GEOHYDROLOGICAL AND MINERALIZATION STUDIES WITH ENVIRONMENTAL ISOTOPES IN A LARGE KALAHARI RANCHING DEVELOPMENT

摘要

Geohydrological, hydrochemical and environmental isotope studies in the Toteng-Sehitwa Grazing Lands area in northern Botswana were aimed at improving groundwater supply, on which the region is exclusively dependent. The area is sand-covered and flat, with the regional piezometric baseline lying inside the study area. It presents difficulties in geohydrological interpretation due to the absence of clear regional trends and the generally poor (total dissolved solids = 500 to 52 000 mg·L~(-1)) groundwater quality. Environmental isotope data on a wide spread of some 100 samples were able to clarify the regional geohydrology considerably. The sluggishness of the hydrological system is reflected in the observed isotope values and hydrochemical types. Stable isotope values for groundwater in both the Kalahari and bedrock aquifers reflect evaporation before infiltration, and an increase in dissolved solid concentration, probably due to ponding in numerous small surface pans. The ~(14)C frequency distribution suggests a continuum, ranging from unconfined conditions to completely confined groundwater, where mineralization and δ~(13)C values become more uniform. In general, mineralization shows no age dependence. Rain recharge is proven, in spite of poor ground-water quality and lack of regional drainage. These apparently conflicting conclusions can be reconciled by invoking evapotranspirative losses from the saturated zone, resulting in an ongoing increase in overall groundwater mineralization. Such closed basin conditions without hypersalinity suggest that the aquifers are periodically, if partially, flushed during 'pluvial' episodes. Recharge can only be quantified in general terms, as aquifer porosity and saturated thickness, as well as initial radiocarbon activity, can only be estimated. Results from the study predict that exploitation should improve groundwater quality with time. This was confirmed by the historical records of a number of supply boreholes.