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Abstract-Realistic and reliable methods are needed for the accurate characterization of the potential ecological risks posed by pesticides. This study presents a sequence of risk assessment methods that are based on the comparison of exposure concentrations to laboratory-derived toxicity data for representative species of aquatic organisms exposed to pesticides. The sequence of methods progresses from single-value quotient calculations to comparison of cumulative distributions of exposure and toxicity data and culminates with the use of Monte Carlo methods to estimate distributions of quotients. To illustrate the sequence of increasingly quantitative methods, we used a case study of the ecological risks posed by diquat dibromide in U.S. regional lakes and farm ponds. Diquat dibromide is a well-studied aquatic herbicide and is one of only a few pesticides registered for direct use in aquatic systems. The exposure concentration for the farm pond scenario used in the U.S. Environmental Protection Agency reregistration eligibility decision for diquat dibromide was used. In addition, exposure concentrations were developed for several regional lake and farm pond scenarios using the Exposure Analysis Modeling System (EXAMSII) methodology with separate, region-specific parameter values and simulated diquat dibromide application rates. Except for the results obtained in the conservative quotient method, the calculations for diquat dibromide were consistent with its demonstrated high safety. This suggests that a minimal ecological impact to benthic invertebrates and fish exists. Aquatic plants in the vicinity of application to surface waters appear to be at risk, and this is expected, as diquat dibromide kills aquatic plants. Comparisons of cumulative distributions of exposure and toxicity data provided the most quantitative approach to characterizing risks. The probabilistic quotient approach does not specifically estimate ecological risk. However, the implicit correlation between increasing values of quotients and increased potential risk does suggest the utility of these methods for identifying pesticides as acceptable for registration and use (i.e., minimal risk compared to benefits) or unacceptable.

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Relatively little is known about contaminants in reptiles, particularly snakes. The concentrations of arsenic, cadmium, chromium, lead, manganese, mercury, and selenium were examined in blood and skin of 46 northern water snakes (Nerodia sipedon) in Tennessee and correlated with concentrations in internal tissues (liver, kidney, muscle) to determine if blood or skin could serve as a nonlethal indicator of internal metal exposure or body burden. Snakes were collected from the East Fork Poplar Creek (EFPC) within the United States Department of Energy's Y-12 National Security Complex (part of the Oak Ridge National Laboratory) and from a reference stretch of the Little River in East Tennessee. For blood, the only consistent positive correlations with internal organs were for mercury, and correlations were low except for muscle. Skin showed significant positive correlations with all three organs for mercury, chromium, selenium, and lead. For manganese and cadmium, skin level was positively correlated with liver level. Blood generally reflects recent exposure, not necessarily body burden, but in water snakes it correlates with body burden for mercury. Skin proved useful for more metals, although patterns were not necessarily consistent across sex and locality subgroups. The most consistent pattern was for mercury, the metal of greatest concern in many aquatic ecosystems, including EFPC.

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We compared the levels of arsenic, cadmium, chromium, lead, manganese, mercury, and selenium in the blood, kidney, liver, muscle, and skin of northern water snakes (Nerodia sipedon) collected from the upper reach of East Fork Poplar Creek (EFPC) within the United States Department of Energy's (USDOE's) Y-12 National Security Complex with concentrations in tissues of northern water snakes from a reference reach of the Little River downstream from the Great Smoky Mountains National Park in East Tennessee. Our objectives were to determine whether concentrations of these metals were higher in tissues of water snakes collected from EFPC compared with the reference site and if northern water snakes were suitable bioindicators of metal contamination. Except for chromium, metal levels were significantly higher in tissues (kidney, liver, muscle, and skin) of EFPC northern water snakes compared with those in tissues of snakes from the reference site. Although female northern water snakes were significantly larger than male snakes, their tissues did not contain significantly higher metal concentrations compared with those from male snakes, possibly because of maternal transfer of metals to eggs. This study was the first to examine the accumulation of contaminants resulting from the operations of the USDOE's Oak Ridge Reservation in snakes.

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