Figure: 30 TAC §290.115(c)(5)(C)(ii)(V)

Frequency of Initial Distribution System Evaluation (IDSE) Monitoring¹
Population and Type of Water	Sampling Frequency and Timing²
Systems distributing surface water or groundwater under the direct influence of surface water (GUI)
less than 500 that purchase treated surface water or GUI	one (during peak historical month²)
less than 500 with no purchased water source	one (during peak historical month²)
500 to 3,300 that purchase treated surface water or GUI	four (every 90 days)
500 to 3,300 with no purchased water source
3,301 to 9,999
10,000 to 49,999	six (every 60 days)
50,000 to 249,999
250,000 to 999,999
1,000,000 to 4,999,999
5,000,000 or more
Systems that only use groundwater not under the direct influence of surface water
less than 500 that purchase treated groundwater	one (during peak historical month²)
less than 500 with no purchased water source nonconsecutive systems	one (during peak historical month²)
500 to 9,999	four (every 90 days)
10,000 to 99,999
100,000 to 499,999
500,000 or more

¹A dual sample set with both a total trihalomethanes (TTHM) and an haloacetic acids (group of five (HAA5) sample must be taken at each monitoring location during each monitoring period.
²The peak historical month is the month with the highest TTHM or HAA5 levels or the warmest water temperature month. Monitoring must be conducted during the peak historical month for TTHM levels or HAA5 levels. Available compliance, study, or operational data must be reviewed to determine the peak historical month for TTHM or HAA5 levels.