PURPOSE: This rule establishes the maximum contaminant level and monitoring requirements for total trihalomethanes.

1. (1) The maximum contaminant level (MCL) for total trihalomethanes (TTHM) is:

   Level,

   Milligrams

Contaminant Per Liter Total Trihalomethanes 0.10

1. (2) The MCL for TTHM applies to community water systems which add a disinfectant to the water in any part of the drinking water treatment process and serve ten thousand (10,000) or more persons. Compliance with the MCL is calculated pursuant to section (3).

2. (3) A supplier of water must collect samples of his/her product water for analyses as follows:

   1. (A) Community water systems must perform sampling at quarterly intervals.

      1. 1. Analyses for TTHM shall be per-

formed at quarterly intervals on at least four (4) water samples for each treatment plant used by the system.

1. 2. The minimum number of samples

required shall be based on the number of treatment plants used by the system except that multiple wells drawing raw water from a single aquifer, with the department’s approval, may be considered one (1) treatment plant for determining the minimum number of samples.

1. 3. Community water systems serving

fewer than ten thousand (10,000) persons, at the discretion of the department, may be required to submit fewer samples; and

1. (B) All samples taken within an established frequency shall be collected within a twentyfour (24)-hour period.

1. (4) At least twenty-five percent (25%) of the samples shall be taken at locations within the distribution system reflecting the maximum residence time of the water in the system. The remaining shall be taken at representative locations in the distribution system, taking into account the number of persons served, different sources of water and different treatment methods employed.
2. (5) The results of all analyses per quarter shall be arithmetically averaged and all samples collected shall be used in the computation of the average.

3. (6) Upon a community water system’s written request, the department may reduce the TTHM analysis monitoring frequency to a minimum of one (1) sample per quarter.

   1. (A) The sample shall be taken at a point in the distribution system that reflects the maximum residence time of the water in the system.
   2. (B) The department shall provide, in writing, a determination that local conditions and data from at least one (1) year of monitoring in accordance with section (3) of this rule demonstrate that TTHM concentrations will be consistently below the MCL.

   3. (C) The supplier of water immediately shall begin monitoring in accordance with the requirements of section (3) of this rule upon finding that—

      1. 1. At any time during the reduced mon-

itoring, the results from any analysis for TTHM exceed 0.10 milligrams per liter (mg/l) and the results are confirmed by at least one (1) check sample taken promptly after the results are received; or

1. 2. The system makes any significant

change(s) to its source of water or treatment process; and

1. 3. This monitoring shall continue at

least one (1) year before the frequency may be reduced again.

1. (7) Upon the written request of a community water system that utilizes only groundwater sources, the department may allow the water system to substitute a minimum of one (1) sample per year for maximum TTHM potential in place of quarterly sampling for TTHM.

   1. (A) This monitoring frequency applies separately to each treatment plant used in the system.
   2. (B) The sample shall be taken at a point in the distribution system that reflects the maximum residence time of the water in the system.

   3. (C) The department shall provide, in writing, a determination that—

      1. 1. The system has a maximum TTHM

potential of less than 0.10 mg/l based upon data submitted by the water supplier; and

1. 2. Based upon an assessment of local

conditions, the system is not likely to approach or exceed the MCL for TTHM.

1. (D) A water supplier immediately shall begin monitoring in accordance with the requirements of section (3) of this rule upon finding that—

   1. 1. The results from any analysis taken

by the water supplier for maximum TTHM potential are equal to or greater than 0.10 mg/l; and

1. 2. The results are confirmed by at least

one (1) check sample which was taken promptly after the results were received; and

1. 3. This monitoring shall continue for at

least one (1) year before the frequency may be reduced again.

1. (E) If the system makes any significant change(s) in the raw water or treatment program at any time during the period of reduced monitoring frequency, the water supplier immediately shall collect an additional sample to be analyzed for maximum TTHM potential. The sample shall be taken at a point in the distribution system that reflects the maximum residence time of the water in the system. The results of the analysis shall be used to determine whether the system must comply with the monitoring requirements of section (3) of this rule.

1. (8) Compliance with section (1) of this rule shall be determined based on a running annual average of quarterly samples collected by the supplier of water as prescribed in section (3). If the average of samples covering any twelve (12)-month period exceeds the MCL, the supplier of water shall report to the department pursuant to 10 CSR 60-7.010 and notify the public pursuant to 10 CSR 60- 8.010. Monitoring after public notification shall be at a frequency designated by the department and shall continue until a monitoring schedule as a condition to a variance, exemption or enforcement action shall become effective.
2. (9) Samples for TTHM shall be dechlorinated upon collection to prevent further production of trihalomethanes. Samples for maximum TTHM potential shall not be dechlorinated and must be held for seven (7) days at twenty-five degrees Celsius (25°C) prior to analysis.
3. (10) At the option of the department, monitoring frequencies may be increased above the minimum where this is necessary to detect variations of TTHM levels within the distribution system.

4. (11) Before a community water system makes any significant modifications to its existing treatment process for the purposes of achieving compliance with this rule, the system must obtain departmental approval of its proposed modifications and those safeguards that it will implement to ensure that the microbiological quality of the drinking water served by the system will not be adversely affected by the modifications. At a minimum, the department shall require the system modifying its disinfection practice to—

   1. (A) Evaluate the source water for microbiological quality;
   2. (B) Evaluate its existing treatment practices and consider improvements that will minimize disinfectant demand and optimize finished water quality throughout the distribution system; and
   3. (C) Conduct additional monitoring and studies as required by the department to assure continued maintenance of optimal biological quality in finished water.

AUTHORITY: section 640.100, RSMo (1994).* Original rule filed April 14, 1981, effective Oct. 11, 1981. Amended: Filed Feb. 1, 1996, effective Oct. 30, 1996. *Original authority 1939, amended 1978, 1981, 1982, 1988, 1989, 1992, 1993, 1995.