Groundwater contamination by landfill leachates in a karstic aquifer

The contamination of a karstic aquifer by the leachate from the urban solid waste landfill of La Mina (Marbella, south of Spain) has been monitored. A characterization of the karstic media and the different storages of water in the carbonate rock have been deduced by the study of the hydrodynamic and hydrochemical variations in water points of the study network. The chemical compositions of four springs, three pumping wells and five piezometers in the surroundings of the landfill, yield two patterns of hydrochemical behaviour at the contaminated points: (1) the contamination at the piezometers, measured by the concentration of Cl- and by electrical conductivity, increased over time, but was associated with the precipitation of calcite, evidenced by a reduction in the concentration of HCO3- and Ca2+; (2) at points near the landfill, contamination also increased, but the CO2 from the landfill provoked an additional dissolution of carbonate minerals, a process reflected in the high concentrations of hydrogen carbonate, calcium and magnesium. The contaminated points were irregularly distributed, the most distant piezometer presenting the greatest impact, whereas no traces of contamination were detected at one piezometer close to the landfill. The irregular distribution of these processes is explained by the heterogeneity of the karstic media, with different types of storage (conduits, fractures/fissures and matrix) and a difference in density between the leachate and the groundwater.