Trihalomethanes (THMs) are caused by which of the following?

Trihalomethanes (THMs) are byproducts formed when chlorination, a common practice for disinfecting drinking water, reacts with organic materials that are typically found in surface water sources. These organic compounds may come from decayed plant matter, algae, and other natural materials which can introduce carbon-based contaminants into the water. When chlorine is added to the water for disinfection, it can react with these organic materials, leading to the formation of THMs, which are a concern due to their potential health risks when consumed over long periods.

The understanding of THMs being linked to organic material reactions emphasizes the importance of monitoring water quality, particularly in systems that utilize surface sources. Thus, controlling the levels of organic materials prior to chlorination can help minimize the formation of these undesired byproducts and ensure safer drinking water.