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Thanks to the carelessness of a teenager last week who couldn't find a proper bathroom and instead chose to urinate in a reservoir, the city of Portland, Ore., has opted to flush out 38-million gallons of otherwise drinkable water into the Columbia River.

Although no one would ever be keen on grabbing a glass of contaminated water, the amount of urine present in the vast reservoir is somewhere on the order of three per billion. Could such a small amount of an essentially harmless contaminant merit draining so many millions of gallons of water?

The Environmental Protection Agency (EPA) sets thresholds for different compounds beyond which water is no longer safe to drink. These are known as Maximum Contaminant Level (MCLs). If three parts per billion is enough for Portland to drain a reservoir, you might be surprised to find out acceptable levels of other contaminants that could be lurking in your drinking water.

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Found in mercury, fluorescent light bulbs and other electronic equipment, mercury has an MCL of two parts per billion. Mercury enters groundwater as a result of runoff from landfills or farmlands, wastewater and industrial discharges, according to the EPA.

Mercury can come in different forms. Elemental mercury, the kind found in thermometers and used at gold mine sites, can be ingested and simply passes through the digestive system. Methylmercury, on the other hand, is much more toxic, negatively affecting the immune system, the nervous system, and the senses of touch, taste and sight among other health effects, according to the U.S. Geological Survey (USGS).

Excessive levels of mercury in the water affects not only humans, but also fish, which we then may also consumer. Exposure to methylmercury comes almost entirely from seafood.

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Atrazine is a weedkiller used in fields growing corn, sugarcane, wheat and more. In fact, it happens to be the most popular herbicide in the country today. Because of its agricultural use, atrazine often enters the water supply as run-off from farmers' fields.

Although research has yet to show negative health outcomes in humans, numerous studies have linked atrazine exposure to abnormalities in the sexual development of amphibians, fish, reptiles and other animals. While the EPA has set an MCL of three parts per billion for the herbicide, the agency has been criticized repeatedly for not taking further action to limit the transmission of the chemical into drinking water.

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Benzene is a clear, colorless, aromatic and highly flammable compound. Formed in natural processes, such as forest fires and volcanoes, Benzene is also part of crude oil, gasoline and cigarette smoke, according to the EPA.

Discharged by factories or leaching from gas storage containers, benzene has an MCL of five parts per billion. Acute exposure to benzene in humans can damage the central nervous system and lead to death, according to the World Health Organization (WHO) (PDF). Studies have also shown that excessive benzene exposure may lead to leukemia.

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Cadmium is used as an anticorrosive agent in steel. As a compound, cadmium can be used as a pigment in plastics or in batteries, electronic components or even a nuclear reactor. Fertilizers made from phosphate ores are also a major source or cadmium contamination, according to the WHO (PDF).

Cadmium can enter the water supply through corrosion of galvanized steel or runoff from batteries or paints, according to the EPA. It has an MCL of five parts per billion.

Excessive exposure to cadmium can cause kidney damage, and limited evidence has suggested a possible link to cancer in humans.

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A well-known poison, arsenic also has a wide range of applications, from agriculture to smelting to mining and more. It also turns up in a number of consumer products, including paints, dyes, drugs, soaps and more, according to the EPA.

Just as numerous as the applications of arsenic are the potential devastating health consequences of acute exposure in drinking water, including "skin lesions, neurological effects, hypertension, peripheral vascular disease, cardiovascular disease, respiratory disease, diabetes mellitus, and malignancies including skin cancer," as noted in a paper on arsenic published in 2004 in the journal Toxicology and Applied Pharmacology. Arsenic can, of course, also be fatal.

Arsenic has an MCL of 10 parts per billion. It enters the drinking water through agricultural and industrial runoff, and as a result of natural erosion.

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Lead has been used in plumbing for thousands of years. In fact, the word "plumbing" comes from the Latin "plumbum," which means lead.

As long as there is lead, however, there is lead poisoning. At acute levels of exposure, lead poisoning can be fatal. Common symptoms of lead poisoning may include fatigue, abdominal pain, headache, mood disorders and more.

Before a law passed in 1986 banning lead for plumbing in the United States, houses were built around the country at least up until World War II. Those same plumbing systems with lead pipes can still contaminate drinking water today, although the source of most lead poisoning these days is due to the presence of lead in household paints.

The EPA sets a target of zero parts per billion for lead. When contaminant levels reach 15 parts per billion, the environmental agency employs an additional treatment technique to control for corrosion.

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You might think that a compound as notoriously deadly as cyanide has no place in your drinking water, but the EPA has actually set an MCL of 200 parts per billion for this organic chemical.

Commonly used in compounds and other synthetic fibers and resins as hydrogen cyanide, the chemical enters the groundwater as discharge from sheet metal, plastics and fertilizer factories, according to the EPA.

Acute levels of cyanide can be fatal. It can also cause nerve damage and lead to thyroid dysfunction.

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Nitrates are chemicals used in fertilizers, which is why water contaminated with excessive levels of these nitrogen-compounds are near agricultural centers.

These compounds have an MCL of 10,000 parts per billion. Excessive nitrates intake through contaminated water is primarily a health risk to infants, who may exhibit symptoms including shortness of breath and blue baby syndrome, according to the EPA. In some cases, acute exposure can be fatal.

Nitrates from agricultural run-off can also encourage the growth of algae blooms, which can deprive water of oxygen and potentially be fatal to fish and other sea life.

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Every entry on the list so far has included an element or compound whose safety level in drinking water are regulated by the EPA. There are also a whole host of other contaminants that are unregulated and may pose their own health risks.

Last year, a joint study carried out by the U.S. Geological Survey and the Environmental Protection Agency found traces of 18 unregulated chemicals in a third of the nation's water supply. These chemicals included "11 perfluorinated compounds, an herbicide, two solvents, caffeine, an antibacterial compound, a metal and an antidepressant," according to Scientific American.

In 2009, as part of a five-month investigation, the Associated Press also revealed that the drinking water supplies of at least 41 million Americans are contaminated with a range of pharmaceuticals, including "antibiotics, anti-convulsants, mood stabilizers and sex hormones."

All the potential unseen chemicals that could be lurking in our drinking water seems to beg the question: How safe is U.S. drinking water?