Abstract
Abstract. Throughout the Mediterranean, salinization threatens water quality, especially in coastal areas. This salinization is the result of concomitant processes related to both seawater intrusion and water–rock interaction, which in some cases are virtually indistinguishable. In the Nurra region of northwestern Sardinia, recent salinization related to marine water intrusion has been caused by aquifer exploitation. However, the geology of this region records a long history from the Palaeozoic to the Quaternary, and is structurally complex and comprises a wide variety of lithologies, including Triassic evaporites. Determining the origin of the saline component of the Jurassic and Triassic aquifers in the Nurra region may provide a useful and more general model for salinization processes in the Mediterranean area, where the occurrence of evaporitic rocks in coastal aquifers is a common feature. In addition, due to intensive human activity and recent climatic change, the Nurra has become vulnerable to desertification and, in common with other Mediterranean islands, surface water resources periodically suffer from severe shortages. With this in mind, we report new data regarding brackish and surface waters (outcrop and lake samples) of the Na-Cl type from the Nurra region, including major ions and selected trace elements (B, Br, I, and Sr), in addition to isotopic data including δ18O, δD in water, and δ34S and δ18O in dissolved SO4. To identify the origin of the salinity more precisely, we also analysed the mineralogical and isotopic composition of Triassic evaporites. The brackish waters have Cl contents of up to 2025 mg L−1 , and the ratios between dissolved ions and Cl, with the exception of the Br / Cl ratio, are not those expected on the basis of simple mixing between rainwater and seawater. The δ18O and δD data indicate that most of the waters fall between the regional meteoric water line and the global meteoric water line, supporting the conclusion that they are meteoric in origin. A significant consequence of the meteoric origin of the Na-Cl-type water studied here is that the Br / Cl ratio, extensively used to assess the origin of salinity in fresh water, should be used with care in carbonate aquifers that are near the coast. Overall, δ34S and δ18O levels in dissolved SO4 suggest that water–rock interaction is responsible for the Na-Cl brackish composition of the water hosted by the Jurassic and Triassic aquifers of the Nurra, and this is consistent with the geology and lithological features of the study area. Evaporite dissolution may also explain the high Cl content, as halite was detected within the gypsum deposits. Finally, these Na-Cl brackish waters are undersaturated with respect to the more soluble salts, implying that in a climate evolving toward semi-arid conditions, the salinization process could intensify dramatically in the near future.
Highlights
In the Mediterranean, the demand for good quality water is rapidly increasing, but the process of salinization (e.g. Petalas and Lambrakis, 2006; El Yaouti et al, 2009; Ghiglieri et al, 2012; Sdao et al, 2012) threatens the exploitation of additional water resources such as groundwater
The ratios between dissolved ions and Cl, with the exception of the Br / Cl ratio, are not those expected from the simple mixing of rainwater and seawater
The δ18O and δD data indicate that most of the waters fall between the regional meteoric water line and the global meteoric water line, supporting the conclusion that they are meteoric in origin
Summary
In the Mediterranean, the demand for good quality water is rapidly increasing, but the process of salinization (e.g. Petalas and Lambrakis, 2006; El Yaouti et al, 2009; Ghiglieri et al, 2012; Sdao et al, 2012) threatens the exploitation of additional water resources such as groundwater. Chemical data available for the Nurra aquifers (Ghiglieri et al, 2009) show that these groundwaters are affected by significant chemical variability (e.g. TDS (total dissolved solid) values from 600 to 4000 mg L−1 , Cl concentrations from 3 to 76 mg L−1 , and SO4 concentrations from 0.2 up to 40 mg L−1) This variability indicates that various geochemical processes may affect the composition of the resource. Ghiglieri et al (2009) suggested that the initial chemical composition of source water was conditioned by water–rock interactions, including ion exchange with hydrothermal minerals and clays, and incongruent dissolution of dolomite These findings, in addition to the importance of the water resource and its role as a strategic reserve in a climate evolving towards semi-arid conditions, indicate the need for a detailed study of the processes that determine the hydrogeochemistry of the Nurra groundwater and its quality, based on identification of the sources of the ions responsible for the high salinity. To determine the origin of the salinity more precisely, we analysed the mineralogical and isotopic compositions (δ34S and δ18O) of Nurra Triassic evaporites
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