Which of the following is not a typical constituent of groundwater contamination?

Carbon dioxide is not typically classified as a constituent of groundwater contamination. While it is present in natural groundwater systems and can influence water chemistry, especially in relation to acidity and alkalinity, it is not generally regarded as a contaminant. Contaminants are usually defined as substances that can cause harm to health or the environment when present in higher-than-normal concentrations.

In contrast, heavy metals, pathogens, and nutrients like nitrates are well-known contaminants. Heavy metals can enter groundwater through industrial activities, mining, and improper disposal of waste. Pathogens, which include bacteria and viruses, can contaminate groundwater through sewage leaks and agricultural runoff, posing serious health risks. Nutrients, particularly nitrates from fertilizers, can lead to issues such as eutrophication in connected surface water bodies.

Therefore, while carbon dioxide plays a role in the natural processes and chemistry of groundwater, it does not fit the typical definition of a contaminant that poses a risk at significant levels.