26.08.02.05

.05 Surface Water Mixing Zones.

A. General.

(1) Effluents may be mixed with surface waters in the mixing zone.

(2) Effluents may not be treated in the mixing zone.

(3) Lethality to passing organisms may not occur in any mixing zone.

(4) Surface waters outside the mixing zones shall meet the water quality criteria for that particular body of water.

(5) Mixing zones may be designated by the Department provided that the following requirements are met outside the mixing zones:

(a) There shall be no interference with biological communities or populations of indigenous species to a degree which is damaging to the aquatic life or ecosystem;

(b) There shall be no diminishing of other legitimate beneficial uses;

(c) Mixing zones may not form barriers to the migratory routes of aquatic life;

(d) Mixing zones shall be designated and located to protect surface waters and shallow water shoreline areas;

(e) The general water quality criteria set out in Regulation .03B(1)—(3) of this chapter apply within the mixing zones.

(6) Complete mixing within the mixing zone shall be assumed for toxic substance discharges to streams, rivers, and estuaries unless site-specific information indicates that another mixing pattern is more appropriate.

(7) Stream flows other than the design flow values set forth in §§B—E of this regulation may be used, at the Department's discretion, on a case-by-case basis for mixing zones associated with noncontinuous discharges.

(8) Toxic pollutants shall be treated as conservative substances when calculating instream waste concentrations. The assumption of conservatism may be waived based on pollutant-specific and site-specific information.

(9) Unless a later time is stipulated by the Department, the discharger shall submit to the Department, at the time of permit application, the mixing zone technique preferred for each of its discharges, and actual mixing zone calculations together with supporting documentation.

(10) A mixing zone may not cause a significant human health risk, considering likely pathways of human exposure.

(11) Except when the applicant can demonstrate to the satisfaction of the Department that adverse aquatic life or human health effect does not occur, overlapping mixing zones are not permitted. Demonstration may include chemical monitoring, ambient toxicity testing, or examination of benthic communities or fish tissue.

(12) A mixing zone may not be granted in water body segments with documented occurrences of any endangered or threatened species listed under §4 of the federal Endangered Species Act, if that discharge would likely have an adverse effect on those species.

B. Mixing Zones for Conventional Pollutants. The following requirements apply to the calculation of the regulatory mixing zones for conventional pollutants as identified in the Federal Act:

(1) Except for thermal mixing zones established in accordance with COMAR 26.08.03.03—.05 and toxic substance mixing zones established in accordance with §§C—E of this regulation, any mixing zone may not exceed the following maximum limits:

(a) In freshwater streams and rivers, a mixing zone width may not exceed 1/3 of the width of the surface water body;

(b) In lakes, the combined area of all mixing zones may not exceed 10 percent of the lake surface area; and

(c) In estuarine areas, the maximum cross-sectional area of the mixing zone may not exceed 10 percent of the cross-sectional area of the receiving water body; and

(2) The flows used shall be:

(a) For freshwater streams and rivers, the design stream flow; and

(b) For estuaries and the open ocean, determined from:

(i) Specific data, when available, for the mean water level and average tidal velocity and, where appropriate, the design stream flow,

(ii) Specific data on waste dispersion or dilution, when available for a specific discharge, or

(iii) Dispersion or dilution studies required at the Department's discretion.

C. Application of Toxic Substance Acute Criteria for the Protection of Aquatic Life.

(1) In intermittent streams, the acute criterion shall be applied at the end of the discharge pipe.

(2) In other water bodies, achievement of the acute criterion to protect aquatic life shall be provided:

(a) Within a very short distance from the outfall using:

(i) A high velocity discharge with an initial velocity of 3 meters per second or more, and

(ii) A mixing zone limited to 50 times the discharge length scale in any direction, where the discharge length scale is defined as the square root of the cross-sectional area of any discharge outlet;

(b) Without a high velocity discharge, within a short distance from the outfall using the most restrictive of the following conditions:

(i) Meeting the acute toxicity criterion within 10 percent of the distance from the edge of the outfall structure in any direction to the edge of the mixing zone used for application of toxic substance chronic criteria,

(ii) Meeting the acute toxicity criterion within a distance of 50 times the discharge length scale in any direction, when the discharge length is defined as the square root of the cross-sectional area of any discharge outlet, or

(iii) Meeting the acute toxicity criterion within a distance of five times the local water depth in any horizontal direction from the discharge outlet, where appropriate; or

(c) By demonstration or calculation that a drifting organism may not be exposed to a 1-hour average concentration exceeding the acute aquatic life criterion.

(3) For the application of the acute criteria, any mixing zone may not exceed the following maximum limits:

(a) In freshwater streams and rivers, a width equal to 1/3 the width of the surface water body;

(b) In lakes, for all discharges combined, 5 percent of the lake surface area; and

(c) In estuarine areas, a cross-sectional area equal to 5 percent of the cross-sectional area of the receiving water body.

(4) The flows used shall be:

(a) For freshwater streams and rivers, the design stream flow; and

(b) For estuaries and the open ocean, determined from:

(i) Specific data, when available, for the mean low water and minimum daily average 1-hour tidal velocity and, when appropriate, the design stream flow,

(ii) Specific data on waste dispersion or dilution, when available for a specific discharge, or

(iii) Dispersion or dilution studies required at the Department's discretion.

D. Application of Toxic Substance Chronic Criteria for the Protection of Aquatic Life.

(1) Any mixing zone may not exceed the following:

(a) In freshwater streams and rivers, a mixing zone width may not exceed 1/3 of the width of the surface water body;

(b) In lakes, the combined area of all mixing zones may not exceed 10 percent of the lake surface area; and

(c) In estuarine areas, the maximum cross-sectional area of the mixing zone may not exceed 10 percent of the cross-sectional area of the receiving water body.

(2) The flows used shall be:

(a) For freshwater streams and rivers, the 30Q5 value; and

(b) For estuaries and the open ocean, determined from:

(i) Specific data, when available, for the mean water level and average tidal velocity and, when appropriate, the 30Q5 stream flow,

(ii) Specific data on waste dispersion or dilution, when available for a specific discharge, or

(iii) Dispersion or dilution studies required at the Department's discretion.

E. Application of Toxic Substance Criteria for the Protection of Human Health. The flow used to determine impacts to human health shall be the mean annual flow value.