Section 620.APPENDIX B   Procedures for Determining Hazard Indices for Class I: Potable Resource Groundwater for Mixtures of Similar-Acting Substances

a)         This appendix describes procedures for evaluating mixtures of similar-acting substances that 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 this appendix, a "mixture" means two or more substances present in Class I:  Potable Resource Groundwater that may or may not be related either chemically or commercially but are not complex mixtures of related isomers and congeners produced as commercial products (e.g., PCBs or technical grade chlordane).

c)         The substances specified in Section 620.Appendix E are similar-acting substances.

d)         When two or more substances occur together in a mixture, the additivity of the toxicities of some or all of the substances must be considered when determining health-based standards for Class I:  Potable Resource Groundwater. This is done by using 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:

HI = [A]/ALA + [B]/ALB +. . . [I]/ALI

Where:

e)         For a substance with a threshold mechanism of toxicity, the substance's acceptable level in subsection (d) is:

1)         The substance's standard specified in Section 620.410; or

2)         The substance's Human Threshold Toxicant Advisory Concentration (HTTAC) as determined in Appendix A if the substance has no standard specified in Section 620.410.

f)         For a carcinogenic substance, the substance's acceptable level in subsection (d) is:

1)         The substance's standard specified in Section 620.410; or

2)         If a substance has no standard specified in Section 620.410, the substance's one-in-one-million cancer risk concentration, unless that concentration is less than the substance's lowest appropriate LLOQ specified in "Test Methods for Evaluating Solid Wastes, Physical/Chemical Methods", EPA Publication No. SW-846, incorporated by reference in Section 620.125, or the substance's lowest appropriate LCMRL specified in the drinking water methods incorporated by reference in Section 620.125.  If the one-in-one-million cancer risk concentration is less than the lowest appropriate LLOQ or LCMRL, the acceptable level is the lowest appropriate LLOQ or LCMRL.

g)         Because the assumption of dose addition is most properly applied to substances that induce the same effect by similar modes of action, a separate Hazard Index 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.

(Source:  Amended at 49 Ill. Reg. 4488, effective March 28, 2025)