Temporal and Spatial Variability of DBPs in a Chloraminated Distribution System

Abstract

Assessments of disinfection by‐product (DBP) exposure to consumers of drinking water currently are based on infrequent measurements at the point of entry (POE) to, and a few points in, the distribution system. In this article the authors report considerable short‐term temporal variability in the DBP levels (measured in terms of trihalomethanes, haloacetic acids, and total organic halide) at the POE to a chloraminated distribution system that appears to be related to the control of the addition of ammonia at the treatment plant. Samples collected every few hours over several consecutive days from the POE and from several distribution system points showed that this temporal variability at the POE most likely was dampened in the distribution system because of a combination of innate storage within the system and dynamic mixing behavior induced by variations in flow and demand. The variations in measured DBP concentrations in the system are relatively small and appear to be normally distributed, whereas the variations in measured DBP concentrations at the POE are much larger. The mean of the measured DBP concentration values in the system matches the mean of the measured values leaving the treatment plant. This suggests that more accurate characterization of weekly DBP concentrations throughout the distribution system for purposes of exposure assessment can be obtained from the analysis of a single grab sample collected during the daytime at a representative point in the chloraminated distribution system rather than from the POE to the distribution system.