SEDIMENT AND TRACE-ELEMENT TRAPPING IN A FORESTED WETLAND, CHICKAHOMINY RIVER, VIRGINIA

The Chickahominy River, arising near Richmond, Virginia, flows southeast toward Newport News, which impounds the river for much of its water supply. Much of the bottomland between the two cities is flooded for extended periods annually. Sediment-deposition rates estimated from tree rings were used in conjunction with multi-element analyses of sediments and of selected growth rings from oak trees to estimate amounts of trapped sediment and trace elements. Mean rates of deposition at eight study sites range from 0.7 to 5.7 mm/yr and are related to stream gradient, stream power, percent wetland, hydroperiod, and land use. Deposition rates are highest downstream from the confluence of upper basin tributaries near Richmond, where stream power is low and there is a high percentage of emergent/shrub-scrub wetlands; rates decrease along downstream reaches toward the Chickahominy reservoir. Tree-ring data suggest that mean sedimentation rates were greater during the last 50 years than during the previous 30-year period, possibly because of urban expansion in the upper basin. Sites nearest the urban area have the highest rates of sedimentation and the highest concentrations of most trace elements in sediments. Trace elements concentrated in sediment include zinc, lead, chromium, copper, nickel, tin, and cadmium. Concentrations in tree rings of zinc, copper, nickel, and lead were generally proportional to those in sediment at a site, and some inter-site correlations were also observed. Unusually high concentrations of zinc were detected in some tree rings, including some that formed before 1950. Concentrations of zinc and lead in the most recently formed rings of those trees suggest that sediment concentrations of those elements may have declined relative to earlier periods. The trapping of substantial amounts of sediment and trace elements by these forested wetlands demonstrates their importance in the maintenance of water-quality.