Algal production and trihalomethane formation potential: an experimental assessment and inter-river comparison

Trihalomethanes (THMs) are byproducts produced during the disinfection of drinking water. We combined survey and experimental approaches to identify factors that influence THM formation potential (THMFP) in the Ohio River drainage basin. Two surveys of the Ohio River and its five principal tributaries were conducted to characterize spatial variation in THMFP in relation to algal abundance and suspended organic matter. We performed three experiments by placing Ohio River water in 2000-L outdoor mesocosms and manipulating algal senescence and bloom development by shading. Increases in THMFP among high- and low-light and dark tanks suggest that algal production, algal senescence, and possibly photolysis increased THMFP by as much as 50% over 3–6 days. Comparable yields of THMs (per unit of chlorophyll) were observed in both survey and experimental settings. Comparison of input waters with outputs indicates that the Ohio River at times acts to attenuate downstream transport of THM precursors. Our findings suggest that both watershed-scale and internal processes regulating THMFP should be considered as utilities develop strategies to meet new drinking water guidelines.