ADMINISTRATIVE CODE TITLE 35: ENVIRONMENTAL PROTECTION SUBTITLE F: PUBLIC WATER SUPPLIES CHAPTER I: POLLUTION CONTROL BOARD PART 611 PRIMARY DRINKING WATER STANDARDS SECTION 611.357 MONITORING FOR WATER QUALITY PARAMETERS
Section 611.357 Monitoring for Water Quality Parameters
All large suppliers and all medium suppliers with corrosion control treatment (unless deemed optimized under Section 611.351(b)(3)), and all small and medium suppliers that exceed the lead or copper action levels must sample and monitor water quality parameters in addition to lead and copper in compliance with the requirements of this section. Any supplier may be required to monitor water quality parameters as determined by the Agency, including as provided in this section.
a) General Requirements
1) Distribution system samples for water quality parameters.
A) Distribution system samples collected at water taps must represent water quality throughout the supplier's distribution system, considering the number of persons served, the different sources of water, the different treatment methods employed by the supplier, and seasonal variability. Sites selected for sampling in the supplier's distribution system under this section can be the same or different from tap sampling sites the supplier targeted for lead and copper sampling under Section 611.356(a). Suppliers may consider selecting sites used for total coliform sampling under 40 C.F.R. 141.21(a)(1). The supplier must include sites it selects for sampling in the distribution system under this Section in the site sample plan under Section 611.360(a)(1). The supplier must update the site sample plan before changing sampling locations.
B) Samples collected in the supplier's distribution system must be analyzed for the following parameters, when applicable, as specified:
i) pH;
ii) Alkalinity;
iii) Orthophosphate (as PO4), when an inhibitor containing an orthophosphate compound is used;
iv) Silica, when an inhibitor containing a silicate compound is used; and
v) Any parameters specified by the Agency under Section 611.352(a)(1) or 611.352(f)(6).
2) Entry point samples for water quality parameters.
A) Samples collected at the entry point(s) to the supplier's distribution system must be from locations representative of each source water after treatment. If a supplier draws water from more than one source water and the source waters are combined before distribution, the supplier must sample at an entry point to the distribution system during periods of normal operating conditions when water is representative of all sources typically being used.
B) Except as provided in subsection (b)(3)(B) for ground water systems, the following parameters must be measured at each entry point to the distribution system, when applicable, as specified:
i) pH;
ii) When alkalinity is adjusted as part of corrosion control, a reading of the dosage rate of the chemical used to adjust alkalinity, and the alkalinity concentration;
iii) When a corrosion inhibitor is used as part of corrosion control, a reading of the dosage rate of the inhibitor used, and the concentration of orthophosphate (as PO4) or silica (whichever is applicable); and
iv) Any parameters specified by the Agency under Section 611.352(a)(1) or 611.352(f)(6).
b) Standard monitoring for water quality parameters.
1) Number of samples
A) Distribution system samples. Suppliers must collect two distribution system samples for applicable water quality parameters during each monitoring period specified under subsections (b)(2) through (b)(4) from each of the minimum number of sites listed in table 1 to this subsection (b)(1)(A). Suppliers that collect distribution system samples for water quality parameters from additional sites as a result of the Distribution System and Site Assessment requirements in Section 611.352(j) must add those sites to the minimum number of sites listed in table 1 to this subsection (b)(1)(A) up to a maximum of not more than twice the minimum number of sites.
B) Samples at entry points.
i) Suppliers without installed or re-optimized OCCT and without Agency-designated optimal water quality parameters required to collect entry point samples must collect a minimum of two entry point samples for each applicable water quality parameter at each entry point to the supplier's distribution system at least once during each monitoring period specified in subsection (b)(2).
ii) Suppliers with installed OCCT or re-optimized OCCT and/or Agency-designated optimal water quality parameters required to collect entry point samples, including as provided in subsection (b)(3)(C), must collect one entry point sample for each applicable water quality parameter at each entry point to the supplier's distribution system at least once every two weeks during each monitoring period the supplier is required to conduct sampling as specified in subsections (b)(3) and (b)(4) and (c).
2) Initial sampling for suppliers. A large supplier without corrosion control treatment must begin monitoring for water quality parameters as specified in subsections (b)(2)(A) and (b)(2)(B) during the first two six-month monitoring periods beginning no later than January 1 of the calendar year after the supplier either becomes a large supplier or exceeds the practical quantitation limit for lead. Any medium supplier without corrosion control treatment that exceeds the lead action level or the copper action level must begin monitoring for applicable distribution system and entry point water quality parameters as specified in subsections (b)(2)(A) and (b)(2)(B) for two consecutive six-month monitoring periods beginning the month immediately following the end of the tap monitoring period in which the action level exceedance occurred. Any small supplier that exceeds the lead or copper action level must begin monitoring for applicable distribution system and entry point water quality parameters as specified in subsections (b)(2)(A) and (b)(2) B) for two consecutive six-month monitoring periods beginning the month immediately following the end of the tap monitoring period in which the action level exceedance occurred. Suppliers must continue monitoring as described by subsections (b)(3) and (b)(4).
A) At sites in the supplier's distribution system, collect two samples for:
i) pH; and
ii) Alkalinity.
B) At each entry point to the supplier's distribution system, collect all the applicable parameters listed in subsection (a)(2)(B).
3) Monitoring after installation of OCCT or re-optimized OCCT.
A) A supplier that modifies or installs OCCT in compliance with Section 611.351(d)(5) or Section 611.351(e)(5) and is required to conduct follow-up monitoring for lead or copper in compliance with Section 611.351(d)(6) or (e)(6) must monitor for applicable distribution system and entry point water quality parameters as specified in subsections (a)(1) and (a)(2) every six months until the Agency designates new water quality parameter values for OCCT in compliance with Section 611.352(f). Suppliers must collect these samples at a regular frequency throughout the six-month monitoring period to reflect seasonal variability.
B) Any ground water supplier can limit entry point sampling described in subsection (a)(2) to those entry points that are representative of water quality and treatment conditions throughout the supplier's system. If water from untreated ground water sources mixes with water from treated ground water sources, the supplier must monitor for water quality parameters both at representative entry points receiving treatment and representative entry points receiving no treatment. Prior to the start of any monitoring under this subsection (b)(3)(B), the supplier must provide to the Agency, written information and documentation identifying the selected entry points, including information on seasonal variability, sufficient to demonstrate that the sites are representative of water quality and treatment conditions throughout the supplier's system.
C) The Agency may require small suppliers with corrosion control treatment for which the Agency has not designated optimal water quality parameters that do not exceed the lead or copper action level to conduct water quality parameter monitoring as described in this subsection (b) or the Agency can develop its own water quality parameter monitoring structure for these systems.
4) Monitoring by suppliers with Agency-designated optimal water quality parameter values for OCCT. Monitoring must occur at a regular frequency throughout the monitoring period to reflect seasonal variability and be consistent with the requirements in subsections (a)(1) and (a)(2).
A) Medium suppliers with corrosion control treatment and all large suppliers must sample for the applicable water quality parameters designated by the Agency and determine compliance with the requirements of Section 611.352(g) every six months with the first six-month monitoring period to begin on either January 1 or July 1, whichever comes first, after the Agency specifies the optimal values under Section 611.352(f).
B) A small supplier with corrosion control treatment that exceeds the lead or copper action level must begin monitoring during the standard six-month tap monitoring period immediately following the tap monitoring period in which the action level exceedance(s) occurs and continue monitoring until the supplier no longer exceeds the lead action level and/or copper action level and meets the Agency-designated optimal water quality parameters in two consecutive six-month tap monitoring periods under Section 611.356(c). For any small supplier that is subject to a reduced monitoring frequency in compliance with Section 611.356(d) at the time of the action level exceedance, the start of the six-month monitoring period under this subsection (b)(4)(B) must coincide with the start of the tap monitoring period under Section 611.356(c).
C) Compliance with Agency-designated optimal water quality parameter values must be determined as specified in Section 611.352(g).
D) The Agency has the discretion to require suppliers described in subsection (b)(4)(B) to continue to monitor optimal water quality parameters.
c) Reduced monitoring.
1) A medium or large supplier that maintains the range of values for the water quality parameters reflecting OCCT specified by the Agency under Section 611.352(f) and does not exceed the lead action level or copper action level in either of the two consecutive six-month monitoring periods under subsection (b)(4) must collect two distribution system samples for applicable water quality parameters specified in subsection (a)(1)(B) from each of the minimum number of sites listed in table 1 to this subsection (c)(1) during each six-month monitoring period. These suppliers must collect these samples at a regular frequency throughout the six-month monitoring period to reflect seasonal variability. A supplier meeting the requirements of this subsection (c)(1) must continue to monitor at the entry point(s) to the distribution system as specified in subsection (a)(2). Suppliers with sites added as a result of the Distribution System and Site Assessment requirements in Section 611.352(j) must continue to sample at the added sites up to a maximum of not more than twice the minimum number of sites specified in table 1 to subsection (b)(1)(A).
A) A supplier that maintains the range of values for the water quality parameters reflecting OCCT specified by the Agency under Section 611.352(f) and does not exceed the lead or copper action level during three consecutive years of monitoring may reduce the frequency with which it collects distribution system samples for applicable water quality parameters specified in subsection (a)(1)(B) from each of the minimum number of sites listed in table 1 to subsection (c)(1) from every six months to annually. This sampling must begin during the calendar year immediately following the end of the monitoring period in which the third consecutive year of six-month monitoring occurs.
B) A supplier may reduce the frequency with which it collects distribution system samples for water quality parameters specified in subsection (c)(1) to every year if the supplier demonstrates during two consecutive monitoring periods that its tap water lead level at the 90th percentile is less than or equal to the practical quantitation limit for lead of 0.005 mg/L, that its tap water copper level at the 90th percentile is less than or equal to 0.65 mg/L as calculated in compliance with Section 611.350(c)(3), and that the supplier has maintained the range of values for the water quality parameters reflecting OCCT specified by the Agency under Section 611.352(f).
3) A supplier that conducts sampling at taps for water quality parameters annually must collect these samples at a regular frequency throughout the year to reflect seasonal variability.
4) A supplier monitoring at a reduced frequency that fails to operate at or within the range of values for the optimal water quality parameters designated by the Agency in Section 611.352(f) for more than nine cumulative days, as specified in Section 611.352(g), in any six-month period under subsection (b)(4) must resume distribution system sampling in compliance with the number and frequency requirements in subsection (b)(4). Such a supplier may resume annual monitoring for water quality parameters in the distribution system at the reduced number of sites specified in subsection (c)(1) after the supplier completes two subsequent consecutive six-month rounds of monitoring that meet the criteria of subsection (c)(1) and/or may resume annual monitoring for water quality parameters in the distribution system at the reduced number of sites after the supplier demonstrates through subsequent rounds of monitoring that it meets the criteria of either subsection (c)(2)(A) or (c)(2)(B).
5) Any supplier monitoring at a reduced frequency that exceeds the lead action level or copper action level must resume standard water quality parameter monitoring beginning with the six-month period immediately following the tap monitoring period in which the action level exceedance(s) occurs. When the supplier no longer exceeds the lead action level and/or copper action level and meets the Agency-designated optimal water quality parameters in two consecutive six-month tap monitoring periods, the supplier may then reduce monitoring in compliance with subsections (c)(1) and (c)(2).
d) Additional monitoring by suppliers. The results of any monitoring conducted in addition to the minimum requirements of this section must be considered by the supplier and the Agency in determining concentrations of water quality parameters under this section or Section 611.352.
BOARD NOTE: This Section derives from 40 CFR 141.87.
(Source: Amended at 50 Ill. Reg. 2531, effective February 17, 2026) | ||||||||||||||||||||||||||||||||||||