Effects of a Reservoir Water on the GW Quality in a Coastal Aquifer of Semi-arid Region, North-east of Tunisia

Abstract:

This study investigated effects of a reservoir water in a salinized shallow aquifer based on spatial distribution of geochemical properties in groundwater (GW). In many coastal shallow aquifers of arid and semi-arid regions, groundwater table (GWT) depression and salinization have occurred due to GW overexploitation. In Korba aquifer, north-east of Tunisia, after a dam reservoir has been constructed in order to assure a water resource for irrigation, improvement of GW level and quality have been observed in the downstream area of the dam (area-A), while the GW in the other area (area-B) still has high salinity. This study, therefore, aimed to investigate the effects of the reservoir water on the GW quality. In June 2013, water quality survey and sampling were carried out at 60 wells (GW), a dam reservoir, river and the sea. Major ions, boron, bromide, and oxygen-18 and deuterium in collected samples were analyzed. From the results, in the area-B, the GWT was lower than the sea level and the high salinity were observed. The Br^- concentration of the GW was correlated with the Cl^- concentration, and the values of B/Cl^- and Br^-/Cl^- of the GW were similar to the seawater. Since the GWT depression allowed the seawater to intrude into the aquifer, the GW salinization occurred in this area. On the other hand, in the area-A, GWT was higher than the seawater level, and the Na⁺ and Cl^- concentrations were lower than the area-B. Especially, in the irrigated areas by using the reservoir water, the isotopic values, B/Cl^- and Br^-/Cl^- of the GW were relatively higher than the others. The reservoir water has high isotopic values due to evaporation effect, and the B/Cl^- and Br^-/Cl^- values become higher due to organic matters in sediment of the reservoir or soil in the filtration process. Thus, in addition to the direct infiltration from the reservoir into the aquifer, irrigation using a reservoir water probably has a positive impact on the GW quality in this area.