MANAGING SALTWATER INTRUSION IN A POORLY-CONSTRAINED ARID AQUIFER, UTILIZING ISOTOPE AND GEOCHEMISTRY, NORTH WESTERN COAST, EGYPT

Ras El Hekma is located on the northwestern coast of Egypt as a triangular headland inside the Mediterranean. The scarcity of rainfall and high pumping rates degraded groundwater quality in the main water-supply wells. Saltwater intrusion is the major concern in coastal aquifers around the world explicitly in arid regions, where recharge is limited and groundwater withdrawals is the main source for potable water. In such arid areas, groundwater is being pumped from the aquifer that is hydraulically connected with the sea, causing lateral movement of seawater toward land and upconing of the underlying saline water toward the pumping wells. In Ras El Hekma area, the groundwater occurs in the fractured limestone aquifer of Miocene age and a friable O-Olitic limestone aquifer of Pleistocene age, where structure settings, dissected elongated ridges as well as sequent local depressions act as barriers and favorable recharge sites for groundwater entrapments. Water chemistry, stable isotopes (δ ¹⁸O and δ²H), and multi isotope tracers were utilized to estimate the percent of seawater mixing in groundwater and the source of recharge for different aquifers. An analytical model for seawater intrusion was used to predict the extent of seawater intrusion along the coast, to determine the optimal pumping rates and to evaluate the recharge source for the coastal unconfined Pleistocene aquifer. Steady flow in an isotropic and homogeneous medium with a sharp interface between the freshwater and seawater wedge is assumed. The seawater-freshwater interface was determined through the explicit equations assuming fuzzy and globally distributions of total pumping and recharge rates in the entire region. The conservative mass balance equation and the historical records of water level, groundwater salinity and conservative ions were used for model calibration. The models estimate average annual groundwater recharge and seawater intrusion along the coast for different pumping scenarios.