Tracing water and nitrate movement through soils with bromide
[摘要] English: The pollution of surface and subsurface water is a serious problem worldwide. To cleanup 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 zoneof the subsurface. A better understanding of the movement of chemicals in the vadosezone under different conditions can therefore contribute significantly to prevent thepollution of groundwater resources. One particularly important pollutant of groundwatersources in agricultural areas is nitrogen. However, it is very difficult to study themovement of this chemical in agricultural soils, because of its complex andinterdependent transformations in the soil. The major aim of this thesis was to comparethe movement of bromide and nitrogen in the Bainsvlei soil of South Africa and a soil ofEthiopia, under steady state, transient state and cropped soil conditions, with the view touse bromide as a substitute tracer for nitrogen.The hydraulic properties of the Bainsvlei soil were determined in situ. Differentsimplified models were used to determine the hydraulic conductivity of the soil, and theperformance of the models was evaluated. A steady state experiment was conducted usingbromide tracer and a rainfall simulator to determine solute transport parameters usingdeterministic and stochastic models. Parameters determined from the deterministicconvective-dispersive equation and the stochastic stream tube model were similar. It wasobserved that this soil did not exhibit preferential flow. From the experiments conductedto 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 increasesleaching in this soil. The experiments conducted on a bare plot under natural rainfallconditions suggested the leaching of a fertilizer could be minimized if the fertiliser is notapplied in batch mode at the beginning of the growing season of a crop, but split over thegrowing season of the crop.The study of the movement of bromide and nitrate was repeated with the alluvial sandyloam of Dire Dawa, Ethiopia. Two sets of plots were used for this purpose. One set wasleft bare and the other planted with maize. No clear conclusions could be reached fromthis study because of the high natural background nitrate concentration of the soil, whichobscured the positions of the concentration peaks. Nevertheless, the experiment did showthat the maize uptake attenuated the movement of both chemicals in the soil and that therate 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 appliesthe fertilizer not in batch mode at the beginning of the growing season, but split it overthe growing season, dependent on the vegetative phase of the crop and weatherconditions.
[发布日期] [发布机构] University of the Free State
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