九龙江流域地表水锰的污染来源和迁移转化机制

锰是人体必需微量元素,但近年来河流湖库等地表水锰超标现象时有发生,威胁供水安全.本文于2016—2017年在福建省九龙江流域开展水系沿程梯度调查及机理实验,结合历史监测资料综合研究,探明九龙江锰含量的时空分布与迁移转化规律,揭示九龙江锰的污染来源、超标成因与调控机制.结果表明,溶解锰高值及超标站位集中在北溪上游矿区支流和干流部分水库,且主要发生在枯水期;上游矿区支流颗粒锰含量最高,随后从上游到下游沿程递减,且与总悬浮颗粒物(TSM)和pH显著正相关(p0.05).基于锰形态与pH值之间的密切关系和沿程变化,及培养实验中沉积物锰释放主要受控于pH的变化而非溶解氧的变化,推测九龙江地表水锰污染主要来自红壤颗粒的流失(特别是矿区和坡地),流域风化形成高pH(pH7.8)环境促进亚热带红壤颗粒富集锰,大量富锰颗粒进入pH逐渐下降的河流下游和电站库区后向溶解锰转化,从而导致锰超标.河流下游及库区pH值下降的主要原因包括酸沉降、酸性废水排放、富营养化条件下有机物分解等.研究结果为我国地表水锰污染防控、饮用水安全保障与流域水环境综合管理提供科学依据.

Manganese pollution in the Jiulong River watershed: Sources and transformation

Metal manganese (Mn) in trace amounts is essential for human being. Over the past decades, dissolved Mn occasionally increased in freshwater rivers and reservoirs, which potentially deteriorated the water quality of nearby waters for drinking purposes. Here we investigated Mn concentrations along with other hydrochemical parameters in the whole Jiulong River watershed (Fujian province, Southeast China), from the upper headwaters until the river mouth, and a series of laboratory experiments were further conducted to examine the factors influencing the dissolved Mn dynamics. Combined with historic data, we explored the temporal and spatial distributions of dissolved and particulate Mn, and especially the sources, and transformation of the metal along the whole river. The study aimed to better understand the mechanism leading to the high levels of dissolved Mn in the river water system. Our results show that the high levels of dissolved Mn occurred in the upper tributaries near the mining zones, and some reservoirs in the main channel of the North Jiulong River during dry season. Particulate Mn showed a generally decreasing pattern with decreasing pH (p<0.05) downward along the tributary and mainstream. The laboratory experiments further provide evidence that the releases of dissolved Mn ions from sediment particles are mainly controlled by the environmental factor of pH instead of dissolved oxygen. Summarily we hypothesize that in the upper tributary, subtropical red soil particles strongly scavenge Mn ions under the condition of relative high pH (>7.8). Once the Mn-enriched particles flow into lower reaches of the river and dam reservoirs, the low-pH environment there promotes a substantial release of dissolved Mn ions from particles into the water column, probably as a result of acidic deposition, acidic sewage discharges, and organic matter decomposition in the eutrophic waters locally. The study therefore provides essential scientific evidence for mitigating manganese pollution, securing drinking water and integrating watershed management.