Water Quality Impacts of Long-Term Effluent Disposal Strategies in Southeast Florida

The Southeast Florida Ocean Outfall Experiment II (SEFLOE II, 1991—1994) project is the result of a cooperative effort of several government agencies and Hazen and Sawyer, P.C. The project was designed to satisfy bio-monitoring concerns and provide site specific information to allow the U.S. EPA Regional Administrator to evaluate if four open ocean outfalls located off the Southeast Florida coast were contributing to "unreasonable degradation" of the local marine environment. During the project’s field studies, tremendous efforts were made to collect physical, chemical and biological data. These data were analyzed to characterize outfall plumes and associated environmental conditions.

Four major open ocean outfalls (Miami-Central, Miami-North, Hollywood and Broward County) were investigated in the SEFLOE II project. All four outfalls are located off the east coast of South Florida and in the western boundary region of the Florida Current (i.e., the Gulf Stream). These outfalls discharge effluents of secondary treated domestic sewage. Two of the outfalls, Miami-Central and Miami-North, have multi-port diffusers; the other two, Hollywood and Broward, have only single port outlets. The water depth at diffusers or outlets of these outfalls ranges from 28 to 34 meters. The distance from the nearest shoreline to the discharge sites ranges from 2,100 to 5,700 meters. Table 1 summarizes the characteristics of these outfalls.

Objectives of the project were to

• Scientifically characterize the physical oceanographic conditions associated with each of the four open ocean outfalls on a year-round basis to identify critical oceanographic conditions.

• Characterize and define the area of rapid dilution and the mixing zones.

• Determine the characteristics of the receiving water/effluent mixture both with and without chlorination using bioassay techniques.

• Examine the dilution properties and natural disinfection capability of the open ocean on secondary effluents dispersed through open ocean outfalls.

• Determine the nutrient concentrations in the discharged effluent, background and outfall plumes.

• Collect samples for priority pollutant and oil and grease determinations.

 

Data Acquistion and Sampling Methods

To achieve the project objectives, field investigations were designed to include

• Moored current meter arrays,

• Intensive cruises by NOAA research vessels to each outfall during extreme oceanographic conditions (one winter and one summer cruise), and

• Semi-monthly small boat operations at each outfall from July 1991 to October 1992 (except the months when intensive cruises were underway).

 

Physical oceanographic data

Current monitoring: Current meter mooring systems, each containing two Aanderaa RCM-4 or RCM-5 current meters, were deployed near each outfall discharge site during several periods within a one-year span (from August 1991 to October 1992). These measurements provide long-term records of current speeds and directions. In addition to the mooring systems, an Acoustic Doppler Current Profiler (ADCP) was deployed near the Miami-Central outfall diffuser. The ADCP data provide information for current profiles through the water column.

Temperature, salinity and density: During both intensive cruises and small boat operations, water column temperature, conductivity and depth were measured using CDTs (Sea-Bird Seacat Model SBE19). Salinity and density profiles through the water column then were obtained from CDT data.

 

Plume characterization

Dye measurements: During the two intensive cruises, red dye (Rhodamine-WT) liquid was injected into effluent. Dye concentrations in the effluent were measured using fluorometers. In the field, a research vessel made tracks crossing surface plumes, and dye concentrations were measured continuously by a deck-mounted fluorometer along the ship’s track. Dye concentrations also were measured from collected water samples. Plume mixing and dilution behavior can be obtained from dye concentration data.

Salinity measurements: During each of the small boat operations, a boat towed a CDT at one meter or less beneath the water surface through surface plumes. The CDT data were later processed to obtain salinity distributions in surface plumes. This was demonstrated to be an effective method to characterize plumes. Both initial dilution and subsequent dilution of plumes can be estimated using salinity deficit as a tracer.

Acoustic measurements: During the field investigations, a high-frequency back scattering acoustic system (200 kHz Raytheon Model DE719B Echosounder) was used to aid in initial plume detection, to guide plume sampling and to provide two-dimensional acoustic images of outfall plumes.

Bacteriological measurements: Water samples collected during field operations were analyzed for total coliforms, fecal coliforms and enterococcus bacteria. These bacteria groups have been suggested as appropriate indicators for water quality criteria.

Nutrient measurements: Water samples were analyzed for ammonia, TKN, total phosphorous and nitrates.

Other measurements: Water samples also were analyzed for oil and grease, priority pollutants such as silver, zinc and phenols, and total suspended solids (TSS).

 

Effluent characterization

Effluent samples were collected in treatment plants and analyzed for salinity, bacteria, nutrients, priority pollutants, oil and grease, BOD5, TSS, etc.

 

Bioassay tests

Bioassay work was conducted to characterize the acute and chronic potential toxicity of the effluent/receiving water mixture in the zone of initial dilution and the mixing zones and to evaluate the exposure potential of the effluent to the indigenous ocean population. Ocean bioassay samples were collected by either a 10-liter Niskin bottle lowered to a depth of one meter, a portable suction pump attached by a diver to the end of the outfall pipe, or surface sampling bottles. The samples then were transferred to sample containers provided by the certified laboratories performing the bioassay tests and stored on ice until they could be transferred from the ship. All test species, test procedures and quality assurance were in accordance with EPA-approved procedures and methods for acute bioassay tests. A total of 1,727 acute static bioassay tests and 109 short-term chronic bioassay tests were performed. Table 1 summarizes the measurements and their densities made during the SEFLOE II project.

 

Findings

Physical oceanographic conditions

Three current regimes are found present at the four outfall sites

• Northerly-directed flows that are thought to be associated with western meanders of the Florida Current.

• Southerly flows that are expected parts of an extensive eddy current.

• Rotary-like flow that consists of groups of rotations interspersed between northerly and southerly flows. The rotations are irregular with periods of rotation as short as three hours.

Although all three current regimes are present, the entire regional currents continue to be from the south to the north. Easterly and westerly currents are infrequent and of short duration. Therefore, the potential for the highly diluted effluent plumes reaching the shore is extremely remote. The continued northerly flow greatly reduces the superposition effect often encountered in tidal rivers where effluent concentrations are additive over time.

The ADCP data revealed that non-linear profiles of current rapidly move through the water column. In general, the current magnitude is greater near the water surface than near the bottom. Although current speed measured at a curtain depth could be zero, the vertically averaged current speed was not zero at any time during the measurement period.

Density profiles were found to vary seasonally. Uniform profiles are present during winter months and stratifications during summer months. The maximum density difference observed was 0.001 g/cm3 over the water column. The typical density (near the middle depth) is 1.023 g/cm3 during summer and 1.025 g/cm3 during winter.

 

Outfall plume characteristics

Field investigations revealed that surfacing plumes were present at all four outfalls throughout a year (i.e., even in summer months when the water column density stratification was present). This was because the density stratifications were weak enough to allow the surfacing of plumes. However, some trapping of portions of rising plumes was detected by the acoustic system during strong stratification conditions.

Field dye and salinity data were processed to obtain initial dilutions and subsequent dilutions. The initial dilution data together with current meter data and effluent discharge data then were analyzed using a dimensional analysis method and regression techniques to establish semi-empirical relations. Total physical dilutions as a function of distance from the surface boil were generated from dye concentration data and from salinity data.

Using this data, it was found that within the 100 meter range, the Broward and Hollywood outfall plumes undergo an enhanced dilution. This rapid dilution may be attributed to an internal hydraulic jump. Subsequent mixing of plumes may be dominated by buoyant spreading for several hundred meters from the boil, since the positive buoyancy of effluent plumes has not been dissipated even after the internal hydraulic jump. For the Miami-North and Miami-Central outfalls, effluent was initially distributed over a wide area because of multi-port diffuser discharges. However, the total physical dilutions of these outfall plumes did not increase rapidly as the Hollywood and Broward outfall plumes. Both buoyant spreading and oceanic turbulence were expected to be dominant mixing mechanisms for subsequent mixing of these plumes.

 

Bacteria

The indicator bacteria data were plotted as a function of distance from the outfall discharge point. Table 2 presents the range at which the Class III bacteriological criteria are met for each outfall. The data represent unchlorinated effluent conditions. Bacteriological decay rates also were developed for unchlorinated effluent conditions.

 

Nutrients

The ammonia, TKN, total phosphorous and nitrate concentrations as a function of distance from the outfall discharge point were obtained. In general, the nutrient concentrations in outfall plumes reach background levels within 400 meters from the discharge points. This finding suggests that the outfall nutrients are not involved with the presence of Codium algae outbreaks on the coral reefs.

 

Oil and grease

In general, oil and grease data did not show a correlation with other measured parameters. The accuracy of the data is questionable since during many of the sampling days the background concentration was higher than the concentration measured within surface plumes. The visual field observations indicated that no surface slices of oil and grease were present within plumes at the surface. It appears that neither oil nor grease are a concern of these outfall discharges.

 

Priority pollutants

The plant effluents were analyzed for all (126) priority pollutants. None of the pollutants detected exceeded the acute toxicity criteria listed under the State of Florida Maximum Allowable Effluent Level. Therefore, the pollutants detected present only chronic toxicity issues that can be addressed through suitable dilution and mixing zones.

 

Bioassays

The bioassay test results indicate that the treated effluent is not toxic. It is believed that the bioassay test is extremely conservative. The receiving water criteria is based on a concentration that is one-half the concentration causing observed laboratory reactions. In addition, the required test procedure assumes a Eulerian exposure of organisms where the subject is fixed in the concentrated effluent plume for a 48- or 96-hour period. Actual outfall conditions exhibit Lagrangian exposures only where an organism moves with the plume that continues to be diluted by mixing processes. The safety factors associated with bioassay tests of outfall plumes were estimated to range from 143 to 360 for using a 30 percent effluent and from 476 to 1,147 for using a 100 percent effluent. These safety factors are much higher than those typically used in many more sensitive freshwater environments with high-rate diffusers.

 

Mixing zones

Mixing zones are needed for those chemicals where concentrations in effluent are greater than Class III receiving water criteria. Three types of mixing zones are recommended.

• A "zone of rapid dilution" with a radius of 162 meters for pollutants where concentrations meet the Class III criteria outside the area under the specified worst-case conditions.

• A circular mixing zone with a radius of 400 meters for pollutants that exceed the dilution requirements of a zone of rapid dilution but within the dilution requirements of the 400 meter radius mixing zone under the specified worst-case conditions.

• A non-circular mixing zone that meets the FAC requirements and is based on a probabilistic approach including an exceedance probability concept and a continuous simulation technique.

Recognition of the initial dilution of the outfall, as proposed in a rule change petition submitted to the Florida DEP, would eliminate the need for the vast majority of the mixing zone applications the outfall utilities have submitted.

 

Conclusions

The SEFLOE II project met all of its objectives. The data demonstrate that the open ocean outfall discharges are environmentally acceptable and create "no unreasonable degradation" to the ocean environment.

 

About the Authors:

Ghislaine B. Carr, P.E., and Patrick A. Davis, P.E., are engineers for Hazen and Sawyer, Hollywood, Florida.

Robert E. Fergen, P.E., is an engineer for Hazen and Sawyer, Raleigh, North Carolina.

Frederick Bloetscher, P.E., is the director of engineering, operations and planning for the Florida Governmental Utility Authority, Tallahassee, Florida.

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