Aggregation of fine particles at the sediment‐water interface

Abstract

The presence of a bottom sediment layer agitated by mechanical stirring or by resident organisma (tubificid oligochaetes) significantly increases the rate at which fine (1 ,um) cohesive particles are removed from suspension in laboratory columns. Measured rates of particle removal are equivalent to deposition velocities ranging from 0.23 m day-1 to 0.41 m day1. These rates are an order of magnitude faster than deposition by gravitational settling or coagulation with larger particles in the water column as observed in experimental controls. It is hypothesized that the increased removal rate is the result of aggregation in a sediment layer at the bed-water interface characterized by loosely bound (fluffy), porous material hydrodynamically coupled to the water column. According to this hypothesis particle removal occurs when motion of the overlying water or organism activity causes suspended fine particles to collide with and stick to the interfacial sediment. This new hypothesis is supported by the mass and size distribution of tracer particles recovered in cores and sediment traps at a coastal site and by theoretical estimates of interfacial aggregation rates. INTRODUCTIONA significant fraction of the total mass of suspended material in natural water bodies is composed of particles smaller than 10 µm [Lal and Lerman, 1975;McCave, 1975;Clegg and Whitfield, 1990]. The transport of these fine particles is important in the geochemical cycling of natural elements and the fate of anthropogenic contaminants. Of particular significance is the rate at which fine particles are deposited in bottom sediments. McCave [1985a, b] concludes that particle settling exceeds other possible deposition modes such as Brownian diffusion through a benthic boundary layer. However, particles smaller than 10 µm settle so slowly (less than 1 m day-1 for marine sediments [McCave, 1984], cells [Smayda, 1970], and sewage [Hunt and Pandya, 1984;Wang, 1988]) that in the absence of other deposition mechanisms, they would remain in the water column for weeks to years in lakes and coastal waters and for decades in the deep ocean. For this reason, the dominant process for the deposition of fi ne particles has been thought to be their incorporation into larger, faster-settling aggregates by coagulation or biogenic repackaging in the water column [Edzwald et al., 1979;McCave, 1984;Hunt and Pandya, 1984;Farley and Morel, 1986;Fowler and Knauer, 1986].In this article we present laboratory experiments in support of a new hypothesis that fine particles are removed from suspension by aggregation in a hydrodynamically active "fluff" layer at the sediment-water interface. Measured rates of removal by this mechanism are significantly faster than rates of removal by settling of primary particles or by coagulation with larger particles in the water column. We conclude that interfacial aggregation may regulate the deposition of fi ne particles, particularly in the sub-micron size range. Copyright 1992 by the American Geophysical Union.Paper number 92JC01827. 014...

The Permeability of Synthetic Fractal Aggregates with Realistic Three-Dimensional Structure

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