Source, transport and fate of nitrate in shallow groundwater in the eastern Niger Delta

Abstract

The eastern Niger Delta region in Nigeria is a hotspot for reactive nitrogen pollution due to extensive animal husbandry, pit latrine usage and agricultural practices. Despite the high level of human activity, the sources and processes affecting nitrogen in groundwater remain understudied. Groundwater nitrate (NO₃⁻) concentrations are highly variable, with some areas recording values well above the safe drinking water threshold of 50 mg/L. This is particularly true near municipal sewage systems. Elevated nitrite (NO₂⁻) and ammonium (NH₄⁺) concentrations were also detected in the study area. Sewage analysis revealed NO₃⁻ concentrations ranging from 1 to 145 mg/L, NO₂⁻ from 0.2 to 2 mg/L, and notably high NH₄⁺ concentrations. A comparison of major ions indicated that 71%, 90%, 87%, and 92% of groundwater samples surpassed reference site levels for calcium (Ca²⁺), sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻), respectively, pointing to sewage as a likely source of contamination. The NO₃⁻/Cl⁻ ratios at several sites suggested that most groundwater NO₃⁻ originates from human waste. Stable isotope analysis of NO₃⁻ showed a general enrichment in ¹⁵N and, in some cases, a depletion in ¹⁸O, indicating that the NO₃⁻ originates from sewage-derived NH₄⁺ nitrification. Although denitrification, a process that reduces NO₃⁻, is present, the high dissolved oxygen (DO) and NO₃⁻ levels in the groundwater suggest that denitrification is insufficient to fully mitigate NO₃⁻ pollution. Consequently, there is a risk of NO₃⁻ leaching from shallow aquifers into the Niger Delta’s surface waters and ultimately into the coastal ocean.