§ 141.623
Reduced monitoring.
(a) You may reduce monitoring to the level specified in the table in this paragraph (a) any time the LRAA is ≤0.040 mg/L for TTHM and ≤0.030 mg/L for HAA5 at all monitoring locations. You may only use data collected under the provisions of this subpart or subpart L of this part to qualify for reduced monitoring. In addition, the source water annual average TOC level, before any treatment, must be ≤4.0 mg/L at each treatment plant treating surface water or ground water under the direct influence of surface water, based on monitoring conducted under either § 141.132(b)(1)(iii) or § 141.132(d).
Source water type
Population size category
Monitoringfrequency 1
Distribution system monitoring location per monitoring period
Subpart H:
<500
monitoring may not be reduced.
500-3,300
per year
1 TTHM and 1 HAA5 sample: one at the location and during the quarter with the highest TTHM single measurement, one at the location and during the quarter with the highest HAA5 single measurement; 1 dual sample set per year if the highest TTHM and HAA5 measurements occurred at the same location and quarter.
3,301-9,999
per year
2 dual sample sets: one at the location and during the quarter with the highest TTHM single measurement, one at the location and during the quarter with the highest HAA5 single measurement.
10,000-49,999
per quarter
2 dual sample sets at the locations with the highest TTHM and highest HAA5 LRAAs.
50,000-249,999
per quarter
4 dual sample sets—at the locations with the two highest TTHM and two highest HAA5 LRAAs.
250,000-999,999
per quarter
6 dual sample sets—at the locations with the three highest TTHM and three highest HAA5 LRAAs.
1,000,000-4,999,999
per quarter
8 dual sample sets—at the locations with the four highest TTHM and four highest HAA5 LRAAs.
≥ 5,000,000
per quarter
10 dual sample sets—at the locations with the five highest TTHM and five highest HAA5 LRAAs.
Ground Water:
<500
every third year
1 TTHM and 1 HAA5 sample: one at the location and during the quarter with the highest TTHM single measurement, one at the location and during the quarter with the highest HAA5 single measurement; 1 dual sample set per year if the highest TTHM and HAA5 measurements occurred at the same location and quarter.
500-9,999
per year
1 TTHM and 1 HAA5 sample: one at the location and during the quarter with the highest TTHM single measurement, one at the location and during the quarter with the highest HAA5 single measurement; 1 dual sample set per year if the highest TTHM and HAA5 measurements occurred at the same location and quarter.
10,000-99,999
per year
2 dual sample sets: one at the location and during the quarter with the highest TTHM single measurement, one at the location and during the quarter with the highest HAA5 single measurement.
100,000-499,999
per quarter
2 dual sample sets; at the locations with the highest TTHM and highest HAA5 LRAAs.
≥ 500,000
per quarter
4 dual sample sets at the locations with the two highest TTHM and two highest HAA5 LRAAs.
1 Systems on quarterly monitoring must take dual sample sets every 90 days.
(b) You may remain on reduced monitoring as long as the TTHM LRAA ≤0.040 mg/L and the HAA5 LRAA ≤0.030 mg/L at each monitoring location (for systems with quarterly reduced monitoring) or each TTHM sample ≤0.060 mg/L and each HAA5 sample ≤0.045 mg/L (for systems with annual or less frequent monitoring). In addition, the source water annual average TOC level, before any treatment, must be ≤4.0 mg/L at each treatment plant treating surface water or ground water under the direct influence of surface water, based on monitoring conducted under either § 141.132(b)(1)(iii) or § 141.132(d).
(c) If the LRAA based on quarterly monitoring at any monitoring location exceeds either 0.040 mg/L for TTHM or 0.030 mg/L for HAA5 or if the annual (or less frequent) sample at any location exceeds either 0.060 mg/L for TTHM or 0.045 mg/L for HAA5, or if the source water annual average TOC level, before any treatment, >4.0 mg/L at any treatment plant treating surface water or ground water under the direct influence of surface water, you must resume routine monitoring under § 141.621 or begin increased monitoring if § 141.625 applies.
(d) The State may return your system to routine monitoring at the State's discretion.