62-4.244. Mixing Zones: Surface Waters

(1) Zones of mixing for non-thermal components of discharges.

(a) The Department may allow the water quality adjacent to a point of discharge to be degraded to the extent that only the minimum conditions described in subsection 62-302.500(1), F.A.C., apply within a limited, defined region known as the mixing zone. Under the circumstances defined elsewhere in this section, a mixing zone may be allowed to provide an opportunity for mixing and thus to reduce the costs of treatment. However, no mixing zone or combination of mixing zones shall be allowed to significantly impair any of the designated uses of the receiving body of water.

(b) A zone of mixing shall be determined based on the following:

1. The condition of the receiving body of water including present and future flow conditions and present and future sources of pollutants.

2. The nature, volume and frequency of the proposed discharge including any possible synergistic effects with other pollutants or substances which may be present in the receiving body of water.

3. The cumulative effect of the proposed mixing zone and other mixing zones in the vicinity.

(c) Except for the thermal components of discharges and nitrogen and phosphorus acting as nutrients, mixing zones which do not adhere to all of provisions paragraphs (1)(d) through (j) shall be presumed to constitute a significant impairment of the designated uses of surface waters of Classes I, II and III. An applicant for a mixing zone may obtain an exemption from these limitations as follows:

1. The applicant shall provide public notice, which shall be prepared or approved by the Department, in a newspaper of general circulation in the area in which the mixing zone is proposed. The notice shall identify the specific exemption it is seeking and notice the time, date and place of a public meeting at which, if the meeting is requested, the Department will consider comments to the requested exemption. The notice shall allow a person to request such a public meeting by contacting the Department within 14 days of the publication of the notice. If there is no such request, a public meeting is not required.

2. The applicant shall arrange for a public meeting which will be held if requested at which the Department will consider public comments on the exemption that is being sought. The Department shall also provide for public notice of the meeting in the Florida Administrative Register.

3. The applicant shall affirmatively demonstrate to the Department that the mixing zone exemption will not produce a significant adverse effect on the established community of organisms in the receiving body of water or otherwise significantly impair any of the designated uses of the receiving body of water.

4. The applicant shall affirmatively demonstrate to the Department that the requirements of paragraph (5)(c) of this section will be met.

(d) A mixing zone shall not include an existing drinking water supply intake or any other existing water supply intake if the constituents of such mixing zone would significantly impair the purposes for which the supply is used.

(e) A mixing zone shall not include a nursery area of indigenous aquatic life or any area approved by the Department of Environmental Protection for shellfish harvesting.

(f) In canals, rivers, streams, and other similar water bodies, the maximum length of a mixing zone shall be no more than 800 meters. In no case shall a mixing zone be larger than is necessary for the discharge to completely mix with the receiving water to meet water quality standards, and in no case shall a mixing zone significantly impair the designated use of the water body other than within the boundaries of the mixing zone.

(g) In lakes, estuaries, bays, lagoons, bayous, sounds, and coastal waters, the area of mixing zone shall be 125,600 square meters unless a lesser area is necessary to prevent significant impairment of a designated use. In no case shall a mixing zone be larger than is necessary to meet water quality standards.

(h) In open ocean waters, the area of a mixing zone shall be 502,655 square meters unless a lesser area is necessary to prevent significant impairment of a designated use. In no case shall a mixing zone be larger than is necessary to meet water quality standards.

(i) The mixing zones in a given water body shall not cumulatively exceed the limits described below:

1. In rivers, canals, and streams, and tributaries thereto and other similar water bodies: 10% of the total length;

2. In lakes, estuaries, bays, lagoons, bayous and sounds: 10% of the total area.

(j) Within mixing zones in Class I, Class II, and Class III waters, the turbidity shall not average greater than 41 Nephelometric Turbidity Units above natural background.

(k) Mixing zones in Class IV and V waters are subject only to the provisions of paragraph (d) above and of Rule 62-302.500, F.A.C., and shall not significantly impair the designated uses of the receiving body of water.

(2) There shall be no mixing zone for any component of any discharge unless a Department permit containing a description of its boundaries has been issued for that component of the discharge.

(3)(a) Waters within mixing zones shall not be degraded below the minimum standards prescribed for all waters at all times in Rule 62-302.500, F.A.C. In determining compliance with the provisions of subsection 62-302.500(1), F.A.C., the average concentration of wastes in the mixing zone shall be measured or computed using generally accepted scientific techniques; provided that, the maximum concentration of wastes in the mixing zone shall not exceed the amount lethal to 50% of the test organisms in 96 hours (96 hr. LC50) for a species significant to the indigenous aquatic community, except as provided in paragraph (b), (c) or (d) below. The dissolved oxygen value within any mixing zone shall not be less than 1.5 milligrams per liter at any time or place, except for an open ocean discharge, which must be above 1.5 milligrams per liter within 20 feet of the outfall structure.

(b) Except for open ocean discharges described in paragraph (c) and ionic imbalanced demineralization concentrate discharges, described in paragraph (d) below, the maximum concentration of wastes in the mixing zone may exceed the 96 hr. LC50 only when all of the following conditions are satisfied.

1. Dilution ratio of the effluent exceeds 100:1 under critical conditions. That is, flow in the receiving waters exceeds 100 units for every unit of effluent flow under critical conditions. Critical conditions are defined as those under which least dilution of the effluent is expected, e.g., maximum effluent flow and minimum receiving stream flow.

2. High rate diffusers or other similar means are used to induce rapid initial mixing of the effluent with the receiving waters such that exposure of organisms to lethal concentrations is minimized.

3. Toxicity must be less than acute [as defined in subsection 62-302.200(1), F.A.C.] no more than a distance of 50 times the discharge length scale in any spatial direction. The discharge length scale is defined as the square root of the cross-sectional area of any discharge outlet. In the case of a multiport diffuser, this requirement must be met for each port, using the appropriate discharge length scale for that port. This restriction will ensure a dilution factor of at least 10 within this distance under all possible circumstances, including situations of severe bottom interaction, surface interaction, or lateral merging.

4. The effluent when diluted to 30% of full strength, shall not cause more than 50% mortality in 96 hours (96 hr. LC50) in a species significant to the indigenous aquatic community.

5. If the following pollutants are present in the effluent, their concentrations (in the effluent) shall not exceed the values listed:

Acrylonitrile 65 ug/l

Aldrin 7.5 ng/l

Dieldrin 7.5 ng/l

Benzene 4 mg/l

Benzidine 53 ng/l

Beryllium 6.4 ug/l

Cadmium 100 ug/l

Carbon Tetrachloride 694 ug/l

Chlordane 48 ng/l

Chlorinated ethanes:

1,2-dichloroethane 24.3 mg/l

1,1,2-trichloroethane 4.2 mg/l

1,1,2,2-tetrachloroethane 1 mg/l

Hexachloroethane 874 ug/l

Chloroalkyl Ethers:

bis(chloromethyl) ether 184 ng/l

bis(2-chloroethyl) ether 136 ug/l

Chloroform 1.57 mg/l

Chromium (hexavalent) 0.5 mg/l

DDT 2.4 ug/l

Dichlorobenzidine 2 ug/l

1,1-Dichloroethylene 185 ug/l

Dinitrotoluene 11 ug/l

Diphenylydrazine 56 ug/l

Ethylbenzene 33 mg/l

Fluoranthene 540 ug/l

Halomethanes 1.6 mg/l

Heptachlor 29 ng/l

Hexachlorobenzene 74 ng/l

Hexachlorocyclohexane

Hexachlorocyclohexane 310 ng/l

Hexachlorocyclohexane 547 ng/l

Hexachlorocyclohexane 625 ng/l

Lead 0.5 mg/l

Mercury 1.5 ug/l

Nickel 1 mg/l

Nitrosamines 124 ug/l

Polynuclear aromatic hydrocarbons 3 ug/l

Polychlorinated biphenyls (PCBs) 8 ng/l

Selenium 100 ug/l

Tetrachloroethylene 885 ug/l

Thallium 480 ug/l

Toxaphene 73 ng/l

Trichloroethylene 8 mg/l

Vinyl Chloride 52 mg/l

(c) For open ocean discharges:

1. The effluent, when diluted to 30% full strength with water having a salinity representative of the average receiving-water’s salinity, shall not cause more than 50% mortality in 96 hours (96 hr. LC50) in a species significant to the indigenous aquatic community.

2. Rapid dilution shall be ensured by the use of multiport diffusers, or a single port outfall designed (by a professional engineer registered in Florida) to achieve a minimum of 20:1 dilution of the effluent prior to reaching the surface. This dilution shall be determined using the appropriate plume model described in the EPA document, “Initial Mixing Characteristics of Municipal Ocean Discharges: Volume 1. Procedures and Applications,” using the “Single plume, stagnant ambient” procedures or current speeds as established by field measurements. Miami-Dade Central District, Miami-Dade North District, City of Hollywood, and Broward County may use 12.3 cm/sec as a default value for ambient current speed at the present location of their respective outfalls. Alternatively, dilution studies for facilities not using the “Single plume, stagnant ambient” procedures or the 12.3 cm/sec default ambient current speed (as appropriate) shall be conducted in accordance with a site-specific Department approved Plan of Study. The Plan of Study shall be approved upon a demonstration by the applicant that the plan will produce data to characterize the daily, seasonal, and annual fluctuations in current speed and direction. The discharge shall otherwise comply with federal law.

3. For open ocean dischargers that comply with the requirements of subparagraphs 62-4.244(3)(c)1. and 2., F.A.C., compliance with applicable water quality criteria specified in Rule 62-302.530, F.A.C. (criteria), must be achieved by the point the discharge attains 20:1 dilution rather than at the point of discharge. Mixing zones shall not be necessary for any parameter that requires 20:1 dilution or less to attain criteria. However, effluent limitations will be set by permit, and dilutions will be granted up to 20:1 in these limitations, for parameters that exceed criteria at the end-of-pipe.

a. The demonstration of required dilution shall be determined by the ratio of the worst case effluent concentration (WCEC) minus the worst case background concentration to the criterion minus the worst case background concentration, i.e.:

(Worst case effluent concentration – Worst case background concentration)

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(Criterion – Worst case background concentration)

b. The WCEC for parameters that exceed criteria in the effluent shall be the 95th percentile effluent concentration (of DMR or other data collected in accordance with the sampling requirements of the permit measured for the most recent 3-year monitoring period, at the time of permit renewal) for each such parameter and not based on the maximum amount of dilution available. The WCEC used to demonstrate the required dilution for a parameter shall also be used as a facility performance check for each such parameter. Any exceedance of the WCEC shall provide sufficient cause for the Department to re-evaluate the applicability of this section and revise the permit. Additionally, any measured value(s) of sufficient concentration to require greater than 20:1 dilution to attain criteria shall be considered as a violation of the permit.

4. Subparagraph 62-4.244(3)(c)3., F.A.C., does not apply to bacterial criteria or silver in marine waters.

(d) Discharges of demineralization concentrate, as defined in Section 403.0882(2)(a), F.S., for which ionic imbalance is demonstrated, may exceed the 96 hr. LC50 in a mixing zone no greater than the area defined in sub-subparagraphs 62-4.244(3)(d)1.b. and 2.a., F.A.C. Ionic imbalance is defined as the failure of whole effluent toxicity tests caused predominantly by the presence of major ionic constituents naturally occurring in the source water (limited to calcium, potassium, sodium, magnesium, chloride, bromide, and other constituents designated by the Department). Ionic imbalance is shown using the protocols contained in “Technical Guidance Document for Conducting the Florida Department of Environmental Protection’s Protocols for Determining Major-Seawater-Ion Imbalance Toxicity (MSIIT) in Membrane-Technology Water-Treatment Concentrate” (FDEP, Bureau of Laboratories, May 4, 2004), which is adopted and incorporated by reference herein. Mixing zones for toxicity caused by ionic imbalance, as described in this paragraph, are allowed under the following conditions:

1. For all demineralization concentrate discharges defined in Section 403.0882(2)(a), F.S., except for small water utility businesses defined in Section 403.0882(2)(b), F.S.:

a. The effluent, when diluted to 20% full strength with laboratory pure water having a salinity representative of the receiving water’s salinity, shall not cause more than 50% mortality in 96 hours (96 hr. LC50) in a species significant to the indigenous aquatic community; and,

b. Under all ambient receiving water flow conditions, the effluent, mixed with receiving waters, must meet water quality standards within a distance not in excess of two times the natural water depth at the point of discharge. The natural water depth is defined as either the depth at Mean Lower Low Water (MLLW) Level in tidally affected waters or the depth at the seven-day, ten-year low flow (7Q10) conditions for non-tidal rivers, streams, canals, or ship channels. In no case shall the depth be artificially changed from its existing depth for the purpose of extending the area for complying with water quality standards and the acute toxicity requirements of paragraph 62-4.244(3)(d), F.A.C.

2. For small water utility businesses, as defined in Section 403.0882(2)(b), F.S.;

a. The discharge must achieve a minimum of 4-to-1 dilution within a distance not in excess of two times the natural water depth, as described in subparagraph 62-4.244(3)(d)1.b., F.A.C., at the point of discharge; and,

b. The effluent, when diluted to 20% full strength with laboratory pure water having a salinity representative of the receiving water’s salinity, shall not cause more than 50% mortality in 96 hours (96 hr. LC50) in a species significant to the indigenous aquatic community.

(4) Except for the minimum conditions of waters as specified in Rule 62-302.500, F.A.C., and the provisions of Rule 62-4.244, F.A.C., no other water quality criteria apply within a mixing zone.

(5) Mixing zones for dredge and fill permits shall not be subject to the provisions in paragraphs (1)(c) through (j), subsection (2), (3), or (4) of this section, provided that applicable water quality standards are met at the boundary and outside the mixing zone.

(a) The dimensions of dredge and fill mixing zones shall be proposed by the applicant and approved, modified or denied by the Department.

(b) Criteria for departmental evaluation of a proposed mixing zone shall include site-specific biological and hydrographic or hydrological considerations.

(c) In no case shall the boundary of a Joint Coastal Permit mixing zone be more than 1000 meters from the point of discharge into the waterbody or the boundary of a dredge and fill mixing zone be more than 150 meters downstream in flowing streams or 150 meters in radius in other bodies of water, where these distances are measured from the cutterhead, return flow, discharge, or other points of generation of turbidity or other pollutants.

(d) When determining the appropriate size of a turbidity mixing zone for a Joint Coastal Permit, the Department shall also use the following criteria:

1. Measures will be implemented to minimize the magnitude and duration of turbidity to the maximum extent practicable;

2. Mixing zones shall be kept to the minimum size necessary to meet the turbidity standard; and,

3. Mixing zones shall not encompass hardbottom communities, coral resources, or submerged aquatic vegetation beds outside of the authorized impact sites unless those areas are also evaluated as impact sites.

(6) Where a receiving body of water fails to meet a water quality standard for pollutants set forth in department rules, a steam electric generating plant discharge of pollutants that existed or was licensed on July 1, 1984, may be granted a mixing zone, provided that:

(a) The standard would not be met in the water body in the absence of the discharge; and,

(b) The discharge is in compliance with all applicable technology-based effluent limitations; and,

(c) The discharge does not cause a measurable increase in the degree of noncompliance with the standard at the boundary of the mixing zone; and,

(d) The discharge otherwise complies with the mixing zone provisions specified in this section.

(7) Additional relief from mixing zone restrictions necessary to prevent significant impairment of a designated use is through:

(a) Reclassification of the water body pursuant to Rule 62-302.400, F.A.C.;

(b) Variance granted pursuant to Section 403.201, F.S., and Rule 62-103.100, F.A.C.

(c) Modification of the requirements of this section for specific criteria by the Secretary upon compliance with the notice and hearing requirements for mixing zones set forth in paragraph (1)(c) above and upon affirmative demonstration by an applicant that the applicant’s discharge from a source existing on the effective date of this rule complies with best technology economically achievable, best management practices, or other requirements set forth in Chapter 62-600, F.A.C., and the economic, environmental and social costs of compliance with the existing criteria outweigh the social, environmental, and economic benefits of compliance with more stringent discharge limitations necessary to comply with mixing zone requirements of subsection 62-4.244(1), F.A.C., and the provisions relating to dissolved oxygen in Rule 62-4.244, F.A.C.

1. No discharger may be issued more than one permit or permit modification or renewal which allows a modification pursuant to this subsection unless the applicant affirmatively demonstrates that it has undertaken a continuing program, approved by the Department, designed to consider water quality conditions and review or develop any reasonable means of achieving compliance with the water quality criteria from which relief has been granted pursuant to this subsection.

2. With respect to paragraphs 62-4.244(1)(c) and 62-4.244(7)(c), F.A.C., the applicant must affirmatively demonstrate the minimum area of the water body necessary to achieve compliance with either subsection. Within a minimum area determined by the Secretary to be necessary to achieve compliance, the discharger shall be exempt from the criterion for which a demonstration has been made.

(d) Whenever site specific alternative criteria are established pursuant to Rule 62-302.800 or paragraph 62-302.510(2)(g), F.A.C., a mixing zone may be issued for dissolved oxygen if all provisions of Rule 62-4.244, F.A.C., are met with the exception of subparagraph 62-4.244(1)(j)1., F.A.C.

Rulemaking Authority 403.051, 403.061, 403.062, 403.087, 403.0882, 403.804, 403.805 FS. Law Implemented 403.021, 403.051, 403.061, 403.087, 403.088, 403.0882, 403.101, 403.121, 403.141, 403.161, 403.182, 403.201, 403.502, 403.702, 403.708 FS. History–Formerly part of 17-3.05, Revised and Renumbered 3-1-79, Amended 10-2-80, 1-1-83, 2-1-83, 2-19-84, 4-26-87, 8-31-88, 10-17-90, Formerly 17-4.244, Amended 3-26-00, 12-13-05, 8-1-13, 2-17-16.