Models for Describing the Diffusion of Oxygen and Other Mobile Constituents Across the Mud-Water Interface

Abstract

Transfer of material across the mud-water interface in lakes and ponds has an important influence on biological activity in both the aquatic and the mud environment. Oxygen is one of the important components influencing biological activity in both media. The amount of oxygen in any micro-volume has an influence on total biological activity, the nature of the biological population and the products of biological activity. Thus, any description of an environment must include a description of the concentration of oxygen and oxygen transfer. In many aquatic media with a limited amount of suspended, decaying organic matter, or in shallow bodies of water, the oxygen content of the water is high enough that, biologically, the aquatic medium would be described as aerobic. The mixing action of wind and temperature gradients and production of oxygen by photosynthesis maintain a reasonable supply of oxygen in relation to consumption by organisms. However, in the porous (mud) medium, appreciable mixing and phbtosynthesis do not occur and diffusion of oxygen through the liquid phase of the mud is too slow to provide sufficient oxygen for more th'an a thin layer of the surface of the mud if biological activity is at all appreciable, and, as a consequence, the mud environment can usually be described as anaerobic, except for the surface layer. Characteristically, the surface, aerobic layer of the mud will contain little Fe + and Mn+ +, and CO2 will be the principle end product of biological activity. However, only a few millimeteres away the anaerobic zone begins where high concentrations of Fe' + and Mn+ + and the various organic by-products of anaerobic respiration will be found. Thus, drastic chemical changes in the mud occur over a distance of a few millimetres. In some deep lakes with temperature stratification, consumption of oxygen by organisms in the water and in the mud together deplete the hypolimnion of oxygen. The mud then becomes anaerobic to the interface with the water and large amounts of Fe' +, Mn+ + and phosphate, among other ions, are released to the water. With turnover of the lake and mixing of the water, the lower part of the lake becomes oxygenated and oxidation of the mud surface proceeds until the oxygen in the water is again depleted. The quantity of a material transferred by diffusion phenomena per unit area per unit time is proportional to the diffusion coefficient (characteristic of the medium and the diffusing material) and the concentration gradient. In the mud near the water-mud interface, concentration gradients can be relatively large and hence diffusion of oxidants from