Opinion ID: 2035709
Heading Depth: 1
Heading Rank: 3

Heading: Mixing Regulations

Text: Two other features of the amended regulations are their provisions for allowed mixing (35 Ill.Adm.Code §§ 302.102(a), (b) (1992)), and mixing zones (35 Ill.Adm. Code § 302.102(d) (1992)). Under these provisions, a discharger may be allowed to comply with the Board's water quality standards by mixture of an effluent with its receiving waters. (35 Ill.Adm.Code § 302.102(a) (1992) (allowed-mixing concept).) Pursuant to the allowed-mixing concept, the regulations provide that, if a discharger complies with certain regulatory requirements, an opportunity shall be allowed for compliance with 35 Ill.Adm.Code 304.105 by mixture of an effluent with its receiving waters. (Emphasis added.) (35 Ill.Adm.Code § 302.102(a) (1992).) The allowed-mixing regulations further set forth 12 factors which limit the allowed-mixing area. 35 Ill.Adm.Code §§ 302.102(b)(1) through (b)(12) (1992). In contrast to the allowed-mixing concept, the regulations also provide for mixing zones and zones of initial dilution (ZID). (35 Ill.Adm.Code §§ 302.102(d), (e) (1992).) A mixing zone is an area for allowed mixing which is formally defined by the Agency in the NPDES permitting process and, if granted, is included as a condition in the permittee's NPDES permit. A ZID is likewise formally defined and granted by the Agency during the permitting process and, if granted, is included in the discharger's mixing-zone permit condition. The significance of having an area of allowed mixing or a mixing-zone permit condition is that it alters the point in the receiving waters at which the Board's various water quality standards must be met. Outside of the allowed-mixing area or the mixing zone, all water quality standards, both acute and chronic as well as the subpart criteria, must be met. Inside the allowed-mixing area or the mixing zone, only the acute toxicity standards and acute narrative criteria must be met by the discharger, subject to the existence of a ZID. If a ZID is granted, the discharger is excused from compliance with the Board's acute toxicity standards and the acute criteria within the ZID. The Agency's permit decisions regarding mixing-zone and ZID conditions control the permittee's opportunity to utilize mixing as a means of compliance with the Board's water quality standards. (35 Ill.Adm.Code §§ 302.102(g), (h) (1992).) If there is no NPDES permit in effect or the permit is silent as to mixing, the discharger may invoke the general allowed-mixing provisions as a defense to any enforcement action brought against it for violations of the Board's water quality standards and has the burden to prove that it is in compliance with the allowed-mixing regulations. (35 Ill.Adm.Code § 302.102(i) (1992).) If the Agency has made a mixing-zone or ZID determination, the permittee may appeal that determination to the Board pursuant to section 40 of the Act (Ill.Rev.Stat.1989, ch. 111½, par. 1040 (permit appeals)). 35 Ill.Adm.Code § 302.102(f) (1992).