Illinois Administrative Code › Title 35 - ENVIRONMENTAL PROTECTION › Part 620 - GROUNDWATER QUALITY › Subpart F - HEALTH ADVISORIES › Appendix B - Procedures for Determining Hazard Indices for Class I: Potable Resource Groundwater for Mixtures of Similar-Acting Substances

Current through Register Vol. 48, No. 28, July 12, 2024
a) This appendix describes procedures for evaluating mixtures
				  of similar-acting substances which may be present in Class I: Potable Resource
				  Groundwaters. Except as provided otherwise in subsection (c), subsections (d)
				  through (h) describe the procedure for determining the Hazard Index for
				  mixtures of similar-acting substances.
b) For the purposes of this appendix, a "mixture" means two
				  or more substances which are present in Class I: Potable Resource Groundwater
				  which may or may not be related either chemically or commercially, but which
				  are not complex mixtures of related isomers and congeners which are produced as
				  commercial products (for example, PCBs or technical grade chlordane).
c) The following substances listed in Section 620.410 are
				  mixtures of similar acting substances:
1) Mixtures of ortho-Dichlorobenzene and
				  para-Dichlorobenzene. The Hazard Index (HI) for such mixtures is determined as
				  follows:
HI = [ortho-Dichlorobenzene]/0.6 +
				  [para-Dichlorobenzene]/0.075
2) Mixtures of 1,1-Dichloroethylene and
				  1,1,1-trichloroethane. The Hazard Index (HI) for such mixtures is determined as
				  follows:
HI = [1,1-Dichloroethylene]/0.007 +
				  [1,1,1-trichloroethane]/0.2
d) When two or more substances occur together in a mixture,
				  the additivity of the toxicities of some or all of the substances will be
				  considered when determining health-based standards for Class I: Potable
				  Resource Groundwater. This is done by the use of a dose addition model with the
				  development of a Hazard Index for the mixture of substances with similar-acting
				  toxicities. This method does not address synergism or antagonism. Guidelines
				  for determining when the dose addition of similar-acting substances is
				  appropriate are presented in Appendix C. The Hazard Index is calculated as
				  follows:
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e) For substances that are considered to have a threshold
				  mechanism of toxicity, the acceptable level is:
1) The standards listed in Section 620.410;
				  or
2) For those substances for which standards have
				  not been established in Section 620.410, the Human Threshold Toxicant Advisory
				  Concentration (HTTAC) as determined in Appendix A.
f) For substances that are carcinogens, the acceptable level
				  is:
1) The standards listed in Section 620.410;
				  or
2) For those substances for which standards have
				  not been established under Section 620.410, the one-in-one-million cancer risk
				  concentration, unless the concentration for such substance is less than the
				  lowest appropriate PQL specified in "Test Methods for Evaluating Solid Wastes,
				  Physical/Chemical Methods," EPA Publication No. SW-846, incorporated by
				  reference at Section 620.125, for the substance, in which case the lowest
				  appropriate PQL shall be the acceptable level.
g) Since the assumption of dose addition is most properly
				  applied to substances that induce the same effect by similar modes of action, a
				  separate HI must be generated for each toxicity endpoint of concern.
h) In addition to meeting the individual substance
				  objectives, a Hazard Index must be less than or equal to 1 for a mixture of
				  similar-acting substances.