Between March 2003 and January 2006, 863 water samples were collected from streams in seven urban watersheds with varying land uses within or near the City of Atlanta, Georgia. Sixty-four sampling sites representing three site types were established in those watersheds. The first type consisted of sites within three watersheds not affected by combined sewer overflows; these were designated as the control basins. The second and third site types were established in four watersheds and were designated as sites upstream or downstream from combined sewer outfalls.
Stream samples collected during the study were analyzed for major ions, nutrients, trace metals, and 60 organic compounds commonly found in wastewater (organic wastewater-indicator compounds, OWICs). Inorganic constituents were analyzed to discern possible relations between OWICs and urban runoff, sewage effluent, or combined sewer overflows (CSOs). The OWICs were grouped into nine compound classes based either on an already accepted class of compounds (such as pesticide) or the manner in which the compounds are used (such as automotive uses). The compounds benzo(a)pyrene, 4-cumylphenol, 3-tert-butyl-4-hydroxyanisole (BHA), isophorone, isoquinoline, metolachlor, metalaxyl, and 4-octylphenol were not detected in any sample collected during the study.
As many as 33 OWICs were detected above study reporting levels in water samples collected from streams in the Intrenchment Creek, Peachtree Creek, Proctor Creek, and South River watersheds (basins with CSOs), a number markedly higher than the number detected in water samples from the control basins (watersheds without CSOs). Several compounds known to disrupt the endocrine systems of aquatic biota were among the compounds detected.
The median numbers of OWICs detected in base-flow samples from the control basins ranged from 3 to 4 and 7 to 9 in stormflow samples, while the median in base-flow samples from the four CSO-affected watersheds ranged from 4 to 16 and 11 to 19 in stormflow samples. The detection frequencies and concentrations of OWICs in water samples varied depending on flow conditions during sample collection; however, regardless of flow condition, the total OWICs concentrations were strongly related to the numbers of OWICs detected in these samples. In addition, the median number of detectable OWICs and total OWIC concentrations increased linearly with increasing impervious area and stream flashiness (flashiness indicates the rapidity with which streamflow responds to high rainfall amounts).
Four compounds-tris(2-butoxyethyl) phosphate (TBEP), tris(2-chloroethyl) phosphate (TCEP), bromacil, and cholesterol-were detected at concentrations greater than study reporting levels in at least 45 percent of all samples collected during the study. On a broad scale, the seasonal distributions of OWICs detected in samples collected during the study period were consistent with use patterns or urban activity, but were markedly different and more variable within individual watersheds.
Seven of the nine OWIC classes were detected with greater frequency in base-flow samples from sites downstream from CSOs than from those upstream from CSOs or from control-basin sites; these OWICs also were detected in a greater percentage of base-flow samples from upstream than control-basin sites. Polycyclic aromatic hydrocarbon (PAH) and automotive-use compounds were detected with similar frequency in base-flow samples from all sites. The pesticides and industrial-use compounds were detected with similar frequency in base-flow samples from sites upstream and downstream from CSOs. The compounds 7-acetyl-1,1,3,4,4,6-hexamethyl tetrahydronaphthalene (AHTN), 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethyl cyclopenta-g-2-benzopyran (HHCB), and triclosan were detected with greater frequency in base-flow than in stormflow samples from all sites. The detection frequencies of these three compounds were particularly high in base-flow samples from downstream sites, especially those from the Intrenchment Creek watershed.
In stormflow samples, only the industrial-use compounds were detected with similar frequency among samples from control basins, and upstream and downstream sites. Caffeine, camphor, and menthol were detected in a greater percentage of stormflow than base-flow samples from all sites. The disinfectant byproduct bromoform was detected with the highest frequency in base-flow samples from the upstream sites, particularly those from the South River watershed.
Typically, compounds in the pesticide class were detected with similar frequency in base-flow and stormflow samples from upstream and downstream sites, although bromacil and carbaryl were the exceptions. Bromacil was detected with greatest frequency in base-flow samples from all sites, but was detected in a larger percentage of base-flow samples from upstream and downstream sites, especially upstream and downstream sites in the Proctor Creek and South River watersheds. Carbaryl, however, was detected in a greater percentage of stormflow samples from all sites. Although collectively the industrial-use compounds were detected in more stormflow than base-flow samples, tetrachloroethene (PCE) was detected in more base-flow than stormflow samples at all sites. More specifically, PCE was detected with the highest frequency in samples from the upstream sites, particularly those from the Proctor Creek watershed.
The similarity in the pattern and distribution of OWICs in samples at sites upstream and downstream from known CSO outfalls indicates that CSOs were not the dominant source of OWICs during the study period. Other sources may include non-sewage discharges-both permitted, permitted but out of compliance, and non-permitted, contaminated groundwater from leaking sewer lines or septic systems, sanitary-sewer overflows, or dry-weather runoff from outdoor water use. These OWICs may be better suited for identifying sewage-contaminated groundwater than sewage-contaminated surface water because groundwater is not typically affected by the OWICs that are more common in urban runoff.