Seasonal Dynamics and Interannual Variability in Mercury Concentrations and Loads through a Three-Reservoir Complex

The Hells Canyon Complex (HCC) along the Snake River (Idaho–Oregon border, U.S.A.) encompasses three successive reservoirs that seasonally stratify, creating anoxic conditions in the hypolimnion that promote methylmercury (MeHg) production. This study quantified seasonal dynamics and interannual variability in mercury concentrations (inorganic divalent mercury (IHg) and MeHg) and loads at four reservoir inflow and outflow locations through the HCC (2014–2017). We observed (1) that the HCC is a net sink for both IHg and MeHg, (2) interannual variability in IHg and MeHg loads largely reflecting streamflow conditions, and (3) seasonal variability in particulate IHg loading at the inflow (greatest from February to April) and MeHg export from the outflow (greatest from September to December) of the HCC. Seasonal export of MeHg was evidenced by increases in monthly mean concentrations of unfiltered MeHg (approximately 2-fold) and the percentage of total mercury (THg) as MeHg (≥4-fold) coincident with reservoir destratification. Despite evidence of seasonal export of MeHg from the HCC, annual loads indicate a 42% decrease in unfiltered MeHg from HCC inflow to outflow. Results from this study improve the understanding of seasonal variability in mercury transport through and transformation within a reservoir complex.

坐落于美国爱达荷州与俄勒冈州界内蛇河沿岸的地狱峡谷综合体（Hells Canyon Complex, HCC）包含三座依次串联的水库，这些水库会发生季节性分层，在湖下层（hypolimnion）形成缺氧环境，进而促进甲基汞（methylmercury, MeHg）的生成。本研究量化了2014至2017年间，HCC范围内4处水库入流与出流点位的汞浓度（涵盖无机二价汞（inorganic divalent mercury, IHg）与甲基汞）及其负荷的季节动态与年际变异。研究结果显示：（1）HCC是无机二价汞与甲基汞的净汇；（2）无机二价汞与甲基汞的负荷年际变异主要与径流条件的变化相一致；（3）入流点位的颗粒态无机二价汞负荷于2月至4月达到峰值，而HCC出流点位的甲基汞输出通量则于9月至12月达到峰值，二者均存在显著季节波动。当水库分层消失时，未过滤甲基汞的月均浓度提升约2倍，总汞（total mercury, THg）中甲基汞的占比提升≥4倍，这一现象证实了HCC存在甲基汞的季节性输出。尽管观测到HCC存在甲基汞的季节性输出，但年度负荷数据显示，未过滤甲基汞从HCC入流到出流的通量减少了42%。本研究结果提升了我们对水库复合体中汞迁移与转化过程的季节变异的认知水平。