Understanding groundwater chemistry using multivariate statistics techniques to the study of contamination in the Korba unconfined aquifer system of Cap-Bon (North-east of Tunisia)
The Korba aquifer of Cap-Bon peninsula (North-east of Tunisia), which extends over 40 km and occupies an area of 438 km2, is of great economic importance. Its location in an often narrow plain with intense human activity (agriculture, industry, tourism, etc.) makes it particularly vulnerable from both a qualitative and quantitative alteration.The high salinization in some parts of the study area has been attributed to seawater intrusion process, because of the high and increasing contents of chloride ions and electric conductivity value distribution. However, recent studies of this aquifer have shown that the chemical characteristics of groundwaters are the result of different components: intruding seawater, direct cation exchange linked to seawater intrusion, dissolution processes associated with cations exchange and solute recycling through irrigation return flow. In this paper, we describe the hydrological processes in the Korba coastal plain using hydrochemical data, correlation matrices and factor analysis to provide evidence of salinization processes. Based on historical hydraulic heads data (1996-2005), salinization of fresh groundwater is highly associated with groundwater withdrawal. Thus, the piezometric survey confirmed the inversion of the groundwater flow in Diarr El Hojjaj and Tafelloun villages where a piezometric depression of 12 m was observed at 3000 m from the shoreline which accelerate seawater intrusion by reversing the hydraulic gradients. Based on the hydrochemistry, the groundwater was classified into three types: Ca-Cl, Na-Cl and SO4-mixed types. The groundwaters showed paths of hydrochemical evolution, from Ca-SO4 type to Na-Cl type; or from Ca- SO4 type directly to Na-Cl type. Geochemical data reveal frequent participation of seawater in the coastal and central areas water samples, showing a very high salinity waters which are not suitable for most domestic and irrigation purposes. Also, we conclude that salinization has its origin in the dissolution of gypsum, dolomite and halite, as well as contamination by nitrate caused mainly by extensive irrigation activity. The application of Multivariate Statistics Techniques based on Principal component Analysis and Hierarchical Cluster Analysis has lead to the corroboration of the hypotheses developed from the previous hydrochemical study. Two factors were found that explained major hydrochemical processes in the aquifer. These factors reveal the existence of an intensive intrusion of seawater and mechanisms of nitrate contamination of groundwater.