Ill. Admin. Code tit. 35 § 304.103
Because the effluent standards in this Part are based on concentrations achievable with conventional treatment technology that is largely unaffected by ordinary levels of contaminants in intake water, they are absolute standards that must be met without subtracting background concentrations. However, these rules do not intend to require users to clean up contamination caused essentially by upstream sources or to require treatment when only traces of contaminants are added to the background. Compliance with the numerical effluent standards is not required when effluent concentrations greater than the standards result entirely from influent contamination, evaporation, or the incidental addition of traces of materials not used or produced in the activity that is the waste source.
Ill. Admin. Code tit. 35, § 304.103