Development and mapping of seleniferous soils in northwestern India.

Periodic surveys were undertaken to identify and characterize Se-contaminated soils in northwestern India. Total Se content varied from 0.023 to 4.91mgkg(-1) in 0-15cm surface soil and 0.64-515.0mgkg(-1) in samples of vegetation. Selenium-contaminated land occupying an area of 865ha was classified into different categories based on total Se content of soils as moderately contaminated (0.5-2.0mg Sekg(-1)) and highly contaminated (>2.0mg Sekg(-1)). The normal soils contained <0.5mg Sekg(-1). The soil map was prepared using village level cadastral maps. Se-contaminated soils were silty loam to silty clay loam in texture and tested pH 7.9-8.8, electrical conductivity 0.3-0.7dSm(-1), calcium carbonate 0.1-4.1% and organic carbon 0.4-1.0%. Selenium was present throughout the soil profile up to 2m depth; 0-15cm surface soil layer contained 1.5 to 6.0 times more Se than in subsurface layers. Selenium content in rock samples collected from lower and upper Shiwalik sub-Himalayan ranges varied from 1864 to 2754 and 11 to 847µgkg(-1), respectively. The sediments transported through seasonal rivulets linking the Shiwalik ranges to affected sites contained 0.57-2.89mg Sekg(-1). The underground water containing 2.5-69.5µg SeL(-1) used for irrigating transplanted rice grown in Se-contaminated area resulted in a net Se addition in soil up to 881gha(-1)y(-1); possibly further aggravating the Se-toxicity problem. Presence of substantial amount of Se in rock samples and sediments of seasonal rivulets suggests that Se-rich materials are being transported from Shiwalik hills and deposited in regions where seasonal rivulets end up.

Selenium concentrations of common weeds and agricultural crops grown in the seleniferous soils of northwestern India.

The plants grown in seleniferous soils constitute a major source of toxic selenium levels in the food chain of animals and human beings. Greenhouse and field experiments were conducted to study selenium concentrations of weeds, forages and cereals grown on seleniferous soils located between 31.0417 degrees to 31.2175 degrees N and 76.1363 degrees to 76.4147 degrees E in northwestern India. Eleven winter season (November-April) weed plants were grown in the greenhouse in a soil treated with different levels of selenate-Se. Selenium concentrations of weed plants increased progressively with the levels of selenate-Se in soil. The highest Se concentration was recorded by Silene gallica (246 mgkg(-1)) and the lowest by Avena ludoviciana (47 mgkg(-1)) at 2.5 mg Sekg(-1) soil. A.ludoviciana and Spergula arvensis proved highly tolerant to the presence of 1.25 and 2.5 mg selenate-Sekg(-1) soil and the remaining weeds were sensitive to Se. Dry matter yield of Se-sensitive weed plants was 25 to 62% of the yield in the no-Se control at 1.25mg selenate-Sekg(-1) and 6 to 40% at 2.5mg selenate-Sekg(-1) soil. Other symptoms like change in leaf colour and size, burning of leaf tips and margins, and delayed flowering were also observed due to Se. Dry matter yield of Se-sensitive weed plants expressed as percentage of yield in the no-Se control at both the Se levels was inversely correlated with their Se content (r=-0.731, p<0.01, N=17). Among the weed plants grown in seleniferous soils under field situations, Mentha longifolia accumulated the highest Se (365 mgkg(-1)) and Phalaris minor the lowest (34 mgkg(-1)). Among agricultural crops grown on a naturally contaminated soil in the greenhouse, Se concentrations were the highest for oilseed crops (19-29 mgkg(-1)), followed by legumes (6-13 mgkg(-1)) and cereals (2-18 mgkg(-1)). Helianthus annuus among the oilseed crops, A.ludoviciana among the winter season weeds, M.longifolia among the summer season (May-October) weeds and Cirsium arvense among the perennial weeds can be used for phytoremediation of seleniferous soils as these accumulate the highest amounts of Se.