Which MCL varies with temperature?

Fluoride is the correct answer because its maximum contaminant level (MCL) can indeed vary with temperature due to its solubility and reactivity in water. As water temperature increases, the solubility of fluoride can lead to higher concentrations in the water supply if it is not carefully monitored and controlled. This variability necessitates a consideration of temperature when establishing safe levels for fluoride in drinking water, as different temperatures can influence how much fluoride remains in solution and how it interacts with other substances in the water.

In contrast, the MCLs for chlorine, nitrate, and pesticides are generally stable across temperature variations. Chlorine remains effective as a disinfectant across a wide temperature range, while nitrate concentrations tend to remain constant regardless of temperature changes due to their presence from agricultural runoff and wastewater. Pesticides can also have a more straightforward regulatory framework that does not inherently change with temperature, focusing on their persistence and degradation in the environment rather than the temperature-dependent solubility. Thus, fluoride's unique relationship with temperature is what makes it the choice that varies in terms of its MCL.