About Arsenic

• A metalloid of Group V A of the Periodic Table.
• Exists in oxidation state in the natural environment including +5, +3, +1 and -3 valence and rarely occurring in elemental form In various oxidation states, can form many inorganic and organic compounds.
• The common valences of arsenic in unpolluted ground water of geogenic origin are +3 and +5.
• In the natural environment, arsenic occurs in soils at an average concentration of about 5 to 6 mg/KG
• Arsenite [As(III)], the reduced state in inorganic arsenic, is a toxic pollutant in natural environment and is more soluble and mobile than the oxidized state of inorganic arsenic, arsenate [As(V)].
  
  ARSENIC IN GROUND WATER
  

• Arsenic in ground water is generally present as an oxy-anion i.e., as arsenite (H3AsO3) or arsenate (H3AsO4) or both.
• The dominant aqueous species vary depending on pH and Eh (redox potential) condition.
• Occurrence of arsenic in ground water is mostly due to leaching of geological material, mineral precipitation, dissolution of unstable arsenic minerals adsorption - desorption, chemical transformation within the formation, and input from geothermal sources. Factors such as pH, Eh, solution composition, competing and complexing ions, aquifer mineralogy, reaction kinetics and hydraulics of ground water system influence the arsenic concentration in ground water.
• Environmental conditions of the sediments have a greater influence on arsenic speciation and mobility than the total concentration of arsenic in sediments.
• As per WHO standards, permissible limit of arsenic in drinking water is 0.01 mg/litre and as per BIS, permissible limit is 0.05 mg/litre.

  HEALTH HAZARDS OF ARSENIC IN GROUND WATER

  • Arsenic is poisonous in doses significantly larger than 65 mg/litre and a single large dose or repeated small doses of arsenic can cause poisoning.
  • Very large doses of arsenic can cause death.
  • Excess intake of arseniccauses irritation of the digestive tract leading to gastro-intestinal disorders like pain, nausea, vomiting and diarrhea.
  • It can lead to cardio-vascular disorders - decreased production of red and white blood cells, abnormal heart function and blood vessel damage.
  • Prolonged ingestion of arsenic can cause liver and/or kidney injuries and can lead to neurological disorder causing a "pins and needles" feeling in feet and hands.
  • Inorganic arsenic when inhaled increases risk of lung cancer.
  • Ingestion of inorganic arsenic may also cause skin disorders like hyper-pigmentation, hyper-keratosis, warts or corns on the palms and soles. Sometimes warts or corns may develop into skin cancer.

  
  
  EXTENT OF ARSENIC PROBLEM IN WEST BENGAL, INDIA
  
  Occurrence of arsenic in ground water has been reported from all over the globe viz. USA, Mexico, Chile, China, Mongolia, Germany, Japan, Bangladesh, etc. In India, arsenic contamination in ground water came to light in early eighties when a few cases of arsenic dermatosis were reported from parts of Nadia, 24 Parganas (North and South), Murshidabad, Hugli , Haora and Bardhaman districts of West Bengal. Subsequently it was established that prolonged use of arsenic contaminated ground water was the cause of the disease.

POPULATION AFFECTED

At present, arsenic contamination is limited to isolated patches spread over 68 blocks of eight districts affecting about 4.4 million population.

PROBABLE SOURCES AND CAUSES FOR ARSENIC MOBILIZATION

A number of organizations and academic institutions are involved in the Research & Development studies in order to ascertain the source of arsenic and cause for arsenic mobilization in ground water. The studies conducted so far indicate that :

• Ground water occurring mainly within shallow zones (20-60 mbgl) are characterized by high iron, arsenic (>0.5 to 1 mg/l or above), calcium, magnesium and bicarbonate with low chloride, sulphate, pH being above 7.
• Sand grains in the aquifers in the composite meander belt of the upper delta plain are coated with iron and arsenic rich material.
• Clay, silty clay partings between the aquifers contain relatively higher arsenic compared to sand - gravel sequence of the aquifer.
• Arsenic rich pyrite detected from borehole samples of a few locations is the major source of arsenic contamination, on the assumption that large scale ground water withdrawal leads to changes in the geochemical environment which in turn decomposes pyrite to ferrous sulphate, ferric sulphate and sulphuric acid.
• Presence of clay, peat etc. in the sediments having iron and arsenic sulphides deposited in reducing environment is the principal source of elevated arsenic concentration in ground water.
• Changes of pH, oxidation - reduction potential and aqueous chemistry may mobilize the contaminant arsenic species already present in the mineral matrix.
• Phosphate and arsenic content in the vadose zones are negligible.
• Upper sandy clay being tapped by dug wells has arsenic below detection limit.
• Pond, tank or river water in these areas have been found to contain arsenic within detection limit.
• Some researchers believe that leaching of arsenic in ground water has been influenced by a number of factors like ground water withdrawal for irrigation, percolation of oxygenated water through soil, application of fertilizers containing phosphate and microbial reactions within the soil. Some, however, consider that as the sulphate content of ground water is very low and as sulphides are present in more clayey strata which may not be taking part in the water turn over, it seems less likely that arsenic comes from oxidized pyrite and arsenopyrite as aerated ground water is drawn into the aquifer. According to them, the aquifers are in a reduced state allowing the reduction of ferric iron from oxides - hydroxides and with the release of ferrous iron, adsorbed arsenic on the secondary iron and aluminium phases is simultaneously released. High arsenic concentration is generally associated with high iron content in alkaline ground water.
• A combination of oxidation and reduction phenomena coupled with changes introduced by organic matter present in the sediments might have been responsible for such high mobilization of arsenic. Large scale ground water withdrawal for irrigation during summer months causes lowering of water table and enhances oxidized zone which results into oxidation phenomena and As(III) is converted into As(V), which is mobile and is transported along ground water. However, during monsoon months, the sediments are subjected to anaerobic conditions and reduction of As(V) to As(III) takes place. It has been observed that such conditions prevail 4 - 6 months in a year. The release of arsenic may be 10 - 13 times more in the reduction process as compared to that under oxidation process.
  
  STUDIES CONDUCTED BY CENTRAL GROUND WATER BOARD

The main objective of the studies being conducted by the Central Ground Water Board in the arsenic affected areas of West Bengal is two-fold :

• Understanding the ground water regime condition and mechanism of arsenic mobilization Finding remedial measures to combat arsenic menace.

The activities to fulfill the above objectives include :

• Periodic monitoring of water levels and arsenic concentration from selected monitoring stations.
• Determination of arsenic content in lithological samples and vadose water.
• X-ray diffraction studies to ascertain mineral composition of borehole samples.
• Isotope studies to ascertain source and age of ground water and different aquifers and to study aquifer - aquifer and surface water - ground water interaction.
• Hydrological tests on arseniferous aquifers.
• Study of ground water flow using mathematical modelling.
• Deep exploratory drilling to delineate arsenic free aquifers in the affected areas.
• Study impact of artificial recharge on shallow arseniferous aquifer.
• Study efficacy of different arsenic removal plants and filters.
• Study of arsenic rich ground water in food chain.

PROGRESS OF VARIOUS ACTIVITIES
1. Periodic monitoring of ground water regime

Monthly monitoring of water levels was carried out from 62 key observation wells during the period 1996-98 in the Yamuna sub-basin covering parts of North 24 Parganas and Nadia - The depth to water levels were 5-6 mbgl during pre-monsoon and 2-3 mbgl during post-monsoon. Arsenic concentration is higher in pre-monsoon than in the post-monsoon period.

44 key observation wells are being monitored on monthly basis in Barasat - I block - Water levels range from 5.4 to 10.86 mbgl during pre-monsoon period and from 1.81 to 6.59 mbgl during post-monsoon period. Arsenic in ground water is maximum (1.75 ppm) in pre-monsoon period.

2. On-site and laboratory studies for arsenic determination in lithological sample

Water samples were analysed in the field for different chemical parameters viz. pH, EC, DO, Eh, As and Ferrous iron followed by detailed analysis in the chemical laboratory - Ground water is found to be characterized with low DO, low Eh, near neutral pH, high carbondioxide, high bicarbonate and high ferrous iron thereby signifying the reducing environment in aquifers containing arsenic.

Samples of water and sediments were collected from vadose zone and analysed - Arsenic concentration is below detection limit of 0.001 mg/l, iron concentration is low being of the order of 1 mg/l, phosphate concentration ranges from 0.25 to 1.25 mg/l and sulphate is low. In sediment samples from the vadose zone, concentration of iron ranges between 8-112 mg/Kg and that of arsenic between 0.05-6.5 mg/Kg.

Litho-samples collected down to depth of 250 mbgl at Birohi, Nadia district were chemically analysed - Arsenic concentration is limited down to a depth of 120 mbgl . Maximum arsenic concentration of 1.75 mg/Kg has been noticed at depth of 9-12 and 90-100 mbgl.

In South 24 Parganas, deep ground water is old. Carbon - 14 age : 5000 to 13000 years. Interconnection between shallow and deep aquifers is remote.

In shallow aquifers in North 24 Parganas, ground water flow velocity has been computed to be 17 cm per day. In Nadia and North 24 Parganas districts around Yamuna river, interconnection between shallow aquifer and river water is possible. Isotope study in districts of Nadia, Bardhaman, Hugli districts shall be taken up once the proposed piezometers are constructed and are ready for use.

5. Hydrological tests on arseniferous aquifers

Pumping tests have been conducted at 22 sites to determine aquifer parameters and to monitor impact of pumping on arsenic concentration - Arsenic concentration has been found to reduce progressively consequent to pumping in most of the cases. The wells tested yielded between 21-43 m3 /hr of ground water. Transmissivity of aquifer was found to vary from 400 - 538 m2/day and storativity varies from 1.3 x 10-2 - 0.33 x 10-5.
To understand the mobilization of arsenic in ground water, compressed air was injected into shallow aquifer (55m) at Birohi, Nadia district - In a span of 75 minutes, arsenic concentration changed from 0.02 ppm to below detection limit of 0.001 ppm. Iron content reduced from 10.6 to 1.10 mg/litre. The pH increased from 6.1 to 6.9, EC increased from 640 to 680 and dissolved oxygen from 1.9 to 7.0.

6. Mathematic modelling

A three dimensional finite difference ground water flow model has been developed by NIH. The model was tested in CGWB and certain modifications have been suggested in ground water draft component.

7. Ground water exploration

Exploratory drilling has been taken up in Murshidabad, Nadia, and North 24 Parganas districts with suitable well design and proper technique to seal off contaminated aquifer - Arsenic free deep aquifers have been delineated and successfully tapped in the depth range of 77 and 270 mbgl at 16 sites. The work is still continuing.

8. Artificial recharge study

Experimental studies are being carried out in North 24 Parganas district to study the impact of artificial recharge on arsenic concentration in ground water. At Ashoknagar, the artificial recharge measures could help in reducing concentration of arsenic from 0.128 mg/l to <0.001 mg/l in a span of 3 months during October to December. At Joypur village, the work is in progress.

9. Evaluation of arsenic removal plants and domestic filters

16 community based arsenic removal plants and domestic filters using different removal techniques have been tested three times in three different periods - The efficacy of these equipment was found to be 75 % .

10. Study of the effect of arsenic rich ground water on food chain

Studies to assess the effect of arsenic rich ground water on food chain are under progress.

FUTURE WORK PROGRAMME

1. Continuation of the exploration to delineate deep arsenic free aquifer in arsenic infested area and construction of suitably designed tubewell for providing arsenic free water.
2. Opening an Arsenic Mitigation Cell within Central Ground Water Board to expedite the Research & Development work on arsenic mitigation.
3. Continuation of isotope study to precisely differentiate shallow arseniferous aquifer from the deep arsenic free aquifer.
4. Experimental study to ascertain the impact of artificial recharge of arsenic contaminated shallow aquifer.
5. X Ray diffraction studies of the undisturbed core samples to identity arsenic bearing minerals in depth & space.
6. Chemical analysis of the lithological samples to decipher the extension of arsenic bearing zone in space and time.
7. Study on food chain in the arsenic infested area.
8. Details study on the efficacy of the arsenic removal filters/ plants introduced in arsenic infested areas of West Bengal.

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