RECONSTRUCTING THE HISTORY OF NATURAL AND ANTHROPOGENIC SEDIMENT INPUTS BEHIND THE FORMER HEAD-OF-TIDE DAM IN THE BELLAMY RIVER, DOVER, NH

In New England, numerous studies have documented the occurrence of heavy metal enrichments in river sediment associated with past industrialization. As dams are being removed to restore rivers to their natural conditions, these sediments pose potential environmental risks. In 2010, we collected a meter-long sediment core from the former head-of-tide dam in the Bellamy River to reconstruct the history of natural and anthropogenic sediment deposition and to assess the potential for heavy metal remobilization after dam removal. The recovered sediments consist of inter-bedded sand and silty clay with sawdust-rich horizons, including an anomalous maroon-colored sawdust layer. Calibrated radiocarbon dates in stratigraphic succession (1772 A.D., 2 sigma 1735-1805 A.D and 1652 A.D., 2 sigma 1472-1693 A.D.) establish a colonial age for these sediments. X-ray fluorescence (XRF) elemental core scanning documents the relative abundance of 31 common elements and heavy metals throughout our core. To determine element concentrations of the leachable/exchangeable fraction in this record, downcore samples were measured for metal abundance via Inductively Coupled Plasma Mass Spectrometry. We removed an exchangeable fraction with ammonium chloride to determine the environmentally mobile fraction and subsequently employed nitric acid leaching to determine more tightly bound elements. Peaks determined from the XRF core-scanning data are coherent with the nitric acid leaching results for Cr, Mn, Fe, Cu, Zn, Rb, and Sr. Sand and silty clay layers are characterized by high, relative amounts of Al, Si, K, Sr, Rb, Cu, Fe and Ca. The exchangeable fraction shows most abundant Cr and Mn in the sawdust rich layers. Additionally, peak Cr abundances (> 700 ppb) were located in the anomalous maroon-colored sawdust layer. Zn and Pb are both enriched at the surface, likely a result from modern overprinting. Comparison of the industrial history of the Bellamy River with the leachable/exchangeable element concentrations and the ages of the sediments suggests the sediments behind the dam were initially deposited, then infiltrated by heavy metals from upstream sources that leached down into the substrate. Since the removal of the last remnants of the dam at this site in 2004, these metal-laden sediments are now eroding into the Great Bay Estuary.