Assessment of the groundwater quality status of the phreatic aquifer in Grombalia (Tunisia) for irrigation via geostatistical modeling

This study is the first attempt to characterize the suitability of the Grombalia phreatic aquifer for irrigation use by combining traditional classification diagrams with different geostatistical interpolation methods. The Wilcox and USSL diagrams suggest that only 20% of the groundwater samples satisfy the recommended sodium standards, whereas 50% of the groundwater samples are classified as unsuitable for irrigation because of salinity constraints. The spatial interpolation of four parameters, particularly electrical conductivity (EC), chemical oxygen demand (COD), chloride, and piezometric levels, is performed via ordinary kriging (OK). The results reveal the presence of high values of EC in the central and northern parts of the aquifer, and that chloride concentrations exceed the recommended irrigation standards in a large portion of the aquifer, mainly due to the presence of excessive industrial discharge. High levels of COD are observed in the northwestern part, probably due to anthropogenic sources. Indicator kriging (IK) revealed that a large portion of the northern part is likely to experience salinity and toxicity adversities, whereas the majority of the southern part is considered to be suitable for irrigation use. In particular, 38% of the study area is expected to be suitable for irrigation use concerning salinity (i.e., electrical conductivity and total dissolved solids (TDS)), whereas an even lower portion (16%) is expected with respect to chloride.