Influence of groundwater on the degradation of irrigated soils in a semi-arid region, the inner delta of the Niger River, Mali

The problem of soil degradation through alkalinization/salinization in an i rrigated area with a semi-arid climate was examined in the inner delta of t he Niger River, Mall, by the study of groundwater hydraulics and hydrochemi stry in an area recharged by irrigation water. On the basis of data analysi s on various scales, it is concluded that the current extent of the surface saline soils is due to a combination of three factors: (1) the existence o f ancient saline soils (solonchaks) resulting from the creation of a broad sabkha west of the former course of the Niger River, now called the Fala of Molodo. These saline crusts were gradually deposited during the eastward t ilting of the tectonic block that supports the Niger River; (2) the irrigat ion processes during the recent reflooding of the Fala of Molodo (river div ersion in 1950). These used very poorly mineralized surface water but reint roduced into the alluvial groundwater system - generally of a low permeabil ity (K = 10(-6) m s(-1)) - salts derived from the ancient solonchaks; and ( 3) the redeposition of the dissolved salts on the surface due to the intens e evapotranspiration linked to the present Sahelian climate. In this contex t, only efficient artificial draining of subsurface alluvial groundwater ca n eliminate most of the highly mineralized Row and thus reduce the current saline deposits.