Arsenic groundwater contamination in parts of middle Ganga plain, Bihar

Arsenic contamination of groundwater in low-lying Bhagarathi–Ganga deltaic plain is known for more than two decades. The problem was recognized in 1984 (ref. 1) in West Bengal, when 16 patients from a village of 24 Parganas district reported arsenicosis. Subsequent investigations revealed extensive arsenic contamination in groundwater in the Bengal Basin affecting the eastern part of West Bengal and a major part of Bangladesh. About 40 million inhabitants of this densely populated part of the world are residing in the risk-zone having groundwater arsenic level >0.5 mg/l (ref. 4). If the guideline value of 0.01 mg/l of World Health Organisation is to be considered, the affected population would be much more. In the Asian countries, contamination has been recorded from Hanoi city and the upper end of the Red River delta. In addition, the reported high load from flood and delta plains of the Mekong valley in Cambodia and Vietnam, Irrawaddi delta in Mynamar and in the Indus basin, indicated that the lower flood plains and delta regions of southeastern Asia are prone to arsenic groundwater contamination. The middle Ganga plain covering about 89% geographical area of Bihar (~94,000 km) holds potential alluvial aquifers. The tract is known for surplus food production and intensive groundwater extraction for drinking, irrigation, and industrial uses. The middle and upper Ganga plains covering the upstream part from Rajmahal Hills, were earlier considered to be free from arsenic groundwater contamination. Initially, the contamination (>0.05 mg/l) was reported in 2002 from two villages, Bariswan and Simaria Ojhapatti of Bhojpur district of Bihar. The area is located in the middle Ganga plain, amid the flood-prone belt of Sone-Ganga interfluve region. Studies by the Central Ground Water Board and the Public Health Engineering Department, Government of Bihar have indicated the contamination as high as 0.178 mg/l in the surrounding villages, affecting the hand pumps which are generally of 20– 40 m depth. The dug wells (depth 8– 12 m) have been marked with low arsenic (max 0.008 mg/l). Arsenic in groundwater exhibited a wide spatial variation, even more than 90 times within a distance of 150 m. The area is underlain by a multi-layer sequence of sand (aquifer) alternating with aquitards like sandy-clay and clay, down to depth of 300 m. In an affected village Bariswan, an aquiferspecific groundwater analysis revealed a rapid decline in arsenic load with depth, from 0.095 mg/l at 19 m to 0.006 mg/l at 194 m below ground. Hydrostratigraphic analysis based on drill cutting samples at four locations (Shahpur, Paharpur, Karnamipurpur and Bharauli) in arsenic affected areas in Son–Ganga interfluve, reveals that the Quaternary deposits within 300 m below ground can be divided into two-tier aquifer system. The shallow aquifer system is confined within 120–130 m depth, followed by a laterally continuous 20–30 m thick clay/sandy-clay zone forming the aquitard. The deeper aquifer system exists below this aquitard, which continues down to 240–260 m below ground. The upper part of the shallow aquifer (within ~50 m below ground) is affected by groundwater arsenic contamination. The lowland Terai belt in Nepal, where groundwater is an important source for agriculture and drinking, recorded high load of arsenic (0.02–2.6 mg/l) in shallow tube wells, where cases of arsenicosis have also been reported. Arseniccontaminated areas are reported from the areas close to the debouching zones of Himalayan rivers, like Jamuna River flood plain, in northern parts of Bangladesh. In nine districts of north Bihar bordering Nepal, an extensive arsenic testing (total sample analysed ≈3100) by UNICEF has revealed arsenic contamination below 0.05 mg/l (ref. 9). In districtwise sum-up, the highest number of samples exceeding 0.01 mg/l were in east Champaran district (8.3% of total samples). Initial detection of high arsenic in Bhojpur district has entailed apprehension that a major part of the Ganga Alluvial Plain in Bihar may be polluted by arsenic, whereas Acharya had pointed out that the contamination may not be widespread. Detailed hydrogeochemical study in Bhojpur district reveals that in the southern part of the Ganga River, arsenic contamination is confined in the post-Holocene deposits in the oscillation zone of the river Ganga and in the older flood plain of the newer alluvium. The older alluvium (Upper Holocene to Lower Pleistocene) aquifers recorded concentration in the range of 0.001–