Evaluating the Implications of End-of-Pipe Effluent Limits for Achieving Site-Specific Instream Water Quality Criteria: An Example From Wisconsin

An analysis of proposed thermal discharge regulations in Wisconsin was conducted to assess the effluent flow conditions required to attain site- and month-specific default acute and sublethal aquatic life criteria as a function of a state-wide end-of-pipe human health-based effluent temperature limit of 120°F and site-specific monthly default receiving water background temperatures. Scenarios relevant to the pulp and paper industry were evaluated for two Wisconsin rivers (U.S.A.), the Upper Wisconsin River (UWR) and the Lower Fox River (LFR). As in the regulation, temperature is treated conservatively such that only cooling by dilution is considered. Dilution ratio calculations based on forms of a heat balance equation quantify the limiting effect of the end-of-pipe limit on the dilution volume needed in each month to attain the water quality criteria (WQC). The discharge of effluent at 120°F is restricted by the instream WQC to flow rates that are a few percent of the 7Q10 flow, particularly in the warm summer months. Calculated maximum effluent flow rates required to meet acute criteria are similar for both rivers, ranging from 5.5% to 8% of 7Q10 during summer months to around 12% in winter months. Values based on sub-lethal criteria range between 1.3% and 5% of the 7Q10. The volume of effluent at 120°F that can be discharged while attaining the sublethal WQC is most restricted in May in both rivers. The most restrictive month based on attainment of the acute criteria in UWR is July and in LFR is August. Using a representative 7Q10 for UWR of 400 MGD, both the acute and sublethal WQC temperatures are attained for an effluent temperature of 120°F in all months at effluent flow rates of less than roughly 10 MGD. All of the default acute WQC are attained at this effluent temperature at effluent flow rates less than roughly 25 MGD. Effluent cooling considering dilution alone is expected to occur most rapidly during the initial stages of dilution, helping to minimize the potential for the exposure of aquatic life to harmful temperatures. This paper illustrates a screening-level assessment of the portion of the receiving stream downstream of an effluent discharge within which the acute and sublethal WQC are exceeded as a function of an end-of-pipe limit. Where this portion is small, risks of adverse thermal impacts to aquatic life are likely to be small.