IN-SITU HIGH-RESOLUTION MEASUREMENTS OF FLUXES OF NI, CU, FE, AND MN AND CONCENTRATIONS OF ZN AND CD IN POREWATERS BY DGT

The technique of diffusion gradients in thin films (DGT) was used to measure in situ fluxes of metals at fine spatial resolution (1.25 mm) in the surface sediments (0-9 cm) and immediately overlying water of Esthwaite Water, UK. DGT measures labile metal species in situ by immobilizing them in a layer of Chelex resin after diffusion through a 0.4 mm polyacrylamide gel. The measured mass per unit area which accumulates in a known time can be used to calculate an in situ flux from the porewaters to the resin. This flux to the resin can be interpreted to provide porewater concentrations or in situ fluxes of metal from solid sediments to porewaters. Concentrations of Zn and Cd in the porewaters were well buffered by rapid (< min) equilibria with the solid phase, allowing them to be measured by DGT. Particulate Mn was so tightly bound that it was not released to the porewaters within a timescale of hours, supply to the DGT device being solely by diffusion. Nickel, Cu, and Fe represented intermediate cases where there was partial resupply from solid phase to porewaters. In these cases the results from DGT provided a direct measurement of the mean in situ flux (mol cm(-2) s(-1)) from solid to solution phase during 24 h deployment: Nickel (0.5-1 X 10(-15)), Cu (0.5-2.5 x 10(-16)), Fe (1.5-2.5 x 10(-8)). The generally good in situ availability suggests that porewater concentrations are controlled by adsorption/desorption processes rather than solubility or coprecipitation. These first measurements of trace metals at mm resolution showed tightly defined (<1.5 mm) maxima at the sediment water interface. Their location, 1.5-3.0 mm above the broader Fe and Mn maxima, supports previous evidence which indicated they are due to release of metals from rapidly decomposing organic material. Increased spatial resolution will be required to resolve fully the surface maxima to define the zone of metal release, the concentration gradients to overlying water, and underlying sediments and remobilization fluxes.