Determining the residence time of mercury-contaminated fine-grained sediment in the hyporheic zone of a gravel bed river using radionuclide dating methods
Date
2011
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Publisher
University of Delaware
Abstract
Fine-grained sediment and associated contaminants mediate important geochemical cycles in the hyporheic zone of gravel-bed rivers, but the residence time of fine particles in these environments has rarely been measured. The activity of 210Pb, 137Cs and 7Be was measured in samples from four cores obtained on November 2, 2009 from a representative section of the bed composed of a mixture of sand, pebbles, and cobbles. The median grain size is 25.5 mm, the 84th percentile grain diameter is 57.8 mm, and 5.7% of the bed is composed of sediment smaller than 2 mm (sand sized or smaller sediment). Sediment cores were sampled at five centimeter depth increments and each sample was sieved to extract the silt- and clay-sized particles. After freeze-drying the samples, equivalent depth intervals from all the cores were combined to yield a spatially averaged sample with depth intervals of 0-5 cm, 5-10 cm, 10-15 cm, 15-20 cm and 20-25 cm. Radionuclide activities were measured using a Canberra low energy germanium detector (model GL2020R). The activity of 210Pb and 7Be at the time of deposition was estimated from suspended sediment samples collected during a high flow event (recurrence interval 0.24 years) that occurred on November 13, 2009. A two-component age model was used to rectify the difference in ages obtained from single samples that contained both 'young' sediment (age determined from 7Be activity) at depth and 'old' sediment (age determined from 210Pb activity). Modeling results indicate that the residence time of the silt-clay fine fraction of the hyporheic zone is approximately 38 years. Scour chains and bed elevation measurements were also used to document relative bed elevation change. Even if all ongoing sources of mercury to the South River are removed, several decades will be required for the South River to cleanse its hyporheic zone of contaminated silt and clay through episodic scour and fill.