620 Groundwater Quality

On April 11, 2025, the Illinois Pollution Control Board published amendments to 35 Illinois Administrative Code (IAC) Part 620 Groundwater Quality in the Illinois Register. The updated amendments, effective March 28, 2025, include adding groundwater quality standards for:  

• Six PFAS
  • Perfluorooctanoic acid (PFOA)
  • Perfluorooctanesulfonic acid (PFOS)  
  •  Perfluorononanoic acid (PFNA)
  • Perfluorohexanesulfonic acid (PFHxS)   
  • Perfluorobutanesulfonic acid (PFBS)    
  • Hexafluoropropylene oxide dimer acid (HFPO-DA or GenX)
• Molybdenum
• Lithium
• Aluminum
• 1-methylnaphthalene
• Three new atrazine metabolites whose concentrations are combined with atrazine when determining compliance

In addition to setting numerical groundwater quality standards for the newly added chemicals, the Part 620 amendments:

• Update existing Class I and Class II standards for 34 chemical constituents;
• Add site-specific groundwater standards for designated Class III Special Resource Groundwater;
• Update groundwater management zone (GMZ) requirements;
• Update exposure factors used for calculating Human Threshold Toxicant Advisory Concentrations (health-based guidance levels for non-carcinogens);
• Update methods for calculating Human Nonthreshold Toxicant Advisory Concentrations (health-based guidance levels for carcinogens), including a separate calculation for chemicals that are listed as mutagens;
• Add tables listing similar-acting chemicals for assessing mixtures of contaminants in the groundwater;
• Update procedures for determining Class I groundwater;
• Update definitions, incorporations by reference, and analytical requirements; and
• Establish in 620.410(f) that facilities subject to Part 811 or 814 are not required to comply with the Part 620.410 standards for HFPO-DA, PFBS, PFHxS, PFNA, PFOA, or PFOS.

This table constructed by the Agency depicts the updated numeric groundwater quality standards with the previous standards for comparison.

Full rulemaking case details for amendments to IAC Part 620 can be found at https://pcb.illinois.gov/.  Select "Search by Case" and enter Case # R2022-018 in the "Search Criteria."