EFFECTS OF MILL EFFLUENTS ON COMMUNITIES AND POPULATIONS

Discharge of pulp mill effluents into aquatic environments has resulted in measurable responses and a relatively large body of historical information on the effects of effluents produced from chlorine bleaching is available. The first major study on impacts on fish and other populations in receiving waters was that developed under the Swedish "Environment/Cellulose I" project. This study examined the ecological, pathological, and physiological impacts of primary treated bleached kraft mill effluent (BKME) discharged from a mill at Norrsundet on the fish community in the Gulf of Bothnia in the Baltic (Andersson et al., 1987; 1988a). Some of these responses have been observed in fish and other organisms at other locations (McMaster et al., 1991) including unbleached mills (Carey et al., 1993; Robinson et al., 1993).

From a number of studies that have brought greater understanding to these effects (Kautsky et al., 1988; Kautsky, 1992; Hansson, 1987; Sandström et al., 1991; Hall et al., 1991), it is now clear that physical (fibre deposition), toxic substances and nutrient components (and nutrient mimics such as chlorate) of the discharges from pulp mills have been the major source of population shifts and community effects in the past. Responses mediated by toxicity (such as from high concentrations of resin acids and chlorinated phenols) were predictable from laboratory tests, the results of which, if used correctly, could have been used to mitigate the responses. These effects continue to occur in environments where turnover or dilution rate in the receiving water body is low and/or controls are not implemented in the effluent treatment process. Impacts resulting from nutrient addition or physical effects such as the deposition of fibre mats can mask and confound toxicity-mediated effects and risk assessment must take this into consideration. In the absence of clear measures of changes in population structure (changes in numbers resulting from death, sickness or reproductive impairment), impacts appear to be minor or absent. The isolation of specific process-related effects at the population level from this web of causality is difficult and has not been carried out in the field studies conducted to date.

Most of the information available in the published literature on the toxicity of final effluents from pulp mills using high chlorine dioxide substitution is based on toxicity tests with effluents from full-scale mill trials (e.g. Axegård, 1986b, Pryke, 1989), calculated toxicity equivalents using chemical analysis results from pilot to full-scale mill trials (Holloran et al., 1992), or evaluation of effects observed in the receiving water. It is generally accepted that toxicity of bleaching effluent to fish and other aquatic organisms is reduced with substitution of chlorine dioxide for elemental chlorine in pulp bleaching. This is consistent with the other evidence that high chlorine dioxide substitution results in lower organochlorine production and a shift in the chlorinated chemicals produced to less toxic compounds (e.g. less chlorine substitution). It is also generally accepted, however, that there is little difference in the toxicity of whole mill effluents from chlorine dioxide bleached and unbleached pulp mills since contributions from other pulp mill streams may overshadow toxicity from the bleach plant. This implies that, although chlorine dioxide substitution reduces production of potentially toxic chlorinated compounds, this alone cannot remove all toxicity from the effluents. However, effective secondary treatment has a marked effect on toxicity reduction in any mill effluent (Carey et al., 1993; Robinson et al., 1993).

Few studies have addressed population responses resulting from changes to chlorine dioxide bleaching in a consistent manner. From the results of a series of studies conducted before and after the introduction of increased chlorine dioxide substitution in the Grande Prairie mill on the Wapiti river in Western Canada (Swanson et al., 1992a; 1992b; 1993a; 1993b; Owens et al., 1994), it would appear that the original mill process used at Grande Prairie had little effect on fish populations or the community in the river. In this context, it is interesting to note that, at this time, P4501A levels were highly induced but have declined since changes in process have been introduced (Pryke et al., 1994). If introduction of chlorine dioxide had an effect, this was probably minimal or was masked by natural events (floods or fish migration) in the system. Due to the relatively short time of observation after the introduction of chlorine dioxide, it is unlikely that major differences in population response would be immediately observable unless these were intense. Thus, the effects of the introduction of a less hazardous process (such as 100% chlorine dioxide substitution) may not be detectable. However, as shown in Figure 6, the data on fish tissues from the Grande Prairie ecosystem study did show reductions in levels of organochlorines in response to the change to 100% chlorine dioxide substitution (Owens et al., 1994). This shows that exposure has declined, even though any effects at the population or community level may not be demonstrable at this time.

The Finnish and Swedish microcosm studies (Rosemarin et al., 1990) utilized experimentally manipulatable systems in which population and community effects were assessed in the absence of a number of confounding environmental factors. Results demonstrated several of the effects noted at the Norrsundet mill in the Baltic. High chlorate levels had significant negative impacts on the bladder wrack (Fucus vesiculosus). The change in this habitat and food supply had a significant secondary impact on crustaceans (Lehtinen et al., 1991). However, since this time, chlorate emissions have been controlled and this source of impacts is no longer significant. Results from more recent studies (Tana et al., 1993) suggested that the population effects resulting from differences in process technology were not well correlated with chemical parameters in the effluents and suggest that responses observed in fish growth may be due to the effects of naturally occurring substances present in the wood but which are passed through current bleaching and effluent treatment process in sufficient concentrations to cause biological responses.

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