Ill. Admin. Code tit. 35 § 406.103
Because the effluent standards in this Part are based upon 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. This Part is not intended to require users to clean up contamination caused by upstream sources or to require treatment when only traces of contaminants are added to the background. Complying with the numerical effluent standards is not required when effluent concentrations exceeding the standards result entirely from influent contamination before it enters the affected land. Background concentrations or discharges upstream from affected land are rebuttably presumed not to have caused a violation of this Part.
Ill. Admin. Code tit. 35, § 406.103