A modeling study was conducted to examine the distribution of concentrations of the antimicrobial triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether) in rivers following discharge from wastewater treatment plants (WWTPs). Most uses of triclosan are disposed of down residential drains and ultimately reach WWTPs. A modeling analysis was conducted to simulate the discharge of triclosan in WWTP effluents to rivers and calculate the expected concentrations based on characteristics of the reach where the discharge occurred, the estimated concentration of triclosan in the WWTP effluent, and the physicochemical properties of triclosan. A probabilistic exposure assessment was conducted based on data on the characteristics of U.S. reaches receiving wastewater discharges and the physicochemical characteristics of triclosan. A risk assessment was conducted by comparing the estimated concentrations with toxicity endpoint concentrations for species representative of key ecological groups. For fish and invertebrates, neither acute nor chronic risks are of concern, and no concerns exist for vascular aquatic plants. However, certain types of algae are the most sensitive species to triclosan by more than an order of magnitude than other algal or aquatic plant species. For these algae, the potential exists for some risk from triclosan exposure near the WWTP discharge location during low-flow-rate periods for some WWTPs with small dilutions. The risks downstream are lower because of dissipation of triclosan.

Arctic seawater concentrations of two currently used pesticides, endosulfan and gama-HCH, were collated from a variety of cruises undertaken throughout the 1990s up to 2000 for different regions of the Arctic Ocean. Surface seawater concentrations for alpha- and beta-endosulfan ranged from <0.1-8.8 (mean 2.3) pg/L and 0.1-7.8 (mean 1.5) pg/L, while gamma-HCH concentrations were approximately 100 fold higher than alpha-endosulfan, ranging between <0.70 and 894 (mean 250) pg/L. Geographical distributions for alpha-endosulfan revealed the highest concentrations in the western Arctic, specifically in the Bering and Chukchi Seas with lowest levels toward the central Arctic Ocean. In contrast, gamma-HCH revealed higher concentrations toward the central Arctic Ocean, with additional high concentrations in the coastal regions near Barrow, Alaska and the White Sea in northwest Russia, respectively. A fugacity approach was employed to assess the net direction of air-water transfer of these two pesticides, using coupled seawater and air concentrations. For alpha-endosulfan, water-air fugacity ratios (FR) were all <1 indicating net deposition to all regions of the Arctic Ocean, with the lowest values (0.1-0.2) evident in the Canadian Archipelago. Given the uncertainty in the temperature-adjusted Henry's Law constant (factor approximately10), it is plausible that equilibrium may have been reached for this compound in the western fringes of the Arctic Ocean where the highest water concentrations were observed. Similarly, FR values for gamma-HCH were generally <1 and in agreement

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Groundwater is becoming an increasingly important resource, but it is apparent that it is a resource that is increasing under threat from unsustainable utilization and contamination. It is principally because of its importance as a drinking water resource that measures have been taken by regulatory authorities to protect groundwater from contamination from pesticides and other chemicals. Therefore, significant improvements in model complexity access to chemical and environmental data have increased the quality of risk assessment carried out by both regulatory and industry bodies.