RELATIONS BETWEEN THE STRUCTURE OF STORAGE AND THE TRANSPORT OF CHEMICAL-COMPOUNDS IN KARSTIC AQUIFERS

Study of the movement of chemical compounds naturally present in the w ater, or which result from pollution, are examined according to the re servoir structure in karstic aquifers. Structure is represented by a s imple geometrical model; slow Row takes place in blocks with a network of low-permeability cracks. The blocks are separated by highly permea ble karstic conduits that allow rapid flow, and these form the aquifer drainage system. The karat studied covers 110 km(2). It is fed by an interrupted stream draining a 35 km(2) non-karstic basin, contaminated at the entry to the karst by effluents from a sewage treatment statio n. The underground water reappears as a resurgence with an annual aver age flow of approximately 1 m(3) s(-1), after an apparent underground course of 8 km in the karst. Several local sources of pollution (efflu ent from septic tanks) contaminate the underground water during its co urse. Sixteen measurement operations were performed at 12 water points , between the interrupted stream and the spring. Some sampling points were at drains, and others were in the low-permeability fissured block s. Comparison at each point of the concentrations of 14 chemical compo unds gave the following results: when pollutant discharge occurs in a permeable zone, movement is rapid in the drainage network formed by th e karstic conduits, and does not reach the less permeable fissured blo cks which are thus protected; however, if discharge is in a low-permea bility zone, the flow does not allow rapid movement of the polluted wa ter, and this increases the pollutant concentration at the discharge, This simple pattern can be upset by a reversal of the apparent piezome tric gradient between a block and a conduit during Floods or pumping; this may reverse flow directions and hence modify the movement of cont aminants. The study made it possible to site five boreholes whose posi tions in the karstic structure were unknown, showing the interest of s uch an approach for the forecasting of the impact of potential polluti on. (C) 1997 Elsevier Science B.V.