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Implications to Human-Health and Ecosystems

Do detections automatically mean that harmful effects will occur?
No. The presence of "detectable" organic compounds does not mean that harmful human-health effects will occur because detections are often at low concentrations that were well below benchmarks that are protective of human health. USGS analytical methods are designed to measure low levels of compounds—typically as low as 0.02 part per billion and sometimes as low as parts per trillion—which is commonly 100 to 1,000 times lower than drinking-water standards. For perspective, reporting limits for public drinking-water commonly are set through Federal regulations at 0.5 microgram per liter, and water utilities generally are not required to measure below this limit. Detections reported in this study, therefore, do not necessarily indicate a concern to human health, but rather provide a characterization of the low-level environmental occurrence of a wide variety of chemicals not commonly monitored in sources of drinking water. The USGS low-level approach helps to identify emerging issues; to track changes in concentrations over time; and to provide a comprehensive screening for the types of compounds we can expect to see in source waters in different environmental settings across the country.

Is this a risk assessment? If not, why is USGS reporting on human-health effects?
No. The USGS report is NOT a risk assessment and findings should not be confused with risk assessments for specific compounds by the U.S. Environmental Protection Agency (USEPA). The USEPA and States have primary responsibility for risk assessment and risk communication at the Federal and State levels, respectively. Since the implementation of NAWQA in 1991, the USGS has been asked with increasing frequency about the public-health implications of its findings. Therefore, to help place findings in a context for understanding potential effects on human health, measured concentrations by NAWQA were compared to water-quality benchmarks derived from standards and guidelines established by the USEPA, toxicity values from USEPA risk assessments, and selected guidelines from other sources. The water-quality benchmarks are estimates of the concentrations below which adverse effects on humans are not expected to occur. The screening-level assessment is primarily intended to identify and prioritize needs for further investigation. It is not designed to evaluate specific effects of contaminants on human health and is not a substitute for comprehensive risk assessment, which includes consideration of many more factors, such as additional avenues of exposure.

What benchmarks are used to assess potential human-health effects?
Potential human-health effects are not directly assessed in this study, but a screening level assessment was done to identify and prioritize compounds that may warrant further investigation. To help place detections of compounds in a human-health context, annual mean concentrations were compared to the U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds, and to U.S. Geological Survey's (USGS) Health-Based Screening Levels (HBSLs) for unregulated compounds. HBSLs are non-enforceable guidelines that were developed by the USGS in collaboration with USEPA and others using USEPA methodologies and the most current USEPA peer-reviewed, publicly available human-health risk assessments and toxicity information (see USGS Fact Sheet 2005-3059, and the Web site, USGS Health-Based Screening Levels.

Were human-health benchmarks available for all compounds included in this study?
No, only about one-half have human-health benchmarks. Specifically, 38 of the compounds monitored in this study have an established MCL (U.S. Environmental Protection Agency) and 110 have an HBSL (USGS Health-Based Screening Levels).  HBSLs have not been developed for the remaining 129 unregulated compounds because of a lack of toxicity information. The potential human-health significance of these compounds, therefore, cannot be evaluated at this time.

How do detections in this study compare to existing human-health benchmarks?
Concentrations of detected compounds generally were less than 1 part per billion, and annual mean concentrations of all compounds were less than human-health benchmarks, which are available for about one-half of the compounds. As such, adverse effects to human health are unlikely to be caused by exposure to individual contaminants, subject to limitations of available human-health benchmarks.

What are the uncertainties and limitations to using a screening-level assessment to evaluate what detections may mean to human health?
Uncertainties are associated with screening-level assessments such as used in this study. Specifically, concentrations less than human-health benchmarks, such as USEPA Maximum Contaminant Levels (MCLs) and USGS Health-Based Screening Levels (HBSLs), indicate that adverse effects are unlikely to occur, even if water with such concentrations were to be ingested over a lifetime. Water containing concentrations greater than these benchmarks might be of potential human-health concern if the water were to be ingested as the primary drinking-water source without treatment for many years. The likelihood for adverse effects generally increases as concentrations increase above their benchmark values. If water containing concentrations greater than their benchmarks is ingested, however, it does not mean that adverse human-health effects will occur because: (1) human-health benchmarks are intentionally conservative (protective) and incorporate safety factors and conservative assumptions to account for uncertainty in the underlying toxicity information; and (2) models used to develop human-health benchmarks assume lifetime exposure of 2 liters of tap water per day, whereas actual exposure may be lower and for less than a lifetime.

Several limitations also are associated with screening-level assessments. First, some compounds analyzed in this assessment (total of 129) do not have human-health benchmarks (MCLs and HBSLs) because of a lack of peer-reviewed health risk assessments and toxicity information. Second, most of the available human-health benchmarks (MCLs and HBSLs) are based on toxicity information for individual chemicals, whereas NAWQA results indicate that mixtures of compounds occur in many samples. The long-term cumulative effects of low concentrations of multiple contaminants on human health currently are unknown.

What are the potential implications to aquatic health and ecosystems?
This 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. However, this study 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.

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