Evaluating equilibrium and non‐equilibrium transport of ammonium in a loam soil column |
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| Authors: | Lizhu Hou Bill X Hu Zhiming Qi Huan Yang |
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| Institution: | 1. School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing, China;2. MOE Key Laboratory of Groundwater Circulation and Evolution, China University of Geosciences (Beijing), Beijing, China;3. Institute of Groundwater and Earth Science, Jinan University, Guangzhou, China;4. Department of Bioresource Engineering, McGill University, Montreal, QC, Canada |
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| Abstract: | Release of nitrogen compounds into groundwater, particularly those compounds from excessive agricultural fertilization, is a major concern in an aquifer recharge. Among the nitrogen compounds, ammonium (  ) is a common one. In order to assess the risk of agricultural fertilizer contamination to an aquifer through infiltration,  adsorption onto a loamy agricultural soil profile (0–0.60 m depth) was studied using a soil column experiment and modelling simulation. The soil used in the experiment was drawn from an agricultural field in Xinzhen, Fangshan district, Beijing, China, and reconstituted in laboratory soil columns. Column experiments were conducted using bromide (conservative tracer) and  ‐bearing aqueous solutions. The ammonium concentrations in the soil water samples were measured, and their values were plotted as the  breakthrough curves. The chemical's soil–water distribution coefficients (Kd) were calculated using breakthrough curves. Then the retardation factor (R) in saturated soil was calculated. For the  ‐bearing aqueous solutions, the strongest  adsorption occurred at the soil depth of 0.30–0.45 m. The convection–dispersion equation model and chemical non‐equilibrium model in Hydrus‐1D were used to simulate  transport in the loamy soil. The two‐site chemical non‐equilibrium model in Hydrus‐1D was best to simulate  transport through the soil column. Parameter sensitivity study was conducted to investigate the influences of solute transport by Kd, the fraction of exchange sites assuming to be in equilibrium with the solution phase (f), the longitudinal dispersivity (λ), and the first‐order rate coefficients (ω). The sensitivity analysis results indicate Kd is the most critical parameter. |
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| Keywords: | ammonium Hydrus‐1D parameter sensitivity soil column vadose zone |
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) is a common one. In order to assess the risk of agricultural fertilizer contamination to an aquifer through infiltration, 
adsorption onto a loamy agricultural soil profile (0–0.60 m depth) was studied using a soil column experiment and modelling simulation. The soil used in the experiment was drawn from an agricultural field in Xinzhen, Fangshan district, Beijing, China, and reconstituted in laboratory soil columns. Column experiments were conducted using bromide (conservative tracer) and 
‐bearing aqueous solutions. The ammonium concentrations in the soil water samples were measured, and their values were plotted as the 
breakthrough curves. The chemical's soil–water distribution coefficients (Kd) were calculated using breakthrough curves. Then the retardation factor (R) in saturated soil was calculated. For the 
‐bearing aqueous solutions, the strongest 
adsorption occurred at the soil depth of 0.30–0.45 m. The convection–dispersion equation model and chemical non‐equilibrium model in Hydrus‐1D were used to simulate 
transport in the loamy soil. The two‐site chemical non‐equilibrium model in Hydrus‐1D was best to simulate 
transport through the soil column. Parameter sensitivity study was conducted to investigate the influences of solute transport by Kd, the fraction of exchange sites assuming to be in equilibrium with the solution phase (f), the longitudinal dispersivity (λ), and the first‐order rate coefficients (ω). The sensitivity analysis results indicate Kd is the most critical parameter.