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Selected Findings

Are man-made organic compounds in source and treated water?
Yes. About one-half (134) of the organic compounds analyzed in this study were detected in at least one source-water sample collected at the nine source-water intakes. A total of 119 compounds were not detected at all. The most commonly detected compounds in source water were the disinfection by-product, chloroform; the herbicides simazine, atrazine, metolachlor, prometon, 2,4-D, and deethylatrazine (DEA); and the fragrance hexahydrohexamtheylcyclopentabenzopyran (HHCB).

About two-thirds of the compounds detected in 10 percent or more of source-water samples also were detected in treated-water samples. However, concentrations in source and treated water generally were low—generally less than 1 part per billion (microgram per liter) (which is equivalent to one thimble of water in an Olympic pool), and annual mean concentrations of all compounds were less than human-health benchmarks. On the basis of the NAWQA screening-level assessment of concentrations detected in this study, adverse effects to human health are not expected, subject to limitations of available human-health benchmarks.

Are the findings surprising?
No. Many of the compounds detected most commonly in source- and treated-water samples at the community water systems also are among those most commonly detected in ambient stream water sampled across the Nation by NAWQA during 1992-2001 (Gilliom and others, 2006, http://pubs.usgs.gov/circ/2005/1291). Many of the compounds detected generally are associated with streams that drain considerable agricultural and urban land use (as reported in Gilliom and others, 2006) and with upstream wastewater facilities.

What factors complicate the patterns of occurrence?
Occurrence is complex because of mixtures or the “co-occurrence” of many compounds in individual samples. More than 75 percent of source- and treated-water samples contained five or more organic compounds, often including parent compounds and degradates. The common occurrence of compound mixtures means that the total combined toxicity in source water may be greater than that of any single compound that is present. Continued research is needed because human-health benchmarks are based on toxicity data for individual compounds, and the additive or synergistic effects of mixtures of compounds at low levels are not well understood.

Do compounds occur all year?
Multiple samples were collected to characterize variability in source- and treated-water samples. Although the sampling strategy can be characterized as fixed frequency, a range of flow conditions were sampled at most sites.  Specifically, samples generally were collected monthly with as many as four additional samples collected during high flow or base-flow conditions when water-quality changes might be expected or when higher concentrations of some compounds are most likely to occur.

Findings indicate that variability depends on the compound. Some compounds were detected year-round in source- and treated-water samples, such as, chloroform, and the musk fragrance hexahydrohexamtheylcyclopentabenzopyran (HHCB). Wastewater discharge from upstream may be a relatively constant source for these types of compounds. Selected herbicides, like atrazine and simazine, also were detected year-round, but the magnitude and timing varied considerably. For example, concentrations of atrazine at sites with significant agriculture upstream (such as the White River) varied by tenfold or more during the year. The highest concentrations of these herbicides generally occurred in the spring following chemical applications in row-crop areas.

Is the water getting better or worse?
This study is not designed to look at trends. Repeated sampling and re-assessments for trends at the individual sites are not planned.

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