The Canadian water quality guidelines for the protection of aquatic life as
sume that (1) the external water concentration is an effective measure of t
he concentration at the active sire in organisms, which is ultimately respo
nsible for a toxic response, and that (2) the safety factor accounts for an
y differences between laboratory and field conditions as well as the extrap
olation from the effect concentration to a long-term no-effect concentratio
n. This study examines these assumptions and assesses potential errors that
environmental managers can make when applying the guidelines. The methodol
ogy is based on assessing the probability that internal concentrations of s
everal contaminants are greater than or less than the "safe" concentration,
assumed by the guideline, when the water concentration is at the water qua
lity guideline. Results derived from empirical observations and a food-web
bioaccumulation model show that a high probability (62-100%) exists that sa
fe internal concentrations are exceeded for polychlorinated biphenyls, 1 2
4,5-tetrachlorobenzene, and hexachlorobenzene in Lake Ontario when the aque
ous concentrations are at the water quality guideline values. This is due t
o field bioaccumulation factors bring greater than the bioaccumulation fact
ors in laboratory toxicity tests used for the water qualify guideline devel
opment. Factors contributing to the exceedence of safe internal concentrati
ons at the water quality guideline values are identified. Recommendations f
or improvement of the water quality guideline process are provided.