Modeling Solute Mass Exchange between Pore Regions in Slurry-Injected Soil Columns during Intermittent Irrigation

Animal slurry application to agricultural land can be a threat to the quality of groundwater and nearby surface water bodies by percolation of solutes from slurry sources. We hypothesized that local-scale processes, such as mass exchange between preferential flow paths and matrix pore regions, can play a substantial role in relation to slurry application and nutrient leaching. To improve understanding of these mass exchange mechanisms, soil column leaching data of nonreactive slurry components after injection of dairy slurry were analyzed under different initial and boundary conditions with single- and double-porosity model approaches. The data set was from nine intact soil columns (20-cm i.d., 20-cm height) of the plow layer of arable loamy topsoil that were percolated under unsaturated steady-flow conditions with a suction of 5 cm applied at the bottom. Both single- and double-porosity water flow and mobile-immobile solute transport models could describe these experimental breakthrough curves. Rainfall interruptions mimicking more natural conditions and variably saturated intermittent flow led to higher leaching of injected slurry compounds than steady-flow conditions. These observations could be explained by an increased mass exchange of dissolved injected slurry components from the immobile to the mobile pore water regions during interruptions. The results suggest that column tests under steady-flow conditions could lead to false predictions of solute leaching after slurry injection in structured soils. Furthermore, local-scale processes, such as mass exchange between pore regions, should be included in larger scale model predictions of nutrient losses from agricultural fields.