Conduct effluent plume delineation: chapter 1

Purpose of Plume Delineation

Effluent plume delineation is required in the design phase of the EEM program for each pulp and paper mill. The objective of plume delineation is to understand how the mill effluent behaves in the receiving environment and to identify effluent boundaries describing exposure areas and reference areas within which to establish sampling locations.

The exposure area(s) for EEM studies is the area where the effluent concentration is 1% or greater, reflecting a dilution of no more than 1:100. It is important to understand the spatial distribution of effluent in the water column to determine areas for fish collection, as well as to understand where the effluent comes in contact with the bottom substrate to determine areas for sampling the benthic invertebrate community. This is particularly important for effluent that may not exhibit complete vertical or horizontal mixing throughout the receiving environment.

Selection of sampling locations within the reference area(s) for EEM studies requires an understanding of the extended dilution of effluent beyond the 0.1% (1:1000 dilution) effluent concentration limit. This understanding is particularly important for mills discharging into water bodies where flow is not unidirectional.

Delineation of effluent plumes will typically involve field work to track plume movement during a single time period, coupled with the use of numerical modeling to determine target dilution zones over a broader range of environmental conditions. It is recommended that the effluent exposure zone be predicted for the:

• maximum extent, reflecting the zone within which effluent is periodically detectable at a concentration of 1% or greater; and
• long-term average conditions, reflecting the zones within which effluent concentrations of 1% or greater, and 0.1% or greater would be regularly detectable.

Areas beyond the maximum extent delineation under worst case conditions would be expected to be minimally affected by the discharge and may be suitable as "far field" or "reference" areas, depending on the sampling design (e.g., control/impact design or gradient design). The long-term average conditions define what would be considered the "normal" envelop of plume extent and can be use to design an EEM sampling program that will assess the long-term effect of the effluent discharge. It may also be useful to determine the long- term average conditions of 10% or greater effluent concentration to identify areas that may be most impacted by exposure to effluent. It is important to evaluate what are the "normal" environmental conditions to which the effluent will be subjected and what are the extremes that may, on occasion, override the "normal".

For discharge environments with high receiving water flow, in which the effluent is expected to be rapidly mixed, it is important to determine whether the effluent is diluted to less than 1% within 250 m of the discharge, which would remove the EEM requirement to conduct a fish survey.

The EEM program requires that effluent plume delineation be conducted only once, provided there are no substantive changes in effluent characteristics, discharge quantity, discharge method or location, or in the hydraulic or hydrographic features of the receiving environment. Plume delineation must be reviewed in the design phase of each subsequent cycle of EEM to evaluate the need for a new delineation. The onus is on the mill to ensure that they have an understanding of the hydrographic nature of the receiving waters, sufficient data and numerical modeling to meet the objectives of plume delineation for EEM.