PNAS2013-11-10 3:52 AM

Enduring legacy of a toxic fan via episodic redistribution of California gold mining debris 加州金矿开采废墟碎片的不时转移形成的有毒平原研究

Abstract 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 >104 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. 论文摘要 在过去的一百年,我们对大型开采废墟下游水文演变和低地生态系统污染这两者之间的关系知之甚少。 这篇文章证明了在加利福尼亚州内华达山脉的山麓河谷,19世纪金矿挖掘时期形成的人工扇形冲击平原已经成为向下游低地一次次输送含汞泥沙的含汞污染源。这一现象是通过大量历史和现有数据建模证明发现的。在这个巨大的标志性的扇形冲击平原,我们想重点指出:(i) 冲积平原演变过程中一个明显的从倾斜的垂直沟渠到高低有错的扇形梯田的变化。 (ii) 平原内系统的重新分布以及将平原泥沙向下游推进到中心谷地都是由长达100年的洪水侵蚀梯田形成的。 每一次大型的洪水都明显地侵蚀了堆积的泥沙并且将它们推送到下游敏感的低地。这些被带到旧金山三角湾区的沉积物相当于整个被开采后的内华达山区的汞总含量的10-30%。这一绵延但断续的对废墟含汞泥沙的侵蚀很有可能将持续10000年。它在旧金山湾区上游的大块流域,产生了新的被汞甲基化污染的河滩冲击表面并为湾区带来了过量的汞。 有毒泥沙传送的预期,预测和管理,以及相关的低地生态学和人类社会全球化的风险,都和人工扇形平原演变的地形动力学阶段以及特大洪水的频率和持久度相关。

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