Organic Compounds in Clackamas River Water Used for Public Supply near Portland, Oregon, 2003-05

Prepared as part of the National Water-Quality Assessment Program, Source Water-Quality Assessment

Introduction Occurrence of Organic Compounds in Source Water

Figure 1. About 937 square miles of the Clackamas River basin are upstream of the community water system intake. Most of the basin is forested (82 percent of the area, mostly within the Mount Hood National Forest). Agricultural and urban land in the lower basin composes 7 and 3 percent of the area, respectively, and can potentially affect source water quality. Four public water utilities in the lower Clackamas River provide drinking water for about 300,000 people. The Clackamas River Water plant provides drinking water to about 25 percent of these customers.

What “Detections” May Mean to Human Health

U.S. Environmental Protection Agency (USEPA). Many of the Comparisons between source water and finished water were not intended to characterize treatment efficacy (see inset, “Finished Water Sampling, Water Treatment, and Significance of Comparison to Source Water”), but do provide a preliminary indication of the tendency for contaminants to pass through the treatment process.

Five of the 15 compounds commonly detected in source water were commonly detected in finished water, often at similarly low concentrations (fig. 2). Five compounds were commonly detected in finished water but not commonly detected in source water. These include bromodichloromethane and dibromochloromethane (disinfection by-products), tris(2-chloroethyl)phosphate and tributyl phosphate (flame retardants), and chloromethane.

Although chloroform was detected in 23 percent of source-water samples during the first phase of the study, it and one other disinfection by-product (bromodichloromethane) were detected in 100 percent of finished water samples. These compounds form when organic carbon present in source water reacts with disinfectants such as chlorine and (or) bromine, and their presence in finished drinking water is an expected outcome of the water treatment process (Krasner and others, 2006).

A Closer Look at Gasoline Hydrocarbons and Finished Water Sampling, Water Treatment, and Pesticides Significance of Comparisons to Source Water

Gasoline hydrocarbons and pesticides were the most commonly detected organic compounds in the Clackamas River during the Source Water-Quality Assessment (SWQA) study. These compounds originate from a variety of sources and occur in source and finished water.

Gasoline hydrocarbons—Compared with the other eight community water systems sampled during the USGS SWQA study, concentrations of several gasoline hydrocarbons, including toluene, xylene, and benzene, were highest in the Clackamas River (Kingsbury and others, 2008). In all, 14 gasoline hydrocarbons were detected, and the frequent occurrence of these compounds indicates a persistent source during much of the year (fig. 3). Sources of these compounds include vehicle emissions and fumes from fueling stations, runoff from roads and parking lots, ground-water discharge contaminated by leaky underground gasoline storage tanks, and exhaust from water craft, particularly two-stroke engines. The frequent occurrence of benzene likely is related to the naturally high concentrations in crude oil from Alaska, which is the primary source for gasoline refineries in the Pacific Northwest (Oregon Department of Environmental Quality, 2007). New USEPA legislation will require most refineries to reduce benzene levels before 2012, which should help reduce the amount of benzene from gasoline that enters the Clackamas River.

Pesticides—Although 15 pesticide compounds were detected during the SWQA study, 63 pesticides have been detected by the USGS in the Clackamas River basin since 2000 (Carpenter and others, 2008), with most detections occurring during storms. The greatest number of compounds detected and highest pesticide concentrations were found in tributaries of the lower Clackamas River that drain predominantly agricultural and (or) urban land. Pesticide concentrations and loads were highest in Deep and Rock Creeks, which drain mostly nursery and rural residential land. Pesticide yields (loads per unit area) were highest in streams draining urban and industrial areas (Cow, Carli, and Sieben Creeks). Potential uses of pesticides in the Clackamas River basin include applications to Christmas trees, nursery stock and other agricultural crops, landscaping and lawns, roads and other right-of-ways, golf courses, and forestland.

Figure 2. Comparisons between source and finished drinking water. Some pesticides (diuron, for example), flame retardants, and organic synthesis compounds were detected at similar concentrations in source and finished water (data points along the 1:1 line). Some compounds commonly detected in source water were removed or transformed during treatment and, therefore were not detected in finished water (data points on horizontal x-axis). Concentrations of disinfection by-products (compounds formed during treatment) plot along the vertical y-axis. Chloroform, which was occasionally detected in source water during the first phase of the study, was not detected in source water during the second phase, when both source and finished water were analyzed.

Potential Effects on Human Health

With the exception of gasoline hydrocarbons, disinfection by-products and the herbicide diuron, concentrations in source and finished water were less than or equal to 0.1 microgram per liter and always less than human-health benchmarks, which are available for about 60 percent of the compounds detected. On the basis of this screening-level assessment, adverse effects to human health are expected to be negligible (subject to limitations of available human-health benchmarks, see inset “Human-Health Benchmarks Used in This Assessment”).

Concentrations of organic compounds generally were less than 0.1 microgram per liter (μg/L) in finished drinking water; however, concentrations of 14 compounds commonly detected in source and (or) finished water were greater than or equal to 0.1 microgram per liter (table 2). In general, compounds with concentrations greater than 0.1 microgram per liter, such as gasoline hydrocarbons and the herbicide diuron, reflect their widespread use in the Clackamas River basin and their physical properties that allow them to persist in the environment (Gilliom and others, 2006; Zogorski and others, 2006).

Concentrations did not exceed USEPA drinking-water standards (Maximum Contaminant Level; MCLs) for regulated compounds in any sample (table 2). Concentrations also were less than USGS Health-Based Screening Levels (HBSLs)

Figure 3. Fourteen gasoline hydrocarbons (benzene, toluene, ethylbenzene, and xylene, plus ten others) were detected in the Clackamas River.

Human-Health Benchmarks Used in This Assessment

Concentrations of regulated compounds were compared to USEPA Maximum Contaminant Levels (MCLs), and concentrations of unregulated compounds that have USEPA published toxicity information were compared to USGS Health-Based Screening Levels (HBSLs; Toccalino and others, 2007). About 40 percent of the detected compounds do not have human-health benchmarks or adequate toxicity information for evaluating results in a human-health context. Additionally, human-health benchmarks are developed for individual compounds and not mixtures. The screening-level assessment provides an initial perspective on the potential importance of “man-made” organic compounds in source water; it is not a substitute for a comprehensive risk assessment, which includes many more factors, such as additional avenues of exposure.

established for selected unregulated compounds (see inset “Human-Health Benchmarks Used in This Assessment”). The NAWQA screening-level assessment identified two compounds (diuron and chloroform) that occurred at concentrations within a factor of 10 of their USEPA-MCL or USGS-HBSL human-health benchmarks (table 2). Concentrations of benzene in source water approached this “10 times below the benchmark” screening level, and given the frequent detection of this and other gasoline hydrocarbons, may warrant their consideration in low-concentration trends monitoring. Human-health benchmarks are not available for the remaining five compounds that were commonly detected at concentrations greater than 0.1 microgram per liter (table 2).

An important consideration in assessing potential effects for human health is the occurrence of mixtures of organic compounds in source and finished water. The median number of organic compounds detected in source- and finishedwater samples from the Clackamas River was four and five compounds, respectively. This was typical of findings at comparable community water systems sampled during the SWQA study having relatively small amounts of agricultural and urban land (Kingsbury and others, 2008). The potential human-health effects of mixtures of co-occurring organic compounds are largely unknown and have not been extensively studied. The effect of one compound on another’s toxicity may be additive, synergistic (Hayes and others, 2006), antagonistic, or may have no effect. With a few exceptions for pesticides with common modes of action, human-health benchmarks generally are not available for specific mixtures. Continued research is needed because human-health benchmarks generally are based on toxicity data for individual compounds, and the effects of low-level mixtures are not well understood (Gilliom and others, 2006).

Clackamas River Findings in a National Source Water-Quality Assessments by the NAWQA Context, and Possible Implications Program Conducted Across the Nation

• Increased emphasis on watershed management and source water protection strategies to help minimize the sources and transport of compounds to resources used for drinking water, and ultimately to finished water.
• Continued research to enhance toxicity information for commonly occurring unregulated compounds and mixtures that are detected frequently in source water and finished water.
• Further monitoring and assessment of compounds not commonly monitored in water supplies, but frequently present in finished water, which may ultimately identify or lead to the development of treatment technologies for their removal.

References Cited

USGS promotes public access to water-quality information This fact sheet, the USGS national data and investigations reports, and other information are available on the World Wide Web at http://water.usgs.gov/nawqa/swqa. Included at this web site are downloadable data on organic compound occurrence, information on sampling designs and methodology, background on data analyses, and frequently asked questions.

Contacts for additional information

Where this page came from

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