Ill. Admin. Code tit. 35, § 620.310 – Preventive Response Activities

a) The following preventive assessment must be undertaken:

1) If a preventive notification under Section 620.305(c) is provided by a community water supply:
1. A) The Agency must notify the owner or operator of any identified potential primary source, potential secondary source, potential route, or community water supply well that is within 2,500 feet of the wellhead.
2. B) The owner or operator notified under subsection (a)(1)(A) must, within 30 days after the date of issuance of that notice, sample each water well or monitoring well for the contaminant identified in the notice if the contaminant or material containing the contaminant is or has been stored, disposed of, or otherwise handled at the site. If a contaminant identified under Section 620.305(a) is detected, then the well must be resampled within 30 days after the date on which the first sample results are received. If a contaminant identified under Section 620.305(a) is detected by the resampling, preventive notification must be given as specified in Section 620.305.
3. C) If the Agency receives analytical results under subsection (a)(1)(B) that show a contaminant identified under Section 620.305(a) has been detected, the Agency must:
  1. i) Conduct a well site survey under Section 17.1(d) of the Act [415 ILCS 5/17.1(d)] if one has not been conducted within the last 5 years; and
  2. ii) Identify those sites or activities that represent a hazard to the continued availability of groundwaters for public use unless a groundwater protection needs assessment has been prepared under Section 17.1(d) of the Act.
2) If a preventive notification is provided under Section 620.305(c) by a non-community water supply or for multiple private water supply wells, the Department of Public Health must conduct a sanitary survey within 1,000 feet of the wellhead of a non-community water supply or within 500 feet of the wellheads for multiple private water supply wells.

3) If a preventive notification under Section 620.305(b) is provided by the owner or operator of a regulated entity and the applicable standard of Subpart D has not been exceeded, the appropriate regulatory agency must:

A) Determine if any of the following occurs for Class I: Potable Resource Groundwater:

i) The levels specified below are exceeded or are changed for pH:

CASRN	Constituent	Criteria (mg/L)

95-50-1	o-Dichlorobenzene (1,2-dichlorobenzene)	0.01
1634-04-4	MTBE methyl tertiary butyl ether	0.02
108-95-2	Phenols	0.001
100-42-5	Styrene	0.01
108-88-3	Toluene	0.04
1330-20-7	Xylenes	0.02

ii) A statistically significant increase occurs above background (as determined under other regulatory procedures (e.g., 35 Ill. Adm. Code 616, 724, 725, or 811)) for the following inorganic constituents (except due to natural causes) or organic constituents:

CASRN Inorganics	Constituent
7429-90-5	Aluminum
7440-38-2	Arsenic
7440-41-7	Beryllium
7440-43-9	Cadmium
7440-47-3	Chromium (total)
143-33-9	Cyanide
7439-92-1	Lead
7487-94-7	Mercury (mercuric chloride)
7439-98-7	Molybdenum
7440-28-0	Thallium
7440-62-2	Vanadium
Organics
83-32-9	Acenaphthene
67-64-1	Acetone
116-06-3	Aldicarb
120-12-7	Anthracene
319-84-6	alpha-BHC (alpha-benzene hexachloride)
1912-24-9	Atrazine and metabolites DEA, DIA, DACT
71-43-2	Benzene
56-55-3	Benzo(a)anthracene
205-99-2	Benzo(b)fluoranthene
207-08-9	Benzo(k)fluoranthene
50-32-8	Benzo(a)pyrene
65-85-0	Benzoic acid
78-93-3	2-Butanone (methyl ethyl ketone)
1563-66-2	Carbofuran
75-15-0	Carbon disulfide
56-23-5	Carbon tetrachloride
12789-03-6	Chlordane
108-90-7	Chlorobenzene
67-66-3	Chloroform
218-01-9	Chrysene
94-75-7	2.4-D (2.4-dichlorophenoxy acetic acid)
75-99-0	Dalapon
96-12-8	1,2-Dibromo-3-chloropropane (dibromochloroorooane)
1918-00-9	Dicamba
106-46-7	p-Dichlorobenzene (1,4-dichlorobenzene)
75-71-8	Dichlorodifluoromethane
75-34-3	1,1-Dichloroethane
75-35-4	1,1-Dichloroethylene
107-06-2	1,2-Dichloroethane
156-59-2	cis-1,2-Dichloroethylene
156-60-5	trans-1,2-Dichloroethylene
75-09-2	Dichloromethane (methylene
78-87-5	1,2-Dichloropropane
117-81-7	Di(2-ethylhexyl)phthalate
84-66-2	Diethyl phthalate
84-74-2	Di-n-butyl phthalate
99-65-0	1,3-Dinitrobenzene
121-14-2	2,4-Dinitrotoluene
88-85-7	Dinoseb
123-91-1	1,4-Dioxane (p dioxane)
145-73-3	Endothall
72-20-8	Endrin
100-41-4	Ethylbenzene
106-93-4	Ethylene dibromide (1,2-dibromoethane)
206-44-0	Fluoranthene
86-73-7	Fluorene
58-89-9	gamma-HCH (gamma-hexachlorocyclohexane lindane)
13252-13-6	HFPO-DA (hexafluoropropylene oxide dimer acid, GenX)
2691-41-0	HMX (octahydro-1,3,5,7-tetranitro-1, 3, 5, 7-tetrazocine)
76-44-8	Heptachlor
1024-57-3	Heptachlor epoxide
77-47-4	Hexachlorocyclopentadiene
193-39-5	Indeno(l,2,3-c,d)pyrene
98-82-8	Isopropylbenzene (cumene)
72-43-5	Methoxychlor
90-12-0	1-Methylnaphthalene
91-57-6	2-Methylnaphthalene
95-48-7	2-Methylphenol (o-cresol)
91-20-3	Naphthalene
98-95-3	Nitrobenzene
1336-36-3	PCBs (polychlorinated biphenyls as decachloro-biphenyl)
375-73-5	PFBS (perfluorobutanesulfonic acid)
355-46-4	PFHxS (perfluorohexanesulfonic acid)
375-95-1	PFNA (perfluorononanoic acid)
335-67-1	PFOA (perfluorooctanoic acid)
1763-23-1	PFOS (perfluorooctanesulfonic acid)
87-86-5	Pentachlorophenol
1918-02-1	Picloram
129-00-0	Pyrene
121-82-4	RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine)
122-34-9	Simazine
118-96-7	TNT (2,4,6-trinitrotoluene)
93-72-1	2,4,5-TP (silvex)
127-18-4	Tetrachloroethylene
8001-35-2	Toxaphene
120-82-1	1,2,4-Trichlorobenzene
71-55-6	1,1,1-Trichloroethane
79-00-5	1,1,2-Trichloroethane
79-01-6	Trichloroethylene
75-69-4	Trichlorotluoromethane
99-35-4	1,3,5-Trinitrobenzene
75-01-4	Vinyl chloride

iii) For a chemical constituent of gasoline, diesel fuel, or heating fuel, the constituent exceeds the following:
Constituent Criterion (mg/L)
BETX 0.095
iv) For pH, a statistically significant change occurs from background.

B) Determine if, for Class III: Special Resource Groundwater, the levels as determined by the Board are exceeded.
C) Consider whether the owner or operator reasonably demonstrates that:
1. i) The contamination is a result of contaminants remaining in groundwater from a prior release for which appropriate action was taken in compliance with laws and regulations in existence at the time of the release;
2. ii) The source of contamination is not due to the on-site release of contaminants; or
3. iii) The detection resulted from error in sampling, analysis, or evaluation.
D) Consider actions necessary to minimize the degree and extent of contamination.

b) The appropriate regulatory agency must determine whether a preventive response should be undertaken based on relevant factors, including the considerations in subsection (a)(3).
c) After completion of preventive response under the authority of an appropriate regulatory agency, the concentration of a contaminant specified in subsection (a)(3)(A) in groundwater may exceed 50% of the applicable numerical standard of Subpart D if the following conditions are met:
1. 1) The exceedance has been minimized to the extent practicable;
2. 2) Beneficial use, as appropriate for the class of groundwater, has been assured; and
3. 3) Any threat to public health or the environment has been minimized.
d) Nothing in this Section limits the authority of the State or the United States to require or perform any corrective action process.

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

Constituent	Criterion (mg/L)

BETX	0.095