THE RELEASE OF PARTICLES, METALS AND NUTRIENTS DUE TO SEDIMENT RESUSPENSION IN THE GREAT BAY, N.H

The fine-grained sediments in coastal areas close to population centers are often large stores of organic carbon and contaminants that have built up over centuries. These sediments however, do not necessarily permanently store this material, and determining the mechanisms that drive release of contaminants from sediments is important for understanding the controls on coastal water quality. To understand the release of particles, nutrients and trace metals from the sediments in the Great Bay, NH we have investigated release due to both diffusion and sediment resuspension with a combination of field and laboratory techniques. Resuspension was simulated with an EROMES laboratory-based erosion chamber and the release of solutes and particles as a function of imposed shear stress was determined for a variety of sites and seasons. Resuspension leads to an enhanced release of ammonium, silica and particulate metals, but not phosphate or nitrate. To verify the accuracy of the erosion chamber, and to observe resuspension in the field, a comprehensive array of instruments was deployed at one site. The instruments included two velocimeters, a current profiler and a sonar. Deployments were undertaken over a summer tidal cycle and during Tropical Storm Irene so that observations spanned a large range of shear stresses. The observed critical shear stress for erosion was 0.10 N/m² and agreed well with erosion chamber estimates. This value was not exceeded during the summer tidal cycle deployment, but during Tropical Storm Irene shear stresses reached up to 0.45 N/m² and the sediment was eroded by 1 mm. Continuing work is aimed at combining erosion chamber and field observations to improve estimates of nutrient, metal and particle release from Great Bay sediments over a wide range of flow conditions so that the impact of sediment release can be quantified for both the conditions today and for future conditions when more extreme events are predicted.