LABELING OF PRECIPITATION BY STABLE ISOTOPES (O-18, H-2) OVER THE JOSPLATEAU AND THE SURROUNDING PLAINS (NORTH-CENTRAL NIGERIA)

Stable isotopes oxygen-18 and deuterium were studied in precipitation at five meteorological stations on the Jos Plateau and the surrounding plains. Rainwater is progressively depleted in heavy isotopes from th e beginning of the rainy season towards the peak of the season, with t he most depleted values being recorded in the August rains. Isotopic v ariation with altitude is very evident between the stations on the pla teau and that on the plain. The isotope-altitude gradient is probably concealed by the mass-effect of the Jos Plateau. A Local Meteoric Line (LML) of the type partial derivative D=7.8 partial derivative(18)O+10 .6 was obtained with all the available isotopic data. When only the da ta for the rain for the months of June through August are used, a rela tion partial derivative D=8.4 partial derivative(18)O+14, close to the Global Meteoric Water line but with a higher deuterium excess (d) is obtained. This suggests a contribution of continental vapour mass to p recipitation over the region. On the basis of the stable isotope relat ionships, three categories of recharging rainwater are defined, which may have implications in groundwater tracing: i) precipitation of the early and perhaps late rainy season, enriched in heavy isotopes ii) pr ecipitation of the advanced stage of the rainy season, marked by non-e vaporated rains (slope 8, d>10) iii) precipitation of the peak of the rainy season (August, September), depleted in heavy isotopes.