汉江襄阳段主要入江支流沉积物营养盐和重金属风险特征研究

采集汉江襄阳段主要入江支流沉积物柱状样,分析沉积物常规理化指标、上覆水和孔隙水中营养盐含量、以及表层沉积物重金属含量,采用一维孔隙水扩散模型(Fick定律)计算营养盐扩散通量,评估沉积物营养盐释放风险,并评价表层沉积物重金属生态风险.结果表明:所有采样点氨氮及磷酸盐均从沉积物向上覆水释放,表明水质存在内源营养盐(氨氮和磷酸盐)释放风险.氨氮、磷酸盐扩散通量范围分别为42.431~81.369 mg·m-2·d-1、0.745~1.437 mg·m-2·d-1,均以航空路样点最大.表层沉积物重金属污染指数SPI值为6.1,总体表现为中度风险;其中小清河样点SPI值为11.8,达到高风险.小清河Cu、Pb和Zn平均含量分别为131.17 mg·kg-1、57.97 mg·kg-1和398.59 mg·kg-1,均超过相应的SEL值(最高效应阈值).富集系数研究表明汉江支流表层沉积物中Cd富集现象严重,平均富集系数高达4.57.汉江襄阳段入江支流沉积物存在内源营养盐释放风险,中度重金属生态风险,且Cd累积风险突出.因水文调控减少汉江干流流量进而导致河流稀释自净能力减弱,从而对汉江水质安全构成较大威胁.

Risk characteristics of nutrients and heavy metals in the sediments from the branches of Xiangyang section, Hanjiang River

Riverine sediment cores from three branches of Xiangyang section, Hanjiang River, were taken to analyze physical and chemical properties, vertical distribution characteristics of both ammonia (NH₄⁺-N) and orthophosphate (PO₄^3--P) from the overlying water to pore water, and heavy metal contents in the surface sediments. Fluxes of NH₄⁺-N and PO₄^3--P were calculated from pore water concentration gradients with a one-dimensional transport-reaction model, and the environmental risk of heavy metals was assessed by effects limits, sediment enrichment factor (SEF) and sediment pollution index (SPI). The results showed that NH₄⁺-N concentrations were far greater in the pore water than that in the overlying water, and both NH₄⁺-N and PO₄^3--P were released from sediments to overlying water, clearly revealing an internal nutrient releasing risk in the selected branches of Xiangyang section, Hanjiang River. The NH₄⁺-N influxes across the sediment-water interface varied from 42.431 to 81.369 mg · m^-2 · d^-1, and for PO₄³-P, that were from 0.745 to 1.437 mg · m^-2 · d^-1. Sampling site of Hangkong Road had the maximal levels both for NH₄⁺-N and PO₄^3--P influxes. The contents of Cu, Cr, and Ni in the surface sediment from Xiaoqing River were 131.17 mg · kg^-1, 57.97 mg · kg^-1, and 398.59 mg · kg^-1, respectively, all of which exceeded their SEL (severe effect level, NYSDEC guidelines) values, denoting severely sedimentary pollution. It had been further substantiated by the SPI value of 11.8 in Xiaoqing River, significantly higher than 6.10, the SPI value for a moderate risk. SEF for Cd in surface sediments from three selected branches was 4.57, revealing Cd in sediments was the most vital contributor to riverine sediment pollution. With the sharp decrease of hydrological flow of the Hanjiang River and the resulted weakened purification ability, the internal nutrient releasing risk, burdened by a moderate risk of heavy metals, could result in potential threats to the water security and ecosystem biodiversity and therefore should be paid more attention.