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Water Quality Definitions, Concepts and Analytical Measurements 1.1 Key Concepts Organic matter in aquatic systems is a complex mixture of molecules such as carbohydrates, amino acids, hydrocarbons, fatty acids and phenolics, natural macromolecules and colloids (e.g., humics), sewage and industrial particulates, soil organic matter, living phytoplankton and other plant material. These materials are of interest for several reasons. Their transport contributes significantly to the global carbon cycle. For example, the transport of soil-derived organic matter by rivers and its subsequent burial in marine sediments is an important global sink for carbon. The more reactive constituents of organic matter (e.g., carbohydrates) make a significant contribution to heterotrophic metabolism in streams, lakes, estuaries and coastal areas. Fulvic acids and other humic substances affect the behaviour and transport of metals by complexing them. These compounds also interact with organic pollutants and adsorb to the surfaces of mineral solids thus affecting surface chemistry and rates of aggregation. Most waters contain organic matter that can be measured as total organic carbon (TOC). Sources of organic carbon in fresh and marine waters include living material and waste materials and effluents. Organic matter from living material may arise directly from plant photosynthesis or indirectly from terrestrial organic matter. An indication of the amount of organic matter present can be obtained by measuring related properties, principally the biochemical oxygen demand (BOD), chemical oxygen demand (COD), turbidity and colour. The COD usually includes the BOD as well as other chemical demands producing the relationship, COD greater than BOD greater than TOC. If the sample contains toxic substances, however, this relationship may not hold true. Turbidity is a function of light scattering by suspended particles, while colour is related to the quantity of dissolved (true colour) and particulate substances (apparent colour) present. As a result, both parameters are often highly correlated with organic carbon levels. The total organic carbon in water can be a useful indication of the degree of pollution, particularly when concentrations can be compared upstream and downstream of potential sources of pollution. In surface waters, total organic carbon concentrations are generally less than 10 mg/L, and in ground water less than 2 mg/L, unless the water receives wastes or is highly coloured due to natural organic material (e.g., swamps, bogs). Total organic carbon consists of dissolved (DOC) and particulate organic carbon (POC) and is therefore affected by pronounced fluctuations in suspended solids in riverine systems. The DOC and POC levels are determined separately after filtering the sample through a filter approximately 0.4 to 0.7 micron pore diameter. Typically, DOC levels exceed POC levels in the range 6:1 to 10:1, except during river floods or in highly turbid waters where POC dominates (Wetzel 1975).
The particulate organic carbon fraction of total organic carbon has three major sources: allochthonous inputs from the drainage basin (e.g., leaf litter), and autochthonous inputs from the littoral and pelagic zones of carbon flux. Much of the metabolism and decomposition of particulate organic carbon takes place in the sediments or en route during sedimentation. The characteristics and rates of transport of organic carbon from the drainage basin depend on the composition of soils and parent materials, local climatic conditions, topography, hydrology, and vegetation of the watershed (Mitchell and McDonald 1995; Håkanson 1993; Heikkinen 1994; Midgley and Schafer 1992). In addition, some land use activities (e.g., animal husbandry) tend to accelerate soil erosion, thereby increasing the potential for problematic levels of particulate and total organic carbon in the water column, while other activities (e.g., clear cutting) tend to decrease organic carbon inputs (Shields and Sanders 1986; Forsberg 1992; France 1995a,b).
Dissolved organic carbon is composed primarily of two categories of substances: (i) non-humic substances, a class of compounds that includes carbohydrates, proteins, peptides, fats, pigments and other low molecular weight compounds, and (ii) humic substances which form most of the organic matter in waters, and consist of coloured hydrophilic and acidic complexes ranging in molecular weight from the hundreds to thousands (Wetzel 1975). Non-humic substances are easily utilized and degraded by microorganisms (i.e., substances are labile) and exhibit rapid flux rates in aquatic systems. Their instantaneous concentrations are usually very low as a result, although they may play an important role in system metabolism. Humic substances are formed largely as a result of microbial activity on plant and animal material and are more persistent than non-humic substances.
Total organic carbon (TOC) is determined without filtration of the sample. Samples for TOC determination should be stored in dark glass bottles, with minimum exposure to light or air, at 3 to 4 C for no more than seven days prior to analysis. Alternatively, samples can be acidified with nitric, phosphoric or sulphuric acid to pH 3 or less for longer term storage and to eliminate inorganic carbon. Samples for DOC determination follow a protocol similar to that for TOC, except that they are filtered (pore diameter = 0.4 to 0.7 micron, 0.45 micron being the most common) to remove particulate organic carbon. There are several methods available for determining organic carbon depending on the type of sample to be analyzed. Methods are based on the principle of oxidation of the carbon in the sample to carbon dioxide (e.g., combustion, chemical reaction, ultraviolet radiation) which is then determined by one of several methods (e.g., volumetric determination, thermal conductivity, specific CO2 electrode). The US EPA (1983) describes these methods in detail. Wet oxidation methods (e.g., UV persulphate) have been shown to underestimate total combustion techniques by 15 to 30 percent, most likely because of incomplete oxidation of organic carbon to CO2 (Clair 1992). Several specialized techniques are available to characterize the different forms of organic matter in water and wastewaters. For example, Thomas et al. (1993) and Huber and Frimmel (1992, 1994) describe a technique that involves separating out the organic matter fractions by molecular weight and other physical chemical properties using 3D low pressure gel chromatography, and characterizing the chromatographic fractions with high sensitivity DOC and UV detection. The detection limit for chromatographic techniques is in the range of several milligrams per liter and thus a pre-concentration step is necessary. Chromatographic techniques can be used to determine, for example, the fraction of humics that make up the DOC in the sample.
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