Source and migration of dissolved manganese in the Central Nile Delta Aquifer, Egypt

•Nile Delta aquifers can be contaminated with manganese, the source of which was previously unknown.•Pumping records, mineralogy, and sequential chemical extraction tested possible sources.•Carbonate minerals in the surficial Bilqas Formation are identified as the Mn source.•Leakage along well casings and gravel packs transports Mn to the underlying aquifer.•Modified pumping schedules and well construction procedures can ameliorate the problem.

Dissolved metals in waters in shallow deltaic sediments are one of the world’s major health problems, and a prime example is arsenic contamination in Bangladesh. The Central Nile Delta Aquifer, a drinking water source for more than 6 million people, can have high concentrations of dissolved manganese (Mn). Standard hydrochemical analyses coupled with sequential chemical extraction is used to identify the source of the Mn and to identify the probable cause of the contamination. Fifty-nine municipal supply wells were sampled and the results compared with published data for groundwaters and surface waters. Drill cuttings from 4 wells were collected and analyzed by sequential chemical extraction to test the hypothesized Mn-generating processes. The data from this research show that the Mn source is not deep saline water, microbial reduction of Mn oxides at the production depth, or leakage from irrigation drainage ditches. Instead, Mn associated with carbonate minerals in the surficial confining layer and transported down along the disturbed well annulus of the municipal supply wells is the likely source. This analysis provides a basis for future hydrogeological and contaminant transport modeling as well as remediation–modification of well completion practices and pumping schedules to mitigate the problem.