Tracing water and nitrate movement through soils with bromide

English: The pollution of surface and subsurface water is a serious problem worldwide. To clean up a once polluted groundwater source is very difficult and costly, if not impossible. Almost all the sources responsible for groundwater pollution originate in the vadose zone of the subsurface. A better understanding of the movement of chemicals in the vadose zone under different conditions can therefore contribute significantly to prevent the pollution of groundwater resources. One particularly important pollutant of groundwater sources in agricultural areas is nitrogen. However, it is very difficult to study the movement of this chemical in agricultural soils, because of its complex and interdependent transformations in the soil. The major aim of this thesis was to compare the movement of bromide and nitrogen in the Bainsvlei soil of South Africa and a soil of Ethiopia, under steady state, transient state and cropped soil conditions, with the view to use bromide as a substitute tracer for nitrogen. The hydraulic properties of the Bainsvlei soil were determined in situ. Different simplified models were used to determine the hydraulic conductivity of the soil, and the performance of the models was evaluated. A steady state experiment was conducted using bromide tracer and a rainfall simulator to determine solute transport parameters using deterministic and stochastic models. Parameters determined from the deterministic convective-dispersive equation and the stochastic stream tube model were similar. It was observed that this soil did not exhibit preferential flow. From the experiments conducted to determine the effect of intensity and continuity of water application on solute transport, it was observed that increasing the intensity and continuity of rainfall/irrigation increases leaching in this soil. The experiments conducted on a bare plot under natural rainfall conditions suggested the leaching of a fertilizer could be minimized if the fertiliser is not applied in batch mode at the beginning of the growing season of a crop, but split over the growing season of the crop. The study of the movement of bromide and nitrate was repeated with the alluvial sandy loam of Dire Dawa, Ethiopia. Two sets of plots were used for this purpose. One set was left bare and the other planted with maize. No clear conclusions could be reached from this study because of the high natural background nitrate concentration of the soil, which obscured the positions of the concentration peaks. Nevertheless, the experiment did show that the maize uptake attenuated the movement of both chemicals in the soil and that the rate at which maize plants take up nitrogen depends on the vegetative stage of the plants. When combined with the results of the leaching experiment on the bare plot, these results indicate that it would be more economical, and environmental friendly, if a farmer applies the fertilizer not in batch mode at the beginning of the growing season, but split it over the growing season, dependent on the vegetative phase of the crop and weather conditions.