Stream humic substances

Quantification of each spectral region was carried out by cut and weigh methods (35) and are shown in Table VI for Como Creek. The aliphatic region I increased from 24.5% in stream humic substances to 43 and 40% in foam fulvic and humic acids, respectively. In contrast, the aromatic peak decreased from 34% in stream humic substances to 21 and 27% in foam fulvic and humic acids, respectively, while the... 

Surface Activity Measurements. The surface activity displayed by solutions of humic substances and raw foam samples from Como Creek and Suwannee River stream and foam samples was compared to the surface activity of an impure standard of commercial surfactant sodium dodecyl sulfate (SDS) and surface-tension measurements for both sites are shown in Figures 3a and 3b. Como Creek raw foam and foam-extract humic acid showed the greatest surface activity, with foam humic acid contributing to a lesser extent (Figure 3a). In contrast, Como Creek foam and foam-extract fulvic acid and stream humic substances showed little surface activity. Fulvic and humic acids from Suwannee River foam and foam extract showed comparable surface activity to the raw foam, and all samples were less surface active than the SDS (Figure 3b). Stream humic substances showed little surface activity and were comparable to Como Creek stream humic substances. 

In summary, the chemical characterization data show that humic substances in foam are an aliphatic, carboxyl-poor fraction, that are enriched in humic acid compared to stream humic substances. The carboxyl moiety appears to be the dominant ionic species affecting the surface activity of the humic substances, and foaming ability and foam stability are strongly dependent on pH as the average pKa of the organic acids is 4.2 (25). 

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. 

In this chapter, the three decades from the early 1950s to the present are arbitrarily divided into three periods based upon the philosophy of studying stream humic substances, the principal methods of their study, and instrumental capabilities. The three periods are the Awakening Period from 1950-1964, the Sephadex Period from 1964-1973, and the Resurgence Period from 1973 to the present. 

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... 

Gupta, 1964 Bartholomew and Clark, 1965) and became familiar with the previous pioneering soil organic research (Mulder, 1862 Schreiner and Sho-rey, 1914 Oden, 1919 Waksman, 1938). Therefore, this period was dominated by the application of methods, approaches, instrumentation, and philosophies used by soil scientists in studying soil humic substances to characterize stream humic substances. Stream humic substances were assumed to be the same as soil humic substances (Ponomareva and Ettinger, 1954 Shapiro, 1957 Wilson, 1959 Black and Christman, 1963a). 

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. 

The works of Packham (in England), Black, Christman, and Shapiro (in the United States) are frequently associated with this period. The extensive investigations on stream humic substances by Russians, such as Goryunova (1952,1954), Ponomareva and Ettinger (1954), Fotiyev (1970), and numerous others are generally unappreciated outside the USSR, because their works were not commonly translated into English. 

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. 

The humic-fulvic acid separation is believed to be a useful and meaningful characterization and separation procedure for stream humic substances. The clear-cut differences between stream humic and fulvic acids as discussed in the next section of this chapter strongly support the conclusion that it is a valid and meaningful separation. 

Because an exact structure or family of structures cannot be written for humic substances, we must continue to characterize these substances on the basis of several different chemical and physical properties. No one or group of these characteristics rigidly define stream humic substances, but the greater the number of general characteristics which fit an idealized conceptual model of the majority of researchers is the present nonexacting standard. A minimally recommended characterization would include the... 

Weber and Wilson (1975) faced this issue when they stated that it is merely an assumption that stream humic substances are the same as soil humic substances and that such has never been proven. There is no doubt, as Waksman (1938) and others (Malcolm and Durum, 1976 Reuter and... 

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 ... 

The first needs no theoretical discussion it merely assumes that stream humic substances consist of soil fulvic acid which has been leached or eroded from soils. 

A second theory, closely aligned with the first, assumes stream humic substances to be formed within the stream by the same processes as soil humic substances, whatever they may be. 

A third theory speculates that stream humic substances are soil fulvic acids leached from soil in the initial stages of humification and then modified, transformed, or aged by stream humification processes which result in humic substances unique to this aquatic environment. 

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