Department of Earth and Environmental Sciences, University of St. Andrews, St. Andrews KY16 9AL, United Kingdom.
Proc Natl Acad Sci U S A. 2013 Nov 12;110(46):18436-41. doi: 10.1073/pnas.1302295110. Epub 2013 Oct 28.
The interrelationships between hydrologically driven evolution of legacy landscapes downstream of major mining districts and the contamination of lowland ecosystems are poorly understood over centennial time scales. Here, we demonstrate within piedmont valleys of California's Sierra Nevada, through new and historical data supported by modeling, that anthropogenic fans produced by 19th century gold mining comprise an episodically persistent source of sediment-adsorbed Hg to lowlands. Within the enormous, iconic Yuba Fan, we highlight (i) an apparent shift in the relative processes of fan evolution from gradual vertical channel entrenchment to punctuated lateral erosion of fan terraces, thus enabling entrainment of large volumes of Hg-laden sediment during individual floods, and (ii) systematic intrafan redistribution and downstream progradation of fan sediment into the Central Valley, triggered by terrace erosion during increasingly long, 10-y flood events. Each major flood apparently erodes stored sediment and delivers to sensitive lowlands the equivalent of ~10-30% of the entire postmining Sierran Hg mass so far conveyed to the San Francisco Bay-Delta (SFBD). This process of protracted but episodic erosion of legacy sediment and associated Hg is likely to persist for >10(4) y. It creates, within an immense swath of river corridor well upstream of the SFBD, new contaminated floodplain surfaces primed for Hg methylation and augments/replenishes potential Hg sources to the SFBD. Anticipation, prediction, and management of toxic sediment delivery, and corresponding risks to lowland ecology and human society globally, depend on the morphodynamic stage of anthropogenic fan evolution, synergistically coupled to changing frequency of and duration extreme floods.
在百年时间尺度上，人们对主要矿区下游遗留景观的水文驱动演化与低地生态系统污染之间的相互关系知之甚少。在这里，我们通过新的和历史数据（由建模提供支持）证明了，在加利福尼亚州内华达山脉的山前山谷中，19 世纪采金产生的人为扇形物是低地沉积物吸附汞间歇性持续的来源。在巨大的标志性尤巴扇形物中，我们重点介绍了（i）扇形物演化的相对过程明显从逐渐的垂直河道侵蚀转变为扇形阶地的间歇性侧向侵蚀，从而在单个洪水中夹带大量含汞沉积物，以及（ii）扇形阶地侵蚀触发的扇形沉积物在内部的重新分配和向低地的下游推进，进而触发扇形阶地侵蚀，在越来越长的 10 年洪水事件中，引发了扇形沉积物的系统性内部分布和向低地的下游推进。每次大洪水显然都会侵蚀储存的沉积物，并将相当于迄今为止输送到旧金山湾-三角洲（SFBD）的整个采后山地汞质量的 10-30%输送到敏感的低地。这种遗留沉积物和相关汞的长期但间歇性侵蚀的过程可能会持续超过 104 年。它在 SFBD 上游巨大的河道走廊范围内创造了新的受污染的洪泛平原表面，为汞甲基化做好了准备，并增加/补充了 SFBD 的潜在汞源。对有毒沉积物输送的预测、预测和管理，以及对全球低地生态和人类社会的相应风险，取决于人为扇形物演化的形态动力学阶段，与极端洪水的频率和持续时间的变化协同耦合。
