NITRATE ENRICHMENT IN THE VADOSE ZONE AND DEEP GROUNDWATERS OF NORTHERN AFRICA

Nitrate is widely preserved under water-table, phreatic conditions in arid and semi-arid climates of the Sahara and Sahel and the distribution across northern Africa is described in the paper. The transfer of nitrate from the unsaturated zone to the water table may be followed in the interstitial water and the NO₃/Cl ratios may be used indicate the increase of nitrate above evaporative enrichment. Examples illustrate the changes in nitrate over the past 500 years related to natural environment, land use and human intervention.

The NO₃ remains inert in the presence of oxygen where it may be used to investigate the palaeoenvironmental conditions up to the limits of radiocarbon age determination (ca 35 kyr). The available data from Libya, Niger , Sudan and Algeria show that in many of the dated groundwaters nitrate concentrations lie close to or above the 11.3 mg l^-1 limit. The frequency of high nitrate groundwaters appears to be equivalent through the Late Pleistocene and Holocene. Moreover these concentrations are close to the baseline values observed for the region in modern groundwaters, although the cultivation of leguminous crops leads to still higher NO3 concentrations. The results are compatible with studies of North African palaeo-vegetation, which indicate no major changes in plant communities, rather a shift northwards of the Sahelian vegetation zones some 500 km during the Holocene. Thus it seems that the high nitrate concentrations are an intrinsic property of these groundwaters supporting models of vegetation change and providing an important baseline with implications for water quality standards.