Clay minerals in soil as a controlling factor in distribution of surface pollutants – a case study of the Zagreb aquifer (Croatia)

In the groundwater protection zone around Zagreb, at 14 localities the 1.5 meters thick soil cover were investigated in order to determine the vertical distribution of surface pollutants and mineral composition of soil as possible protective factor for sandy-gravel aquifer situated below it. In the near-surface samples that were taken from the upper part of the A-horizon, increased content of potential pollutants is found. These are the pesticides, total oils and fats, mineral oils, detergents, nitrates, nitrites and trace elements (especially Cu and Pb). A comparison with the more deeply located samples revealed the reduced concentrations, which means that all the potential pollutants are being retarded in the surface horizon. The highest clay content was measured in the surface samples, especially at the locations where either the agricultural or forest soil is developed (45-58%). The principal mineral components of the clay fraction are the smectite, randomly interstratified clay minerals, mica minerals (illite, muscovite), kaolinite and chlorite. The reduction of pollution with depth is explained by the adsorptional-sorptional characteristics of clay minerals (especially of smectite and interstratified clay minerals) and their capability to retain the pollutants by keeping them in the near surface layers. The illegal waste dumps in abandoned gravel pits are the most dangerous source of pollution, since there is only the thin and immature soil cover, unable to prevent the percolation of pollutants directly into the aquifer. Small contents of clay minerals minimize the pollutant retention potential, and penetration of pollutants into the aquifer can be expected.