Conduct effluent plume delineation: chapter 2

Effluent Dispersion

Plume delineation requires information on effluent characteristics, discharge conditions and the nature of the receiving environment.

Initial Concept of Effluent Dispersion

An initial concept of effluent dispersal should be developed to help plan the field studies. This "first cut" at understanding effluent behaviour in the receiving water requires some basic information, including:

  • effluent characteristics, such as density and velocity;
  • number of discharges, location, orientation, depth, type (e.g., diffuser, ditch);
  • receiving water characteristics including density, flow characteristics, seasonal or lunar factors (e.g., water level, tidal cycle); and
  • estimation of the initial effluent dilution when the plume surfaces; this can be estimated using a simple numerical model such as the U.S. EPA's Visual Plumes or the Cormix model.

A sketch of the expected plume behaviour should be made, showing expected initial dilution and subsequent dilution in relation to site features near the discharge location and farther away. It is important at this stage to determine the type of numerical modeling (i.e., one-dimensional, two-dimensional, three- dimensional) that will be needed to analyze the field data and extrapolate these data to describe maximum extent and long-term average concentrations in the receiving waters. The type of numerical modeling required may dictate what data will have to be collected for the field study.

Effluent dispersion in the receiving environment is a two-stage process comprising initial dilution near the point of effluent introduction, followed by subsequent dilution farther from the discharge. Initial dilution of the effluent is determined by the method and dynamics of introduction of effluent and by differences in density between effluent and receiving waters. The introduction of effluent is usually visualized as a rising jet (not necessarily vertical) to the water surface where it encounters an upflow boundary and forms a streaming plume moving down stream, carrying the effluent away. Illustrations of initial dilution of effluent are shown in Figure 2.1; depictions such as these are useful in developing an initial concept of effluent dispersion.

Initial dilution near the discharge can be approximated using numerical models (e.g., Cormix) or nomographs (i.e., graphical representations of equations with multiple variables, such as may be found in Wood et al., 1993). Further dilution of the plume occurs by horizontal and vertical mixing. In most cases, horizontal dispersion of the effluent occurs at least an order of magnitude more rapidly than vertical mixing, such that the plume may disperse horizontally for some distance without being fully mixed in the water column. It is therefore important to consider the depth component of dispersion during the field studies and to incorporate this into numerical modeling to determine the plume location within the water column and where it comes into contact with the bottom substrate.

Discharged effluent usually has higher velocity than the receiving water, which results in shear stress with the receiving water. This shear stress results in turbulent mixing. Initial dilution continues until the energy in the discharge dissipates and the velocity of the plume matches that of the receiving water. Once this occurs, the "natural" turbulence in the receiving water causes further dilution or mixing of the effluent with the receiving water.

In addition to velocity differences, most receiving water and effluents differ in density. The effluent is typically less dense than the receiving water (often due to being warmer, or freshwater effluent discharging to marine waters) and therefore tends to rise in the water column. This results in another shear stress, similar to that resulting from the velocity difference.

In most cases, the combination of velocity and density shearing provides sufficient upward momentum to cause the effluent plume to break the water surface. If the density of the plume mixture is still lighter than the receiving water, then the plume will stay at the surface. If the plume mixture is slightly heavier than the receiving water, it will plunge down to the level at which there is a water mass of equal density and then be transported by and mix with that body of water.

After initial dilution, the effluent plume typically moves horizontally with the receiving waters. Subsequent dilution and dispersion depends on the receiving environment and climatic conditions (see Section 2.4).

Additional resources for guidance on effluent dispersion conceptualization include: Bishop (1984), Day (1975), Jirka et al. (1996), Neshyba (1987), Roberts (1989), Roberts and Ferrier (1996), Sorensen (1978), Thomann and Mueller (1987), Tsanis and Valeo (1994), Williams (1985), and Wood (1993).

Figure 2.1 Examples of plume behaviour in receiving environments (modified from Jirka et al. 1996)

Examples of plume behaviour in receiving environments

Effluent Characteristics

The most important effluent characteristics that influence initial dispersion are density and velocity differences compared to receiving water (see Section 2.1). Velocity will influence the degree of shear and therefore mixing that occurs when effluent is discharged. Effluent density will influence the rate of rise and position of the plume in the water column. Velocity may be measured as effluent flow rate (as a daily average), including whether the discharge is continuous or discontinuous (e.g., batch release). The velocity of flow through each discharge pipe port should be considered, in comparison to the receiving water. Density of the effluent should be determined. Additional information on the effluent for plume delineation may include the presence of tracers, which are substances occurring naturally in the effluent, such as resin acids, sodium, and magnesium. These tracers may be used to track dispersion. Effluent values for the two most recent years should be considered.

Effluent Discharge Design

The discharge configuration and performance should be described. Using existing data, such as the most recent underwater inspection reports of the discharge, the performance should be compared to the design or the "as built" drawings. The location, length and orientation of the discharge should be known when determining the width of the plume. It is also important to consider the depth of the discharge within the water column in relation to flows and density gradients that may exist.

Receiving Environment Factors Affecting Plume Dispersion

The receiving environment should be described in terms of flow and currents, physical and chemical water quality and the spatial and temporal variations of these factors. This information is necessary to develop an initial concept of effluent dispersion, as well as to plan the field study. Climatic conditions should be summarized, with a view to their possible influence on plume behaviour.

Field parameters and their general importance in delineating plumes are described below. More detailed guidance for specific receiving environments is provided in Section 5.

Freshwater Flows

Minimum, maximum and average freshwater flows should be described in the receiving environment. This is important for all receiving environments, except those that are strictly marine with no local freshwater inputs. Freshwater flows will influence the initial dilution of the plume as well as subsequent horizontal and vertical mixing. The direction of flow, which may vary with depth and location, will influence the orientation of the plume. Typically, field studies are conducted at near minimum annual flow when effluent dilution will be low and the plume large relative to other times of the year. Field results can be extrapolated to reflect average and maximum flow scenarios for effluent dispersion.

Water Levels

Minimum, maximum and average water levels should be described for all receiving environments. The water level will influence initial dilution and the volume of water available for subsequent dispersion. Fluctuations in water level may occur daily (i.e., tidal areas) or seasonally.

Water Quality

Water quality measures for receiving water that may be useful for plume delineation studies include temperature, density or specific gravity, salinity (for estuarine and marine studies), colour, suspended solids, and substances that may be used as effluent tracers. All of these measures can be used to track the movement of effluent within the receiving environment, as described in Sections 3 and 5.

Variation in Temperature and Salinity

Temperature and salinity both affect the density of the receiving water, and therefore their structure and variation within the water mass over space and time is important in all aspects of conducting a plume delineation study. Plumes that are warmer or less saline than the receiving water will be thermally buoyant when discharged and will rise towards the surface, creating shear that will generate initial mixing and dilution. Subsequent dilution and dispersion will also be influenced by temperature and salinity in the receiving water. Temperature and salinity may vary horizontally and vertically over short (e.g., tidal areas) or long (e.g., seasonal) time frames.

Tides and Seiches

The timing of tides and magnitudes is important to understand when planning the field study for estuarine and marine waters. In addition to marine and estuarine receiving waters, large lakes may also display a tidal cycle, albeit minor by comparison. Large bodies of water, such as lakes, estuaries and fjords, may also display the effects of storm surges and seiches, both in the surface elevation and in internal waves. All of these will influence the direction and pattern of effluent mixing.

Climatic Conditions

Wind and ice may significantly affect effluent dispersion, air temperature, ice conditions and wave action can all influence plume behaviour. Wind acting on large bodies of water may induce currents and waves. Ice may affect dispersion in two ways: by reducing wind-driven currents, and by increasing turbulence by providing a solid rough boundary to flow. Climatic conditions are discussed in more detail in Section 5.

Confounding Conditions

Although plume delineation is intended to capture normal discharge and receiving environment conditions, there are some potentially confounding conditions that may influence interpretation of field study results. Confounding conditions arise from events that are outside of the operational or environmental norms or are transitory events, and may result in a temporary change in the more "normal" location of plume boundaries. These conditions may affect effluent dispersion and therefore, their possibility should be considered when conducting a plume delineation study. Examples of such conditions include the following:

  • pulp mill system upsets in the effluent treatment and discharge process that result in a temporary change in effluent quality or quantity;
  • adverse weather, in particular wind conditions, that generates currents that are not typical for the receiving environment;
  • seasonal events, such as ice conditions or thermal stratification, which can lead to a misleading representation of the plume behaviour; and
  • flow regulation for hydro-electric power production.
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