked Na⁺–Cl⁻ chemical composition. Most waters were found either at near equilibrium with respect to calcite or slightly saturated, but under saturated with respect to gypsum. Isotopic values of δ²H and δ¹⁸O in the water samples indicated a meteoric origin. Particular attention was paid to potential contaminants, which should be evaluated when thermal waters are used in spa treatments and balneology. Concentrations of NO₃⁻ and NH₄⁺ above the Italian limits established for drinking water (50 mg L⁻¹ and 0.5 mg L⁻¹, respectively) occurred in one oxygenated groundwater and five reduced groundwater samples, respectively. Fluoride concentrations exceeding the Italian limit of 1.5 mg L⁻¹ were observed in three groundwater samples. The mean value of As was 3.2 µg L⁻¹, with one groundwater exceeding the 10 µg L⁻¹ of the legal value. The groundwater with the highest temperature (42 °C), an artesian well, was characterized by relatively high concentrations of Cl⁻, F⁻, Li, B, Ge, Rb, Mo, Cs, W, Sc and Ga. Overall results allowed to identify the area most suitable for geothermal exploitation. Deep fluids, probably located at a depth > 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.