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In 2006, about 80% of the population received tap water from a state-regulated public water supply. During the past five years, 99% of larger water systems met all health-based water quality standards. The most common drinking water contaminants are microorganisms, nitrate, and arsenic. Water quality monitoring has improved over the past five years.

Bacteria, viruses, and protozoa (such as Giardia lamblia and Cryptosporidium) are drinking water contaminants that can rapidly cause widespread and serious illnesses. These microbes primarily come from human or animal wastes that wash into lakes and rivers or that rain or irrigation water carry into shallow groundwater aquifers. An aquifer is an underwater storage area of rock, sand, or gravel. Water systems treat water from reservoirs or rivers before distributing it as drinking water, so microbiological contamination is rare. Water systems that use groundwater from shallow aquifers, which may be susceptible to contamination, are required to chlorinate (disinfect) the water. Water from systems supplied by deep, protected wells is less likely to be contaminated. Water systems test for the presence of total coliform and E. coli, two kinds of bacteria that signal the presence of human or animal wastes. When these bacteria are found in a water sample, the water supplier must immediately conduct further testing, look for the source of contamination, and in some cases, increase water treatment. If the problem appears serious, the water supplier must inform all customers about the problem and instruct them to use bottled water or boil their tap water before they drink it.

Nitrate is a chemical found in most fertilizers, manure, and liquid waste discharged from septic tanks. Natural bacteria in soil can convert nitrogen into nitrate. Rain or irrigation water can carry nitrate through the soil into groundwater. Nitrate most often affects water from shallow, poorly constructed, or improperly located wells. Nitrate contamination is particularly hazardous for infants and pregnant women. Nitrate impairs the ability of red blood cells to carry oxygen. In most exposed adults and children, red blood cells rapidly return to normal. But infants can experience a serious health condition called methemoglobinemia or “blue-baby syndrome.” There is limited information on the number of cases that occur in Washington because it is not a “notifiable condition.” (Washington State requires health care providers to report suspected or confirmed cases of selected diseases or conditions to public health authorities.) There is mixed evidence as to whether nitrate exposure is linked to cancer and spontaneous abortions.

Most arsenic in drinking water comes from rock formations that contain arsenic. Groundwater that flows through these rock formations can dissolve the arsenic and carry it into underground aquifers, streams, or rivers. High levels usually occur only in groundwater. The level of arsenic in groundwater varies widely.

Microorganisms and chemical compounds in the environment can and will for the immediate future, contaminate drinking water supplies. These contaminants can cause serious illness, and in some circumstances, death. Infants, children, the elderly, and immune-compromised people are particularly susceptible to serious health effects from contaminants in drinking water.

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