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Legacy Lead vs. Newer Mercury: Atmospheric Pollutants Linger in Streams

Core samples were analyzed to determine the lead and mercury content of peat. Photo by Sebestyen, USDA Forest Service

Trace metal pollutants from the atmosphere may affect water resources on vastly different timescales. Lead from before the 1980s still leaches from peatlands, while mercury appears to largely originate from recent deposition.

Trace metals in rain, snow, and particles from the atmosphere enter forests and drain to streams. These metals bioaccumulate through the aquatic food chain, with adverse effects on humans and top predators. Past research at the Marcell Experimental Forest has provided foundational knowledge on how much lead and mercury move through the environment. Northern Research Station scientists and collaborators have recently revisited these pollutants to determine the relative timescales of lead and mercury effects in landscapes. Despite a dramatic reduction in emissions since the 1980s, when leaded gasoline was banned, and a 90 percent reduction in upland runoff water and surface peat, lead concentrations have only decreased by 50 percent in stream waters. These findings indicate that legacy lead will continue to drain from peatlands where they were previously deposited. In contrast, mercury in streams was mostly from particle deposition (not rain or snow). The detection of mercury from dry deposition in streams suggests relatively rapid transport, rather than long-term storage and later release of the mercury. Together, this information shows lasting effects of lead and hints that lowering mercury emissions to the atmosphere may lead to more immediate reductions of mercury in streams.