Lucie M.J. Lhesque and Dirk H. de Boer’
Abstract
Contaminants are often associated with fine-grained suspended sediment (~63 pm) because of their large surface area, high cation exchange capacity and high surface charge.
Traditionally, fine-grained sediment (~63 pm) was believed to remain continuously in suspension. Flocculation, the formation of larger, composite particles known as ‘floes’, however, facilitates the deposition of tine suspended sediment on the streambed as surticial fine-grained laminae (SFGL). The fine-grained nature of SFGL makes them potentially significant for the short-term storage of sediment-associated contaminants in streams. Despite the potentially important role of SFGL as a short-term sink for trace elements, there are few studies of trace elements concentrations and accumulation rates of SFGL. A study in southern Ontario found that trace metal concentrations were lower in SFGL than in suspended and bed sediment, even though the SFGL primarily consisted of particles between 3 and 20 pm, a size range typically associated with high levels of trace metal adsorption. The low trace metal concentrations in SFGL in this study can possibly be explained by a low capacity for adsorption and by a low trace metal availability. A study of SFGL on the Canadian prairies found enrichment of trace elements in SFGL downstream of an urban center on the Canadian prairies, even though trace metal concentrations in the dissolved phase did not indicate contributions from urban sources. Current results suggest that SFGL can be a useful indicator of water quality when suspended sediment concentrations and trace metal concentrations in the dissolved phase are low. The ease of sampling of SFGL, as opposed to the labour- intensive sampling of suspended particulate matter with a continuous flow centrifuge, makes SFGL an attractive alternative for trace metal studies, especially in remote areas and in developing countries. Further research, however, is required to determine to relationship between trace metals concentrations in the dissolved phase, in the suspended particulate phase and in SFGL.
1 INTRODUCTION
Human activity often results in a deterioration of water quality. Since the early, 1980s there has been an increased emphasis on the role of suspended sediment in transporting nutrients and contaminants through aquatic systems. Contaminants are often associated with fine-grained suspended sediment (~63 pm) because of its large surface area, high cation exchange capacity and high surface charge (Horowitz, 1991). It is now well established that in fluvial environments, suspended solids are at times a concentrated source of bioavailable nutrients and contaminants, and may adversely affect invertebrates and vertebrates.
’ Department of Geography, University of Saskatchewan, 9 Campus Drive, Saskatoon, SK S7N 5A5 CANADA
1.1 Surficial fine-grained laminae (SFGL)
Traditionally, tine-grained sediment (~63 urn) was believed to remain continuously in suspension. In the 1960s and 70s however, attention turned to the dispersal and storage of fine sediment within and on channel beds during low stages. The initial interest in the deposition of fine-grained sediment within the channel arose from concerns about deterioration of spawning grounds in graveled streams (e.g. Einstein, 1968). Fine-grained sediment deposits on the bed and in the pore space within the bed prevents water circulation through the gravel, thereby limiting the availability of oxygen to the eggs. More recently, however, the potential role of fine-grained sediment deposits within the channel in short-term storage and transport of contaminants has been recognized (Packman and Brooks, 1995. The association of fine-grained sediment and contaminants was the motivation for a review of earlier research in fine-grained sediment deposition in channels by Jobson and Carrey (1989) who use the terminology of McDowell-Boyer et al. (1986) to describe the surficial fine- grained deposits on the bed as ‘surface caking.’ Banexjee (1977) describes a l-8 mm thick surficial layer of loosely bound material, and refers to it as a ‘suspension blanket’.
Droppo & Stone (1994) and Stone & Droppo (1994) studied similar depositional features in three agricultural catchments in southwestern Ontario, and call these deposits
‘surficial fine-grained laminae’ (SFGL). They consist of fine-grained, flocculated sediment, temporarily deposited in a thin layer on the riverbed during periods of low flow, and readily resuspended as discharge increases. The deposits developed up to 5 mm thick over bedforms in shallow areas, and were readily resuspended. Droppo & Stone (1994) state that although surticial fine-grained sediment laminae may not constitute a large portion of total sediment load, they may be a “significant in-channel source of fine-grained sediment” (Droppo &
Stone, 1994). The fine-grained nature of SFGL makes them potentially significant for the storage and transport of sediment-associated contaminants.
1.2 Flocculation
SFGL consists of fine particles that in theory should remain in suspension.
Flocculation, however, facilitates the formation of SFGL through the formation of larger, composite particles known as ‘ floes’. Droppo & Stone (1994) and Droppo & Ongley (1992), define a ‘floe’ as a particle that consists of two or more primary, inorganic particles. Floes are complex matrices of both organic and inorganic material, and are fragile, physically unstable and dynamic (Liss et al., 1996; Droppo & Ongley, 1992). They continually undergo aggregation and disaggregation, the rate of which depends on physical, chemical and biological factors. Stone & Droppo (1994) present a schematic overview of the factors controlling flocculation and the formation of surficial tine-grained laminae (Figure 1).
The number and size of floes generally increase with suspended sediment concentration. Higher sediment concentrations result in increased collision frequencies so that there is a greater chance that a particle collides with other particles to form a floe. The extent of flocculation is also affected by flow conditions. Increases in shear stress have been related to an increase in floe size, likely as a result of an increased frequency of collision between particles. Further increases in shear stress, however, result in a break-up of the floes because
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their strength is not adequate to withstand the shear stress exerted by the flowing water. Thus, there appears to be an optimum value of the shear stress for floe formation. Stone &
Krishnappan (1997) report that the D,, of the floe size distribution reached a maximum value at a bed shear stress of 0.169 N mm2. In two small streams in southern Ontario, De Boer et al.
(in press) found that an increase in discharge and shear stress resulted in a decrease in floe size, as shown by effective particle size distribution and by the mean length of the 28 to 30 largest floes. Droppo & Ongley (1989) report similar findings for a river in southern Ontario.
The optimal shear stress for flocculation is a function of stream chemistry, floe composition, concentration and composition of dissolved organic carbon and numerous other factors, all of which vary spatially and temporally.
BIOLOGICAL FACTORS fL4CJRlAL SPECIES & METABOLSM E&TAlfON
BK3LOGlCAL -E
INTERFACE
!iUSPENOEO PARllCUUVE MATfER
Figure 1. Factors and processes controlling flocculation and deposition of surficial fine-grained laminae in streams (from Stone and Droppo, 1994).
Thus, the optimal value of the bed shear stress determined by Stone & Krishnappan (1997) should be interpreted as an indication of the order of magnitude rather than as a
precise prediction. Within the water column, shear stress increases from the surface down to the bed. Consequently, flocculation and settling may result in the break-up of the floes in the zone of high shear stress near the bed. Droppo et al. (1998) call this phenomenon ‘floe recycling’ (Figure 2) and point out that SFGL can only form when floe strength is sufficient to withstand the bed shear stress.
Figure 2. Floe recycling resulting from flocculation and settling followed by floe break-up in the high shear stress zone near the stream bed (from Droppo et al., 1998).
In marine environments, electrochemical processes play an important role in floe formation. In freshwater systems, however, biological processes appear to be much more significant to floe formation than electrochemical forces (Liss et al., 1996; Droppo & Ongley, 1992). Bacteria secrete extra-cellular fibrillar polymers, which bind and trap inorganic and organic particles, stabilizing floe structure and creating complex matrices within pore spaces of floes (Udelhoven et al., 1998; Liss et al., 1996; Droppo & Ongley, 1992, 1989; Ongley et al., 1992). These fibrils are a common component of freshwater ecosystems (Liss et al.,
1996).
1.3 Deposition
The realization that fine-grained sediment is frequently, and perhaps characteristically, transported as floes, has changed the perception of fine-grained sediment transport in fluvial systems. Whereas Novotny (1980), for example, suggested that there is no significant deposition of fines except with shallow flow, it is now recognized that flocculation alters the dynamics of sediment transport. Flocculation increases particle size and surface area, and decreases particle density, thereby altering particle settling velocities and deposition rates
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(Udelhoven et al., 1998; Lick, 1994; Droppo & Stone, 1994; Ongley et al., 1992; Droppo &
Ongley, 1989) Ongley et al. (1992) constructed a fine-sediment transport model which accounted for flocculation by incorporating Krishnappan’s (1991) flocculation model into Krishnappan & Lau’s (1982) model of dispersion. The model was applied to a stretch of the water saturated layer with many inter-particle and inter-floe spaces. The buoyancy of this layer makes it susceptible to resuspension and downstream transport. Resuspension of SFGL may be caused by turbulence, bioturbation and pore water pressure (Packman and Brooks, 1995; Droppo & Stone, 1994; Jobson and Carey, 1989). Once resuspended, fine particles and floes may be transported downstream and redeposited. Thus, according to Stone & Droppo (1994) there are multiple periods of formation and erosion of SFGL over the length of a river system. SFGL therefore may be considered a transient reservoir of fine-grained sediment and associated contaminants.
1.5 Sampling of surficial fine-grained laminae
Different methods have been used to sample surficial fine-grained laminae. Droppo
&d Stone (1994) sampled SFGL with a settling chamber as described by Droppo & Ongley (1992) to determine effective particle size distributions. The settling chamber was placed parallel to the flow about 30 cm downstream from the spot where the SFGL were resuspended by waving a hand laterally above the sediment surface. Even though the hand did not come into contact with the sediment, the local increase in shear stress was sufficient to resuspend the SFGL. Droppo & Stone (1994) point out that this method of SFGL sampling may cause some floe breakage as a result of the increased shear stress. The sample collected in the settling chamber in this manner is a mixture of resuspended SFGL and suspended sediment, but because sampling occurs in the cloud of resuspended SFGL it can be assumed that the contribution of suspended sediment is negligible. Stone & Droppo (1994) sampled SFGL for geochemical analysis by careful removal of the surficial layer from the sediment concrete squares, and consequently the samples represent accumulation and ageing over a period of one week. It is worth noting that sampling SFGL in this way likely results in changes in the effective particle size distribution as a result of the floe break-up during suction.
Petticrew (1996) sampled SFGL in a gravel bed stream in northwestern British Columbia (Canada) by disturbing the gravel bed, and sampling the resuspended sediment 4 to 5 m downstream of the disturbed site. This process was intended to reproduce replicate the bed disturbance caused by spawning salmon. In addition to collecting the resuspended sediment, an underwater plankton camera was used to record images of the particles in the with the gravel and any deposited fine-grained sediment was removed and transported to the lab for further analysis. Petticrew & Biickert (1998) also used a second sediment trap design consisting of a 45 cm section of 7.5 cm diameter plastic pipe that was filled with gravel and closed at both ends with a 0.64 cm wire mesh. The tube was position on the bed parallel to the flow. After the sampling period, the ends of the tube were closed off and the tube containing the gravel and any fine sediment washed into the gravel was transported to the lab for analysis.
Levesque & De Boer (in review) collected SFGL in the South Saskatchewan River upstream and downstream of the city of Saskatoon, Canada during the summer season.
Samples for trace element analysis were collected using sediment traps that were left on the riverbed for approximately 24 hours. Each trap consisted of a frame made out of plastic tubing. During the sampling period, SFGL accumulated in disposable petri dishes clipped onto the frame. The petri dishes were capped under water and stored in plastic containers for transport to the laboratory. Additional samples of SFGL were collected using the resuspension method of Droppo & Stone (1994). The particles were deposited on 0.45 Mm Millipore HA filters in the field directly after sampling (De Boer & Stone, 1999; De Boer et al., in press) for analysis of the effective particle size distribution with an imaging system.
1.6 Surficial fine-grained laminae and trace element transport
Despite the potentially important role of SFGL as a short-term sink for trace elements, there are few studies of trace elements concentrations and accumulation rates of SFGL. The most extensive study to date is likely that of Stone & Droppo (1994) who investigated the concentrations of Cd, Pb, Cu, and Zn in SFGL during baseflow conditions. A sequential extraction procedure was used to determine metal partitioning, and it was found that carbonates, Fe-Mn oxides and organic matter were the dominant accumulative phases of trace metals in SFGL. Surprisingly, trace metal concentrations were lower in SFGL than in suspended and bed sediment, even though the SFGL primarily consisted of particles between 3 and 20 mm, a size range typically associated with high levels of trace metal adsorption. significantly higher downstream of Saskatoon than upstream, even though trace metal concentrations in the dissolved phase did not show an increase. Conversely, during days
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when flow velocities were high, metal concentrations at the upstream and downstream sites were similar. The increase in metal concentrations can be explained by the trace metal contributions of urban sources of trace metal and by the finer particle size distributions at the downstream site. Research to investigate the trace metal contributions of the urban sources is ongoing.
2 CONCLUSIONS
Despite the fine-grained nature of SFGL, investigations of the significance of SFGL as an accumulating phase for trace metals has led to contradictory results. Stone & Droppo (1994) and Symader et al. (1994b, 1995) found no evidence of trace metal enrichment in SFGL relative to suspended and bed sediment during low flow periods, suggesting that SFGL characteristics during this period may not be conducive to trace metal accumulation.
Levesque & De Boer (in review), however, found enrichment of trace elements in SFGL downstream of an urban center on the Canadian prairies, even though trace metal concentrations in the dissolved phase did not indicate contributions from urban sources.
Current results suggest that SFGL can be a useful indicator of water quality when suspended sediment concentrations and trace metal concentrations in the dissolved phase are low. The ease of sampling of SFGL, as opposed to the labour-intensive sampling of suspended particulate matter with a continuous flow centrifuge, makes SFGL an attractive alternative for trace metal studies, especially in remote areas. Further research, however, is required to determine to relationship between trace metals concentrations in the dissolved phase, in the suspended particulate phase and in SFGL.
3 REFERENCES
Banerjee, I., 1977. Experimental study on the effect of deceleration on the vertical sequence of sedimentary structures in silty sediments. J. Sedim. Petrol., 47, 771-783.
De Boer, D.H. and M. Stone, 1999. Fractal dimensions of suspended solids in streams:
comparison of sampling and analysis techniques. HydroZogical Processes, 13, 239- 254.
De Boer, D.H., Stone, M. and L.M. Levesque, in press. Fractal dimensions of individual particles and particle populations of suspended solids in streams. Hydrological Processes.
Droppo, LG. and E.D. Ongley, 1989. Flocculation of suspended solids in southern Ontario rivers. Int. Assoc. Hydrol. Sci. Pub. 184,95- 103.
Droppo, I.G. and E.D. Ongley, 1992. The state of suspended sediment in the freshwater fluvial environment: a method of analysis. Water Research, 26, 1,65-72.
Droppo, I.G. and M. Stone, 1994. In-channel surficial fine-grained sediment laminae. Part I:
Physical characteristics and formational processes. Hydrological Processes, 8, 1 Ol- 111.
Droppo, LG., Walling, D.E., and E.D. Ongley, 1998. Suspended sediment structure: implications for sediment and contaminant transport modelling. International Association of Hydrological Sciences Publication 249,437-444.
Einstein, H.A., 1968. Deposition of suspended particles in a gravel bed. J. Hydruul. Div.
ASCE, 94, 1197-1205.
Horowitz, A.J., 1991. A Primer on Sediment-Trace Element Chemistry. Lewis Publishers, Michigan.
Jobson, H.E. and W.P. Carey, 1989. Interaction of fine sediment with alluvial streambeds. Water Resources Research, 25,1, 135-140.
Kranck, K., 198 1. Particulate matter grain-size characteristics and flocculation in a partially mixed estuary. SedimentoZogy, 28, 107-l 14.
Krishnappan, B.G., 1991. Modelling of settling and flocculation of fine sediments in still water. Canadian Journal of Civil Engineering, 17, 5,763-770.
Krishnappan, B.G. and Y.L. Lau, 1982. Users manual. Prediction of transverse mixing in natural streams. National Water Research Institute, Burlington, Ontario, Canada.
Levesque and De Boer, (in review). Surficial fine-grained laminae and trace element transport in the South Saskatchewan River, Canada. Submitted, International Association of Hydrological Sciences.
Lick, W., 1994. The flocculation, deposition and resuspension of fine-grained sediments. In., Transport and Transformation of Contaminants Near the Sediment Water Interface , by DePinto, Joseph V., Lick, Wilbert and John F. Paul; Lewis Publishers, London, pp. 35-57.
Lisle, T.E., 1991. Methods to measure sedimentation of spawning gravels. Research Note PSW-411, USDA.
Liss, S.N., Droppo, I.G., Flannigan, D.I. and G.G. Leppard, 1996. Floe architecture in wastewater and natural riverine systems. Environmental Science and Technology, 30, 2,680-686.
McDowell-Boyer, L.M., Hunt, J.R., and N. Sitar, 1986. Particle transport through porous media. Water Resour. Res., 22, 1901-1922.
Novotny, V., 1980. Delivery of suspended sediment and pollutants from nonpoint sources during overland flow. Water Resources Research, 16, 1057- 1065.
Ongley, E.D., Krishnappan, B.G., Droppo, I.G., Rao, S.S. and R.J. Maguire, 1992. Cohesive sediment transport: emerging issues for toxic chemical management. Hydrobiologia, 2351236, 177-187.
Packman, A.I. and N.H. Brooks, 1995. Colloidal particle exchange between stream and stream bed in a laboratory flume. Marine and Freshwater Research, 46,233-236.
Petticrew, E.L., 1996. Sediment aggregation and transport in northern interior British Columbia streams. Int. Assoc. Hydrol. Sci. Pub. 236,313-319.
Petticrew, E.L. and S.L. Biickert, 1998. Characterization of sediment transport and storage in the upstream portion of the Fraser River (British Columbia, Canada). Int. Assoc.
Hydrol. Sci. Pub. 249,383-391.
Stone, M. and I.G. Droppo, 1994. In-channel surficial fine-grained sediment laminae. Part II:
Chemical characteristics and implications for contaminant transport in fluvial systems. Hydrological Processes, 8, 113- 124.
Stone, M. and B.G. Krishnappan, 1997. ‘Transport characteristics of tile-drain sediments from an agricultural watershed’. Water, Air and Soil Pollution, 99,89-l 03.
Symader, W., Schorer, M. and R. Bierl, 1994b. Temporal variations of environmental pollutants in channel sediments. Int. Assoc. Hydrol. Sci. Pub.224,491-498.
Symader, W., Schorer, M. and R. Bierl, 1995. Scale effects of particle-associated contaminants of river-bottom sediment. Int. Assoc. Hydrol. Sci. Pub.226, 113-l 18.
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Udelhoven, T., Symader, W. and R. Bierl, 1998. The particle bound contaminant transport during low flow conditions in a small heterogeneous basin. Int. Assoc. Hydrol. Sci.
Pub.249,423-435.