Elevated nutrient levels in watercourses can be caused by manure or fertilizer entering a watercourse directly, runoff from fertilized fields, or nutrient-rich soil eroding from croplands. Ammonia and organic compounds have the potential to create environmental problems if runoff or water is allowed to enter and leave the composting site, primarily because of their biological or chemical oxygen demand. The potential for leaching of nitrates into groundwater from composting facilities is much less of a concern than surface runoff. Oxygen demand removes oxygen from water that would otherwise be available to fish and other aquatic organisms. If wastes with a high oxygen demand, such as manure, are allowed to enter watercourses, the result may be a fish kill.
Water can be a pathway for the transmission of diseases.
More than 100 diseases are transmissible between animals and human beings. Both surface water and groundwater can be polluted with pesticides. Pollution can occur by runoff, leaching, or erosion of soil that has adsorbed pesticides, direct application of pesticides to water, through drift, or by spills. Polluted water may affect any organism living in or using the water. This includes people, domestic animals, fish, birds, plants, and wildlife.