1. a) Groundwater that is 10 feet or more below the land surface and within:

   1. 1) The minimum setback zone of a well that serves as a potable water supply and to the bottom of the well;
   2. 2) Unconsolidated sand, gravel, or sand and gravel that is 5 feet or more in thickness and contains 12% or less of fines (i.e., fines that pass through a No. 200 sieve tested in compliance with ASTM Standard Practice D2487-06, incorporated by reference in Section 620.125);
   3. 3) Sandstone that is 10 feet or more in thickness or fractured carbonate that is 15 feet or more in thickness;

   4. 4) Any geologic material that is capable of a:

      1. A) Sustained groundwater yield, from up to a 12-inch borehole, of at least 150 gallons per day from a thickness of 15 feet or less; or

      2. B) Hydraulic conductivity of 1 x 10^-4 cm/sec or greater using one of the following test methods or its equivalent:

         1. i) Slug test; or
         2. ii) Pump test;
   5. 5) A wellhead protection area, as defined in Section 620.110, that is a Phase I or Phase II wellhead protection area delineated in compliance with the "Guidance Document for Groundwater Protection Needs Assessments" and "The Illinois Wellhead Protection Program", both incorporated by reference in Section 620.125; or
   6. 6) The maximum setback zone of a community water supply well adopted under Section 14.3 of the Act.

2. b) Groundwater that is determined by the Board, under the procedures specified in Section 620.260, to be capable of potable use.

   BOARD NOTE: In determining whether geologic material meets a subsection (a)(2) or (a)(3) thickness minimum or the subsection (a)(4)(A) thickness maximum, the entire thickness of the geologic material is considered, regardless of whether all or only some of the thickness is 10 feet or more below the land surface. For example, groundwater that is 10 feet or more below the land surface and within any geologic material described in subsection (a)(2), (a)(3), or (a)(4)(A) is designated as Class I: Potable Resource Groundwater, even if some of the geologic material's thickness is within 10 feet of the land surface. In addition, if groundwater that is 10 feet or more below the land surface – and within any region or geologic material described in subsection (a) – also extends upward to within 10 feet of the land surface, then the groundwater 10 feet or more below the land surface is designated as Class I: Potable Resource Groundwater but the groundwater within 10 feet of the land surface is not.

Except as provided in Section 620.230, 620.240, or 620.250, Potable Resource Groundwater is as described in subsection (a) or (b):

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