NR 809.566(3)(b)1.1. A public water system has an acute violation of the MRDL for chlorine dioxide when any daily sample taken at the entrance to the distribution system exceeds the MRDL and on the following day one or more of the 3 samples taken in the distribution system exceeds the MRDL. If both exceedances occur, the public water system is in violation of the MRDL and the water supplier shall take immediate corrective action to lower the level of chlorine dioxide below the MRDL and shall notify the public pursuant to the procedures for acute health risks in s. NR 809.951. Failure to take samples in the distribution system the day following an exceedance of the chlorine dioxide MRDL at the entrance to the distribution system shall also be considered an MRDL violation and the water supplier shall notify the public of the violation in accordance with the provisions for acute violations under s. NR 809.951.

NR 809.566(3)(b)2.2. A public water system has a nonacute violation for chlorine dioxide when any 2 consecutive daily samples taken at the entrance to the distribution system exceed the MRDL and all distribution system samples taken are below the MRDL. A water supplier for a public water system with a nonacute violation shall take corrective action to lower the level of chlorine dioxide below the MRDL at the point of sampling and shall notify the public pursuant to the procedures for nonacute health risks in subch. VII. Failure to monitor at the entrance to the distribution system the day following an exceedance of the chlorine dioxide MRDL at the entrance to the distribution system is also an MRDL violation and the water supplier shall notify the public of the violation in accordance with the provisions for nonacute violations under subch. VII.

NR 809.566(4)(4) Compliance requirements for Disinfection byproduct precursors (DBPP). Compliance with disinfection byproduct precursors shall be determined as specified in s. NR 809.569 (1). Water suppliers for public water systems may begin monitoring to determine whether Step 1 TOC removals can be met 12 months prior to the compliance date for the public water system. This monitoring is not required and failure to monitor during this period is not a violation. However, any water supplier that does not monitor during this period, and then determines in the first 12 months after the compliance date that it is not able to meet the Step 1 requirements in s. NR 809.569 (1) (b) and therefore applies for alternate minimum TOC removal (Step 2) requirements, is not eligible for retroactive approval of alternate minimum TOC removal (Step 2) requirements as allowed pursuant to s. NR 809.569 (1) (c) and is in violation. Water supplier may apply for alternate minimum TOC removal (Step 2) requirements any time after the compliance date. For public water systems required to meet Step 1 TOC removals, if the value calculated under s. NR 809.569 (3) (a) or (b) is less than 1.00, the public water system is in violation of the treatment technique requirements and the water supplier shall notify the public pursuant to subch. VII in addition to reporting to the department pursuant to s. NR 809.567.

NR 809.567(1)(1) Reporting requirements. Water suppliers for public water systems required to be sampled quarterly or more frequently shall report to the department within 10 days after the end of each quarter in which samples were collected, notwithstanding the provisions of s. NR 809.563. Water suppliers for public water systems required to be sampled less frequently than quarterly shall report to the department within 10 days after the end of each monitoring period in which samples were collected.