成都市地表水天然水化学变化特征及影响因素

为揭示大型城市对地表水天然水化学的影响，于2019年春开始对长江上游岷、沱江流域成都段河流进行了每月采样监测，同时采集成都市57个污水处理厂进出口水样，测试所有样品的主要离子等水化学参数，并与搜集的历史岷、沱江数据对比.结果表明，成都地表水天然水化学目前仍是中-低矿化度水，水化学类型为重碳酸盐钙组水，是流域碳酸盐岩风化作用决定的天然水化学特征，同时受硅酸盐和蒸发岩风化的影响.成都地表水天然水化学表现出明显的月变化特征，即枯水期主要离子和矿化度质量浓度高而丰水期质量浓度低，反映出点源影响特征；空间上城市下游主要离子和矿化度高于城市上游，而且支流流域高于干流流域，反映出明显的城市影响.模拟计算等进一步分析显示，城市活动是成都地表水天然水化学变化的主要驱动因素，表现在污水排放对水体Cl^-和Na⁺升高的显著贡献，和人为酸性气体排放导致的水体总硬度/碱度>1.对比岷、沱江20世纪60年代天然水化学数据说明，目前水体Cl^-/Na⁺比已显著升高，尽管水体尚未出现天然水化学性质的根本变化，但已表现出一定的盐渍化趋势.作为距长江源头最近的特大型城市，成都市对长江水系天然水化学的影响及其环境效应值得进一步关注.

Natural Water Chemistry Change in the Surface Water of Chengdu and Impact Factors

To investigate the impact of megacities on the chemistry of surface waters, monthly sampling and monitoring were conducted in the Chengdu section of the Minjiang and Tuojiang River basin, corresponding to the upper reaches of the Yangtze River since the spring of 2019, including the influent and effluent water samples from 57 sewage treatment plants in Chengdu. All the samples were analyzed for major ions and other water chemistry parameters, and compared with the historical data of the Minjiang and Tuojiang River. The results showed that the Chengdu surface water still presented a natural chemistry with medium-low total dissolved solids(TDS), and calcium bicarbonate chemistry type, which is the natural consequence of the weathering of carbonate rocks in the basin effected by the weathering of silicates and evaporites. The natural water chemistry of the surface waters in Chengdu presented monthly variation, i.e.,the concentration of major ions and TDS was higher in the dry season compared to the wet season, reflecting the variations of point source. Spatially, the concentration of major ions and TDS downstream of the city was higher than those in the upper reaches, and the concentration in the tributary was higher than that in the mainstream, which may reflect urban influence. Further analyses, such as simulation calculations, indicated that urban activities were the major driving factor for the chemistry change in the surface waters in Chengdu, which is evidenced by the significant contribution of the sewage discharge to the elevated Cl^- and Na⁺ and the ratio of hardness/alkalinity>1 from anthropogenic acid gas emissions. A comparison with the water chemistry of the Minjiang and Tuojiang River in the 1960s indicated that, the current Cl^-/Na⁺ ratio has significantly increased, which has been evidenced by a salinization trend. As a megacity nearest to the source of the Yangtze River, the impact of Chengdu on the natural water chemistry of the Yangtze River system and its environmental effects deserves more attention.