Tracing groundwater salinization processes in coastal aquifers: a hydrogeochemical and isotopic approach in the Na-Cl brackish waters of northwestern Sardinia, Italy

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 δ 18 O, δD in water, and δ 34 S and δ 18 O in dissolved SO 4 . 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 δ 18 O 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, δ 34 S and δ 18 O levels in dissolved SO 4 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.

Effect of salinity on Echinochloa crus-galli germination as affected by herbicide resistance

“…In Europe, 26 countries (Maggio et al, 2011), in particular Spain, Portugal, Italy, Greece (Ghiglieri et al, 2012) and France (Puard et al, 1999) are interested by salinization phenomena (Maggio et al, 2011). In the coastal areas, salinization of aquifers is usually caused by saltwater intrusion (Mongelli et al, 2013) as a result of groundwater overexploitation (Balia and Viezzoli, 2015).…”

Residence times of groundwater and nitrate transport in coastal aquifer systems: Daweijia area, northeastern China

“…The deterioration of coastal aquifers due to salinization may endanger future water exploitation . There are a number of mechanisms that have the potential to lead to salinity in coastal regions: (i) agricultural return flows from local groundwater extraction (Bouchaou et al, 2008;Cruz-Fuentes et al, 2014); (ii) a high degree of evaporation in residual saline water (McCaffrey et al, 1987;Han et al, 2014); (iii) recharge/leakage of imported salt water from the sea for fishing industries (Chang and Li, 2011); (iv) seawater intrusion/tidal flat/sea level rising (Werner et al, 2013); (v) saltwater upconing from underlying aquifers (Barlow, 2003;Szynkiewicz et al, 2008) or downward movement of shallow saline groundwater from upper aquifers under extensive pumping (Vengosh and Ben-Zvi, 1994;Guo et al, 1995;Cary et al, 2015); (vi) mixing modern recharged water with the palaeo-seawater (brines) (Yechieli et al, 1992;Han et al, 2011Han et al, , 2012; (vii) sewage effluents (Vengosh et al, 1998);and (viii) water-rock interaction (Jones et al, 1999;Vengosh et al, 2007;de Montety et al, 2008;Mongelli et al, 2013;Merchán et al, 2015). For a reliable water management/protection plan and to prevent further deterioration, it is essential to study the major geochemical processes that modify hydrochemical compositions of waters salinized by seawater intrusion and to elucidate the salinity sources and flow paths that exist in the coastal aquifer.…”

Confronting the vicinity of the surface water and sea shore in a shallow glaciogenic aquifer in southern Finland

“…However, there is still no clear understanding of the interactions between surface water and groundwater or the impacts of brackish water intrusion on shallow, low-lying coastal aquifers. Major ion chemistry, as well as the stable isotopic composition of oxygen-18 (δ 18 O) and hydrogen (δ 2 H or deuterium), could be used to assess these interactions, as reported earlier by Allen (2004), Harbison (2007), Kortelainen (2007) and Mongelli et al (2013), among others.…”