Title

Mercury Transport in Stream Sediments of the Colorado Plateau

Document Type

Article

Subject Area(s)

Geography

Abstract

Field, documentary, and laboratory analyses show that geomorphic processes are a central component in explaining the origins and transportation of the 2,200 kg of mercury annually deposited in Lake Powell in Arizona and Utah. Almost all the mercury in the lake is derived from weathering of natural source rocks in the lake's 279,000 km2 drainage area and delivered in fluvial sediments. Of the mean annual mercury input to the lake, 40 percent comes from a single tributary, the Green River. The Colorado River contributes 40 percent of the water to the lake, but only 6 percent of the mercury. Local Canyon Lands streams contribute only 9 percent of the lake's water but 36 percent of the mercury and 40 percent of the sediment. Downstream distribution of mercury in fluvial sediments of ephemeral streams is directly related to exposures of geologic units in the contributing drainage area. In the arid and semi-arid Colorado Plateau, mercury is not generally associated with fine particles, enrichment does not occur in the transition from host rock to stream sediments, and organic materials are largely absent from stream sediments and so do not play a significant role in concentrating mercury. The regional mercury budget shows a definite spatial structure: streams contributing large amounts of mercury to the lake either discharge large quantities of sediment or have extensive outcrops of particular geologic formations (Chinle or Morrison) in their drainage areas. Mercury in the Colorado Plateau streams demonstrates that understanding of chemical cycles in the natural environment depends in part on an appreciation of the spatial characteristics of the physical processes involved.

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