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About the Study

Where can I learn more about the detailed study design and results?
A summary of the study design and objectives is provided in a USGS Fact Sheet, available at http://pubs.usgs.gov/fs/2007/3069/.

Technical findings are presented in a USGS Scientific Investigations Report (http://pubs.usgs.gov/sir/2008/5208/), and an overview of findings is provided in a USGS Fact Sheet (http://pubs.usgs.gov/fs/2008/3094/).

Detailed technical information on the study design and analytical methods are available in Carter and others (2007) (http://pubs.usgs.gov/ds/2007/268/).

Please note that USGS findings and information (as described above) represent drinking-water results at only nine community water systems in the United States. Specific local information on drinking water may be found in a Consumer Confidence Report for which utilities are required to provide general information regarding the source(s) of drinking water, the treatment process, and the levels of detected contaminants that are regulated by a primary drinking-water regulation. In addition, supporting information on drinking water and regulations can be found at http://www.epa.gov/safewater/contaminants/index.html.  Additional information on selected chemicals in the environment can be found at the USEPA’s Integrated Risk Information System (http://www.epa.gov/iris) or at the Agency for Toxic Substances and Disease Registry Web site (http://www.atsdr.cdc.gov/toxfaq.html).

Please also note that this study includes some preliminary comparisons to existing human-health benchmarks for drinking water, such as benchmarks for regulated contaminants by the USEPA. The study did not look at implications to aquatic ecosystems or health and, specifically, no comparisons were done between concentrations in source water and aquatic-life criteria. This study, however, did include many of the same compounds measured in ambient water samples collected from 186 streams during 1992-2001 by Gilliom and others (2006). Information on the potential significance of pesticides to aquatic life can be accessed at http://pubs.usgs.gov/circ/2005/1291.

What sites are included in the study?
Sampling and assessments were completed at community water systems with intakes on nine streams, including on White River in Indiana; Elm Fork Trinity River in Texas; Potomac River in Maryland; Neuse River in North Carolina; Chattahoochee River in Georgia; Running Gutter Brook in Massachusetts; Clackamas River in Oregon; Truckee River in Nevada; and Cache La Poudre in Colorado. Characteristics of the community water systems and their watersheds are provided on Table 1 in the USGS Scientific Investigations Report http://pubs.usgs.gov/sir/2008/5208/.

Specific sampling locations of river intakes are not disclosed in reports to meet policy and source-water security purposes under USGS Homeland Security activities. Participating community water systems, however, do receive USGS results and may use and distribute the data as they choose.

Are sites representative of other community water systems?
A sample set of nine sites cannot possibly serve to represent the population of more than 50,000 community water systems across the United States. However, the findings do provide a preliminary assessment of the kinds of compounds that can be expected in relatively large community water systems with conventional treatment. Specifically, the nine community water systems generally represent conventional treatment, consisting of coagulation, flocculation, sedimentation, filtration, chlorine or sodium hypochlorite disinfection, and clear well storage. Additional treatment steps, such as chemical oxidation, pH adjustment, and use of powdered activated carbon (PAC) for seasonal taste and odor issues or removal of organic compounds, are used at several of the systems. Eight of the nine community water systems are categorized as “large” or “very large” water systems, defined by the USEPA as providing water to more than 10,000 and 100,000 people, respectively (http://www.epa.gov/safewater/pws/factoids.html). The size of the contributing watershed to these eight stream sites ranges from about 480 to 11,500 square miles (mi²), with most between about 1,000 and 1,500 mi².  Land use varies among the sites, ranging from relatively undeveloped and forested to intensely agricultural.

Will additional sites be sampled?
The NAWQA Program is planning an additional 21 surface-water assessments through 2013 (http://pubs.usgs.gov/fs/2007/3069/). The 30 systems, which is a small number relative to the number of community water systems across the country, are not intended to comprehensively portray the quality of our Nation’s source waters. They are, however, intended to improve understanding of ambient resource conditions in a drinking-water-supply context. Specific information on the study design is available in Carter and others (2007) (http://pubs.usgs.gov/ds/2007/268/).

How are the sites selected for study?
In general, the community water systems are selected to meet several criteria. Specifically, sites are located in study areas previously sampled by the NAWQA Program during 1992-2001 where data on source water and treated water can be compared to other NAWQA data collected from streams across the country. The community water systems also are generally located on free-flowing reaches of streams. In addition, they are generally single-source systems, with little or no blending of other source waters and are relatively large (serving more than 10,000 people). The type of water treatment or prior monitoring results, including those for compliance monitoring, are not considered in the selection process.

What compounds were monitored in this study?
This study characterizes the occurrence of about 260 anthropogenic (or man-made) organic compounds, including pesticides, solvents, gasoline hydrocarbons, personal-care and domestic-use products, disinfection by-products, and manufacturing additives. The compounds included in this study do not include pharmaceuticals, hormones, steroids, and various other chemicals often associated with wastewaters (including municipal wastewater discharges and livestock agricultural facilities). A national reconnaissance study for pharmaceuticals and other organic wastewater contaminants in untreated drinking-water sources was completed in a separate USGS study by Focazio and others (FAQsdoi:10.1016/j.scitotenv.2008.02.021).  

Do facilities generally monitor for these compounds?
Most of the compounds are unregulated in drinking water and therefore are not required compounds for monitoring by drinking-water suppliers.

What water was sampled at the community water systems?
Source-water samples were collected as close as practical to the drinking-water intake or at a raw water tap. Water samples also were collected following all water treatment, prior to the water entering the distribution system, and after the source-water samples were collected, to account for the residence time in the water-treatment plants.

Do facilities treat the water for removal of the compounds included in this study?
In general, the types of treatment steps used by the systems (and most across the Nation) are not designed specifically to remove most of the organic compounds monitored in this study. Findings on the occurrence in treated water are therefore not intended to characterize treatment efficacy. Rather, they provide a preliminary indication of the potential significance of the presence of organic compounds most commonly detected in source water to the quality of treated water. Findings are intended to evaluate what is in the source water to help guide those involved in decisions on treatment processes in the future. Treatment used by the systems generally is considered conventional, including steps of coagulation, flocculation, sedimentation, filtration, and disinfection. Three of the systems differ in that one water system uses slow sand filtration and disinfection; one adds ozone to the source water as a preliminary step to conventional treatment; and a third system uses a direct filtration treatment plant that follows steps used in conventional treatment. Four of the systems treat their water intermittently with powdered activated carbon (PAC), primarily to remove taste and odor compounds.

How did samples of source water relate to samples of treated water regarding timing?
The study was designed to allow time between collection of the source-water and treated-water samples and account for the retention time in the treatment plant (which ranged from 1 hour to 5 days). However, the timing is not perfect (for example, retention time can vary as a result of changes in water demand), which can introduce some uncertainty in comparisons between compounds in source and treated water. It also is possible that some compounds detected in source water degrade or transform during the treatment process into other compounds, some of which may not have been monitored as part of this study.

Does the study link findings to specific sources?
No. The study was not designed to examine specific sources and (or) factors causing and affecting the occurrence and concentrations of compounds in source water. However, additional perspective is added by highlighting general patterns and associations as appropriate, including those related to land use, wastewater discharge, streamflow, and seasonality.

Did the study look at public supply wells?
Yes. A companion study is scheduled for release in 2009 that summarizes the occurrence of the same organic compounds in ground water supplying high-production wells for 15 community water systems and the associated finished water.

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