One of the major threats to water quality is chemical pollution from heavy metals, solvents, dyes, pesticides, etc. Such chemicals enter the aquatic medium in several different ways, either by direct dumping, such as in industrial effluents, or from wastewater treatment plants (WWTP) that are unable to eliminate them. Another indirect source is plant health products, such as biocides and fertilizers, in agriculture. Discharge resulting from lax enforcement of legislation, illegal usage and inappropriate application of substances may also be considerable. In general, very water-soluble substances are transported and distributed more easily in the water cycle.