17.30.516    STANDARD MIXING ZONES FOR SURFACE WATER

(1) If a discharge to surface water is small in comparison to the volume of the receiving water or if the mixing is nearly instantaneous and the parameter(s) of concern will not threaten or impair existing uses as determined under ARM 17.30.506, a standard mixing zone may be used.

(2) A standard surface water mixing zone will not be granted for a new or increased discharge to a lake or wetland.

(3) Facilities that meet the terms and conditions in (a) through (e) qualify for a standard mixing zone as follows:

(a) Facilities that discharge a mean annual flow of less than one million gallons per day (MGD) to a stream segment with a dilution ratio greater than or equal to 100:1. For purposes of this procedure, the stream dilution ratio is defined as the seven-day, ten-year (7Q10) low flow of the stream segment without the discharge, divided by the mean annual flow of the discharge. In this case discharge limitations will be based on dilution with the 7Q10.

(b) Facilities that discharge a mean annual flow less than one MGD to a stream segment with a dilution less than 100:1. In cases where dilution is less than 100:1, discharge limitations will be based on dilution with 25 percent of the 7Q10.

(c) Facilities that discharge to surface waters through the ground may qualify for a standard surface water mixing zone.

(d) Facilities whose discharge results in a nearly instantaneous mixing zone. Discharge limitations shall be based on dilution with the seven-day, ten-year low flow of the receiving water except as limited by consideration of the factors listed in ARM 17.30.506. For surface waters, nearly instantaneous mixing will be assumed when there is an effluent diffuser which extends across the entire stream width (at low flow), or when the mean daily flow of the discharge exceeds the seven-day, ten-year low flow of the receiving water. A discharge may also be considered nearly instantaneous if the discharger so demonstrates in accordance with a study plan approved by the department. For the purposes of this demonstration nearly instantaneous mixing will be assumed when there will be not more than a ten percent difference in bank-to-bank concentrations at a downstream distance less than two stream/river widths.

(e) Facilities that discharge the parameters found in Department Circular DEQ-12A to surface water. Discharge limitations must be based on dilution with the entire seasonal 14-day, five-year (seasonal 14Q5) low flow of the receiving water without the discharge.

(4) The length of a standard mixing zone for flowing surface water, other than a nearly instantaneous mixing zone, must not extend downstream more than the one-half mixing width distance or extend downstream more than ten times the stream width, whichever is more restrictive. For purposes of making this determination, the stream width as well as the discharge limitations are considered at the 7Q10 or seasonal 14Q5 low flow. The seasonal 14Q5 low flow may be used only in conjunction with base numeric nutrient standards in Department Circular DEQ-12A. The recommended calculation to be used to determine the one-half mixing width distance downstream from a stream bank discharge is described below.

(a) A_1/2 = [0.4(W/2)²V]/L, where:

(i) A_1/2 = one-half mixing width distance;

(ii) W = width in feet at the 7Q10 or seasonal 14Q5;

(iii) V = velocity of the stream at the 7Q10 or seasonal 14Q5 downstream of the discharge (in ft/second);

(iv) L = lateral dispersion coefficient for the 7Q10 or seasonal 14Q5 downstream of the discharge (in ft²/second), where:

(b) L = CDU, where:

(i) C = channel irregularity factor immediately downstream of the discharge, where:

(A) C = 0.1 for straight, rectangular streams;

(B) C = O.3 for channelized streams;

(C) C = 0.6 for natural channels with moderate meandering;

(D) C = 1.0 for streams with significant meandering; and

(E) C = 1.3 for streams with sharp 90º or more bends;

(ii) D = average water depth at the 7Q10 or seasonal 14Q5 downstream of the discharge (in feet);

(iii) U = shear velocity (in ft/sec), where:

(c) U = (32.2DS)^1/2, where:

(i) 32.2 is the acceleration due to gravity (32.2 ft/sec²);

(ii) D = average water depth at the 7Q10 or seasonal 14Q5 downstream of the discharge (in feet); and

(iii) S = slope of the channel downstream of the discharge (feet/feet).

(5) Monitoring may be required at the downgradient boundary of a surface water mixing zone only when there is a site-specific, impact-related reason to require such monitoring.

(6) A standard surface water mixing zone may be modified by the department on a case-by-case basis depending upon existing uses, flow regime, and the configuration of the stream channel. Where currently available data indicates that modifying a standard mixing zone would threaten or impair existing beneficial uses under ARM 17.30.506, the facility will not qualify for this modification procedure.

History: 75-5-301, MCA; IMP, 75-5-301, MCA; NEW, 1994 MAR p. 2136, Eff. 8/12/94; TRANS, from DHES, 1996 MAR p. 1499; AMD, 2006 MAR p. 528, Eff. 2/24/06; AMD, 2014 MAR p. 1815, Eff. 8/8/14.