The conditions for groundwater recharge using natural stable and radioactive isotopes together with analogue simulation

The use of a combination of natural isotopes (D, 18O, 14C, 234U/238U) is reviewed as applied to a study of the conditions under which, groundwater associated with a strongly fissured and karstic rock massif and drawn into mine inflows is recharged. The isotope study programme was drawn up on the basis of analogue simulation data. In the light of their interpretation of data on the isotopic composition of atmospheric precipitations, river run-off and groundwater tapped by wells and drawn into mine inflows, an assessment is made of the origin of the groundwater (D, 18O, 14C and U), the exchange (residence) time of the water is calculated for the whole exchange reservoir and for individual parts of it (D, 18O), and the age of the water from different aquifers (14C), and then the relative contribution made by water of various origins (18O, D, U) is evaluated; the possible relationship is also assessed between Palaeozoic and Cretaceous deposit waters in the area adjoining the zone in which the conical depression develops (D, 18O, 14C and U). Isotopic composition data obtained with stable isotopes are interpreted on the basis of a hydrogeological model with sinusoidal input and output, and are compared with results of independent investigations using other isotopes, analogue simulation data and analyses of mine outflow hydrographs. Despite the approximate nature of the model adopted, satisfactory agreement was observed between the results obtained. The theoretical model used is generalized to cover the case of several inflow components and that of arbitrary time-variation in the input concentration. After analysing the corollaries of the general solution, theoretical arguments are presented in support of several alternative proposals for assessing the nature and magnitude of groundwater flow across relatively impervious formations. For the first time, experimental data have been obtained on the radiocarbon content of the Yaskhan freshwater lens in the Kara-Kum desert; the radiocarbon distribution pattern supports the theory of a non-recurrent formation of the main freshwater portion of the lens and also suggests that at present it is recharged through a zone of aeration. (author)