Other forms of river pollution that are of particular concern include polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pesticides, other forms persistent organic pollutants, pathogens, and trace elements.

Polynuclear aromatic hydrocarbons (PAHs) are a group of diverse compounds containing two or more fused rings of carbon and hydrogen atoms, and are products of coal, tar, oil and their combustion. Anthropogenic PAHs formed from the combustion of fossil fuels can be pyrolitic compounds formed from incomplete combustion or petrogenic compounds derived from crude oil or unburned fuel and its refined products. Both types of PAHs are lipophilic (fat-loving) chemicals that are widespread pollutants of water and sediments. PAHs can accumulate in fatty tissues of animals, and PAHs with four or more condensed benzene rings are commonly mutagenic and/or carcinogenic to diverse organisms, including aquatic species. The ecotoxicology of PAHs largely depends on the metabolic fate of the compounds once they enter an organism.

Polychlorinated biphenyls (PCBs) are a type of persistent organic pollutant (POP) targeted for elimination by the United Nations Environment Programme as one of the dirty dozen POPs. Commercial PCBs were widely used as additives in oils, industrial fluids and products as diverse as pesticides, paints, copying paper, adhesives, sealants, and plastics. Despite their versatility and usefulness, production and use in commerce of PCBs have been banned in the United States since 1973 because they bioaccumulate, resulting in much higher concentrations in biota than in the surrounding environment, and because they are toxic to a variety of living organisms. The U.S. Clean Water Act requires that rivers in which PCBs exceed water quality standards must be subjected to a process in which a total maximum daily load (TMDL) is calculated and implemented, providing some means for regulating inadvertent production of PCBs.

Johnson, L., B. Anulacion, M. Arkoosh, P. Olson, C. Sloan, S.Y. Sol, J. Spromberg, D.J. Teel, G. Yanagida, and G. Ylitalo. 2013. Persistent organic pollutants in juvenile Chinook salmon in the Columbia River basin: implications for stock recovery. Transactions of the American Fisheries Society, 142, 21-40.

An enormous number of papers have been written on pesticides as river pollution, including case studies of transport, dispersal, deposition and concentration of pollutants, of biotic effects, of human health hazards, of remediation, and of pollution in nearshore, estuary and delta environments that integrate one or more rivers. Given the huge number of chemicals released into the environment (more than 14 million), chemicals must be prioritized for risk assessment and monitoring. Unfortunately, standard practices of risk management are unable to adequately control pesticide pollution in rivers. 

Persistent organic pollutants (POPs) include deliberately produced compounds such as pesticides, PCBs, polychlorinated naphthalenes (PCNs), polychlorinated n-alkanes (PCAs), and brominated flame retardants. POPs also include compounds accidentally formed or released as a byproduct of activities such as industrial or combustion processes (e.g., polychlorinated dibenzo-p-dioxins (PCDDs) or polychlorinated dibenzofurans (PCDFs)). All of these compounds are harmful to living organisms, environmentally persistent, lipophilic, able to bioaccumulate, and semivolatile. The international community has been working to eliminate POPs since 1995, but only a few forms have been banned to date, even as new forms of POPs continue to be created. Scientists have noted that application of pesticides and other POPs over large areas can only be sustainable if the molecules biodegrade naturally, because remediation can only be applied to accidents in restricted areas and is not feasible over large areas. Consequently, the only efficient policy for molecules that do not biodegrade is to restrict their use or ban them completely. Polybrominated diphenyl ethers (PBDEs) are one type of brominated flame retardant that has been widely used worldwide, despite being known endocrine disrupters and neurotoxicants for developing organisms. PBDE manufacture in the United States was phased out starting in 2005, but a large number of products containing these compounds remain in use, resulting in continued release of the compounds into the environment. Concentrations of total PBDEs in marine mammals, birds, and humans have doubled approximately every 4 to 7 years between 1970 and 2000. 

de Wit, C.A. 2002. An overview of brominated flame retardants in the environment. Chemosphere 46: 583-624.

Eljarrat, E. and D. Barceló. 2009. Chlorinated and brominated organic pollutants in contaminated river sediments. In Contaminated sediments. Edited by T.A. Kassim and D. Barceló, and O. Kostianoy, 21-56. Berlin: Springer-Verlag.

Pathogens include any microbial organism that can impair the health of humans or other organisms. Pathogens are a distinct class of river pollutant that can come from the same sources as many other types of pollutants, including wastewater treatment effluent, sewage tanks, and agricultural operations, particularly those involving animals. Poorly treated or untreated wastewater is a major source of pathogens, particularly in developing countries. Even in high-income countries, sewage-associated bacteria and antibiotic-resistant bacteria increase in abundance downstream from wastewater sources, particularly following precipitation. One of the difficulties of wastewater treatment is that bacteria are necessary to promote biochemical reactions critical to this treatment, but these bacteria can also be pathogens harmful to human.

The primary trace elements affecting rivers are arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, and zinc. Each of these elements, which enter river corridors through atmospheric deposition and both point and nonpoint source releases to surface water, is toxic to living organisms. Atmospheric forms come from combustion of municipal solid waste and of fossil fuels in coal- and oil-fired power plants, releases from metal smelters, emissions from automobiles, and burning of biomass (e.g., wildfires). Point sources include sludge from municipal sewage treatment plants, effluent to surface waters from coal-fired power plants, releases from industrial uses, and drainage from acid mines. Nonpoint sources include natural rock weathering, agricultural activities leading to runoff of manure and artificial fertilizers, releases from wear of automobile parts, and irrigation return flow.

Cadmium provides an example of the dissemination and effects of trace elements in river ecosystems. Plants take up cadmium from the soil and concentrate it in their tissues, which humans and other animals can then ingest. Because cadmium can adsorb, or physically attach, to silt, clay, sand, and organic material, river muds can concentrate cadmium by 5,000 to 500,000 times relative to stream waters. Once in an organism, cadmium is a teratogen that causes developmental changes and abnormalities; a mutagen that causes chromosomal changes; and a carcinogen that causes cancerous growths. Cadmium concentrates in the organs – particularly the liver and kidneys – of vertebrates. Although humans gradually excrete cadmium through urine and feces, it may remain in the body for 50 years. Adverse effects on fish and wildlife are either pronounced or probable when cadmium concentrations exceed 3 parts per billion in freshwater or 100 parts per billion in the diet.

Eisler, R. 1985. Cadmium hazards to fish, wildlife, and invertebrates: A synoptic review. US Fish and Wildlife Service Biological Report 85 (1.2).

Wohl, E. 2004. Disconnected Rivers: Linking Rivers to Landscapes. Yale University Press, New Haven, CT.

Wohl, E. 2013. Wide Rivers Crossed: The South Platte and the Illinois of the American Prairie. University Press of Colorado, Boulder, CO.