Aquatic environment fractions

Humic acids (HA) and fulvic acids (FA) are the main components of humic substances (HS), which are the most chemically and biochemically active and widely spread fractions of nonliving natural organic matter in all terrestrial and aquatic environments. They comprise a chemically and physically heterogeneous group of substances with colloidal, polydis-persed, polyelectrolyte characteristics and mixed aliphatic and aromatic nature (Senesi and Loffredo 1999). 

In the (aquatic) environment elements occur in particulate-, colloidal- and dissolved forms. These forms are usually distinguished by filtration or centrifugation. Traditionally, a 0.45 um (membrane)- filter separates the particulate from the dissolved forms. This may result in the passage of colloidal fractions through the filter, classifying colloidal matter incorrectly within the dissolved fraction. Although the interaction between dissolved and particulate (surface) fractions cannot be neglected, it is common in speciation studies to consider the "dissolved" fraction. The dissolved forms of trace elements are mainly present as ... 

To maintain a focus on the use of tracers in DOM fractions, this chapter will present only brief descriptions of studies of bulk DOM properties, and will focus primarily on the use of trace moieties from the fulvic acid fraction in freshwater aquatic environments. In addition to being a major DOM fraction, fulvic acid is biogeochemically reactive in natural waters (see Maranger and Pullin, Chapter 8 Chin, Chapter 7 Moran and Covert, Chapter 10). Furthermore, current fractionation methods allow for relatively straightforward isolation of small quantities of fulvic acid from small volume filtered water samples (100-200 mL) in a reproducible manner, as well as for isolation of larger preparative quantities of material. We present examples to illustrate the use of particular trace moieties but do not present a comprehensive review of each trace moiety. 

FIGURE 5 Relative fraction of bound pyrene ( ) or 2,2, 5-PCB ( ) to DOM isolated from various aquatic environments. 

The hydrophobic nature of CDDs, combined with their great affinity for organic carbon, suggests that a major proportion of CDDs in the aquatic environment is sorbed to organic matter and sediment. Because only a minute fraction of CDDs are dissolved in the natural environment, bioconcentration is not the primary route of exposure for most aquatic organisms. Whereas the term bioconcentration is defined as the uptake of a chemical from water only, the term bioaccumulation refers to the combined uptake of a chemical from both dietary sources (e.g., food) and water. A bioaccumulation factor (BAF) that includes the ingestion route of uptake can be calculated based on fish uptake from water, food, and sediment (Sherman et al. 1992). 

The combined influences of solubilization, evaporation, and oxidation are known as weathering. Weathering preferentially removes the lighter hydrocarbon fractions, leaving a residual material made up of relatively heavy hydrocarbons. Over the shorter term in aquatic environments, this residuum forms a stable water-in-oil emulsion known as mousse, which is the material that usually impacts shorelines after an offshore spill. The mousse combines with sediment particles on the shore to form sticky patties of oil and sand, which eventually form asphaltic lumps. 

Exposure to crude oil can occur through both direct contact with the material and contact with environmental media contaminated with crude oil. The primary route of exposure in the environment to crude oil is direct dermal contact with liquid oil. In the workplace, the primary route of exposure is also dermal contact. Inhalation exposure to crude oil could occur through the production of oil mists or through inhalation of the volatile fraction of the crude oil. Exposure to crude oil through contact with environmental media also constitutes significant routes of exposure. This can occur at spill sites, in former oil fields developed into other uses, or areas of natural oil seeps. Dermal contact or incidental ingestion of soil contaminated with crude oil, as well as the inhalation of dust from crude oil-contaminated soil are common at crude oil contaminated sites. Both human and animal exposures can occur when crude oil is released into the aquatic environment. 

In aquatic environments, the heavier and less vola-tile/soluble compounds in crude oil will adsorb to suspended solids and subsequently settle in the sediments. Some heavy fractions with high density may sink into the sediment. This happens after the initial removal of the smaller and more volatile chemicals by either dissolution or volatilization. This is followed by biodegradation of those crude oil constituents that can serve as a food source for bacteria. Biodegradation is a significant mechanism for removal of hydrocarbons released into the environment. However, this generally occurs on the order of months and years. It is not believed that there is significant bioaccumulation of petroleum hydrocarbons in aquatic organisms. 

Varanasi, U., W.M. Baird and T.A. Smolarek. Metabolic activation of PAHs in subcellular fractions and cell cultures from aquatic and terrestrial species. In Metabolism of Polycyclic Aromatic Hydrocarbons in the Aquatic Environment, edited by U. Varanasi, Boca Raton, FL, CRC Press, 1989, Chapter 6. 

I. 0). The isotopic composition of alkanes and of other classes of compounds can also be used as a general source input indicator. Isotopic fractionation resulting from the metabolic pathways involved in the synthesis of biologically produced compounds, when preserved in a dlagenetlc product, is frequently used to differentiate between terrestrial and aquatic sources. Hydrogen and carbon isotopic compositions of biogenic methanes from shallow aquatic environments is discussed in a later chapter of this volume (R. A. Burke and W. M. Sackett). The applicability of carbon isotopic data to tracing the source of deep-sea Mesozoic sediments is discussed by R.M. Joyce and E. S. Van Vleet. 

Senum and Gaffney (49). Biomass burning and fossil fuels are the only known significant sources of methane more 13c-enrlched than -49 /oo (48), and they apparently account for a relatively small fraction of the total input. Bubble ebullition Is the dominant mode of methane Input to the atmosphere from some shallow freshwater (50) and marine (51) aquatic environments. All of the gas samples analyzed for this work were collected as bubbles from sediments covered by very shallow (Im or less) water. Three of the freshwater lakes (Crescent Lake, Mirror Lake and Kilmer Pond) have been observed to release gas bubbles from their sediments, and It Is very likely that active bubbling also occurs in the other environments studied here. There is a good deal of variation ( v24°/oo) In the data presented here (Table l) ... 

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