Groundwater contamination due to cattle slurry: modelling infiltration on the basis of soil column experiments

Infiltration into soil of contaminants present in cattle slurry was studied. Column experiments were performed in order to characterize the release of contaminants at the slurry-soil interface after surface application of slurry, with subsequent rainfall or irrigation. A gradual decrease of contaminant concentrations was observed at this interface. The shape of the release curves suggests that the release of substances from slurry can be modelled by a single-parameter release function derived from the Lapidus and Amundson solution of the convection–dispersion equation (CDE). The release process is mainly controlled by physical properties of the slurry, the soil, and their interface, on which water flow during infiltration depends. The spatial and temporal heterogeneity of water flow across the slurry-soil interface are the major causes of the observed variations in the release process between column replicates. However, good predictions of breakthrough curves (BTCs) were obtained using the release data as input for simulation of solute transport through the column. The shape of the release curve was best fitted by the release function. We compared prediction of solute transport (a) with input defined by the release function and (b) assuming rectangular-pulse input. The results show that the parameter space (soil Peclet number, release number) can be divided into two regions, one in which the assumption of rectangular pulse gives acceptable prediction of the transport through the column, and another in which this assumption is invalid.