Animal waste, if not properly managed, can be transported over the surface of agricultural land to nearby lakes and streams. Leaching from manure storage lagoons and percolation through the soil of fields, where animal waste is applied can contaminate groundwater resources. According to EPA, the release of waste from animal feedlots to surface water, groundwater, soil, and air is associated with a wide range of human health and ecological impacts and contributes to the degradation of surface waters.
The primary pollutants associated with animal wastes are nutrients (particularly nitrogen and phosphorus), organic matter, solids, pathogens, and odorous/volatile compounds. Animal waste also contains salts and trace elements, and to a lesser extent, antibiotics, pesticides, and hormones. Pollutants in animal waste can impact waters through several possible pathways, including surface runoff and erosion, direct discharges to surface waters, spills and other dry weather discharges, leaching into soil and groundwater, and releases to air (including subsequent deposition back to land and surface waters). Pollutants associated with animal waste can also originate from a variety of other sources, such as cropland, municipal and industrial discharges, and urban runoff.
The most dramatic ecological impacts associated with manure pollutants in surface waters are massive fish kills. In addition, manure pollutants can seriously disrupt aquatic systems by over-enriching water (in the case of nutrients) or by increasing turbidity (in the case of solids), processes that can disrupt aquatic ecosystems. Excess nutrients cause fast-growing algae blooms that reduce the penetration of sunlight in the water column and reduce the mount of available oxygen in the water, thus reducing fish and shellfish habitat and affecting fish and invertebrates.
A variety of pollutants in animal waste can also affect human health. Over 150 pathogens in livestock manure are associated with risks to humans; these include the bacteria E. coli and Salmonella species and the protozoa Giardia species. Contact with pathogens contained in manure during swimming or boating can result in infections of the skin, eye, ear, nose, and throat. Shellfish such as oysters, clams, and mussels can carry toxins produced by some types of algae that are associated with excess nutrients. These can affect people who eat contaminated shellfish. Further, contaminants from manure can also affect human health through drinking water sources and can result in increased drinking water treatment costs. For example, nitrogen in manure and liquid waste can be transported to drinking water as nitrates, which are associated with human health risks and which EPA has identified as the most widespread agricultural contaminant in drinking water wells. Elevated nitrate levels can cause nitrate poisoning, particularly in infants (this is known as methemoglobinemia, or “blue baby syndrome”).