Assessment of seawater intrusion and nitrate contamination on the groundwater quality in the Korba coastal plain of Cap-Bon (North-east of Tunisia)

Abstract

In recent years, seawater intrusion and nitrate contamination of groundwater have become a growing concern for people in rural areas in Tunisia where groundwater is always used as drinking water. The coastal plain of Korba (north-east of Tunisia) is a typical area where the contamination of the aquifer in the form of saltwater intrusion and high nitrate concentrations is very developed and represents the major consequence of human activities.The objective of this study is to evaluate groundwater resource level, to determine groundwater quality and to assess the risk of NO3- pollution in groundwater using hydrogeochemical tools. Groundwater were sampled and analyzed for physic-chemical parameters: Ca2+, Mg2+, Na+, K+, Cl−, SO42-, HCO3-, NO3-, Total Dissolved Solid and of the physical parameters (pH, electrical conductivity and the temperature). The interpretation of the analytical results is shown numerically and graphically through the ionic deviations, Piper Diagram, seawater fractions and binary diagrams. Moreover, electrical conductivity investigations have been used to identify the location of the major intrusion plumes in this coastal area and to obtain new information on the spatial scales and dynamics of the fresh water–seawater interface. Those processes can be used as indicators of seawater intrusion progression.First, the hydrogeochemical investigation of this aquifer reveals the major sources of contamination, represented by seawater intrusion. Thus, the intensive extraction of groundwater from aquifer reduces freshwater outflow to the sea, creates several drawdown cones and lowering of the water table to as much as 12m below mean sea level in the center part of the study area especially between Diarr El Hojjej and Tafelloun villages, causing seawater migration inland and rising toward the wells.Moreover, the results of this study revealed the presence of direct cation exchange linked to seawater intrusion and dissolution processes associated with cations exchange.Second, a common contaminant identified in groundwater is dissolved nitrogen in the form of nitrate. The average nitrate concentration of the aquifer is about 30.44mg/l, but contents as great as about 50mg/l occur in the central region where seawater has been identified. Nitrate survey reveals that nitrate concentration above the drinking water standard (50mg/l) covered an area of 122.64km2, which accounts for 28% of the whole area. Irrigation with the nitrogen fertilizers, domestic sewage, industrial wastewater and movement of contaminants in areas of high hydraulic gradients within the drawdown cones probably are responsible for localized peaks of the nitrate concentration.It is suggested that risk assessment of nitrate pollution is useful for a better management of groundwater resources, preventing soil salinisation and minimizing nitrate pollution in groundwater.