Migration rule of non-point source pollutions from seasonal frozen soil in small watershed scale during thawing period

Abstract: Field experiments were conducted to monitor hydrology and pollutant exudation from frozen soil to river during the thawing period in the Hedingzi watershed located in Shuangyang district, Changchun, China. The sub-watersheds in the Heidingzi watershed were divided into four typical drainage areas. The exudation processes of water and pollutants in the watershed scale were determined using mass balance method. In the drainage area of Type I, the water and pollutants exudation processes were dominated by the flow paths from paddy field to the outlet of the main drains. In the drainage areas of Type II and III where maize was planted, the variation of exudation of water and pollutants became more significant as more land use information was included. Water and pollutant exudation amount per unit area in the rural point sources (drainage area of Type IV) was higher than that in paddy and maize fields significantly. Water and pollutant exudation processes were greatly affected by the initial condition, area, slope of the drainage area. In the drainage areas of Types I, II and III, the NH4+ exudation was mainly from the region above the unthawed layer, and significant changes of water content and NO3- concentration were observed in the region from soil surface to the maximum freezing depth during the thawing process. However, during the freezing process, water and pollutants showed significantly different redistribution processes for the underlying surface of paddy and maize fields; in paddy area, the average soil water content increased from 0.325 to 0.487 cm3/cm3, the NH4+ mass from 0.120 to 0.174 g/m2, the NO3- mass from 0.628 to 0.918 g/m2; in maize area, the average soil water content increased from 0.264 to 0.301 cm3/cm3, NH4+ and NO3- mass still showed significant changes, i.e., the NH4+ mass increased from 0.08 to 0.14 g/m2 while the NO3- mass from 0.636 to 0.766 g/m2. The pollutant fluxes were estimated by the mass difference between input and output of watershed scale and accumulated exudation fluxes of sub-watershed scale, respectively. The differences of NH4+ and NO3- fluxes between the two estimation methods were 20.99% and 0.66%, respectively. The two estimation methods for water and pollutant fluxes were consistent. During the thawing process, the average degradation rates of NH4+ and NO3- were 0.12 and 0.01 d-1, and NH4+ flux showed significant variation in the main stream. During the thawing process, the variation coefficient of NH4+ flux was significant lower than that of NO3- flux. The thawing was downward from the surface and upward from the maximum freezing depth; the exudation amounts of water, NH4+ and NO3- per unit area accounted for 32.9%-74.6%, 96.3%-243% and 28.6%-182% of the changes of soil water content, NH4+ mass and NO3- mass above the unthawed layer, respectively; and 10.6%-59.2%, 26.4%-110%, 17.6%-76.4% at the maximum freezing depth, respectively. The ratio of soil water flux between horizontal and vertical directions decreased from 1.45 to 0.119. The study can provide the reference for understanding the rule of water and pollutant exudation from frozen soil during the thawing period.