Water pollution is contamination of water by harmful wastes. The chief source of pollution is industrial waste. Toxic chemicals, such as polychlorinated biphenyl (PCB), are discharged as effluent and cannot be disinfected with chlorine. The burning of fossil fuels causes acid rain. Untreated or partially treated sewage is another source of water pollution. Sewage treatment is unable to prevent the spread of viruses and of phosphorus-based detergents which cause eutrophication. Agricultural chemicals and wastes, such as pesticides and fertilizers, are another major cause of pollution. Once pollution has affected groundwater it spreads more rapidly. Oil spills and ocean dumping are major causes of marine pollution.