Paper No. 12
Presentation Time: 11:05 AM
RADIONUCLIDE DATING OF MERCURY-CONTAMINATED FINE-GRAINED SEDIMENT STORED IN THE HYPORHEIC ZONE OF THE GRAVEL-BEDDED SOUTH RIVER, VIRGINIA
Mercury from an industrial plant at Waynesboro, Virginia, was released into the South River from 1929-1950. While previous studies examined the floodplain and deposits sequestered by large woody debris on the margins of the channel, here we sought to determine the fate of Hg-rich deposits of the hyporheic zone. We obtained two cores from the point bar platform of a migrating (~0.04 cm/yr) bend where the bed material averages 0.5% silt-clay, 2% fine-very fine sand, 19% medium-coarse sand, and 79% gravel. Suspended sediment was passively collected during 3 sub-bankfull discharges; these samples average 53% silt-clay and 47% fine-very fine sand. Activities of Be7, Cs137, Pb210, and Ra226 were measured using gamma spectroscopy. Activities were normalized for textural differences using a numerical scheme based on two assumptions: 1) data from suspended sediments represents the initial activity of sediments deposited on the bed, and 2) activities can be partitioned into 4 textural components, each represented by a constant partition coefficient, consisting of organic matter, silt-clay, fine-very fine sand, and medium-coarse sand. All suspended sediment samples exhibited measurable Be7 activity. No activity could be measured from the bed sediments, suggesting that bed sediments are older than several of Be7’s 53.3 day half lives. Ra226 profiles in the cores are constant with depth, but excess Pb210 decays exponentially. A distinct peak is apparent in the Cs137 profiles at depths of 10-15 cm, dating these samples to 1963. Ages from Cs137 and Pb210 agree closely, and suggest sediment ages of about 75 years at depths of 20-25 cm. Hg concentrations in the cores are consistent with the radionuclide chronology. Our results suggest virtually no vertical reworking of the sediment, which is unexpected based on many published observations of frequent decimeter-scale scour and fill in gravel-bed rivers. The chronology, however, is consistent with a long-term aggradation rate of about 2 mm/yr, which may be related to long-term lateral migration of the point bar platform. If this hypothesis is correct, Hg-contaminated sediments will eventually be incorporated into the floodplain here, rather than being reentrained into the water column.