Administrative Rules of Montana › Department 17 - ENVIRONMENTAL QUALITY › Chapter 17.30 - WATER QUALITY › Subchapter 17.30.5 - Mixing Zones in Surface and Ground Water › Rule 17.30.517 - STANDARD MIXING ZONES FOR GROUND WATER

Current through Register Vol. 13, July 5, 2024
(1) The following
				criteria apply to determine which discharges qualify for a standard ground
				water mixing zone:
(a) A standard ground water
				mixing zone is generally applicable in unconfined aquifers, but may not be
				appropriate for semi-confined or confined aquifers or in aquifers where ground
				water moves through fractures.
(b)
				Disposal systems that discharge to ground water through infiltration,
				absorption systems, injection through a disposal well, leakage from an
				impoundment, seepage from a land application area, or other methods may qualify
				for a standard mixing zone.
(c) To
				determine if the discharge qualifies for a standard ground water mixing zone,
				the person proposing the discharge must estimate the anticipated concentration
				of pollutants at the downgradient boundary of the mixing zone (aquatic life
				standards do not apply in ground water). If the estimated concentration meets
				the nonsignificance criteria at the boundary of the mixing zone, as specified
				in ARM Title 17, chapter 30, subchapter 7, the discharge qualifies for a
				standard mixing zone.
(d) The
				estimation required in (c) must be based on a calculation of the volume of
				water moving through a standard cross-section of aquifer. The calculated volume
				of water moving through the aquifer cross-section is hypothetically mixed with
				the known volume and concentration of the discharge to determine the resulting
				concentration at the boundary of the mixing zone. The recommended method to
				determine the resulting concentration at the boundary of a standard ground
				water mixing zone is described below:
(i)
				Computations of the volume of ground water available for mixing are based on
				the equation: Q = KIA, where:
(A) Q = volume
				of flow;
(B) K = hydraulic
				conductivity;
(C) I = gradient;
				and
(D) A = aquifer cross-section
				area.
(ii) Values for
				hydraulic conductivity (K) and gradient (I) may be obtained from field
				observations or estimated from other sources.
(iii) A specific depth and width are
				necessary to determine the aquifer cross-section area (A) for a standard mixing
				zone. The aquifer cross-section area prescribed by the following lengths is
				used as the area (A) in the equation:
(A) The
				depth of a standard ground water mixing zone extends from the top of the water
				table beneath the source down to 15 feet below the water table.
(B) The width of a standard mixing zone is
				equal to the width of the source plus the distance determined by the tangent of
				5° times the length of the mixing zone on both sides of the
				source.
(iv) It is
				assumed that mixing between the discharge and the receiving ground water is
				complete at the aquifer cross-section area at the standard distance
				downgradient from the source.
(v)
				It is also assumed that pollutants discharged from the source do not change in
				volume or concentration as they migrate through the unsaturated zone down to
				the water table except as allowed in (vi).
(vi) For total nitrogen in residential
				strength wastewater discharged from a wastewater treatment system that uses
				absorption system pressure distribution in accordance with Department Circular
				DEQ-4 and does not require a Montana Pollutant Discharge Elimination System
				(MPDES) or Montana Ground Water Pollution Control System (MGWPCS) permit, the
				waste load as described in (vii)(B) may be reduced in the vadose zone and
				saturated zone to account for natural attenuation using Montana's Septic
				Trading Method in Appendix A of Department Circular DEQ-13.
(vii) The concentration of the parameter in
				the ground water must be measured to determine the existing load present in the
				ground water. The calculated volume of ground water with a measured
				concentration is hypothetically mixed with the known volume and concentration
				of the discharge using the following procedure:
(A) Volume of ground water times the
				concentration of the parameter = existing load;
(B) Volume of discharge times the
				concentration of the parameter = waste load; and
(C) (Existing load + waste load) /total
				volume = resulting concentration.
(viii) If the resulting concentration does
				not exceed the nonsignificance criteria specified in ARM Title 17, chapter 30,
				subchapter 7, for new or increased sources at the mixing zone boundary, a
				standard mixing zone may be granted.
(ix) The downgradient boundary of the
				standard mixing zone extends:
(A) between 100
				and 500 feet as proposed by the applicant for individual or shared wastewater
				absorption systems that discharge residential strength wastewater;
(B) between 200 and 500 feet as proposed by
				the applicant for multiple-user wastewater absorption systems that discharge
				residential strength wastewater with a design flow greater than 800
				gpd;
(C) between 100 and 500 feet
				as proposed by the applicant for commercial, multiple-user, or public
				wastewater absorption systems that discharge residential strength wastewater if
				the design flow is 800 gpd or less; and
(D) 500 feet for any other source of waste,
				including industrial wastewater, discharging into ground water.
(x) Monitoring may be required at
				the downgradient boundary of the mixing zone to measure compliance for a ground
				water mixing zone established for other than an individual wastewater
				absorption system, if there is an overriding site-specific impact-related
				reason to require monitoring and the mixing zone is within 500 feet of surface
				water, another ground water mixing zone, or a drinking water well, or if there
				is some other overriding site-specific, impact-related reason to require
				monitoring.