The 20th most abundant element in the earth's crust, arsenic chemically binds with other elements, such as oxygen and sulfur, and often replaces iron in the mineral pyrite, forming arsenopyrite (FeAsS). Arsenic contamination in drinking water from groundwater sources is most often naturally occurring and is due to the geology and geochemistry of the aquifer. Geologically, arsenic is often found in combination with mineralized veins of ore containing copper, silver and gold. In many regions of the world, arsenic is used as a pathfinder in gold exploration by tracking increasing concentrations of arsenic in water until the arsenic source, and gold source, are discovered. Some volcanic regions have exhibited elevated, naturally occurring arsenic within basalts, obsidian, and granite aquifers. Some sedimentary shales have also been found to contain arsenic. Alluvial aquifers originating from the erosion of arsenic-containing source rock have been known to contain relatively high concentrations of arsenic-bearing rock. In general, any geologic material that is known to contain pyrite may also contain arsenic. Also, glacial moraine deposits may contain arsenic bearing materials.
An aquifer could contain relatively high concentrations of arsenic, yet arsenic may not be dissolved because of the chemistry of the water. The rate of arsenic dissolution in groundwater is not significantly impacted by mining activities, which are known to mobilize other toxic metals, such as lead, due to acid rock drainage. Alkaline pH will increase arsenic mobility. In locations with limestone aquifer material adjacent to arsenic-bearing alluvium, elevated arsenic concentrations may be observed. A radical change in pH, such as that resulting from shock-chlorination of a well, has been reported to increase arsenic concentrations. A change in oxygen concentrations will also mobilize arsenic. Reports of arsenic concentrations increasing when groundwater elevations have dropped, bringing oxygen in contact with arsenic-rich aquifer minerals, have been found in the literature. In addition, a reduction of oxygen concentration in the aquifer will facilitate the growth of naturally occurring soil bacteria that increases the rate of arsenic solution. Because arsenic mobility is a function of water pH and oxygen content, it is generally true that any change in the geochemistry of an aquifer may elevate arsenic concentrations. The reverse is also true in that arsenic concentrations can be lowered by changing water pH and oxygen concentration, making arsenic contamination of well water readily treatable with technologies available to the well owner.
Arsenic has been used in mining and manufacturing and was a component of some pesticides used in the past. Chromated copper arsenate was used to pressure treat wood for preservation and to prevent insect damage; this wood is commonly known as CCA-treated lumber. Although arsenic use as described above has the potential to result in arsenic contamination of groundwater, primarily as a result of industrial activity, the arsenic in most water supplies is naturally occurring and comes from the aquifer from which the water is pumped.
Scale is commonly deposited in plumbing fixtures and pipes, and arsenic is known to be deposited in pipe scale in some circumstances. Although scale formation essentially removes arsenic from the water, any change in water chemistry could result in the dissolution of arsenic-rich scale and reintroduction of arsenic into the water supply. This could be of concern in a mobile home or in situations where the homeowner changes a water source.
Arsenic in drinking water cannot be detected by taste, sight or smell. The only way to know the concentration of arsenic in water is through sampling and laboratory testing.
Arsenic exposure can cause a variety of adverse health effects. The severity of the effect depends on how much arsenic is in the water, how much water is consumed, how long a person has been exposed to the water and a person’s general health. Arsenic poisoning can be acute or chronic. Acute poisoning can occur when a high concentration (more than 60 mg/L) of arsenic is ingested over a short period of time. Arsenic poisoning is easy to diagnose and treat, with the most common symptoms ranging from garlic breath to pins and needles sensations in the hands and feet. Chronic poisoning can occur when moderate or small amounts of arsenic are ingested over long periods. Chronic arsenic exposure may increase the risk of cancer and mimic the symptoms of diabetes. Because arsenic is readily excreted in the urine, testing for chronic arsenic exposure is done by measuring the concentration of arsenic retained in the hair and/or fingernails. Research has shown that arsenic's effects on human health tend to be variable, depending on gender, with some ethnic populations exhibiting greater tolerance. Factors such as genetics, age, metabolism, diet and overall health also may impact health risks associated with arsenic exposure because they potentially affect one’s ability to remove arsenic from the body. Individuals with chronic Hepatitis B infection, protein deficiency or malnutrition may be more sensitive to the effects of arsenic. Children and older adults may be other groups at special risk.
The quality of water supplied by public water systems is regulated by the U.S. Environmental Protection Agency (EPA). Three expert panel reports on the science, cost of compliance, and benefits analysis on arsenic in drinking water were released in October 2001. With this new information, the EPA revised the arsenic drinking water standard and established a maximum contaminant level (MCL) for arsenic of 0.010 milligrams per liter (mg/L), which also can be expressed as 0.010 parts per million (ppm). This amount is equivalent to 10 micrograms per liter (ug/L), which also can be expressed as 10 parts per billion (ppb). This MCL became effective in January 2006 and applies to all community water systems (CWS) and nontransient, noncommunity water systems (NTNCWS). A CWS is a public water system serving a community such as cities and towns. An NTNCWS is a public water system that is not a CWS but serves at least 25 of the same people more than six months of the year, such as in rural schools, churches, nursing homes or businesses with their own water supplies.
Consumers wanting to know the concentration of arsenic in a private water supply must have the water tested by a state-certified laboratory. Arsenic concentration can vary greatly from well to well; this variability makes the prediction of arsenic concentrations in a specific well very difficult. However, arsenic should be suspected in private wells located near public wells with elevated arsenic, or in geographic regions and geologic formations in which public wells with elevated arsenic are present. In addition, arsenic can be present in any private well, and users may want to consider having the water tested for arsenic concentration.
All public community water systems and nontransient, noncommunity water systems must provide drinking water that is in compliance with the 10 ppb MCL standard. The treatment system or combination of systems that will be best for a private well user depends on several factors, including the level of arsenic in the water, desired level of arsenic removal from the water, the quantity of water to be treated, and the chemistry of the water. Existing treatment systems that can be used for arsenic reduction include reverse osmosis, distillation, absorption (with activated alumina, granular ferric hydroxide or titanium dioxide), and (anion) ion exchange using contaminant-specific absorption media.