Investigation and Analysis of the Fluctuating Brominated to Total Trihalomethane Ratio in the Virginia Beach Distribution System

Abstract

An increasing trend in the brominated-trihalomethane to total-trihalomethane concentration ratio from 2001 to 2016 in the Virginia Beach water distribution system was established and investigated. This study aimed at identifying factors contributing to fluctuations in trihalomethane speciation in finished water. Reservoir management practices, treatment processes, and source water conditions were examined to quantitatively and qualitatively discern the impact of their variations on finished water THM speciation. Data on water quality constituents and parameters from the Moore’s Bridges Treatment Facility and within the Virginia Beach distribution system were examined to interpret spatial and temporal relationships with trihalomethane concentrations using established research as a guide. A linear regression analysis performed on both TTHM concentrations and the brominated-THM to total-THM concentration ratio affirms that TTHM concentrations experienced a decreasing trend while the brominated-THM to total-THM ratio experienced an increasing trend from 2001 to 2016. It was found that TTHM concentrations in the Virginia Beach distribution system experience seasonal periodicity while the brominated-THM to total-THM concentration ratio experiences longer term trends that extend across multiple years, and fluctuations in each match up with trends in source water total organic carbon (TOC) concentrations. A multiple regression analysis using disinfection conditions revealed that TOC concentration in filtered water and raw water temperature at Moore’s Bridges Treatment Facility were the most influential disinfection conditions contributing to finished-water TTHM concentrations. Matching trends between source water TOC and THM concentrations, along with consistent treatment practices at Moore’s Bridges Treatment Facility, suggests that influences on source water composition are the primary drivers of significant fluctuations and long-term trends of the brominated-THM to total-THM concentration ratio. Future monitoring of bromide concentrations upstream of source water intakes and continued measurements of other water quality parameters and constituents (i.e., conductivity, TOC, alkalinity, and pH) in relevant reservoirs can greatly aid in strengthening and interpreting relationships between source water composition and finished-water constituents.