The rest of this section will further discuss the improvements needed in aquatic risk assessment 
methods and identify the research and development efforts needed to realize these improvements. 
This will be done in the context of conceptual models which identify the essential components of 
assessments and emphasize the link of this research to the risk assessment framework. The 
nature of the assessments and the needed research can vary depending on the chemical and 
system of concern. One issue of particular importance is the degree to which chemicals 
bioaccumulate. Risk assessments for highly bioaccumulative toxicants will differ in some key 
features from assessment for chemicals which have low bioaccumulation. For bioaccumulative 
toxicants, dietary exposure will be especially important, residue-based dose-response models will 
be more important, and bioaccumulation models will be needed for dose determinations. 
Toxicokinetics for these chemicals are generally slow, so acute toxicity and short-term temporal 
variation in exposures will often, but not always, be of less importance than for 
nonbioaccumulative toxicants. Biomagnification makes risk relatively more important for 
organisms high in the food chain, and some risk assessments of bioaccumulative chemicals could 
concentrate on populations of such organisms, rather than on the broad aquatic community. 
Because of these differences, the assessment needs will be discussed separately below for 
nonbioaccumulative and bioaccumulative toxicants. 
It is true that assessments of these two groups of chemicals will share many common features and 
principles, and thus these separate discussions will involve some redundancies. For example, 
toxic response for almost all chemicals and endpoints is ultimately related to the amount of 
chemical which accumulates at an internal site of action, so a residue-based framework is of 
value to nonbioaccumulative chemicals as well as bioaccumulative chemicals. Also, the 
consequences to populations and communities of toxic effects on individual organisms will entail 
the use of similar tools for both groups of chemicals. Furthermore, whether a chemical is 
"bioaccumulative" is a matter of degree, not a discrete category. Nonetheless, the relative 
importance of various issues will vary between assessments of chemicals with low versus high 
bioaccumulation. Separate discussions are useful in highlighting how assessments will vary, and 
in identifying needed work. However, the results of the work proposed below will often 
transcend the chemical group and efforts will be made to apply any results as broadly as is 
appropriate. 
Nonbioaccumulative Toxicants 
Nonbioaccumulative toxicants include a wide range of organic and inorganic chemicals that are 
of concern in many aquatic risk assessments. Notable examples include heavy cationic metals 
(such as copper, zinc, lead, cadmium, and silver), which are significant contaminants in mining 
areas and various effluents; and ammonia, which is at high concentrations in sewage effluents, in 
some fertilizer runoff, and in areas with high inputs of nitrogen-containing organic compounds. 
These chemicals have been documented to have substantial impacts on certain aquatic systems 
and are responsible for many instances of noncompliance with WQC. Treatment and 
remediation costs associated with meeting criteria for these chemicals can be high. However, as 
discussed above, current assessment and criteria methodology entail various uncertainties 
regarding the actual risks of these chemicals and thus whether the regulatory controls are 
necessary and sufficient. 
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