Human infectious diseases are among the most serious effects of water pollution, especially in developing countries, where sanitation may be inadequate or non-existent. Waterborne diseases occur when parasites or other disease-causing microorganisms are transmitted via contaminated water, particularly water contaminated by pathogens originating from excreta. These include typhoid, intestinal parasites, and most of the enteric and diarrheal diseases caused by bacteria, parasites, and viruses. Among the most serious parasitic diseases are amoebiasis, giardiasis, ascariasis, and hookworm.
Developed countries are not immune to the problem of infectious waterborne diseases. In 1993, high cryptosporidium levels in Milwaukee's drinking water supply sickened more than 400,000 residents. That was an unusually extreme case, but transmission of disease agents such as bacteria and cysts via contaminated but poorly treated municipal water is more common than it should be. Every year, seven million Americans are sickened by contaminated water. This is only partly due to drinking water—contamination of recreational water (such as beach water) is also a problem.
Every year there are thousands of beach closings and outdated monitoring methods may in some cases leave beachgoers vulnerable to a range of illnesses. Polluted beach water can cause rashes, ear aches, pink eye, respiratory infections, hepatitis, encephalitis, gastroenteritis, diarrhea, vomiting, and stomach aches.
Nutrient Pollution
Nutrient pollution the most widespread, chronic environmental problem in the coastal ocean. The discharges of nitrogen, phosphorus, and other nutrients come from agriculture, waste disposal, coastal development, and fossil fuel use. Once nutrient pollution reaches the coastal zone, it stimulates harmful overgrowths of algae, which can have direct toxic effects and ultimately result in low-oxygen conditions.
Certain types of algae are toxic. Overgrowths of these algae result in "harmful algal blooms," which are more colloquially referred to as "red tides" or "brown tides." Zooplankton eat the toxic algae and begin passing the toxins up the food chain, affecting edibles like clams, and ultimately working their way up to seabirds, marine mammals, and humans. The result can be illness and sometimes death.
Developed countries have started monitoring for toxic algal blooms, closing fisheries as necessary. This has reduced the incidence of related human illness but has had the obvious economic cost of lost income for fishers and related businesses—and it does nothing the solve the problem for the marine life stuck in the middle of the brown tide.
Nutrient-pollution-driven blooms of non-toxic algae and seaweed can also cause problems by reducing water clarity, making it hard for marine animals to find food and blocking the sunlight needed by sea grasses, which serve as nurseries for many important fish species.
Finally, nutrient pollution can trigger unusual outbreaks of fish diseases. For instance, scientists have found that Pfiesteria, a tiny marine pathogen, can thrive in nutrient-polluted waters. In 1991, one million menhaden fish in North Carolina's Neuse River were killed in a Pfiesteria outbreak. In 1997, several tidal creeks in the Chesapeake Bay watershed experienced Pfiesteria outbreaks, and serious fish kills occurred. Nutrient pollution played a role in both cases.
Chemical Contamination
Over the years, many types of chemicals have gotten into our waterways—and they continue to do so today. Chemical water pollution typically occurs because :
the chemicals were dumped into the water intentionally
the chemicals seeped into groundwater, streams, or rivers because of failing pipes or storage tanks
the chemicals catastrophically contaminated waterways because of industrial accidents
the pollution settled out of polluted air (or was precipitated out of polluted air)