Arsenic in Groundwater

Arsenic is both toxic and carcinogenic. Inorganic forms of arsenic dissolved in drinking water are the most significant forms of natural exposure. Organic forms of water of arsenic that may be present in food are much less toxic to humans. Clinical manifestation of arsenic poisoning begins with various forms of skin diseases and proceeds on via diseases to internal organs ultimately to cancer and death. To be safe from these diseases arsenic free water is must. WHO guideline value for arsenic in drinking water is upto .01 mg/l. Bangladesh standard for arsenic in drinking water is 0.05 mg/l.
Groundwater of Bangladesh is contaminated with arsenic, which occurs naturally in alluvial and deltaic sediments. First official detection was in 1993 and subsequent confirmation after 1995 of levels of arsenic in shallow and deep wells in various parts of Bangladesh has raised serious health concerns. Resent statistics says that about 59 districts of Bangladesh have affected by arsenic above maximum permissible limit Total 1.12 million tube-wells are found to be affected. As a result 75 million people are in the risk and 1.2 million people are exposed to arsenic poisoning with 24 million people potentially exposed.
Methods of arsenic determination:
Three methods are used to determination of arsenic. They are as follows:
a. Field Kit method
b. SDDC method
c. Atomic Absorption US method.
Among these methods Field Kit method is an approximate determination. There a calibration curve is used. In SDDC, Spectrophotometer of 535 NM color spectrum is used. We use graphite furnace in our lab. Atomic absorption method is more accurate. Calibration curve is used here. But stability is less here. In our lab we have two computerized machines to determine arsenic. In our lab. We observed atomic absorption machine of old and modern version. We watched testing arsenic by new computerized machine which analyses the sample by burning it at about 2500 C. Then results are shown on screen both in graphically and chart.
Results & comments:
In field Kit method arsenic content of water is found as 225 microgram/l.
In SDDC method arsenic content of water is found as 26 microgram/l.
There is a great difference between the results of the two methods. This may be held due to the approximate analysis of field kit method. Here we use zinc, which is not pure itself; it contains a significant amount of arsenic as impurity. Here we observed color of the solution and then we compare this color with a standard color disk to measure arsenic content. But in our test color was not properly matched with standard disk. We assumed an approximate color, which may be wrong. So result is incorrect. In SDDC method we knew color from color spectrum. So, this result is more reliable than before.
Environmental significance:
Arsenic is naturally occurring element that is tasteless and odorless. As compound of underground rock and soil, arsenic works its way into groundwater and enters food chains through either drinking water or eating plants and cereals that have absorbed the mineral. Daily consumption of water with greater than 0.01 mg/l of arsenic. Less than 2 percent of fatal dose can lead to problems with the skin and circulatory and nervous systems. If arsenic builds up to higher toxic levels, open lesions, organ damages (such as deafness), neural disorders and organ cancer often fatal can result.
Groundwater is the preferred source of drinking water in rural areas, particularly in developing countries, because treatment of the same, including disinfections. The problem of arsenic contamination of groundwater is more serious in Bangladesh, where the groundwater in 59 out of the 64 districts is contaminated with arsenic. Since the removal of arsenic is difficult and an alternate source of water is expensive, people still use tube well water.
According to Bangladesh Environment Conservation rules (1997), drinking water standard for Arsenic is 0.05 mg/l; but for WHO guideline of 0.01 mg/l is adopted.
Available methods of testing:
There are few tests for determination of arsenic concentration in water. Those are as follows:
1. The Field Kit Method
2. Silver Diethyldithiocarbamate (SDDC) Method
3. Atomic Absorption Spectrometric (AAS) Method
The Field Kit Method may be following types:
• Merck Kit
• IAN Kit
• Nipson Kit
• BUET Kit
Silver Diethyldithiocarbamate (SDDC) Method may be following types:
* Using Zinc
* Using Sodium Borohydrate
Atomic Absorption Spectrometric (AAS) Method may be following types:
* Hydride Generation
* Graphite Furnace

Tweet