Of stream humic substances

The focus of this chapter is on the geochemistry of stream humic substances and should provide the reader with an appreciation of the dynamics, importance, and uniqueness of streams within the hydrologic system. Streams should not be envisioned only as arteries connecting lake, ground water, and soil environments (which are considered in previous chapters) with estuaries and oceans (which are presented in following chapters), nor as integrators of humic substances from upgradient, but rather they should be viewed as a different and unique aquatic environment where stream humic substances also have a dififerent and unique character. 

During the Awakening Period many scientists first became aware of the extensive existence of colored waters throughout the world and found the study of stream humic substances to be a relatively virgin area of research. These scientists were influenced by soil humic studies (Flaig and Beutel-spacher, 1954 Kumada and Aizawa, 1958 Kononova, 1%1 Schnitzer and... 

Stream humic substances were isolated from water by vacuum concentration (Shapiro, 1957 Barth and Acheson, 1962 Black and Christman, 1963a Midwood and Felbeck, 1968), freeze concentration (Black and Christman, 1963a Shapiro, 1967), and solvent extraction (Shapiro, 1957 Barth and Acheson, 1962 Black and Christman, 1963a). Almost all humic substances studied were obtained from dilute organically colored waters, because sufficient quantities of stream humic substances for intensive studies could be isolated with the least effort, and because many uncolored waters were not believed to contain humic substances. Because the isolation of stream humic substances was so time consuming it was considered acceptable to use soil humic substances obtained by extraction of soils as representative of stream humic substances in water quality studies. 

During this period, the influence of soil science on the study of stream humic substances began to wane. Many believed that new and innovative tech-... 

In an excellent work by Reuter et al. (1983), serious shortcomings in the interpretation of previous oxidation data were demonstrated. They established that mild oxidation of stream humic substances produced large quantities of oxalic acid indicating stream humic substances to be predominantly aliphatic in nature, whereas strong or severe oxidation produced structures stabilized by a 4n -I- 2 overlapping 7r-electron system, attributed mainly to a series of benzene carboxylic acids. Structural interpretation based on the products of severe oxidation would lead one to falsely conclude a greater degree of aromaticity in stream humic substances than actually exists in the unoxidized material. 

Perdue, 1981) state, that soil is one source of stream humic substances or humic substance precursors, but to assume soil as the source or only source is exaggerated. To do so is to totally ignore other known sources / such as groundwater, decaying vegetation and litter, canopy drip, sewage, i and autochthonous material. 

A third issue is that even though the soil is one source of stream humic substances, it is not necessary that soil and stream humic substances have the same composition. If they were of the same composition, then stream humic substances would be primarily humic acids, because the humic cidjo J fulvic acid ratio in soil is approximately 3 1. However, as previously dis- -cussed in this chapter, stream humic substances are approximately 90% fulvic acids. One may say that fulvic acids are leached from soils in preference to humic acids. This may be true, but no one has shown water leachates of soil to contain fulvic acid of the same composition as in the bulk soil. Beck et al. (1974) state that meteoric waters percolating through soil will selectively mobilize nonrepresentative fractions of the soil organic matter. It should be emphasized that even if stream humic substances are the same as soil humic substances, one can not infer that one is the source of the other, but that the same precursors and humification process are probably operable in both soil and stream environments. 

If we do not know with certainty the potential roles of soil as different sources of stream humic substances, it is speculative to postulate the relative contribution of soil as compared to other sources of stream humic substances. Needless to say, there is an urgent need for experiments to determine the quantitative contribution of all the postulated sources of stream humic substances. 

Two additional sources of stream humic substances which are generally neglected but could be seasonally important are humic substances in ground-water and erosion or scouring of the stream bed during high discharges. As... 

The mechanism(s) for the formation of stream humic substances is not known. Several theories of formation of these substances have been formulated, but none have been supported with adequate systematic data. Most of the theories of formation are common to soil humic substances and few are unique to stream humic substances. Five of the general or overall theories are the following ... 

Other possible theories of stream humic substance formation are the browning reactions (the black substances produced by the action of acids on sugars) (Haworth, 1971) and the melanoidin reaction (the dark pigments produced when glucose, alanine or ammonia, and phenolic substances react with one another) (Hoering, 1973 Filip et al., 1974 Ertel and Hedges, 1983). These theories are discussed in detail by Stevenson in Chapter 2 of this book. 

As stated previously, the exact structure of none of the humic substances has been elucidated. The accumulated data on stream humic substances, the many recent advances in stream humic studies, and the better resolution of C NMR spectrometers enable a few comments to be made about the structure of stream humic substances. 

We know that the core or dominant structures of stream humic substances are primarily aliphatic and not aromatic, contrary to what has been theorized for decades. C NMR data indicate that only 16-20% of the carbon in fulvic acid is aromatic carbon. Even the humic acid is only comprised of approximately 30% aromatic carbon which also indicates that its nature is primarily aliphatic and not aromatic. 

The competitive metal complexation of stream humic substances does not follow the theoretical Irving-Williams stability series therefore, the relative order of complexation and the magnitude of the stability constant are variable. All the methods of stability-constant determination are limited in some way and often yield inconsistent values between and among methods (Saar and Weber, 1982). 

Stream humic substances are similar to other aquatic habitats in that fulvic acids predominate by a factor of approximately 9 1 over humic acids. The average concentration of stream humic substances of 2.2 mg C/L is higher than for most other aquatic environments, but this concentration is sometimes attained in the other aquatic environments. 

The fulvic/humic acid separation appears to be a meaningful, valid, and useful separation of stream humic substances. Stream humic acids are always higher in nitrogen content, aromatic carbons, methoxyl and phenolic reactive groups, higher in molecular weight, and more intense in color per carbon atom than stream fulvic acids. 

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