季节性冻土融化期小流域尺度面源污染物迁移规律

为更好地认识冻土在融化期面源污染物的析出规律,该文以吉林省长春市黑顶子河流域为研究对象,将流域分为4类典型汇流区,监测了冻土融化期不同汇流区以及小流域尺度的水文特性及面源污染物析出入河过程。采用质量平衡法确定了流域尺度的水量及污染物析出量。第I类汇流区水和污染物析出通量主要受稻田排水汇流过程影响;以玉米种植为主的第II和第III类汇流区,水和污染物析出通量变化范围受到土壤利用信息的影响更为显著;农村居民区(第IV类汇流区)单位面积析出通量显著的超过I~III类汇流区。初始条件、汇流区面积、坡度是影响冻土向河道析出过程的重要因素。冻结过程中,水稻和玉米两种下垫面水和污染物质量显著增加,然而表现出显著不同的再分布过程。冻土融化过程中,第I、II和III类汇流区表层融化区主要影响NH4+的析出入河过程,最大融化区内含水量和NO3-浓度发生显著变化。冻土融化期析出流量和NH4+析出通量的标准差与均值比显著的小于NO3-析出通量标准差与均值比。采用质量平衡方法确定的流域水和污染物析出入河过程与通过不同汇流区析出水量和污染物质量叠加所确定的水和污染物析出入河过程一致。平衡分析表明,单位面积冻土中水、NH4+和NO3-析出量占未融化层以上水、NH4+和NO3-析出量的32.9%~74.6%,96.3%~243%,28.6~182%,占土壤最大冻结深度以上水、NH4+和NO3-析出量的10.6%~59.2%,26.4%~110%以及17.6~76.4%。

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.