The longer the contact time between water and chlorine residuals, the ________ the THM level is likely to be?

When considering the relationship between contact time and trihalomethane (THM) levels in water chlorination, longer contact times typically result in higher THM concentrations. This phenomenon occurs because chlorination leads to the formation of THMs when chlorine reacts with organic matter present in the water. Given more time for this chemical reaction to take place, the greater the amount of THMs will be produced as chlorine continues to interact with available precursors in the water.

In environments with prolonged contact time, the chlorine has enhanced opportunities to oxidize organic materials, which effectively increases the formation of THMs. Therefore, the correct answer highlights that with extended contact time, there is likely to be an increase in THM levels, reflecting the direct relationship between chlorine contact duration and THM production in water treatment processes.

Consequently, any misunderstandings regarding chlorine dosage, organic material presence, or reaction kinetics can often lead one to consider alternate effects. However, the established relationship remains that increased contact time promotes greater THM synthesis.