干挖法清淤对南汉垸内沟渠沉积物中氮形态和氨氮扩散通量的影响

为研究干挖法清淤对沉积物中氮形态与氨氮扩散通量的影响规律,以洞庭湖区南汉垸为研究对象,通过对农田灌溉区、生活排污口、养鸭场、水产养殖区和养猪场等典型区域的沟渠沉积物及上覆水进行采样分析,利用Fick定律估算沉积物-水界面氨氮扩散通量,采用连续分级法测定沉积物中各形态氮含量。结果表明:南汉垸内沟渠采用干挖法清淤后沟渠内源污染减小,上覆水质改善。沉积物-上覆水界面氨氮扩散通量为~(-1)1.43～16.19 mg·m~(-2)·d~(-1),沉积物中总氮含量为403.66～1 120.23 mg·kg~(-1),沉积物总氮与游离态氮(FN)、可交换态氮(EN)、酸解态氮(HN)和残渣态氮(RN)含量均随清淤完成时间延长呈增加趋势。清淤36个月后,不同功能区沉积物总氮含量由高到低依次为养鸭场养猪场水产养殖区生活排污口农田灌溉区。沉积物-上覆水界面氨氮扩散通量与沉积物中FN显著相关(P0.05)。与农田灌溉区相比,养猪场、养鸭场、水产养殖区及生活排污口对沟渠氮素的贡献量大。南汉垸内畜禽和水产养殖及生活排污的外源输入是沟渠沉积物及其上覆水中氮含量增加的主要原因。采用干挖法清淤后沉积物-水界面氮的释放短期内明显降低,使得南汉垸内沟渠的地表水环境质量得到明显改善;因沟渠清淤具有时效性,需加强对垸内沟渠实时监控并定期清淤,保障沟渠连通性和维持良好上覆水质。

Effects of dry excavation dredging on the nitrogen species and diffusion flux of ammonia nitrogen in the sediments of ditches in the Nanhan Embankment, China

The aim of this study is to investigate the nitrogen species and diffusion flux of ammonia nitrogen in the Nanhan Embankment in the Dongting Lake area, which has been dredged using the dry excavation method. For this, sediments and overlying water were sampled and analyzed from ditches located in typical areas, including the areas around farmland irrigation, a domestic sewage outlet, a duck farm, a pig farm, and aquaculture. Fick''s law was used to estimate the nitrogen diffusion flux at the sediment-water interface, and sequential extraction methods were employed for determining the nitrogen content in the sediments. Results showed that the dry excavation dredging method could improve the quality of the overlying water and the endogenous pollution load could be suppressed to a certain extent in the Nanhan Embankment during a short period. The diffusion flux of ammonia nitrogen at the sediment-overlying water interface ranged from-11.43 mg·m^-2·d^-1 to 16.19 mg·m^-2·d^-1, while the total nitrogen content in the sediments ranged from 403.66 mg·kg^-1 to 1 120.23 mg·kg^-1. The total nitrogen and four nitrogen species, including free nitrogen(FN), exchangeable nitrogen(EN), hydrolysable nitrogen(HN), and residual nitrogen(RN), in the sediments increased with time. After 36 months of dredging, the CTN in the sediments of ditches from different functional areas ranked as:duck farm > pig farm > aquaculture > domestic sewage outlet > farmland irrigation. The diffusion flux of ammonia nitrogen was significantly correlated with the FN in the sediment(P<0.05). External inputs, including livestock/poultry pollutants and aquaculture/domestic sewage discharge, were the main culprits for the nitrogen increase in both the ditch sediments and overlying water. After dry excavation dredging, the release of nitrogen pollutants from the sediments decreased significantly in the short term, which improved the quality of the overlying water in ditches in the Nanhan Embankment. It is necessary to enhance regular management and dredge works to secure the connectivity of the ditches and maintain good quality for overlying water.