Which of the following is a common radiological contaminant?

Radionuclides are elements that emit radiation as they decay, making them a common form of radiological contamination. This can include a variety of isotopes that may arise from natural sources, such as uranium or radium, or man-made sources from nuclear reactors, fallout from nuclear testing, and waste disposal activities. Their presence in water supplies is a significant concern due to the potential health risks associated with exposure to radiation, which can lead to various medical conditions, including cancer.

In contrast, arsenic, nitrate, and herbicides are primarily classified as chemical contaminants rather than radiological ones. Arsenic is a metalloid known for its toxicity and potential carcinogenic properties. Nitrates are primarily related to agricultural runoff and can affect water quality but do not involve radioactive decay. Herbicides, used for controlling unwanted vegetation, also fall into the category of chemical pollutants. Thus, among the given options, radionuclides are distinctly recognized as radiological contaminants.