Quality of underground water and its contribution towards selenium enrichment of the soil-plant system for a seleniferous region of northwest India

Quality of underground water drawn from 90 tubewells located in the seleniferous region of northwestern India was evaluated on the basis of Se content, electrical conductivity and residual sodium carbonate (RSC). All the water samples were found to be suitable for irrigation on the basis of total dissolved salts. With respect to RSC ratings, 86% of the samples were marginally fit and 12% of the samples were unfit for irrigation purposes. Selenium content of tubewell waters at or near the toxic sites ranged between 0.25 and 69.5 microgram l(-1) with an average value of 4.7 microgram l(-1). The maximum contamination level (MCL) of 10 microgram Se l(-1) for drinking purposes was exceeded by 11.1% of the tubewell waters and the maximum permissible level (MPL) of 20 microgram Se l(-1) for irrigation purposes was exceeded by 4.4% of the waters. In water samples having > 1 microgram Se l(-1), a significant positive relationship (r = 0.287*) of Se with pH was observed and the r values increased to 0.919 as the level of Se in the water increased. Underground water pumped from shallow tubewells (24-36 m depth) contained 2-3 times more Se than that from deep tubewells (76 m depth). Selenium enrichment in the soil obtained by subtracting Se removed by crops from Se additions through irrigation water, was found to be highly positive in the case of rice and sunflower. Selenium balances turned out to be negative for mustard followed by Egyptian clover, pearl millet and wheat. In rice-based cropping sequences, Se balances were positive, suggesting that cultivation of rice may be discouraged if the aim is to reduce accumulation in shallow groundwater. More area should be brought under mustard, as its cultivation results in depletion of Se from the soils. Even irrigation with water containing Se at MPL should result in accumulation of Se in the soil under rice, sunflower, sugarcane, maize or oat.