Biodegradation of organic pollutants such as liquid manure, sewage effluents and sewage treatment sludge can deplete oxygen concentrations in waterbodies, causing fish kills and releasing heavy metals from bottom sediments back into the water column, a process characterized by a high biochemical oxygen demand (BOD) from microbial decomposition of these pollutants. Algal bloom decomposition also can deplete the oxygen content in eutrophic waterbodies, particularly lakes and wetlands.
Based on model analyses, BOD concentrations increased in many parts of Africa, Asia and the Pacific, and Latin America during 1990-2010 from industrial and domestic wastewater discharges, and agricultural and urban run-off, with highest increases in rapidly urbanizing and industrializing countries (Annex 9-1). BOD pollution in most developed countries has significantly decreased with enhanced wastewater treatment (e.g. implementation of the 1991 EU Urban Wastewater Treatment Directive).
Synthetic organic pollutants include pesticides, industrial chemicals and solvents, and personal care and pharmaceutical products. Persistent organic pollutants (POPs) are particularly problematic because they do not readily biodegrade in the aquatic environment. Used in many industrial and agricultural applications, they can impact human health and aquatic ecosystems, persisting in fatty tissues of humans, fish and other organisms, and accumulating in sediments. DDT has human carcinogenic and teratogenic risks, for example, but is still used in many regions to control malaria (Annex 9-1). Other synthetic compounds, including non-POPs, continue to enter the ecological food chain globally, while others, such as endocrinedisrupting chemicals, are considered contaminants of emerging concern (see Section 9.5.7). Neonicotinoid and fipronil systemic insecticides, for example, are water soluble and can leach into freshwater and marine systems. Neonicotinoid insecticides are toxic to most arthropods and invertebrates, while fipronil is toxic to fish and some bird species (Annex 9-1; van Lexmond et al. 2015; Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services 2017).