Chapter 4* - THE USE OF PARTICULATE MATERIAL

4.1. Introduction

Since the publication of the original version of this guidebook in 1978 (UNESCO/WHO, 1978), much new information has been published on the role of particulates in the uptake, release and transport of pollutants, as well as sediment-bound nutrient and contaminant interactions with water and biota, within the aquatic environment. Assessment of the literature on sediments clearly reveals the prominent role that they play in elemental cycling, and this has been used to great effect in environmental monitoring and assessment. For this reason, a separate chapter is now devoted to this topic to provide the basic background and understanding needed to interpret accurately data derived from sediment sampling programmes. More detailed information is also available in Golterman et al. (1983), H鋕anson and Jansson (1983) and Salomons and F鰎stner (1984).

It is common practice to accept, as an operational definition, that particulate matter (PM) refers to particles greater than 0.45 µm. By this definition dissolved matter includes particles finer than 0.45 µm, including colloids. Particulate matter is derived primarily from rock weathering processes, both physical and chemical, and may be further modified by soil-forming processes. Erosion subsequently transfers the sediments or soil particles from their point of origin into freshwater systems. During transport, the sediment is sorted into different size ranges and associated mineral fractions until it is deposited on the bottom of the receiving water body. Sediment may then be resuspended, and transported farther afield, by intermittent storm activity until it comes to its ultimate resting point or sink, where active sediment accumulation occurs. Modification of the composition of sediments may occur as a result of the input of autochthonous organic and inorganic particles (e.g. calcite, iron hydroxides) generated in the water column and by chemical alterations, especially during periods of deposition.

Particle size and mineralogy are directly related because individual minerals tend to form within characteristic size ranges. Sediments may thus be described in terms of discrete compositional fractions, the overall characteristics of which are due to the variation in the proportions of these fractions and the consequent changes in particle size. Four major categories of particle pollutants may be defined as follows:

· Particulate organic matter: either dissolved organic substances adsorbed from solution onto mineral particles or particulate-sized organic detritus of allochthonous (external) or autochthonous (internal) origin (such as algal cells). Organic matter largely originates from plant detritus although some animal debris may also be present. Microbially mediated decay of the organic matter results in the use of oxygen from the water which can, in extreme cases, cause complete anoxia when all the oxygen has been consumed.

· Nutrients: adsorbed nutrient elements required for plant growth (of which the most important are phosphorus and nitrogen) which actively exchange between sediment and water. Sediment-bound nutrients create a reserve pool which, under specific conditions, can be released back to the overlying waters, enhancing nutrient enrichment effects (eutrophication).

· Toxic inorganic pollutants: sorbed heavy metals, arsenic, etc., controlled by various processes, such as adsorption and desorption, uptake and recycling, and redox conditions.

· Toxic organic pollutants: sorbed organochlorine compounds, hydrocarbons, etc., controlled, for example, by hydrophilic/hydrophobic characteristics and liposolubility.

4.2. Composition of particulate matter

4.2.1. Natural sources of particulate matter

Two major natural sources of sediment to rivers and lakes can be considered: (i) products of continental rock and soil erosion, and (ii) the autochthonous material which is formed within the water body and which usually results from the production of algae and the precipitation of a few minerals, mostly calcite (Campy and Meybeck, 1995).

The mechanical erosion of rock and soil results from the combined effects of various erosion agents, i.e. running water, wind, moving ice, mass movements of material on slopes. Where human activities are negligible, natural erosion is maximum in mountainous areas and in active volcanic regions. In particular, it is enhanced when the climate is characterised by alternating wet and dry seasons as in tropical areas (e.g. monsoon climate of South East Asia). Erosion rates may vary from less than 10 t km^-2 a^-1 to more than 10,000 t km^-2 a^-1.

As a result of the combined processes of erosion and river transport, the concentration of river suspended matter (SM) (usually measured after filtration through 0.45 µm or 0.5 µm pore filters and referred to as total suspended solids (TSS)) is one of the most variable characteristics of water quality. The yearly TSS average may range from 1 to > 10,000 mg l^-1, and for a given river it may vary over three orders of magnitude. The lowest TSS values are measured in lowland regions where lakes are abundant, as in Amazonia, the Canadian Shield, Finland and Zaire. Highest levels are encountered in semi-arid regions, as in North Africa, South West USA, South Central regions of the former USSR, etc. (Meybeck et al., 1989).

Concentrations of autochthonous material are usually low in rivers not influenced by human activities. However, autochthonous material is a major source of lake sediments. The production of macrophytes (aquatic plants) and phytoplankton (free-floating algae) leads to organic debris that eventually sinks to lake bottom sediments. In hard-water lakes rich in Ca²⁺ and HCO₃^-, increases in pH from algal productivity can cause precipitation of calcite (CaCO₃), which sinks to the bottom. A third origin of autochthonous material is the debris of algal diatoms which are very rich in silica.

When allochthonous sediment sources to lakes (dust, river inputs and shoreline erosion) are limited, the sediments may be formed mostly by autochthonous material, i.e. diatomite, organic debris and lacustrine chalk.

4.2.2. Chemical composition of river suspended matter

In regions of very high mechanical erosion, the elemental content of river suspended matter reflects the principal origins. The composition is generally close to the composition of the parent rocks and, depending on the lithological nature of the parent rock, the suspended matter may present some variations in major elements (Table 4.1A). When chemical alteration exceeds mechanical erosion, the most soluble elements are carried in the dissolved phase as ions (Ca²⁺, Mg²⁺, Na⁺ K⁺) and dissolved SiO₂, whereas the least soluble ones (Al, Fe, Ti, Mn) remain in the soil which gradually becomes more enriched. As a result of this relative enrichment, the soil particles, which are eventually eroded during heavy rains, are quite different from the parent rock. This is well documented for major elements. Lowland tropical rivers have higher Al, Fe and Ti concentrations than highland temperate rivers (Table 4.1A).

The organic carbon content of river suspended matter, usually expressed as a percentage of TSS, ranges from 0.5 per cent to 20 per cent and is inversely related to the amount of particulate matter found in the river (Meybeck, 1982). The particulate organic nitrogen (PON) is closely linked to particulate organic carbon (POC) and the POC/PON ratio is very constant: between 7 and 10 g g^-1 in unpolluted rivers (Table 4.1B).

The natural trace element concentration of river suspended matter is difficult to determine since many rivers are already subject to anthropogenic influences, particularly in the Northern temperate regions. World averages can be estimated accurately for a few elements (Table 4.1C). For most of these, the averages are close to the world average surficial rock value and to the average content of trace elements in shales (given in Table 4.2C).

Table 4.1. The natural chemical composition of river suspended matter

A. MAJOR ELEMENTS (mg kg^-1)

B. DISTRIBUTION OF ORGANIC CARBON (POC) AND NITROGEN (PON) IN WORLD RIVERS³

C. AVERAGE CONCENTRATIONS OF TRACE ELEMENTS (mg kg^-1)^2,4

POC Particulate organic carbon
PON Particulate organic nitrogen
TSS Total suspended solids

¹ Three unpolluted monolithologic watersheds in France; inorganic fraction of particulate matter (Meybeck, unpublished)

² Sources: Martin and Meybeck, 1979; Meybeck, 1988

³ Source: Meybeck, 1982

⁴ Source: Elbaz-Poulichet, F. and Seyler, P., Ecole Normale Superior, Paris, pers. comm.

4.2.3 Natural composition of lake sediments

The chemical and mineralogical composition of lake sediments may be greatly influenced by the occurrence of autochthonous material in addition to the allochthonous fraction resulting from basin erosion (Table 4.2A). In Lake Geneva (Lake L閙an), for example, the chemical composition of the deepest sediments reflects the combination of its various allochthonous origins (Jaquet et al., 1982). When autochthonous matter is dominant, lake sediments may be either carbonate-rich (e.g. Annecy lake, France) or silica-rich (e.g. Pavin lake, France) due to the accumulation of siliceous diatoms, or they may be mostly organic. In the latter case, the organic carbon content may reach 20 to 25 per cent, but in peat bogs it may be even higher (Campy and Meybeck, 1995).

As a result of these various origins of particulate matter, and of the post-depositional processes (chemical diagenesis), the trace element content of world lake sediments may naturally range over an order of magnitude (F鰎stner and Whitman, 1981). However, the median values of this distribution (usually log-normal) are very close to the content of average shale (fine detrital sedimentary rock) reflecting, therefore, the major influence of allochthonous inputs in most lake sediments (Table 4.2C).

4.2.4. Anthropogenic chemicals in particulate matter

Natural sediment formed during weathering processes may be modified quite markedly during transportation and deposition by chemicals of anthropogenic origin. Major point or diffuse sources of pollutants to sediments have been described in Chapter 1 and are summarised in Figure 4.1. Firstly, it must be noted that anthropogenic chemicals may be scavenged by fine sediment particles at any point from their origin to the final sink or their deposition. Secondly, to compute a geochemical mass balance for sediment-associated elements, it is imperative to derive, by measurement, a mass balance for the sediment in the system under evaluation. This includes deposition of atmospheric particles, total sediment loadings in rivers, accumulation in lakes, and river output to the marine system. These are discussed

4.3. Transport and deposition

As noted previously, sedimentation can be defined in terms of particle size and mineralogical composition, both of which are inter-related. The chemical composition of the sediment at its point of deposition is a product of the composition of the source material, the size of the source material, the sorting during transport, and the physical conditions at the point of deposition.

Table 4.2. Natural chemical composition of lake surficial sediments

A. MAJOR ELEMENTS (% OF INORGANIC FRACTION AFTER IGNITION AT 550° C)

B. INORGANIC CARBON (POC % DRY WEIGHT, TOTAL FRACTION)

C. TRACE ELEMENTS (mg kg^-1 DRY WEIGHT, TOTAL FRACTION)

¹ IL = ignition loss between 550° C and 1,000° C, mostly attributed to the CO₂ of carbonate minerals

² Carbonate sediment mostly derived from autochthonous precipitation

³ Mostly diatomaceous sediment (allochthonous fraction < 10%)

⁴ Mixing of allochthonous fraction resulting from erosion of both crystalline and carbonate rocks plus autochthonous carbonate precipitation

⁵ Source: F鰎stner and Whitman, 1981; 87 lake sediments mostly from remote areas

⁶ The bimodal distribution for Sr reflects its double origin - silicate minerals and carbonate minerals

⁷ Source: Turekian and Wedepohl, 1961

Figure 4.1. Sources of pollutants to sediments and the associated appropriate sampling operations for surveys of particulate pollutants

Transportation occurs in a similar fashion in both rivers and lakes, and is a direct function of water movement. In rivers, water movement is linear, whereas in lakes water movement is mainly orbital or oscillatory, due to the passage of wind-generated waves. In lakes, wind stress also induces major water circulation patterns involving low velocity currents which influence the transport directions of wave-perturbated sediment.

4.3.1. Particle size fractions

The size range (diameter 0) of transported particles ranges upwards from the clay-sized material conventionally defined as 8?(< 4 µm). This fraction consists mostly of clay minerals such as montmorillonite, kaolinite, etc., but may also include some other fine minerals and organic debris. The silt fraction is medium sized (4?-8? 64-4 µm) and the sand (- 1?4? 2 mm-64 µm) and gravel (< - 1? > 2 mm) make up the coarser size fraction. These limits are only conventional and may slightly change from one scale to another (Krumbein and Pettijohn, 1938). There is a marked relationship between the particle size and its origin (rock minerals, rock fragments, pollutants, etc.) as shown in Figure 4.2.

4.3.2. Transport mechanisms

Erosion, transportation and deposition of sediment is a function of current velocity, particle size, and the water content of the materials. These factors have been integrated into a set of velocity curves (the Hjulstrom curves), which set the threshold velocities for erosion, transport and deposition of various particle sizes (Figure 4.3). Two distinct sediment transport systems are functional under hydraulic conditions. These are defined as transport in suspension and transport by traction along the bottom, often termed bedload. The suspended particles normally consist of finer materials, usually clays and colloids, occasionally with a substantial proportion of silt. Under extreme flow conditions sands, and even gravels, may become suspended. This condition, however, is rare and confined to major storms in high gradient rivers and to the breaker zone of large water bodies. The bedload consists of coarser materials, sands, gravels and larger particles, which move along the bottom by rolling and saltation. Saltation is a process in which a particle is plucked from the bed and moves in a series of bounces in the downstream direction.

Transport brings about a separation by particle size of the material introduced into a moving water body, whether a river or a lake. The resultant separation is: (i) fine grained, geochemically active, suspended material, and (ii) a coarse, geochemically (and relatively) inactive bedload.

Figure 4.2. Major origins of particulate matter in aquatic systems and their distribution in class sizes

Figure 4.3. Velocity curves defining erosion, transport and deposition of sediments of differing grain size and water content (After Postma, 1967)

4.3.3. River transport and variations in total suspended solids with water discharge

An idealised vertical profile of the proportional composition of sediment in a river would show the clay fraction dominant in the upper water layer, silt in the middle layer and fine to coarse sand near the river bottom. This situation rarely occurs, mainly due to the composition of the source material which generally tends to be deficient in silt. In reality, in many cases, little change in particle size is observed under different flow regimes and there is a clear separation of the sand and clay materials with only a small proportion of silt.

The concentration of total suspended solids varies dramatically with changes in discharge. This is illustrated by Figure 4.4 for the River Exe in England, where a generalised relationship occurs with the peaks in sediment concentration closely approximating to the peaks in the discharge. In most rivers the sediment peaks slightly precede the hydrography peaks, a condition which is known as advanced (see the flood on 25 December in Figure 4.4). Also, the peak concentration of suspended sediment decreases for each of the five consecutive storms measured in the River Exe. Resuspension of fine grained bottom sediment with increasing discharge is the cause of the major increase in suspended solids. When this happens in series, less sediment is available in the river bed to be remobilised in each subsequent event. This removal process is termed sediment exhaustion. However, during calm periods, bed sediment is replaced by deposition from newly eroded and deposited sediment. As a result, a major scatter is often observed in the short-term relationship between sediment concentration and river discharge. This causes a succession of clockwise hysteresis curves in the data. In many rivers, however, the TSS is generally linked to water discharge Q on an annual basis according to the relationship: TSS = aQ^b. Where b > 1, this corresponds to a linear variation in a log-log diagram.

Two other points can be illustrated by the data from the River Exe. Firstly, the suspended solids range from about 15 mg l^-1 to nearly 2,500 mg l^-1, i.e. somewhat in excess of two orders of magnitude. This TSS variability far exceeds the variability in concentrations of pollutants measured on the sediment particles. Hence, to compute contaminant loadings in river systems, accurate measurements of discharge and sediment concentrations are absolutely essential. Secondly, Figure 4.4 illustrates the process of sediment storage within the river drainage system, or basin, which is a function of river basin size, slope and water discharge regime. Individual events, or event series, remove a proportion of the stored sediment, including associated pollutants. Extreme storm discharges may flush all of the stored sediment.

Figure 4.4. The temporal relationship of total suspended solids to the hydrography of the River Exe, UK (After Walling, 1977)

4.3.4. Lake sedimentation

The sediment input to lakes and reservoirs is derived from:

· River input: fine grained suspended load (inorganic and organic particles), coarse traction load.

· Shoreline erosion: sediment of mixed particle size.

· Lake bed erosion: size determined by the strength of the erosional forces.

· Airborne inputs: fine particulate material of inorganic or organic origin (e.g. pollen grains).

· Autochthonous organic matter and autochthonous inorganic precipitates: usually fine particles, but larger algal aggregates and faecal pellets from zooplankton can occur.

In reservoirs the first two sources of sediment input are dominant.

The different particle sizes, in both lakes and reservoirs, are separated by hydraulic transport in a similar manner to that in rivers. Coarse sediment, derived from large river inputs, is deposited first at the river mouth, forming both emerged and submerged deltaic deposits (e.g. the Selenga delta in Lake Baikal, Russia and the Rhone delta in Lake Geneva, Switzerland).

Fine sediment in lakes is transported in suspension by major lake circulatory currents established by the wind stress. During extended periods of calm, suspended sediment will settle, even in very shallow water. An increase in wind leads to resuspension and the particles then continue to be transported. This intermittent transport occurs until the sediment is deposited in an area where water movements are insufficient to resuspend or remobilise it. Fine grained sediment deposits normally define the areas in the lake where active accumulation is taking place. For pollution studies, these depositional basins are of critical importance since they represent the only areas which can be sampled to determine accurately levels of pollutants in lake sediments. Such basins also preserve, with depth, the history of the influence of man on the composition of the sediment.

Lake sedimentation models are relatively simple since they lack tidal currents, and complexity is more related to lake morphology. Four major models of lake sedimentation (with variants) can be conceptualised. Such models are given in Figure 4.5 and described as follows (Thomas, 1988):

A. Shallow lake: Non-depositional (Figure 4.5 A): The input of fine grained sediment is approximately equal to the output of fine sediment. Coarse sediment normally forms a delta which is progressive and ultimately results in lake infilling. Fine particles may deposit on the open lake bed, but are eventually resuspended under storm conditions. The fine sediment cover remains thin and is intermittently mixed by physical processes. Sediment cores taken in lakes of this type have a thin, modern sediment in which, due to mixing, the profiles of elements are randomly distributed throughout and are, therefore, unfit for pollution assessment.

B. Shallow lake: Depositional (Figure 4.5B): In this model, the fine sediment input load is greater than the output and, hence, net accretion or deposition occurs. Deltas or bars may form depending on the sand input, but the lake essentially fills from the bottom upwards. Excess energy derived from storm waves resuspends and mixes the surface of the fine sediment to depths which sometimes exceed 10 cm. Coring of this type of lake gives random element profiles for the top layers, which have been subjected to physical mixing, with smooth profiles below. These profiles reflect the upward movement of averaged concentrations in a mixing zone of constant thickness, analogous to moving averages in smoothing data trends.

C. Shallow water: Fetch controlled deposition (Figure 4.5C): Fetch controlled deposition occurs in moderate to large lakes in which wind fetch dominates water depth as the controlling factor bringing about the focused deposition of fine material. The fine material input exceeds output, hence net deposition occurs. Resuspension is less significant, therefore, permanent and continuous sedimentation of fine material can be observed. The sediment texture coarsens downwind with increasing wave energy until deposition of fine particles ceases, and erosion and lag deposit formation occurs. Coring in fine material may provide good elemental profiles in quieter upwind reaches relevant to the prevailing winds.

D. Deep water model (Figure 4.5D): This model describes the most common lake condition. Coarse materials occur as bars or deltas and the shallow water periphery is almost exclusively an erosional or non-depositional zone. Fine sediments occur in the deeper water and fan outwards from all sides into the deep water basins. These sediments are not subjected to physical mixing and any disruption of the sediment surface is exclusively due to bioturbation. In deep lakes, the accretion rates of sediments (see section 4.8.4) are commonly between 0.1 and 1.0 mm a^-1. Coring in such deep water sediments tends to produce elemental profiles which are readily interpreted with respect to lake and basin history. However, care still has to be taken to account for any post depositional sediment stirring, or the occurrence of turbidites, slump deposits and bioturbation in the core. Biological mixing, or bioturbation, involves the physical reworking of sediment by a variety of benthic organisms. In lakes, bioturbation may extend downwards for many centimetres. This depth is normally controlled by the oxygen content of the interstitial waters. Turbidites and slump deposits are coarser material which may reach the deepest parts of some lakes during rare events (usually extreme river floods, slumping on steep slopes, etc.). These layers are unfit for determining the pollution history.

Figure 4.5. Lake sedimentation models with special reference to fine particle sedimentation (After Thomas, 1988)

4.4. Environmental control of particulate matter quality

4.4.1 Grain-size influence

The specific surface area is a key particle property which controls adsorption capacity. It is inversely proportional to particle size and decreases over three orders of magnitude from clay-sized particles (10 m² g^-1) to sand grains (0.01 m² g^-1). Therefore, the finest particles are generally the richest in trace elements. This effect is particularly evident when separate chemical analyses are made on different size fractions as shown for Cu and particulate matter in the Fly River Basin, Papua New Guinea (Figure 4.6). When total particulate matter is considered, the trace element content is usually directly proportional to the amount of the finest fraction as shown in the Rhine river for the < 16 mm fraction (Salomons and De Groot, 1977).

4.4.2 The form of pollutants bound to particulate matter

Particulate pollutants and nutrients can be partitioned into different forms or phases (speciations), likely to occur in suspended or deposited sediments. These forms depend on the origin of the substances bound to the particulate matter and on the environmental conditions, such as pH, redox potential, etc. The major forms in which pollutants and nutrients occur in the particulate matter are as follows (approximately ranked from the most reactive to the least reactive):

(i) adsorbed (electrostatically or specifically) onto mineral particles;

(ii) bound to the organic material, which consists mainly of organic debris and humic substances;

(iii) bound to carbonates;

(iv) bound to sulphides;

(v) occluded in Fe and Mn oxides, which occur commonly as coatings on particles;

(vi) within the mineral lattice (e.g. apatite or calcium phosphate for phosphorus; copper oxide or sulphide for Cu); and

(vii) in silicates and other non-alterable minerals.

Figure 4.6 Copper in various grain-size fractions in the Fly River basin, Papua New Guinea. Ok Tedi tributary and the Middle Fly River are reaches influenced by copper mining operations. The Lower Fly reach has concentrations close to the background concentrations which are observed in the Strickland River (After Salomons et al., 1988)

In unpolluted conditions, the majority of the inorganic compounds (i.e. trace elements, phosphorus) are found in the last three categories. In polluted environments, the additional inputs are mainly found adsorbed onto particles and bound to organic material. The great majority of synthetic organic compounds are found in the adsorbed fraction. Particulate organic matter (terrestrial or aquatic organic detritus) has a very high specific area and consequently a high adsorption capacity. As a result, the concentration of pollutants in the particulate matter may also be proportional to the amount of organic particulates or to the amount of carbon adsorbed on mineral surfaces.

The determination of the chemical phases of trace elements is a tedious task, undertaken by successive chemical extractions, which can only give an operational definition of the actual speciation. The analytical procedures are numerous (Salomons and F鰎stner, 1984). Some of the most commonly used are described by Tessier et al. (1979) for trace elements and by Williams et al. (1976) for phosphorus. Most procedures differentiate up to five main phases: sorbed, organic-bound, carbonate-bound, hydroxide-bound and detritus. Criticisms of these methods include the non-selectivity (i.e. some extraction steps may release portions of other forms), the difficulty of inter-comparison of results obtained in various environments (Martin et al., 1987), the time involved (only a dozen samples treated, per week, per person) and the need for highly trained analysts.

Of the chemical analysis techniques currently used the simplest is total digestion (di- or tri-acid attack) which solubilises all material present. However, under natural conditions, only part of the total trace element content is actually reactive to changes in environmental conditions or available for accumulation by biota, since the elements are strongly bound to the minerals or even incorporated within them. Some workers advocate a strong acid attack method which solubilises most specific forms, including oxide coatings, but excluding the lattice-bound elements in aluminosilicates or mineral oxides. Partial leaching at moderate pH values (around pH 2) is often used as an estimate of the maximum content of reactive and available elements. Even though partial leaching is difficult to standardise, it provides very useful information when applied to comparable environmental conditions (e.g. within a lake or river basin). The complete determination of four to six chemical forms should only be undertaken within research programmes.

4.4.3 Effects of changing environmental conditions

As environmental conditions change, the various phases of elements, nutrients, etc., found in particulate matter are altered, and various amounts of these substances may be released into solution. Various forms of organic matter, such as detritus and organic coatings on mineral particles, can be degraded under oxidising conditions, leading to the release of bound substances into solution. The solubility of metals is primarily a function of the oxidation state. For example, reduced forms of iron and manganese (Fe²⁺ and Mn²⁺) are highly soluble under anoxic conditions and, as a result, are released from particulate matter into solution. Particulate phosphates, in the form of Al-phosphates, Fe-phosphates and Ca-phosphates are more soluble at low pH. In general, acidification (pH < 5) results in the solubilisation of Fe, Mn, Al and other metals from most minerals. In contrast, some elements, like Pb, form insoluble sulphides under low pH and redox conditions. The solubility of sulphides is inversely related to the pH.

The adsorption of trace elements, hydrocarbons, organochlorines, as well as of some forms of nutrients (PO₄^3-, NH₄⁺, etc.), onto particulate material has been clearly established. When salinity increases, as in estuarine waters, the major cations cause the release of some of the above substances because the cations have a stronger bonding to adsorption sites. Particulate pollutants may also become soluble within the digestive tract of organisms due to the acidic conditions. As a result, the pollutants become more readily available to the organisms and bioaccumulation in body tissues may occur (see Chapter 5). As noted above, trace elements may also exist in the crystalline matrix of minerals (e.g. silicates). Such trace elements are seldom released into solution under the conditions normally encountered in the aquatic environment.

4.4.4 Internal recycling

As a result of changing environmental conditions there is an internal recycling of pollutants in the aquatic environment which is not yet fully understood. These processes are complex and require specific conditions within a multivariate system. The most studied and best understood elements are mercury and phosphorus. In the case of mercury, the transfer from sediment is mediated by bacteria which convert sediment-bound mercury to soluble, mono-methylmercury or to volatile di-methylmercury, depending on the pH. This methylation process, together with its impact on water quality and aquatic organisms, has been very well described in the English Wabigon river-lake system in north western Ontario, Canada (Jackson, 1980) (see section 6.6.1).

Many studies have been carried out on the recycling, or internal loading, of phosphorus from lake sediments to water. This process is particularly important since it amplifies trophic levels in eutrophic (nutrient rich) lakes by producing significant release of phosphorus to the hypolimnion waters, with subsequent mixing during overturn. The process makes phosphorus directly available to plankton in shallow lakes, and slows the rate of reversal of nutrient enrichment when management action is taken to reduce phosphorus loadings. Many environmental and physical processes are involved in the release of phosphorus. The most common is the release of phosphorus bound to iron oxide under the reducing conditions which occur in the interstitial waters of lake sediments. When bottom waters are oxygenated, the phosphorus release is stopped at the sediment-water interface. However, when bottom waters are anoxic, the redox barrier is no longer effective and interstitial phosphate diffuses to the overlying water, accelerating eutrophication.

4.5. Sampling of particulate matter

Specific systems deployed for the sampling of sediment in rivers and lakes may be sub-divided into two categories, those for suspended sediment and those for bed sediments. Different systems are appropriate for rivers and lakes and for various environmental conditions. The following discussion of equipment is based on the summary in Table 4.3.

Bottom sampling devices

Commonly used sampling equipment for lakes includes grab samplers and simple gravity coring devices (Table 4.4). A complete description of samples and sampling operations can be found in Golterman et al. (1983) and in H鋕anson and Jansson (1983).

The grab samplers used for sampling the beds of large rivers are the same as those used in the sampling of lakes. This equipment must be used from a boat of adequate dimensions to ensure safety. Small, shallow rivers may be sampled by wading into the water and scooping sediment into an appropriate container. In deeper waters, a container attached to a pole may be used. Appropriate bank deposits can be sampled directly below the water surface and should represent recent deposits from the river system. Finer bed deposits can be found behind structures which create backeddies, or in still water conditions and in slack water on the downstream, inside banks of river curves.

River sampling for total suspended solids

Measurement of TSS is now widely employed in river monitoring. Ideally, individual samples should be taken from three to five depths along three to eight vertical profiles at the river station. These samples are then united proportionally to the measured velocity at each depth. When velocities are not measured, special depth integrating samplers can be used: they provide a velocity-averaged water sample for each vertical profile. Once the composite water sample is obtained, it is filtered through a 0.45 mm filter. A full description of these procedures can be found in WMO (1981).

Sampling TSS for chemical analysis requires more precautions in order to avoid contamination. These samples are generally taken at mid-depth in the middle of rivers assumed to be representative of the average quality of river particulates, or with depth integrating samplers. For eventual chemical analysis the TSS samples and filtration kits must be treated in the same manner as laboratory glassware for the same categories of pollutants. For trace metal analysis they must be pre-cleaned with high quality, dilute acids and for trace organics with purified solvents, etc. During field operations, great care must be taken to avoid any contact with rusted devices, greasy wires, etc.

Table 4.3 Sampling methodology for particulate matter in lakes and rivers

Table 4.4 Suitability of bottom sediment samplers

Water may also be collected and transported to a laboratory centrifuge or processed in the field by pumping at a controlled flow rate through a high capacity centrifuge. The Westfalia and Alfa Laval commercial high flow separators are examples of such centrifuges (Burrus et al., 1988; Horowitz et al., 1989). When used at flow rates of up to 6 litres per minute, the efficiency of recovery exceeds 95 per cent of the total solids finer than 0.45 mm. This technique has become progressively more popular as it provides sufficient material for complete particulate matter analysis. When pollutant loads are required, it is essential to link the sampling operations for chemical analysis to the TSS measurement operations and discharge measurements.

Lake sampling devices for total suspended solids

Most lakes are characterised by very low TSS values (generally < 1 ppm). Therefore, TSS sampling is a particularly difficult task. Recovery of sufficient quantity of material to accomplish a wide range of analyses requires either long time period sampling or the processing of large volumes of water. Another strategy is the deployment of sediment traps (Bloesch and Burns, 1980). Most traps consist of vertical tubes with open tops exposed to settling particles. Traps are attached to fixed vertical lines, anchored at the bottom, and attached at the top to buoys. The traps are deployed in the lake for periods of two weeks or one month to allow the capture of sufficient sediment particles without excess decomposition of the organic matter. Another method for collecting suspended solids is filtration of water samples using a 0.45 mm filter. However, only small quantities of sediment can be collected due to filter clogging. As a result, the sediment mass is usually low, permitting only a few chemical analyses. Sediment samples can also be obtained by continuous pumping from the appropriate water depth and processing the water with a continuous flow centrifuge.

4.6. Analysis of particulate matter

Details of sediment analysis procedures are not given here but full details are available in the appropriate texts (e.g. Salomons and F鰎stner, 1984). Desirable analyses are outlined in the following sub-sections in three levels, ranging from a minimum of simple, and essential, variables to a complete analytical scheme which can be carried out by the most sophisticated laboratories. The monitoring of sediment chemistry is expensive. Therefore, such work must only be undertaken based on clear programme objectives and on a specific list of chemicals chosen to meet those objectives. Examples of such lists have been discussed in Chapters 1 and 3 and are summarised in Tables 1.3, 1.4, 3.9 and 3.10. Care must be taken to avoid comprehensive analytical procedures of organic and inorganic pollutants where many of the results are not actually required and merely increase the expense. Such comprehensive analyses are only valid for special investigations and for preliminary or basic surveys of river basin and lake systems.

4.6.1 Chemical analysis schemes for sediment

Table 4.5 shows a scheme for three levels of sophistication (levels A, B and C) of sediment analyses for bottom sediment and total suspended solids in rivers and lakes. For most trace elements, the total or strong acid extractable forms are determined. However, for nutrients and metals sequential chemical extraction techniques are available which provide insight into which of the chemical phases the substances are bound (see section 4.4.2).

Table 4.5 Suggested sediment analyses for three levels of assessment with increasing complexity

TOC Total organic carbon
AAS Atomic absorption spectrophotometry
ICPS Inductively coupled plasma spectroscopy

¹ Level A: basic equipment required
Level B: some specific equipment necessary
Level C: sophisticated equipment necessary

A comprehensive outline for the analysis of the inorganic component of sediment is given for the most sophisticated level of analysis in Figure 4.7. Steps may be omitted to provide the analyses which are necessary at levels A and B defined in Table 4.5.

Figure 4.7 A system for the complete analysis of the inorganic components of sediments (Modified from H鋕anson and Jansson, 1983)

4.6.2 Core dating

More advanced analyses (levels B and C in Table 4.5) may require additional procedures when core samples are being investigated. These may include stratigraphic analyses of the core to investigate internal structure for slumps, turbidites, general homogeneity and bioturbation. To establish an historical record, the cores must be accurately sub-sampled into appropriate depth increments, usually centimetre intervals (as in Figure 7.12), and analysed for the pollutants of interest (nutrients, trace elements or organic pollutants). If possible, the same increments can be dated (see Table 4.6) to provide a chronological interpretation. Any datable event observed in the stratigraphic assessment can be used to provide a sediment accretion rate for the core (see Table 4.6). The most sophisticated determinations include analyses of ¹³⁷Cs and ²¹⁰Pb to establish an accurate chronology of sedimentation (Krishnaswamy and Lal, 1978).

Table 4.6 Methods used for dating lake sediment cores

Source: Thomas, 1988

4.6.3 Analytical compensation for grain size effect

As already discussed, the relationship between concentration of a pollutant and sediment grain size leads to a “grain size effect” which must be eliminated to allow a reasonable inter-comparison between samples either spatially or vertically within a core. This can be carried out in two ways: analysis of the same grain size fraction in all samples, or normalisation procedures.

Analysis of the same grain size fraction in all samples

For chemical analysis, the most commonly used fraction is the silt and clay fraction (less than 50 or 64 mm grain size), obtained by wet sieving of the collected sample. Despite giving improved inter-comparative results, this approach suffers from variations in the relative proportions of silt and clay and in the probability that, in many lake samples, the silt may contain significant quantities of calcite which may dilute the pollutant concentrations.

Normalisation procedures

These include taking the ratio of the concentration of the variable of interest to some other sediment element or component that quantifies the geochemically active and/or geochemically inactive sites. Such ratios can be made using sand (quartz), clay, organic carbon, aluminium or other major, or trace, elements lattice-bound in clay (e.g. scandium, K, Ti). An example for aluminium is given in section 4.8.3.

4.7. Development of a programme for assessing particulate matter quality

4.7.1 Objectives

The objectives of an assessment programme for particulate matter quality can be numerous as indicated below:

· To assess the present concentrations of substances (including pollutants) found in the particulate matter and their variations in time and in space (basic surveys), particularly when pollution cannot be accurately and definitely shown from water analysis.

· To estimate past pollution levels and events (e.g. for the last 100 years) from the analysis of deposited sediments (environmental archive).

· To determine the direct or potential bioavailability of substances or pollutants during the transport of particulate matter through rivers, lakes and reservoirs (bioavailability assessment).

· To determine the fluxes of substances and pollutants to major water bodies (i.e. lakes, reservoirs, regional seas, oceans) (flux monitoring).

· To establish the trends in concentrations and fluxes of substances and pollutants (trend monitoring).

The objectives are listed above in increasing order of complexity, with each step requiring more sampling and measurement effort. The type of information obtained through the study of particulate matter (Table 4.7) is highly variable, depending mainly on the types of studies carried out.

4.7.2 Preliminary surveys

Before establishing a new monitoring programme or extending an existing one, preliminary surveys are recommended to collect information on the present characteristics of the water bodies of interest. These surveys are needed for the selection of sampling sites and devices, establishing sampling periods, and to aid interpretation of results. Table 4.8 summarises the information obtained from different types of appropriate surveys.

4.7.3 Sampling design

Sampling design mostly depends on: (i) the objectives, (ii) the available funds and materials, both in the field and at the laboratory, and (iii) knowledge obtained from preliminary surveys. Some examples of good design are given in section 4.9. A tentative list of possible assessments of the quality of the aquatic environment through the study of particulate material is given in Table 4.9 for the three levels of monitoring discussed earlier.

Table 4.7 Information obtained from chemical analysis of particulate matter in relation to specific assessment objectives

Objectives

a - basic surveys
b - environmental archives
c - bioavailability assessment
d - flux monitoring
e - trend monitoring

Sediment sampling strategies have been discussed in considerable detail by Golterman et al. (1983) and a full discussion is beyond the scope of this guidebook. However, some observations can be made which emphasise certain aspects of river and lake sampling.

Rivers

To establish background levels of particulate matter composition, samples of bottom sediment should be taken in the upper reaches of the river basin. The effects of tributaries on the main river should be covered by sampling tributaries close to their junction with the main river. The possible effects of point sources can be estimated from a sample taken from the point source (effluent or tributary), whereas the impact on the river is determined by taking samples immediately upstream and downstream of the source. These samples must be taken from the same side of the river as the effluent input, since the river flow will maintain an influx to the bank of origin for many kilometres downstream. The impact of land-use (diffuse sources) and the influence of a city should be covered by sampling both upstream and downstream of the city or land-use area. Single bottom sediment samples are adequate provided the objective is to assess only the qualitative impact on the composition of the sediment. This sampling regime is summarised schematically in Figure 4.1.

Table 4.8 Preliminary surveys pertinent to particulate matter quality assessments

TSS Total suspended solids

Lakes

Lakes represent more static conditions than those observed in rivers and, therefore, the sampling intensity is related to the purpose of the study. For example, historical trends can be determined easily by the accurate analysis of a single sediment core recovered from an active depositional basin of fine grained sediment, generally at the deepest point of the lake. For extensive monitoring, surface sediment can be collected from an appropriate grid which is related to the size and shape of the lake, or to a particular region of the lake which is important because of a specific use. Some examples are given in Figure 4.8. To provide mean concentration values, at least five, and preferably ten, samples should be taken for each sediment type observed in the lake. Subsequent sampling episodes should take samples from the same locations.

Table 4.9 Development of participate matter quality assessment in relation to increasing levels of monitoring sophistication