A laboratory characterization of the upward flux of nitrate from a shallow water table in a sandy loam soil
Weldeyohannes, Amanuel Oqbit
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Shallow water tables are common in areas that have been irrigated for several decades and are reported to be one of the causes for increased salinity in large irrigation fields. Upward flux of solutes from a shallow water table can occur as a result of evaporation and plant water uptake. Evaporation-driven fluxes will have positive and negative implications on agricultural production. Thus, characterization of the upward flux of solutes in soils is important for the accurate prediction of arrival times and spatial patterns of solutes coming from shallow water tables. The main objectives of the study were as follows. Firstly to become acquainted with tracing techniques used to quantify water and solute upward fluxes. Secondly to quantify the effect of time, flux rate, and solute concentration on the upward movement of nitrate ions. Thirdly to evaluate prediction procedures for nitrate movement and/or hydraulic properties for the sandy loam soil. Three laboratory experiments on repacked homogeneous sandy loam subsoil columns were conducted with water tables maintained at a depth of 750 mm and using nitrate as an anion tracer. These were, varying time with a constant groundwater N03--concentration and flux rate, varying flux rate at a constant time and groundwater N03--concentration and finally varying groundwater N03--concentration at a constant time and flux rate. The upward mass flow of N03- was measured and calculated by the mass flow component of the convective-dispersion equation (eDE). Results of N03-- concentration and water content showed temporal and spatial variation in all the experiments that agreed with the theoretical approaches found in literature. In all three experiments the theoretically calculated and actual measured N03-- accumulations in the soil column were compared. The theoretically calculated values were higher than the measured. Denitrification losses during the experiments were put forward as the reason for the lower measured N03--concentration. The measured upward mass flow N03--accumulation increased as a function of time, flux rate and N03--concentration level in the groundwater solution with the highest accumulation in the top surface layer. The hydraulic soil properties were determined and fitted to the two-part retentivity function of Hutson & Cass. The hydraulic conductivity vs matric potential and hydraulic diffusivity vs water content relationships were also derived for the experimental soil. It was concluded that higher N03--concentrations in the groundwater, than the 25 mg N03- r' used in this study, should be used in future studies and a concentration of 100 mg N03- 1-1was recommended. The 20 day durations of the experiments were also too short because it allowed for only about 0.6 to 0.8 pore volumes of cumulative flux at rates of 6 to 8 mm d", This was insufficient to reach equilibrium conditions. Longer experiments of up to 60 days were recommended.