Arsenic contamination of groundwater
Arsenic contamination of groundwater is a natural occurring high concentration of arsenic in deeper levels of groundwater, which became a high-profile problem in recent years due to the use of deep tubewells for water supply in the Ganges Delta.
It has occurred in various parts of the world, most notably the Ganges Delta of Bangladesh and West Bengal, India, causing serious arsenic poisoning among large numbers of people. A 2007 study found that over 137 million in more than 70 countries are probably affected by arsenic posioning of drinking water. 
Parts of Thailand, Taiwan, Argentina, Chile and Mainland China have been affected. Approximately 20 incidents of groundwater arsenic contamination have been reported from all over the world. 
 Of these, four major incidents were in Asia.
There are also many locations in the United States where the groundwater contains arsenic concentrations in excess of the new Environmental Protection Agency standard of 10 parts per billion.

# Effects on specific nations and regions

## Bangladesh and West Bengal
The story of the arsenic contamination of the groundwater in Bangladesh is a tragic one.  Diarrheal diseases have long plagued the developing world as a major cause of death, especially in children.  Prior to the 1970s, Bangladesh had one of the highest infant mortality rates in the world.  Ineffective water purification and sewage systems as well as periodic monsoons and flooding exacerbated these problems.  As a solution, UNICEF and the World Bank advocated the use of wells to tap into deeper groundwater for a quick and inexpensive solution.  Millions of wells were constructed, infant mortality and diarrheal illness were reduced in half.  However with over 11 million wells constructed, it has been found over the last two decades that 40% of these wells are contaminated with arsenic.  Arsenic is a carcinogen which causes many cancers including skin, lung, and bladder as well as cardiovascular disease.
Despite this, there is no scientific evidence to suggest that the arsenic in the water was the main source of this disease. Although arsenic is poisonous, the symptoms of arsenic intake and these symptoms don't match up. Another cause could be the unsanitary living conditions in most of these places. 
In the Ganges Delta, the affected wells are typically more than 20 m and less than 100 m deep. Groundwater closer to the surface typically has spent a shorter time in the ground, and so has not absorbed a high concentration of arsenic; water deeper than 100 m is exposed to much older sediments which have already been depleted of arsenic.
Dipankar Chakraborti from West Bengal brought the crisis to international attention in a research paper published in The Analyst in 1995 and reported on by David Bradley (The Guardian, January 5, 1995, "Drinking the water of death").
  Beginning his investigation in West Bengal in 1988, he eventually published, in 2000, the results of a study conducted in Bangladesh which involved the analysis of thousands of water samples and hair, nail and urine samples. They found 900 villages with arsenic above the government limit. Chakraborti described this as "only the tip of the iceberg."
Chakraborti has criticized aid agencies, saying that they denied the problem during the 1990s while millions more tube wells were sunk, and later hired foreign experts who recommended treatment plants which were not appropriate to the conditions, regularly breaking down or not removing the arsenic.
Chakraborti says that the arsenic situation in Bangladesh and West Bengal is due to negligence. However, he also says that in West Bengal, water is mostly supplied from rivers. Groundwater comes from deep tubewells, which are few in the state. So the risk of arsenic patients in West Bengal is comparatively less. 
According to the World Health Organisation, In Bangladesh, West Bengal (India) and some other areas, most drinking-water used to be collected from open dug wells and ponds with little or no arsenic, but with contaminated water transmitting diseases such as diarrhoea, dysentery, typhoid, cholera and hepatitis. Programmes to provide safe drinking-water over the past 30 years have helped to control these diseases, but in some areas they have had the unexpected side-effect of exposing the population to another health problem - arsenic. 
WHO has defined the area under threat: Seven of 16 districts of West Bengal have been reported to have ground water arsenic concentrations above 0.05 mg/L; the total population in these seven districts is over 34 million and it has been estimated that the population actually using arsenic-rich water is more than 1 million (above 0.05 mg/L) and is 1.3 million (above 0.01 mg/L). According to a British Geological Survey study in 1998 on shallow tube-wells in 61 of the 64 districts in Bangladesh, 46% of the samples were above 0.010 mg/L and 27% were above 0.050 mg/L. When combined with the estimated 1999 population, it was estimated that the number of people exposed to arsenic concentrations above 0.05 mg/l is 28-35 million and the number of those exposed to more than 0.01 mg/l is 46-57 million (BGS, 2000). 
The solution according to Chakraborti, By using surface water and instituting effective withdrawal regulation. West Bengal and Bangladesh are flooded with surface water. We should first regulate proper watershed management. Treat and use available surface water, rain-water and others. The way we're doing at present is not advisable.

## United States
There are many locations across the United States where the groundwater contains naturally high concentrations of arsenic.  Cases of groundwater-caused acute arsenic toxicity, such as found in Bangladesh, are unknown in the United States where the concern has focused on the role of arsenic as a carcinogen.  The problem of high arsenic concentrations has been subject to greater scrutiny in recent years because of changing government standards for arsenic in drinking water.
Some locations in the United States, such as Fallon, Nevada have long been known to have groundwater with relatively high arsenic concentrations (in excess of 0.08 mg/l)..  Even some surface waters, such as the Verde River in Arizona, sometimes exceed 0.01 mg/l arsenic, especially during low-flow periods when the river flow is dominated by groundwater discharge.
A drinking water standard of 0.05 mg/l (equal to 50 parts per billion, or ppb) arsenic was originally established in the United States by the Public Health Service in 1942.  The Environmental Protection Agency (EPA) studied the pros and cons of lowering the arsenic Maximum Contaminant Level (MCL) for years in the late 1980s and 1990s.  No action was taken until January 2001, when the Clinton administration in its final weeks promulgated a new standard of 0.01 mg/l (10 ppb), to take effect January 2006.  The incoming Bush administration suspended the new regulation, but after some months of study the new EPA administrator Christine Todd Whitman approved the new 10 ppb arsenic standard, and its original effective date of January 2006.
Many public water supply systems across the United States obtained their water supply from groundwater that had met the old 50 ppb arsenic standard, but exceeded the new 10 ppb MCL.  These utilities searched for either an alternative supply or an inexpensive treatment method to remove the arsenic from their water.  In Arizona, an estimated 35% of water-supply wells were put out of compliance by the new regulation; in California, the percentage was 38%.
The proper arsenic MCL continues to be debated.  Some have argued that the 10 ppb federal standard is still too high, while others have argued that 10 ppb is needlessly strict.  Individual states are able to establish lower arsenic limits; New Jersey has done so, setting a maximum of 0.005 mg/l for arsenic in drinking water.

# Water purification solutions

## Small-scale water treatment
Chakraborti claims that arsenic removal plants (ARPs) installed in Bangladesh by UNDP and WHO were a colossal waste of funds due to breakdowns, inconvenient placements and lack of quality control.
A simpler and less expensive form of arsenic removal is known as the Sono arsenic filter, using 3 pitchers containing cast iron turnings and sand in the first pitcher, and wood charcoal and sand in the second. Plastic buckets can also be used. It is claimed that thousands of these systems are in use, and that they can last for years, while avoiding the toxic waste disposal problem inherent to conventional arsenic removal plants.
In the United States small "under the sink" units have been used to remove arsenic from drinking water.  This option is called "point of use" treatment.  The most common type of domestic treatment unit has operated by reverse osmosis, although ion exchange, activated alumina, and ferric oxide adsorption units have also been marketed.

## Large-scale water treatment
In some places, such as the United States, all the water supplied to residences by water utilities must meet primary (health-based) drinking water standards.  This may necessitate large-scale treatment systems to remove arsenic from the water supply.  The effectiveness of any method depends on the chemical makeup of a particular water supply.  The aqueous chemistry of arsenic is complex, and may affect the removal rate that can be achieved by a particular process.
Some large utilities with multiple water supply wells could shut down those wells with high arsenic concentrations, and produce only from wells or surface water sources that meet the arsenic standard.  Other utilities, however, especially small utilities with only a few wells, may have no available water supply that meets the arsenic standard.
Coagulation/filtration removes arsenic by coprecipitation with iron oxide. Coagulation/filtration using alum is already used by some utilities to remove suspended solids, and may be adjusted to remove arsenic.
Iron oxide adsorption filters the water through a granular medium containing ferric oxide.  Ferric oxide has a high affinity for adsorbing dissolved metals such as arsenic.  The iron oxide medium eventually becomes saturated, and must be replaced.
Activated alumina is another filter medium known to effectively remove dissolved arsenic.  It has also been used to remove undesirably high concentrations of fluoride.
Ion Exchange has long been used as a water-softening process, although usually on a single-home basis.  It can also be effective in removing arsenic with a net ionic charge.  (Note that arsenic oxide, As2O3, is a common form of arsenic in groundwater that is soluble, but has no net charge.)
Both Reverse osmosis and electrodialysis (also called electrodialysis reversal) can remove arsenic with a net ionic charge.  (Note that arsenic oxide, As2O3, is a common form of arsenic in groundwater that is soluable, but has no net charge.)  Some utilites presently use one of these methods to reduce total dissolved solids and therefore improve taste.  A problem with both methods is the production of high-salinity waste water, called brine, or concentrate, which then must be disposed of.

# Dietary intake
Researchers from Bangladesh and the United Kingdom have recently claimed that dietary intake of arsenic adds a significant amount to total intake, where contaminated water is used for irrigation.