Recharge to groundwater in arid and semi-arid regions from the Holocene to the present

Groundwater being exploited in many arid and semi-arid regions at the present day was recharged during former humid episodes of the Pleistocene or Holocene and, in contrast, the amounts derived from modern recharge are small and variable both in space and time. Geochemical and isotopic techniques provide the most effective way to identify and characterize palaeowaters and to calculate recharge, since physically-based water-balance methods are generally inapplicable. The unsaturated zone in unconsolidated sediments where homogeneous, piston flow predominates can provide a record of both the amount of recharge and the recharge history over periods of at least several hundreds of years using especially the accumulation of atmospheric chloride in interstitial water profiles. Examples from Africa (Senegal, Niger, Nigeria, Sudan as well as Cyprus) show that direct recharge rates may vary from zero to around 40% of mean rainfall, dependent primarily on the soil depth and the lithology. Spatial variability presents a real problem in any recharge investigation but results from Senegal show that unsaturated zone profiles may be extrapolated using the chemistry of shallow groundwater. Information on the history of recharge, and therefore recent climatic variations at the decade to century scale, can sometimes be deduced from the distribution of chloride in the unsaturated zone. A persistence time for peaks in chloride before they are removed by dispersion may be defined. In Senegal, for example, the unsaturated zone contains a record of the major decade scale droughts and wet periods of the past 100 years, which correspond to records contained in river flow (Senegal River) and to a lesser extent in rainfall. Unsaturated-zone studies show that there are limiting conditions to direct recharge through soil, but that present day replenishment of aquifers takes place via wadis and channels. This is well shown in hydrogeological studies carried out in Sudan. The preferential recharge via river channels is also an important feature of Holocene recharge. In Libya, distinct freshwater channels may be recognised using isotopic and chemical indicators, representing recharge from a major palaeoriver running from the Tibesti to the Sirte gulf, overlying the main palaeowater resource of Pleistocene age. In the Butana area of central Sudan the regional groundwater was recharged during a mid-Holocene wet phase and is now in decline. The only current recharge sources, which can be recognized distinctly using stable isotopes, are Nile baseflow and ephemeral wadi floods.