The influence of cascade reservoir construction on water chemistry distribution in Karst area

Abstract

为深入了解河流梯级筑坝对喀斯特地区河流水化学分布的影响，于2017年1、4、7和10月别对乌江干流洪家渡水库（多年调节）、乌江渡水库（季调节）和索风营水库（日调节）3个具有不同滞留时间的水库进行水样采集，分析入库水、坝前剖面水和下泄水的水化学特征，探讨河流梯级筑坝对水化学分布及风化速率估算的影响.研究结果表明：3个水库深层水比表层水HCO₃^-浓度分别高12.9%、5.5%和8.0%，Ca²⁺浓度分别高15.9%、2.4%和8.5%.河流梯级筑坝一定程度上改变了水体水化学组成，从而影响碳酸盐岩风化速率估算.整体上，洪家渡水库、索风营水库和乌江渡水库的全年风化速率变化范围分别为：-1.7%~15.4%、-5.6%~1.1%和-0.3%~3.4%.河流筑坝作用对风化速率估算及主量离子浓度的影响：HCO₃^-与Ca²⁺浓度分布均为：洪家水库 > 乌江渡水库 > 索风营水库，这与水体滞留时间长短规律一致，表明水体滞留时间影响着水化学的组成分布.同时水体离子浓度表现出明显的季节性差异，丰水期各水库变化率明显大于枯水期.上述结论表明喀斯特地区河流连续筑坝后水化学组成及分布特征发生了一定程度改变，影响流域化学风化速率的估算，且影响程度随水体滞留时间延长而增大，并受气温影响.因此，今后在估算流域风化速率及探究水化学空间变化时应对筑坝作用加以考虑，以便更加准确地评估喀斯特流域岩石风化在全球碳循环中的作用.;In order to better understand the effect of cascade damming on river chemistry distribution in the karst region, we collected water samples from Hongjiadu Reservoir (multi-yearly regulation), Wujiangdu Reservoir (seasonal regulation) and Suofengying Reservoir (daily regulation) in January, April, July and October of 2017. We analyzed the water chemistry characteristics of the inflow, outflow and stratified water samples of these three reservoirs and investigated the cascade damming effect on water chemistry. The results show that water chemistry distribution has been highly affected by the damming effect. The HCO₃^- concentration of the hypolimnion is 12.9%, 5.5% and 8.0%, and the Ca²⁺ concentration is 15.9%, 2.4% and 8.5% higher than that of the surface water, respectively, for the Hongjiadu Reservoir, Suofengying Reservoir and Wujiangdu Reservoir. Consequently, the chemical weathering rate estimation of carbonate rock has also been affected. The annual weathering rate variation of Hongjiadu Reservoir, Suofengying Reservoir and Wujiangdu Reservoir is -1.7%-15.4%, -5.6%-1.1% and -0.3%-3.4%. The calculated weathering rate and HCO₃^- and Ca²⁺ concentrations in decreasing order is Hongjiadu Reservoir > Wujiangdu Reservoir > Suofengying Reservoir, which is consistent with the water retention time, indicating that water retention time affects the redistribution of water chemistry. At the same time, the ions concentration showed obvious seasonal variations, and the change rates in the three reservoirs were significantly greater during the flood season than that of the dry season. The above conclusions indicate that the water chemistry of rivers in karst area is highly affected by damming, which affects the estimation of chemical weathering rate. Water residence time and temperature are the main controlling factors on water chemistry redistribution. Therefore, we proposed that the damming effects should be taken into account to calculate the weathering rate accurately.