time, the organics in the mixture increasingly shifted toward a distribution characteristic of natural systems.

On the other hand, researchers have identified some important differences between natural and wastewater-derived organics. Some organic compounds found not at all or only at low concentrations in natural systems appear at much higher concentrations in wastewater even after it has been subjected to extensive treatment. Barber et al. (1996) illustrated the use of several of these organic compounds to determine the origins of organic contamination from municipal and industrial wastewater in the Mississippi River (Table 2-8).

In an attempt to identify distinctive features that could be used as indicators of wastewater contributions to a water sample, Peschel and Wildt (1988) compared various characteristics of treated wastewaters with those of natural waters. They reported that differences between wastewater organics and natural organic matter from the Ruhr River were too small to be useful for this purpose. On the other hand, Fujita et al. (1996) concluded that aggregate parameters were useful for following "longer term processes involved in the turnover of organic carbon in aquifers" and that specific organic compounds (such as EDTA and alkylphenoxy ethoxycarboxylates, or APDCs) were useful as markers of wastewater. In a similar study, researchers analyzed ground water in the Montebello Forebay area, which is partially recharged with reclaimed water in spreading basins (Nellor et al., 1984). They found that aliphatic compounds comprised a substantially greater portion of the hydrophobic acids (the "fulvic acid" fraction) and that aromatic compounds existed in smaller proportions in wells containing reclaimed water than in wells less affected by human activity. The distinction apparently persists for several years after reclaimed water is introduced into the aquifer. This relationship probably derives from the fact that, in unperturbed environments, the precursor for much of the natural organic matter in water is thought to be lignin, a highly aromatic polymer with limited solubility that serves as a major structural component of plants. Further, natural organic matter in reclaimed water has been subjected to more microbial activity than natural organic matter in unperturbed water, and microbial activity increases its solubility. Presumably for the same reason, the specific ultraviolet absorbance (or absorbance per unit mass of organic carbon) is lower in wastewater than in natural water without substantial wastewater input (Debroux et al., 1996).

Risks of Nonionic Detergents in Reuse Systems

A group of nonionic detergents known as alkylphenylpolyethoxylates (or APnEOs, where n represents the number of ethoxy groups in

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