GROUNDWATER RECHARGE ESTIMATION USING CHLORIDE, STABLE ISOTOPES AND TRITIUM PROFILES IN THE SANDS OF NORTHWESTERN SENEGAL

A study of environmental chloride, deuterium, oxygen-18, and tritium i n deep sand profiles (35 m) has been carried out in order to estimate their relative value for measuring average groundwater recharge. The i nvestigation was located at a 0.1-km(2) site in Quaternary sands near the northwestern coast of Senegal in a zone of rainfed agriculture. By using a steady-state model for duplicate unsaturated zone chloride pr ofiles, the long-term average recharge at the site was estimated to be 30 mm yr(-1) or around 10% of the average precipitation (290 mm). The chloride concentration of adjacent shallow groundwater was relatively uniform and comparable to the unsaturated zone average, while the spa tial variability in the depth distribution of Cl- in the unsaturated z one was considerable. Stable isotope (deuterium and oxygen-18) data sh ow that there is some isotopic enrichment due to direct evaporation th rough the soil surface. The degree of heavy isotope enrichment is prop ortional to the extent of evaporative loss and there is good correspon dance with the chloride enrichment. Nevertheless, stable isotopes cann ot be used quantitatively to estimate the recharge. The excellent pres ervation of the peak in thermonuclear tritium in precipitation in the unsaturated zone at depths between 12 and 20 m enables an estimated an nual recharge of 24 mm yr(-1) in this area to be calculated, using the piston flow model. Agreement therefore between Cl and H-3 as tools fo r recharge measurement is reasonable over the site.