Evidence for a Reduction of 2,3,7,8-TCDD Toxic Equivalent Concentrations in White Sucker (Catostomus commersoni) Exposed to Bleached Kraft Pulp Mill Effluent, Following Process and Treatment Improvements

Over a 4-year period from 1990 to 1993, the H4IIE bioassay was used to measure 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalent concentrations (TECs) in the liver tissue of white sucker exposed to bleached kraft mill effluent. During two of the sampling periods, the H4IIE bioassay-derived TECs (BD-TECs) were compared directly with BD-TECs measured by a similar bioassay using a rainbow trout liver cell line (RTL-W1). From 1989 to 1993, chemistry-derived TECs (CD-TECs) were calculated directly from measured polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) concentrations in liver tissue. Chemistry-derived TECs were compared to bioassay-derived TECs using toxic equivalent factors (TEFs) developed for these systems. During the study period, the pulp mill began secondary treatment of effluent and increased the substitution of chlorine dioxide for molecular chlorine during bleaching. Significant reductions in H4IIE BD-TECs and CD-TECs were observed in liver tissue of both male and female white sucker during summer and fall sampling periods but not during spring sampling periods. Process and treatment improvements at this particular bleached kraft pulp mill appear to have been successful at reducing the levels of PCDDs and PCDFs found in white sucker exposed to the effluent. Despite the decreases in TECs, relative mixed function oxygenase (MFO) induction in the exposed white sucker did not decrease over the study period, suggesting the PCDDs and PCDFs are currently not the dominant MFO-inducing compounds in the effluent of this mill.