A laboratory characterization of the upward flux of nitrate from a shallow water table in a sandy loam soil

[摘要] Shallow water tables are common in areas that have been irrigated for severaldecades and are reported to be one of the causes for increased salinity in largeirrigation fields. Upward flux of solutes from a shallow water table can occur as aresult of evaporation and plant water uptake. Evaporation-driven fluxes will havepositive and negative implications on agricultural production. Thus, characterizationof the upward flux of solutes in soils is important for the accurate prediction of arrivaltimes and spatial patterns of solutes coming from shallow water tables.The main objectives of the study were as follows. Firstly to become acquainted withtracing techniques used to quantify water and solute upward fluxes. Secondly toquantify the effect of time, flux rate, and solute concentration on the upwardmovement of nitrate ions. Thirdly to evaluate prediction procedures for nitratemovement and/or hydraulic properties for the sandy loam soil.Three laboratory experiments on repacked homogeneous sandy loam subsoilcolumns were conducted with water tables maintained at a depth of 750 mm andusing nitrate as an anion tracer. These were, varying time with a constantgroundwater N03--concentration and flux rate, varying flux rate at a constant time andgroundwater N03--concentration and finally varying groundwater N03--concentrationat a constant time and flux rate.The upward mass flow of N03- was measured and calculated by the mass flowcomponent of the convective-dispersion equation (eDE). Results of N03--concentration and water content showed temporal and spatial variation in all theexperiments that agreed with the theoretical approaches found in literature. In allthree experiments the theoretically calculated and actual measured N03--accumulations in the soil column were compared. The theoretically calculated valueswere higher than the measured. Denitrification losses during the experiments wereput forward as the reason for the lower measured N03--concentration. The measuredupward mass flow N03--accumulation increased as a function of time, flux rate andN03--concentration level in the groundwater solution with the highest accumulation inthe top surface layer. The hydraulic soil properties were determined and fitted to the two-part retentivity function of Hutson & Cass. The hydraulic conductivity vs matricpotential and hydraulic diffusivity vs water content relationships were also derived forthe experimental soil.It was concluded that higher N03--concentrations in the groundwater, than the 25 mgN03- r' used in this study, should be used in future studies and a concentration of100 mg N03- 1-1was recommended. The 20 day durations of the experiments werealso too short because it allowed for only about 0.6 to 0.8 pore volumes of cumulativeflux at rates of 6 to 8 mm d, This was insufficient to reach equilibrium conditions.Longer experiments of up to 60 days were recommended.

[发布机构] University of the Free State