How well can toxicity data be extrapolated among species and endpoints? Finally, research 
efforts should address the need to describe and test a complete methodology for conducting 
assessments and developing criteria. 
Research Needs 
Per the conceptual model presented in Figure 10, fundamental tools of assessments are the 
models linking the state variables. Brief descriptions of each model and its research needs are as 
follows: 
Exposure Model 
The exposure model should produce a description of the spatial/temporal distribution and the 
speciation of the chemical that satisfies the input needs of the effects models. It must also 
describe other physical and chemical properties that can affect toxicological re^nse. Because 
NHEERL research primarily addresses effects assessment, only a brief overview of exposure 
research needs will be given here. These do, however, represent significant knowledge gaps that 
need to be addressed if improved risk-based criteria are to be implemented. 
The exposure model must assess physical transport, degradation, speciation, and partitioning of 
the chemical. Current implementations of WQC and WQS often use simple models that will not 
support more comprehensive characterizations of risks. For physical transport, the primary need 
is to adapt and apply better methods and models that are available. In particular, better risk 
characterizations need dynamic, two- and three-dimensional models, which can more fully 
describe the spectrum of exposures experienced by biological receptors. 
Degradation is not an issue for elemental toxicants such as metals, but is of utmost important for 
ammonia. Decomposition of nitrogen-containing organic matter produces ammcxiia, while 
nitrification in oxygenated water reduces ammonia concentrations. Nitrification is reduced at 
colder temperatures, resulting in higher and more widespread ammonia concentrations during 
winter months. More comprehensive characterization of risk will require more complete 
modeling of the spatial and seasonal distribution of ammonia. Of particular concern is the 
ammonia concentrations in or near the sediment that results from the interaction of organic 
matter decomposition, nitrification, and mixing processes. 
Speciation is well characterized for ammonia, but current models for metal speciation have 
considerable uncertainty, especially regarding complexation by organic matter. There is a need 
both to improve current predictive models and also to develop analytical techniques that can 
reliably and efficiently measure metal speciation in laboratory tests and natural systems. 
Whether by model or measurement, these methods must describe speciation sufficiently for the 
interpretation and application of the effects of various chemical parameters on metals toxicity. 
Partitioning between sediment and overlying water is of utmost importance to assessing the risk 
of both ammonia and metals. Exposure analyses for the application of WQC need to incorporate 
the role of sediment as a source or sink of chemical, processes that entail considerable 
uncertainty. Currently there are no published EPA guidelines for ammonia in sediment, but 
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