When is a violation of the MCL for TTHMs considered to occur?

A violation of the Maximum Contaminant Level (MCL) for Total Trihalomethanes (TTHMs) is considered to occur when the average of quarterly samples exceeds the MCL. This approach is established to ensure that water systems maintain a consistent level of water quality over time rather than relying on sporadic high readings from a single sample. The MCL for TTHMs is based on the potential long-term health effects of these compounds, and averaging multiple samples helps to account for natural variability in water quality and the effects of seasonal changes.

Monitoring quarterly and averaging the results allow water operators to detect trends and determine whether corrective actions are necessary if the water quality is consistently poor. It reflects a more comprehensive assessment of water quality rather than a reactive approach based on isolated instances, which could be due to temporary spikes or anomalies in the water supply. The focus on averages promotes better management and public health safety regarding disinfection by-products like TTHMs.