Determining the source of nitrate pollution in the Niger discontinuous aquifers using the natural [formula omitted] ratios

Abstract

In the semi-arid Niamey area (Niger), more than 10% of the deep wells exploiting the fracture network of the Precambrian aquifer are contaminated by nitrates, with concentrations as high as 10 meq l −1. In order to identify the source(s) of this pollution, nitrate and 15N contents in the polluted wells were monitored over a 20-month period. Potential sources of nitrate contamination were also analyzed for their 15N content. The isotopic compositions of nitrate in polluted waters were > + 12‰ and in rare cases exceeded +17‰. Latrines (∼ + 15‰) may be the major nitrate source for wells showing δ 15N values above +15‰. Below this value, waters may be polluted by a combination of nitrates from both latrine and soil sources (∼ + 10‰). In some cases, the soil may account for up to 85% of the groundwater nitrate load. This mode of groundwater pollution is thought to be a consequence of deforestation. Despite their reputation as polluting agents, fertilizers ( +0.5 < δ 15 N < + 3.6‰ ) which are used in rice paddies close to the contaminated areas, do not appear to be a significant source of nitrate contamination. Denitrification is probably not a significant process in the study area. Results suggest that nitrate contamination of the aquifer is a consequence of unregulated urbanization (home-made latrines) and deforestation. While latrines are limited to the urban zones, intensive cutting of the forest to meet the city dwellers' wood demand occurs in an ever increasing area around the capital, threatening the local water supply.