Section 611.7902  Monitoring Requirements

a)         General requirements

1)         Suppliers must take all samples during normal operating conditions at all entry points to the distribution system.

2)         If the supplier draws water from more than one source and the sources are combined before distribution, the supplier must sample at an entry point to the distribution system during periods of representative operating conditions.

3)         Suppliers must use only data collected under the provisions of this Subpart AZ to qualify for reduced monitoring.

4)         All new suppliers that begin operation after, or suppliers that use a new source of water after April 26, 2027, must demonstrate compliance with the MCLs within a period of time specified by the Agency.  A supplier must also comply with initial sampling frequencies required by the Agency to ensure the supplier can demonstrate compliance with the MCLs.  Compliance monitoring frequencies must be conducted in compliance with this Section.

5)         For purposes of this Section, the trigger levels are defined as shown in the following Table 1.

Section 611.7902 (a)(5) TABLE 1:  Trigger Levels for PFAS Contaminants

6)         Based on initial monitoring results, for each sampling point at which a regulated PFAS listed in Section 611.311(c) is detected at a level greater than or equal to the trigger level, the supplier must monitor quarterly for all regulated PFAS beginning April 26, 2027, in compliance with subsection (b)(2).

7)         For the purposes of this section, each water supplier must ensure that all results provided by a laboratory are reported to the Agency and used for determining the required sampling frequencies.  This includes values below the practical quantitation levels defined in Section 611.7903 (f)(1)(D); zero must not be used in place of reported values.

b)         Monitoring requirements for PFAS

1)         Initial monitoring.

A)        Groundwater CWS and NTNCWS suppliers serving greater than 10,000 persons and all surface water CWS and NTNCWS suppliers must take four consecutive samples 2 to 4 months apart within a 12-month period (quarterly samples) for each regulated PFAS listed in Section 611.311(c).

B)        All groundwater CWS and NTNCWS suppliers serving 10,000 or fewer persons must take two samples for each regulated PFAS listed in Section 611.311(c) five to seven months apart within a 12-month period.

C)        All groundwater under the direct influence of surface water (GWUDI) CWS and NTNCWS suppliers must follow the surface water CWS and NTNCWS supplier monitoring schedule in subsection (b)(1)(A).

D)        All suppliers that use both surface water and groundwater must apply the requirements in subsections (b)(1)(A) through (C) depending on the source(s) of water provided at a given entry point to the distribution system (EPTDS).  If the EPTDS provides surface water, the requirements for a surface water CWS/NTNCWS supplier apply.  If the EPTDS provides groundwater, the requirements for a groundwater CWS/NTNCWS supplier apply, based on supplier size.  If an EPTDS provides a blend of surface water and groundwater, the requirements for a surface water system apply.  For suppliers that change the source water type at an EPTDS during the initial monitoring period (i.e., one part of the year it is surface water and the remaining part of the year it is groundwater), the sampling requirements for a surface water supplier apply.

E)        Suppliers must monitor at a frequency indicated in the following table, though the Agency may require more frequent monitoring on a supplier specific basis:

Section 611.7902(b)(1)(E) TABLE 1  Initial Monitoring Requirements

F)         The Agency may accept data previously acquired by a supplier to count toward the initial monitoring requirements if the data meets the requirements of Section 611.7901(b)(1), samples were collected starting on or after January 1, 2019, and otherwise meet the timing requirements specified in table 1 to subsection (b)(1)(E).  For the purposes of satisfying initial monitoring requirements, acceptable data may be reported to a concentration no greater than the MCLs.  However, a supplier is only eligible for triennial monitoring at the start of the compliance monitoring period if the supplier demonstrates that concentrations in all samples it uses to satisfy the initial monitoring requirements are below the trigger levels defined in subsection (a)(5).

G)        If suppliers have multiple years of data, the most recent data must be used.

H)        For suppliers using previously acquired data that have fewer than the number of samples required in a continuous 12-month period for initial monitoring as listed in table 1 to subsection (b)(1)(E): All surface water suppliers, GWUDI suppliers, and groundwater suppliers serving greater than 10,000 persons must collect in a calendar year one sample in each quarter that was not represented, two to four months apart from the months with available data; All groundwater suppliers serving 10,000 or fewer persons must collect one sample in the month that is five to seven months apart from the month in which the previous sample was taken.

I)         In determining the most recent data to report, a supplier must include all results provided by a laboratory whether above or below the practical quantitation levels.  These results must be used for the purposes of determining the frequency with which a supplier must monitor at that sampling point at the start of the compliance monitoring period.

J)         The Agency may delete results of obvious sampling errors.  If the Agency deletes a result because of an obvious sampling error and the supplier fails to collect another sample this is a monitoring violation described in Section 611.7905(c).

K)        Initial monitoring requirements, including reporting results to the Agency, must be completed by April 26, 2027.

2)         Compliance Monitoring.

A)        Based on initial monitoring results, at the start of the monitoring period that begins on April 26, 2027, suppliers may reduce monitoring at each sampling point at which all reported sample concentrations were below all trigger levels defined in subsection (a)(5), unless otherwise provided for by the Agency.  At eligible sampling points, each supplier must analyze one sample for all regulated PFAS during each three-year monitoring period, at a time specified by the Agency, in the quarter in which the highest analytical result was detected during the most recent round of quarterly or semi-annual monitoring.  If a sampling point is not eligible for triennial monitoring, then the supplier must monitor quarterly at the start of the compliance monitoring period.

B)        If, during the compliance monitoring period, a supplier is monitoring triennially and a PFAS listed in Section 611.311(c) is detected at a level equal to or exceeding the trigger levels defined in subsection (a)(5) in any sample, then the supplier must monitor quarterly for all regulated PFAS beginning in the next quarter at the sampling point.  The triggering sample must be used as the first quarter of monitoring for the running annual average calculation.

C)        For all source water types, the Agency may determine that all regulated PFAS at a sampling point are reliably and consistently below the MCL after considering, at a minimum, four consecutive quarterly samples collected during the compliance monitoring period.  A sampling point that the Agency has determined to be reliably and consistently below the MCL is required to collect annual samples for at least the first three years after that determination is made.  Annual samples must be collected in the quarter in which detected concentrations were highest during the most recent year of quarterly monitoring.  If, after three consecutive years, annual samples all contain results that are below the trigger levels defined in subsection (a)(5), the Agency may allow a supplier to begin triennial monitoring at the sampling point.  The supplier must collect triennial samples in the quarter with the highest concentrations during the most recent round of quarterly sampling.  If an annual sample meets or exceeds an MCL or the Agency determines that the result is not reliably and consistently below the MCL for all regulated PFAS, then the supplier must monitor quarterly for all regulated PFAS beginning in the next quarter at the sampling point.

D)        The three different compliance monitoring sampling schedules that may be assigned and the criteria for each are summarized in the following table:

Section 611.7902 (b)(2)(D) TABLE 1:  Compliance Monitoring Schedules and Requirements

E)        The Agency may require a confirmation sample for any sampling result.  If a confirmation sample is required by the Agency, the supplier must average the result with the first sampling result and the average must be used for the determination of compliance with MCLs as specified by Section 611.7903.  The Agency may delete results of obvious sampling errors from the MCL compliance calculations described in Section 611.7903.  If the Agency deletes a result because of an obvious sampling error and the supplier fails to collect another sample this is a monitoring violation as described in Section 611.7905(c).

F)         The Agency may increase the required monitoring frequency, where necessary, to detect variations within the supplier's system (e.g., fluctuations in concentration due to seasonal use, changes in water source).

G)        Each supplier must monitor at the time designated by the Agency within each monitoring period.

H)        When a supplier reduces its sampling frequency to annual or triennial sampling, the next compliance sample must be collected in the monitoring period that begins the calendar year following Agency approval of a reduction in monitoring frequency.

BOARD NOTE:  This Section derives from 40 CFR 141.902

(Source:  Added at 50 Ill. Reg. 2531, effective February 17, 2026)