Open-incubation, diffusion methods for measuring solute reaction rates in sediments

Sedimentary solute distributions and fluxes are determined in part by the kinetics of production/consumption reactions in a deposit. It is possible to estimate rates and investigate kinetic relations in undisturbed or manipulated sediments by documenting build-up or depletion patterns of solutes allowed to diffuse either between relatively thin sections of sediment and a well-stirred water reservoir or through a large section of sediment without an overlying reservoir. The time-dependent concentrations in the sediment in both cases depend on reaction rates, kinetics, diffusion coefficients, and geometric scaling of the sediment and contacting reservoir. The water reservoir size and sediment thickness can be altered to allow rapid estimates of reaction rates at near steady-state or to examine nonsteady-state behavior. Nonsteady-state models are always required for the whole-core incubation method, which has the advantages that it is less labor-intensive than many other rate measurement methods and the incubations can be performed in situ. Experimental comparisons between open-incubation and more traditional closed-incubation estimates of reaction rates show good agreement for solutes such as HN{sub 4}(+), SO4(--), HPO{sub 4}(-) and I(-). In some cases, e.g., Mn(++), Fe(++), and HPO{sub 4}(-) production, where major back-reactions with sediment occur, open-incubations without substantial build-up of solutes may providemore » the most accurate method for estimating production rates. In principle, the open incubation methods described can be used for any diffusable species. 31 refs., 19 figs., 1 tab.« less