Abstract
A physical model was developed to describe the soil-to-stream export processes of water and pollutants in a small river basin during the soil thawing period. The hydrological and pollution transport and transformation behaviors in paddy- and corn-dominated catchments were numerically simulated; the impacts of the pollution concentrations, interactions between the migrated water and pollutants in the soil, and pollutant transformations during the pollution export were coupled. Experimental field data from the Heidingzi river basin during the soil thawing period were used to calibrate the model parameters and evaluate the performance. The mass of the dissolved pollutants from soil particles in the migrated soil pore water was the key factor affecting the pollution export into the streams; the water content directly affected the pollution export. The concentration of the pollutants peaked when the initial exported water was high. The pollutant transport processes influenced the pollution export more significantly after the soil water was significantly reduced. The N-S efficiency coefficients between the simulated and monitored flow rates and the pollution concentrations at the outlets of the paddy- and corn-dominated catchments were >0.60 and >0.54, respectively. The system deviations between the simulated and monitored flow rates and the pollution concentrations were <10% and <15%, respectively. The proposed model effectively described the water flow, pollution transport and transformation processes.
Highlights
In seasonal freeze–thaw regions, soil freezing affects the biological processes of microorganisms and the transformation of nitrogen and phosphorus
Freezing and thawing affect the migration and transformation of pollutants, primarily in the following three aspects: (1) Freeze–thawing changes the structure of the soil aggregates, which affects the permeability of the soil [5,6]; the contact areas between the soil particles and melted ice increases, resulting in more pollutants adhering to the soil particles which dissolved into the river water [7,8,9]
The model parameters were calibrated using the observations taken during the soil thawing periods in 2016 and 2017
Summary
In seasonal freeze–thaw regions, soil freezing affects the biological processes of microorganisms and the transformation of nitrogen and phosphorus. Freezing and thawing affect the migration and transformation of pollutants, primarily in the following three aspects: (1) Freeze–thawing changes the structure of the soil aggregates, which affects the permeability of the soil [5,6]; the contact areas between the soil particles and melted ice increases, resulting in more pollutants adhering to the soil particles which dissolved into the river water [7,8,9]. The migration and transformation processes of pollutants during the soil freezing and thawing periods can be described by adjusting the soil parameters, the source and sink fluxes, and the kinetic parameters of the convection dispersion equation [15,16]
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