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Disinfection by‐product concentrations in a transmission system can change in response to downstream treatment processes. Semimechanistic models that describe the formation of disinfection by‐products (DBPs) (including trihalomethanes and haloacetic acids) were developed for the Massachusetts Water Resources Authority, an unfiltered surface water system using chlorine. The model's parameters were hydroxide concentration, chlorine decay kinetics or chlorine reacted, ultraviolet light absorbance at 254 nm, and algae concentration. One of the challenges in estimating or predicting DBP concentrations is that treatment processes at various points in the transmission system have substantial effects on the factors that affect DBP formation and rates of DBP formation. This article describes the formation of total trihalomethanes and haloacetic acids in the Massachusetts Water Resources Authority system and takes into account three treatment processes within the system. Coefficients used in the model were based on monitoring data collected from October 1997 to July 1999 and were validated with monitoring data from August 1999 to January 2000.

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Total organic carbon (TOC) is an important indicator of water quality in drinking water supply systems. Prescribed chlorination rates are often based on TOC levels, and levels of disinfection byproducts (DBP) can be subsequently approximated. Unfortunately, TOC is difficult to measure, and some water utilities rely on the more easily measured absorbance of ultraviolet light (specifically at a wavelength of 254 nm, abbreviated UV-254) as a surrogate indicator of organic content. For daily or weekly operating purposes, measurements of UV-254 can provide inexpensive and meaningful prescriptive guidance for disinfection processes. However, for longer term planning periods (monthly, seasonal, or annual) over which the minimization or reduction of TOC is an objective, a modeling approach may be necessary, and in a modeling environment, simulating TOC with simple mass balance relationships is much less data intensive than simulating light absorbance. This study aims to (1) develop a simple mass balance model for TOC in a stratified drinking water reservoir that can be used to help minimize TOC in real time and over longer term planning periods, and (2) demonstrate that TOC and UV-254 are well correlated, so that either value can be used as model input or output. The study shows that TOC and UV-254 are correlated reasonably well in a water supply reservoir for Boston, MA, and that a simple mass balance tool can effectively assist with planning and operations management using UV-254 as a primary water quality indicator. (KEY TERMS: TOC, UV-254 absorbance; water quality modeling; decision support systems; lakes; drinking water.)