三峡库区表层沉积物营养盐时空变化及评价

三峡工程开建以来,三峡水库长江干支流水文形势发生了重大变化,水体流态及悬浮物沉降条件的改变可能导致库区表层沉积物性状的改变.对2000~2015年三峡水库长江干流江津至坝址段和嘉陵江、御临河、乌江、小江、大宁河、香溪河等主要入库支流河口表层沉积物中营养盐含量水平、时空变化及污染状况分析和评价.结果表明:干流表层沉积物中总磷各断面含量平均值在678.2~928.6 mg·kg~(-1)之间,总氮质量分数平均值在0.203%~0.362%,钾元素质量分数平均值在1.74%~2.37%之间,有机质质量分数平均值在0.94%~1.54%之间;支流河口表层沉积物中总磷各断面含量平均值在490.1~832.3 mg·kg~(-1)之间,总氮质量分数平均值在0.257%~0.495%,钾元素质量分数平均值在1.69%~2.32%之间,有机质质量分数平均值在1.21%~2.27%之间.干支流绝大部分断面表层沉积物中总磷、钾均值与背景值基本相当,但总氮均值明显高于背景值、有机质均值则显著低于背景值.干支流各断面沉积物中营养盐含量未表现出明显的岸别差异;不同的营养盐在沉积物中含量沿程变化趋势呈现出较明显的差异.不同水期干支流沉积物中营养盐含量存在一定程度的波动;蓄水对表层沉积物中营养盐含量影响有限,未出现明显地随蓄水进程而进一步富集现象.有机指数和有机氮污染评价结果表明,三峡库区表层沉积物环境状况以较清洁为主,仅个别支流和局部时段会呈现较明显的有机污染现象,但均存在较明显的有机氮污染.三峡库区表层沉积物中营养物质存在一定的生态风险,营养物质生态风险主要来自TP和TN.

Evaluation of Temporal and Spatial Variation Characteristics of Nutrients in Surface Sediment in the Three Gorges Reservoir Area

With the construction of the Three Gorges Dam, the river flow pattern and sediment dynamics are expected to be affected. As a consequence, the sediment traits could be impacted by these changes. The temporal and spatial variation characteristics of the nutrient content from 2000 to 2015 in surface sediment were analyzed in the main stream from Jiangjin to the dam site section of the Yangtze River and in the estuaries of the representative input tributaries of the Three Gorges Reservoir, such as the Jialing River, Yulin River, Wujiang River, Xiaojiang River, Daning River, and Xiangxi River. Then, the state of the pollution was assessed. Results revealed that the average concentrations of total phosphorus (TP) in the surface sediments of the main stream ranged from 678.2 to 928.6 mg·kg^-1, the average contents of total nitrogen (TN) ranged from 0.203% to 0.362%, the average contents of potassium ranged from 1.74% to 2.37%, and the average contents of organic matter ranged from 0.94% to 1.54%. The average concentrations of TP in the estuaries of the representative input tributaries ranged from 490.1 to 832.3 mg·kg^-1, the average contents of TN ranged from 0.257% to 0.495%, the average contents of potassium ranged from 1.69% to 2.32%, and the average contents of organic matter ranged from 1.21% to 2.27%. The concentrations of TP and potassium in the study sections were generally within background concentration ranges, while the concentrations of TN were obviously above background levels, and the concentrations of organic matter were obviously below background levels. The nutrient contents in the surface sediment did not display significant differences among the study sites; however, the distributions of different nutrient contents along the river varied widely. Although the nutrient contents in the surface sediments fluctuated somewhat during different flow periods, they were seemingly unaffected by the impoundment, since no obvious enrichment was observed throughout the impoundment. The evaluation of organic index indicated a clean state for the surface sediments in the majority of the study area, while there was a moderately polluted state in some local sections. The evaluation of nitrogen pollution indicated a polluted state for most sections. Overall, the nutrients in the surface sediments of the Three Gorges Reservoir showed moderate ecological risks, possibly caused by TP and TN.