GROUND WATER QUALITY MONITORING
Monitoring of ground water quality is an effort to obtain information on chemical quality through representative sampling in different hydrogeological units. The chemical quality is being monitored by Central Ground Water Board once in a year through a network of about 15000 observation wells located all over the country, in regular monitoring programme. Apart from these observation wells the quality is also monitored through various studies like ground water management studies, ground water exploration etc. The ground water monitoring activity is aimed at generating background data of different chemicals constituents in Ground water on a regional scale.
GROUND WATER QUALITY SCENARIO IN INDIA
Indian Sub- Continent is endowed with diverse geological formations from oldest Achaeans to Recent alluviums and characterized by varying climatic conditions in different parts of the country. The natural chemical content of ground water is influenced by depth of the soils and sub-surface geological formations through which ground water remains in contact. In general, greater part of the country, ground water is of good quality and suitable for drinking, agricultural or industrial purposes. Ground water in shallow aquifers is generally suitable for use for different purposes and is mainly of Calcium Bicarbonate and Mixed type. However, other types of water are also available including Sodium-Chloride water. The quality in deeper aquifers also varies from place to place and is generally found suitable for common uses. There is salinity problem in the coastal tracts and high incidence of fluoride, Arsenic, Iron & heavy metals etc in isolated pockets have also been reported. The distributions of various constituents present in ground water in different parts of the country have been discussed in following paragraphs.
The main ground water quality problems in India are as follows.
Salinity:- Salinity in ground water can be of broadly categorised into two types, i.e Inland Salinity and Coastal salinity
Inland salinity in ground water is prevalent mainly in the arid and semi arid regions of Rajasthan, Haryana, Punjab and Gujarat, Uttar Pradesh, Delhi, Andhra Pradesh Maharashtra, Karnataka and Tamil Nadu. There are several places in Rajasthan and southern Haryana where EC values of ground water is greater than 10000 mS /cm at 25o C making water non-potable. In some areas of Rajasthan and Gujarat, ground water salinity is so high that the well water is directly used for salt manufacturing by solar evaporation. Inland salinity is also caused due to practice of surface water irrigation without consideration of ground water status. The gradual rise of ground water levels with time has resulted in water logging and heavy evaporation in semi arid regions lead to salinity problem in command areas.
The Indian subcontinent has a dynamic coast line of about 7500 km length. It stretches from Rann of Kutch in Gujarat to Konkan and Malabar coast to Kanyakumari in the south to northwards along the Coromandal coast to Sunderbans in West Bengal .The western coast is charactrised by wide continental shelf and is marked by backwaters and mud flats while the eastern coast has a narrow continental shelf and is characterized by deltaic and estuarine land forms. Ground water in coastal areas occurs under unconfined to confined conditions in a wide range of unconsolidated and consolidated formations. Normally, saline water bodies owe their origin to entrapped sea water (connate water), sea water ingress, leachates from navigation canals constructed along the coast, leachates from salt pans etc. In general, the following situations are encountered in coastal areas i. Saline water overlying fresh water aquifer ii. Fresh water overlying saline water iii. Alternating sequence of fresh water and saline water aquifers In India, salinity problems have been observed in a number of places in most of the coastal states of the country. Problem of salinity ingress has been conspicuously noticed in Minjur area of Tamil Nadu and Mangrol – Chorwad- Porbander belt along the Saurashtra coast.
85 % of rural population of the country uses ground water for drinking and domestic purposes. High concentration of fluoride in ground water beyond the permissible limit of 1.5 mg/l poses the health problem. Nearly The occurrences of fluoride beyond permissible limit (> 1.5 mg /litre) has been observed based on the chemical analysis of water samples collected from the groundwater observation wells . The name of the districts having spot values of >1.5 mg/l are give in following table.
State wise details of distribution of flouride in Ground Water above permissible limit
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The occurrence of Arsenic in ground water was first reported in 1980 in West Bengal in India. In West Bengal, 79 blocks in 8 districts have Arsenic beyond the permissible limit of 0.05 mg/L.The most affected districts are on the eastern side of Bhagirathi river in the districts of Malda, Murshidabad, Nadia, North 24 Parganas and South 24 Parganas and western side of the districts of Howrah, Hugli and Bardhman. The occurrence of Arsenic in ground water is mainly in the intermediate aquifers upto the depth of 100m. The deeper aquifers are free from Arsenic contamination. Apart from West Bengal, Arsenic contamination in ground water has been found in the states of Bihar, Chhatisgarh and Uttar Pradesh &Assam. Arsenic in ground water has been reported in 15 districts In Bihar, 9 districts in U.P and one district each in Chhatisgarh & Assam states. The occurrence of Arsenic in the states of Bihar, West Bengal and Uttar Pradesh is in Alluvium formation but in the state of Chhatisgarh, it is in the volcanic rocks exclusively confined to N-S trending Dongargarh-Kotri ancient rift zone. It has also been reported in Dhemaji district of Assam. The following table shows the occurrence of Arsenic in ground water in some state of India.
Occurrence of High Arsenic in Ground Water of Some States of India
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In West Bengal & Bihar states, Arsenic contamination affected blocks are based on the findings of Task Force/ State Government. In case of Uttar Pradesh, Assam & Chhattisgarh States Arsenic contamination is identified as point source based on findings of Central Ground Water Board and State Ground Water Departments.
High concentration of Iron (>1.0 mg/l) in ground water has been observed in more than 1.1 lakh habitations in the country. Ground water contaminated by iron has been reported from the states of Andhra Pradesh, Assam, Bihar, Chhattisgarh, Goa, Gujarat, Haryana, J& K, Jharkhand Karnataka Kerala. Madhya Pradesh Maharashtra, Manipur, Meghalaya Orissa, Punjab, Rajasthan, Tamil Nadu, Tripura, Uttar Pradesh, West Bengal & UT of Andaman & Nicobar.
Nitrate is a very common constituent in the ground water, especially in shallow aquifers. The source is mainly from anthropogenic activities. High concentration of Nitrate in water beyond the permissible limit of 45 mg/l causes health problems. High Nitrate concentration in ground water in India has been found in almost all hydrogeological formations.