Responses at the Organismal Level
The risk characterization of possible responses at the organismal level was done by comparison of the concentrations of chlorinated compounds found in effluents to the responses reported in the literature (NCASI, 1992; Nielson et al., 1991). As several criteria are used to assess toxicity, these comparisons must be treated differently. For example, where a no-observable-effect-concentration (NOEC) from chronic exposure has been measured, a smaller margin of safety may be considered acceptable than for the LC50 determined from acute toxicity. A risk characterization quotient of greater than one based on a chronic NOEC indicates the potential for adverse effects. In the absence of chronic toxicity data, a quotient of less than 0.01 for an acute test may be considered protective of chronic responses (Sloof et al., 1986). In this risk characterization, these toxicity data were compared to the mean and 95th percentile concentrations of chlorophenols found in final effluent from mills using 50-70% and 100% chlorine dioxide substitution and are presented in Table 1. For those compounds for which toxicology information was available, the margins of safety for the 95th percentile concentration and the most sensitive effect were calculated and are presented graphically in Figure 7.
For the most part, the risk characterization of the compounds found in pulp mill effluents showed high margins of safety between toxic effect and effluent concentration at both levels of chlorine dioxide substitution (Table 1 and Figure 7). The lowest margin of safety calculated from the mean concentrations was 500 (4,5-dichlorocatechol). It is also possible to assess this data from a worst-case point of view using the upper 95th percentile of the concentration distribution. Generally speaking, the margins of safety for the upper 95th percentile concentrations of chlorinated phenols in the effluent of mills with 100% chlorine dioxide substitution were greater than 100. Because the 95th percentile concentration was used, this means that there was a more than hundred-fold margin of safety 95% of the time. This was not the case for those mills where lower levels of substitution were used. In these effluents, margins of safety for 3,4,5-trichloroguaiacol, 3,4,5-trichlorocatechol, 4,5-dichlorocatechol and tetrachlorocatechol were less than 100. However, this level of exposure would only be expected to occur 5% of the time. These margins of safety also need to be considered in relation to the dilution which will occur in the receiving environment. If dilution factors are higher than ten (which is likely in most North American receiving environments), risk quotients for the 95th percentile concentrations of these chemicals will have margins of safety of more than 1000 fold. An additional factor that needs to be considered is that little or no toxicity data are available for several of the chlorinated substances found that may be found in effluents2. It is, however, unlikely that the toxicity of these compounds will differ substantially from their better known homologues.
Effluents from the bleaching of sulphite pulps with 100% chlorine dioxide have not been studied in as much detail as those from kraft pulps but likely have similar levels of the chlorophenols (Axegård et al., 1993). Environmental effects from chlorophenols are therefore likely to also be similar to those discussed above.