Tens of thousands of old dams in the eastern U.S. no longer power machinery, but they still hold the toxic legacy of the agricultural and industrial expansion of non-indigenous settlers. A team of Earth scientists has found that, as the decrepit dams are removed, they release stores of lead, phosphorus, copper and other chemicals into the Chesapeake Bay and other waterways.
Even small streams could be turned into a source of water power for grist, flour, saw, and other types of mills. Sediment samples from mill ponds behind dams in Cumberland County, Pennsylvania, showed the march of development across the area. During the 18th century, iron ore mining and paper mills began to deforest the region. The runoff left sand deposits in the ponds with traces of iron slag and charcoal.
Later with the spread of farms into the area, the runoff began carrying chemical residues from fertilizers and pesticides into the watershed. Over the decades the chemicals built up into a sizable unhealthy heritage.
The dawn of the automobile age left a leaden footprint. Until leaden gasoline was mostly phased out in 1986, the addition of the malleable metal to fuel helped control engine pinging and boost octane. From the 20's until the closing decades of the last century, the lead concentration in the sediment samples built up. The release of lead into the water supply is dangerous for humans and animals because it has been linked to neurological damage and other infirmities.
The scientists noted that the contribution of toxins from mill pond sediments hadn't been measured before, and may be a significant source of pollutants in the area's watersheds. They suggested dredging out the trapped sediments when dams were removed to prevent the pollutants from escaping once again. The results of the sediment sampling were published this week in the journal Geology.
The Hagley Mill Race, used to generate power for the original DuPont powder mills on the Brandywine River in Delaware, is located on over 230 acres preserved at the Hagley Museum and Library. (Hagley Museum and Library)