Final answer:
Chemical Z poses the greatest risk due to its shorter residence time of 17.2 hours, which allows higher concentrations to remain before degrading, compared to Chemical X and Y with longer residence times of 2.8 and 3.5 days respectively.
Step-by-step explanation:
In a scenario where three carcinogens, namely Chemical X, Chemical Y, and Chemical Z, are released into a river following an industrial accident and are known to be equally harmful at equal concentrations, the chemical with the shortest residence time poses the greatest risk of exposure over the course of a week. Given the residence times of 2.8 days for Chemical X, 3.5 days for Chemical Y, and 17.2 hours for Chemical Z, it is Chemical Z that possesses the greatest risk, owing to its significantly shorter residence time which is less than a day. This allows for higher concentrations of Chemical Z to remain in close proximity to the community before degrading, thus potentially leading to greater exposure.
Understanding water pollution involves knowing the concentrations of chemicals, which are often measured in parts per million (ppm) or parts per billion (ppb). Lethal doses for humans have been established, and it is known that even essential nutrients can become toxic in high doses. In cases of chemical exposure, commonly from contaminated water, the intensity of the risk depends on the concentration and the time over which the community is exposed.