Investigation of the probable wastewater infiltration and its impact on groundwater quality by electrical resistivity tomography and hydrochemistry: a case study of the Pleistocene aquifer at El Sadat city, Egypt

Abstract

Electrical resistivity tomography (ERT) and hydrochemical investigations were conducted at the wastewater ponds (oxidation ponds) in El Sadat industrial city, Egypt, to study the possibility of wastewater percolation to the Pleistocene aquifer and its effect on the groundwater chemistry. Pleistocene aquifer is the main groundwater reservoir in this area, where El Sadat city and its vicinities use it as a main source for water supplies required for drinking, agricultural and industrial activities. In this study, seven ERT profiles were measured around the wastewater ponds and ten water samples were collected from the ponds and the nearby groundwater wells. The water samples were analyzed for major cations (Ca²⁺, Na⁺, K⁺, and Mg²⁺); major anions (Cl⁻, ${\text{CO}}_{ 3}^{ - 2}$, ${\text{HCO}}_{ 3}^{ - }$, ${\text{SO}}_{ 4}^{ - 2}$), nutrients (${\text{NO}}_{ 2}^{ - 1}$, ${\text{NO}}_{ 3}^{ - 1}$, and ${\text{PO}}_{ 4}^{ - 3}$) and heavy elements (Cd, V, Cr, Zn, Ni, Cu, Fe, Mn, and Pb). Also, the physical parameters (pH, alkalinity, EC, and TDS) of the water samples were measured. The ERT sections exhibited low resistivity values towards the wastewater ponds and high values in opposite sides. Besides, the water table was detected at shallow depths towards the oxidation ponds. These indications could verify the possibility of wastewater infiltration to the groundwater aquifer around the oxidation ponds. Comparison of the physicochemical parameters of the wastewater samples with those of the groundwater samples indicates that the ionic levels are randomly varying and no specific trend could be obtained. Also, the wastewater samples show some ionic levels lower than those detected in the nearby groundwater samples. Besides, the nitrate level is higher in the water samples collected from the cultivated land than the wastewater samples. From this study, it could be concluded that the wastewater infiltrated from the oxidation ponds is NOT the main controller of the groundwater chemistry of the Pleistocene aquifer in this area. Rather, the variable ionic concentrations could be attributed to local, natural and anthropogenic processes.