عنوان مقاله [English]
Background and objectives: Industrial and agricultural activities may result in aquifer pollution. Nitrogen fertilizer is widely used in agricultural activities. Nitrate ion with negative charge is not absorbed by soil particles; therefore, it is subjected to surface and ground water leaching which is more intensive in sandy loam. Analytical and numerical models applied to investigate nitrate transport between soil and groundwater and its effect on groundwater contamination. For using these models, dispersion, and retardation factors are required. Therefore, quantitative estimation of these factors for solving the problems related to solute and metal transport in the soil, is necessary. The parameters were estimated by comparing laboratory and field data versus theoretical ones. The objective of this study is to determine dispersion and retardation factors of nitrate and ammonium ions with three different methods including breakthrough curve (BTC), least square, and Hydrus models in a saturated sandy loam soil.
Materials and Methods: The study was conducted in the soil columns of centimeter and diameter with three replications. Before leaching, ammonium nitrate fertilizer is added to soil columns with concentration of g per liter. Concentration of nitrate and ammonium in leached water at the end of soil column with time, commonly known as the breakthrough curve (BTC), is determined. BTC as the first method, resulting from a step input of solute is often of sigmoidal shape and the dispersion and retardation factors are determined with this curve. The second method is least square one. In this method an error function model that fits to a breakthrough curve is presented with two unknown parameters. The parameters can be estimated by using laboratory data and a least square method. The last method is Hydrus model. In Hydrus model, the convection-dispersion and mobile-immobile models through inverse modeling were used to estimate the parameters.
Results: Dispersion and retardation factors for nitrate ion were in the range of and and for ammonium ion were in in the range of , and respectively.
Conclusions: For quick and accurate estimation of dispersion and retardation factors from a soil column data, three methods are discussed. All of the models discussed in this study, have approximately the same result in estimating dispersion and retardation factors. Dispersion and retardation factors of nitrate ion was higher than ammonium ion showing nitrate ion was absorbed to soil particles more than ammonium ion which consequently leads to less hazard of leaching to the groundwater.