Total Trihalomethanes (TTHMs) in Wisconsin Drinking Water

When a water utility adds chlorine to kill pathogens, the chlorine reacts with naturally occurring organic matter (leaves, soil, algae) in the source water to form trihalomethanes — chloroform, bromodichloromethane, dibromochloromethane, and bromoform. The EPA regulates the sum of all four as Total Trihalomethanes (TTHMs) with a Maximum Contaminant Level of 80 micrograms per liter, measured as a Locational Running Annual Average (LRAA) — not a single sample. A single sample over 80 is a flag, but not a violation by itself.

Wisconsin systems most affected tend to be smaller utilities in the north and west that draw from surface water or shallow groundwater with high organic content. The Northwoods, the Driftless Area, and Lake Michigan corridor all have elevated TTHM formation potential. Treatment options at the utility level include switching to chloramines, adding granular activated carbon, or adjusting the point of chlorine addition. At the household level, a point-of-use carbon filter removes TTHMs from drinking water.