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Engineered nanomaterials (ENMs) are increasingly entering the environment with uncertain consequences including potential ecological effects. Various research communities view differently whether ecotoxicological testing of ENMs should be conducted using environmentally relevant concentrations—where observing outcomes is difficult—versus higher ENM doses, where responses are observable. What exposure conditions are typically used in assessing ENM hazards to populations? What conditions are used to test ecosystem-scale hazards? What is known regarding actual ENMs in the environment, via measurements or modeling simulations? How should exposure conditions, ENM transformation, dose, and body burden be used in interpreting biological and computational findings for assessing risks? These questions were addressed in the context of this critical review. As a result, three main recommendations emerged. First, researchers should improve ecotoxicology of ENMs by choosing test endpoints, duration, and study conditions—including ENM test concentrations—that align with realistic exposure scenarios. Second, testing should proceed via tiers with iterative feedback that informs experiments at other levels of biological organization. Finally, environmental realism in ENM hazard assessments should involve greater coordination among ENM quantitative analysts, exposure modelers, and ecotoxicologists, across government, industry, and academia.

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The microbial processes that occur in the subsurface under a typical Midwest agricultural soil were studied. A 26-m bore was installed in November of 1988 at a site of the Purdue University Agronomy Research Center. Aseptic collections of soil materials were made at 17 different depths. Physical analysis indicated that the site contained up to 14 different strata. The site materials were primarily glacial tills with a high carbonate content. The N, P, and organic C contents of sediments tended to decrease with depth. Ambient water content was generally less than the water content, which corresponds to a-0.3-bar equivalent. No pesticides were detected in the samples, and degradation of added "4C-labeled pesticides (atrazine and metolachlor) was not detected in * Corresponding author. t Paper no. 12,887 of the Purdue Agricultural Experiment Station series.

Abstract-The molecular complexity of imazaquin and presence of ionizable functional groups limits the ability to predict sorption behavior from single soil parameters such as organic carbon content. Partition coefficients (K p ) for both neutral and anionic forms of imazaquin as well as the effects of solution ionic strength and composition were investigated to more adequately describe sorption of imazaquin in soil. Soils representing a range of characteristics were evaluated, including soils with permanent negative or variable surface charge. Imazaquin retention resulted from combined sorption for the neutral (K oc,n , 1,110 Ϯ 80 L/kg) and anionic (K oc,a , 38 Ϯ 20 L/kg) forms. Imazaquin sorption was best correlated to soil organic carbon content and soil-solution pH. However, results indicated that positively charged Fe 2ϩ and Al 3ϩ oxyhydroxides contribute to sorption of the organic anion; thus mineral surfaces contributed to sorption in soils with low organic carbon content. The effects of electrolyte matrices on imazaquin sorption were accounted for by concomitant changes in pH. However, enhanced imazaquin sorption was observed with increasing ionic strength for soils where pH-induced changes in speciation were negligible, indicating the role of mechanisms other than weak hydrophobic interactions. Addition of H 2 P significantly decreased imazaquin sorption, especially in weathered soils.