Nickson et al.1 suggested that the arsenic in these alluvial sediments is derived from sulphide deposits in the Ganges basin. However, the copper belt of Bihar, which contains small amounts of arsenopyrite, and the coal basins of the Damodar valley, which contain moderate concentrations of arsenic, are drained by rivers that flow far to the south of the Ganges tributary system (Fig. 1). We suggest that there are several more likely sources of sedimentary arsenic, including the Gondwana coal seams in the Rajmahal basin, which contain up to 200 parts per million (p.p.m.) of arsenic; isolated outcrops of sulphides in the Darjeeling Himalayas, which contain up to 0.8% arsenic; and other sources in the upper reaches of the Ganges river system.

Figure 1: Map showing arsenic-affected areas (shaded).
figure 1

Cu, copper belt of Bihar; S and D, Subarnarekha and Damodar river basins; C, coal basins; R, Rajmahal volcanics; H, Himalayas; and SO, sulphide occurrence with arsenic in Darjeeling Himalayas.

The Ganges alluvial tract upstream of Rajmahal, in the states of Bihar and Uttar Pradesh, does not suffer from large-scale arsenic contamination. This indicates that the Quaternary sediments around the Ganges delta have characteristic features that favour the initial retention and subsequent release of arsenic. These sediments have high proportions of clay and contain relatively large amounts of organic carbon, and are thicker towards the south3,4. The average arsenic content in cores from boreholes in West Bengal is higher in layers of clay (9.5–12 p.p.m.) than of sand (3.8–4.8 p.p.m.)5. Quaternary sediments in the Ganges alluvial tract in Bihar and Uttar Pradesh contain more sand and are much narrower than sediments in the Bengal basin, which may explain why they retain less arsenic.

The groundwater of Uttar Pradesh and Bihar has trace concentrations of iron (0 to 0.7 mg per litre) compared with higher values in West Bengal (up to 36 mg per litre) and Bangladesh (30 mg per litre)1. The relatively low values of dissolved iron upstream of the Ganges delta indicate that the environment may not be sufficiently reducing to mobilize iron and arsenic.

Nickson et al.1 reported that arsenic concentration increases with depth in wells at Manikganj, Faridpur and Tungipara in Bangladesh. However, this observation appears to be site specific, as the large database5,6,7 for aquifers in West Bengal indicates that arsenic decreases with depth (Fig. 2).

Figure 2
figure 2

Variation of total arsenic with depth in aquifers in West Bengal.

During the past thirty years, ground-water has been used increasingly for irrigation and the use of phosphate fertilizers has increased threefold. More than 0.5 million tubewells with handpumps, 0.1 million shallow tubewells, and 3,000 deep tubewells have been sunk at depths of 10–20 m, 30–100 m and 50–200 m, respectively8,9. This widespread withdrawal of ground-water may have mobilized phosphate derived from fertilizers and from the decay of natural organic materials in shallow aquifers. The increase in phosphate concentration could promote the growth of sediment biota and the desorption of arsenic from sediments. These combined microbiological and chemical processes might have increased the natural mobility of arsenic.

The efficiency with which arsenic is removed by adsorption onto iron-coated sand10 and by adsorption on and co-precipitation with ferrihydrite11 depends on both the arsenic oxidation state and the ratio of iron to arsenic. The proposed removal of arsenic by simple aeration1 of anoxic water must therefore be approached with caution.

See also Chowdhury et al Reply - McArthur