TRACE METAL GEOCHEMISTRY IN THE CENTRAL NILE DELTA, EGYPT

The central Nile Delta, between the Rosetta and Damietta branches of the Nile river, is a rich agricultural area that has supported a large population for several millennia. Today, the Nile provides irrigation water through the ancient system of canals, while public water supplies are chiefly (70%) groundwater from the Nile Delta aquifer. The water is usually of good quality, but in many areas dissolved manganese (Mn) exceeds drinking water standards. A joint study by The University of Texas and Tanta University analyzed regional hydrogeology and sampled groundwater from 48 municipal water wells, as well as from the Nile and from shallow village wells. Continuous core was also collected from 6 wells and analyzed by XRD and sequential extraction of metals.

The groundwaters are circum-neutral pH with low TDS and are generally reducing, with dissolved oxygen less than 0.25 mg/L. Nitrogen species are dominated by ammonia (0-2 mg/L) while nitrate is almost always less than 0.2 mg/L. Dissolved methane reaches 1.2 mg/L (0.05 atm) with no detectable ethane, suggesting microbial methanogenesis. Initial dissolved Mn concentrations (immediately after well completion) are typically less than 0.1 mg/L, but after months to years of pumping, Mn may range from 0.2 to 1.5 mg/L, substantially exceeding WHO standards. This slow increase of Mn over time in a redox system already methanogenic is unusual; MnO₂ is typically reduced at much higher E_H conditions, and there should be little reduceable Mn remaining in this aquifer. This is supported by the results of the sequential extraction of sediments, where scant reduceable Mn or Fe is found in the aquifer sediments at the well screen depth. The extraction data instead suggest the Mn is derived from carbonates in the shallow flood deposits, now permanently saturated in the nearly constant-level Nile system. These data suggest that well installation and prolonged pumping mobilize Mn from the shallow system, eventually contaminating that well, but with little effect on the regional aquifer.