Surface-water infiltration and groundwater movement in arid zones of Venezuela

Radiocarbon dating of the carbonate species present in groundwater is capable of furnishing information on the recharge of underground deposits as well as directions and rates of movement. The origins of the dissolved carbonates fall under two classifications: modern carbon is provided by carbon dioxide from the atmosphere and the products of plants in the soil, and ancient carbon comes from dissolving limestone. The latter source is essentially free of radiocarbon and, therefore, causes falsely old dates for the total groundwater carbonates. An estimate of the amount of limestone dilution is the main difficulty connected with radiocarbon dating of groundwater. The methods proposed for this correction were applied to several aquifers under different conditions. It is seen, with numerous measurements, that the use of naturally occurring 13C would result in important uncertainties. Adjustments of the dates for the limestone contribution employing bicarbonate and total carbon concentrations is considerably more successful and gives reasonable results in the cases studied. Aquifers of Barquisimeto, Maracaibo, Coro, Paraguana and Valencia in Venezuela were investigated by the method of natural radiocarbon. Ages ranging from modern to 35 000 yr were obtained. In general, the older dates correlate well with arid climatic conditions. However, deep wells providing modern water have been observed in virtual deserts, but these are explained by the re-entrance of irrigation water previously extracted from the ground. Infiltrations from rivers and lakes were also studied using wells surrounding the surface-water bodies. The relationship between the amount and the distribution throughout the year of rainfall and evaporation is seen to be important for arid zones. Radiocarbon ages of the various groundwaters, after correction for limestone, are in agreement with the expected situations as deduced from considerations of climatic conditions of the different regions. The practical importance of these studies for the management of the available water supplies in arid zones of Venezuela is discussed. (author)