Quantifying the Groundwater Mixing Processes under the Land-Use Change and Anthropogenic Impact: A Case Study of the Quaternary Groundwater System underlying the New Reclaimed Lands, the Eastern Fringe of the Nile Delta, Egypt

Abstract:

The stable (δ²H, δ¹⁸O and δ¹³C) and radiocarbon (¹⁴C) isotopic compositions of water and hydrochemical information were analyzed and used to quantify the contribution of different sources, i.e., groundwater in the original Quaternary and the Miocene aquifers, surface water in the Ismailia canal and wastewater, to the Quaternary aquifer system. ¹⁴C activities and isotope data suggest that about 52% of groundwater in the Quaternary aquifer is derived from the past rainfall, i.e., presumably 5000-7800 years B.P. Northward (EC <1000 µS/cm, high HCO₃^- concentration, δ¹³C-depleted, δ¹⁸O/δ²H-enriched, and ¹⁴C ≥ 70 pMC) and southward (EC >6000 µS/cm, low HCO₃^- concentration, δ¹³C-enriched, δ¹⁸O/δ²H-depleted, and ¹⁴C ≤ 42 pMC) spatial changes in chemical and isotopic compositions of groundwater in the Quaternary aquifer are attributed to the contributions of the Ismailia canal in the north and the groundwater of the Miocene aquifer in the south, respectively. Temporal changes of isotopic composition of the Nile water in response to the construction of the Aswan High Dam are also detected and the information is used to evaluate the groundwater recharge processes from the Ismailia canal. Current contribution from the Ismailia canal (25%) is considered to be greatly enhanced through surface water diversion and related irrigation practices, i.e., freshwater ponds. Contribution of the groundwater from the Miocene aquifer was also detected locally, and it was thought to be related to the excessive pumping. Increase in nitrate concentrations, change in the stable isotopic composition of groundwater from wells adjacent to wastewater ponds, along with the information obtained from the analysis of city water balance and recent geophysical data show that local recharge from wastewater ponds (4%) occurs to the Quaternary aquifer system. The results are expected to be helpful for formulating appropriate protection and sustainable water management strategies.