but do not fully account for these effects and do not account at all for other factors. An 
especially important uncertainty (which is also relevant to other chemicals) is the effect of winter 
conditions on chronic ammonia toxicity to life-stages of sensitive organisms present during the 
winter. Aquatic life WQC for metals currently are expressed as dissolved metals, with a 
correction for hardness in fresh water, reflecting empirical observations of reduced toxicity due 
to adsorption by suspended solids and increased hardness. Research has shown, however, that 
several factors other than hardness also are important in determining the toxicity of metals in 
water, and has identified mechanistic bases for their effects. Development of a model that 
incorporates many of these factors, referred to as the biotic ligand model (BLM) is currently 
being supported in part by OW. Applications of current versions of this model to criteria entail 
substantial uncertainties, especially for chronic toxicity and for certain taxonomic groups. 
Significant research in support of this model is underway under the support of OW and various 
industry groups. Such efforts will contribute to the third step in Figure 9. 
As discussed above regarding exposure research issues, organisms dwelling in or on sediments 
can receive chemical exposures with large temporal and spatial variation, due to hydrologic 
events disturbing the sediments or seasonal changes affecting partitioning or processing of 
chemicals. Decomposition of nitrogen-bearing organic matter in the sediment can cause 
substantial gradients between sediment and overlying water. For metals, sediment can be 
relatively more contaminated than overlying water due to past inputs, diagenesis of deposited 
particles, or seasonal fluctuations in loads to systems, leading also to substantial gradients from 
sediment to overlying water that are subject to marked variation. Sediment pore water metal 
concentrations in anoxic layers are greatly affected by AVS, which varies with season and depth 
and can be oxidized when sediment is resuspended. Even in laboratory tests, sediments and 
overlying water are likely at some disequilibrium that varies with time and between tests. 
Regulations based on assumptions of equilibrium therefore can involve considerable uncertainty. 
The toxicological implications of non-equilibrium conditions (such as resuspension and 
seasonality) need to be investigated as part of the extrapolation methods represented in the third 
step in Figure 9. 
Laboratory tests used for aquatic risk assessments of nonbioaccumulative toxicants typically 
include exposures only via chemical dissolved in water, the organisms either not being fed or fed 
food that is not contaminated commensurate with the water concentrations. This is not an issue 
with ammonia, as ammonia per se would not be a significant contaminant in food, although 
catabolism of food is a source of ammonia that organisms must eliminate. However, the 
significance of the dietary route of exposure is an important issue for metals assessments. 
Aquatic animals certainly can accumulate substantial amounts of metals from their diets, and 
diets contaminated with high concentrations of metals can cause adverse effects. However, the 
importance of this route of exposure when there is also commensurate amounts of metal 
dissolved in the water is uncertain. Some studies have presented evidence that dietary uptake 
results in increased uptake and/or risk compared to water exposure alone, while other studies 
have suggested the opposite. The uncertainty regarding the significance of dietary uptake to 
metal risk is a fundamental barrier to good risk assessments for these chemicals and could have 
substantial implications to regulatory programs. 
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