Chemosphere 99 (2014) 56–63
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Development and mapping of seleniferous soils in northwestern India Karaj S. Dhillon, Surjit K. Dhillon ⇑ Department of Soil Science, Punjab Agricultural University, Ludhiana 141 004, Punjab, India
h i g h l i g h t s Periodic surveys to identify and characterize Se sources and contaminated soils. Se in soil, plant varied from 0.02-4.9, 0.6-515 mg/kg; present in profile 2 m depth. Developed map showing different categories of seleniferous soils in northwest India. Shiwalik rocks and underground water are the main sources of Se contamination. Se-rich sediments deposited by rain water lead to development of seleniferous soils.
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Article history: Received 7 May 2013 Received in revised form 30 August 2013 Accepted 19 September 2013 Available online 7 November 2013 Keywords: Seleniferous soils Distribution map Rocks Sediments Seasonal rivulets Shiwaliks
a b s t r a c t Periodic surveys were undertaken to identify and characterize Se-contaminated soils in northwestern India. Total Se content varied from 0.023 to 4.91 mg kg1 in 0–15 cm surface soil and 0.64–515.0 mg kg1 in samples of vegetation. Selenium-contaminated land occupying an area of 865 ha was classified into different categories based on total Se content of soils as moderately contaminated (0.5–2.0 mg Se kg1) and highly contaminated (>2.0 mg Se kg1). The normal soils contained 0.5 mg Se kg1 are,
in fact, associated with plant samples containing >5 mg Se kg1 and hence were designated as seleniferous soils. This critical level should, however, be used cautiously in differentiating seleniferous from nonseleniferous soils. The plant species differ greatly in their capacity to absorb Se from soil (Dhillon and Dhillon, 2009). In general, plants belonging to the Cruciferae family absorbed the largest amounts of Se, followed by those of the Leguminoseae and Gramineae families. In the present investigation, 88% of the plant samples belonged to 30–40 day-old wheat (T. aestivum) shoots and the rest belonged to fresh fully developed leaves of sugarcane (Sachharum officinarum). Only about 8% of the plant samples from the soils containing 2.0 mg Se kg1 as ‘highly toxic’. The limits for moderately toxic and highly toxic levels of Se in soil were fixed arbitrarily. The fields showing snow-white chlorosis on more than 50% of wheat plants (Fig. 1) were also included in the ‘highly toxic’ category. All the values for soil-Se were plotted on the unified Cadastral map and different categories were demarcated (Fig. 2). The seleniferous soils comprising 865 ha are sporadically distributed in a number of villages located in Hoshiarpur (300 ha) and Nawanshahar (565 ha) districts of the state of Punjab (Table 1). No significant differences were observed among the physical and chemical characteristics of soils in the seleniferous region. The soils are alkaline in reaction (pH 7.9–8.8) with normal electrical conductivity (0.3–0.7 dS m1), calcium carbonate (0.1–4.1%), organic carbon (0.4–1.0%), cation exchange capacity (2.6–36.7 cmol kg1), silty loam to silty clay loam in texture and moderately well drained. There are no major difference in the productivity potential of seleniferous and normal soils except that the farm produce obtained from contaminated soils is always rich in Se and is thus not fit for animal and human consumption. Selenium contents in contaminated soil and plants (Table 1) are several times higher than the minimum safe levels permissible in normal soils and plants. This is also true for the Se content in cereal grains (5–66 mg kg1), vegetables (1–51 mg kg1) and forages (4–41 mg kg1) produced in the contaminated region (Dhillon
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K.S. Dhillon, S.K. Dhillon / Chemosphere 99 (2014) 56–63 Table 1 Variation in Se content of plant and soil samples from contaminated land demarcated into different categories. Se concentration (mg kg1)
Soil category
Plants
Normal soil (0.5–2.0 mg Se kg1 soil) Highly toxic soil (>2.0 mg Se kg1 soil)
Seleniferous area (ha)
a
Soil
No. of samples
Range
Mean ± SD
No. of samples
Range
Mean ± SD
66 51 25
0.64–16.7 2.8–351.0 43.8–515.0
3.31 ± 3.59 61.32 ± 76.13 259.1 ± 152.1
65 60 49
0.023–0.47 0.53–1.95 2.05–4.91
0.31 ± 0.11 1.22 ± 0.45 2.75 ± 0.78
–b 602 263
a 88% of the plant samples belonged to 30–40 day-old wheat (Triticum aestivum) shoots and the rest belonged to fresh fully developed leaves of sugarcane (Sachharum officinarum). b Only 865 ha were affected by selenium toxicity problem in this region. The remaining area was found to be normal.
Fig. 2. Map showing the distribution of highly and moderately Se-toxic soils in the Punjab state of northwestern India.
and Dhillon, 1997). The greatest risk of Se exposure to domestic animals and humans living in the affected region appears to be by the possible consumption of forages and grains containing Se above the maximum permissible level of 4–5 mg kg1 (Underwood and Suttle, 1999). In fact, typical symptoms of Se poisoning resembling chronic selenosis (Rosenfeld and Beath, 1964) have been observed in both animals and humans due to inadvertent consumption of Se-rich farm produce (Dhillon and Dhillon, 1997). The farmers are experiencing substantial losses in cattle wealth due to Se poisoning. Thus it is quite obvious that the animals and humans consuming farm produce from the fields belonging to the highly toxic category will always be at higher risk as compared to others with adjoining fields. In addition to Se content, redox potential and pH of the soils determine the extent of toxicological or environmental risks due to Se. In the well aerated alkaline seleniferous soils of this region, selenate-Se is the dominant species (Dhillon and Dhillon, 2004) which is the most mobile and readily available for plant uptake. In semiarid western USA, Se toxicity problems are usually associated with alkaline soils where Se is present in the selenate form (Jump and Sabey, 1989). When alkaline soils are subjected to reduced conditions, the available fraction of Se is drastically reduced due to conversion of selenate to selenite species (Dhillon and Dhillon, 2004). Due to strong adsorption on clays and hydrous
oxides of Fe, selenite-Se becomes fairly insoluble and thus, largely unavailable to plants (Mikkelsen et al., 1989). There is also risk of spreading Se contamination to the normal soils situated on the periphery of the seleniferous region through moving water as a consequence of heavy rainfall and irrigation events. Studies under simulated rainfall conditions revealed that significant amounts of Se could be lost from seleniferous to the adjoining soils through run-off and drainage processes The losses however were controlled by soil properties like total and water soluble Se content, calcium carbonate and texture of the soils (Dhillon et al., 2008). Highly significant and positive relationship of total Se have been recorded with hot water soluble (available) Se (r = 0.83), CaCO3 content (r = 0.42), electrical conductivity (r = 0.39) and silt content (r = 0.31) of seleniferous soils (Dhillon et al., 1992). Although Se levels in soil and plants (Table 1) indicate the potential risk of Se toxicity problem in animals and humans, but no incidence of selenosis have been reported in the recent years from northwestern India. This suggests that either the local population may have adapted to the high Se intake or have substituted their diet with foods from outside the high Se areas. Selenium accumulation in plant parts may also affect the ecological interactions of plants. Results of several laboratory and field studies indicate that elevated Se concentrations in Se accumulators and hyperaccumulators are toxic to a wide variety of herbivores
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K.S. Dhillon, S.K. Dhillon / Chemosphere 99 (2014) 56–63
Fig. 3a. Distribution of total selenium in soil profiles representing different seleniferous sites.
Fig. 3b. Distribution of hot water soluble selenium (HWS-Se) in soil profiles representing different seleniferous sites.
Table 2 Physical and chemical characteristics of soil profiles representing seleniferous sites in northwestern India.
a
Soil profile parameters
Nazarpur
Simbly
Jainpur
Barwa I
Barwa II
Depth (cm) Total Se content (lg kg1)a Water-soluble Se (lg kg1)a pH Electrical conductivity (dS m1) Organic carbon (%) Calcium carbonate (%) Available P (mg kg1) Available S (mg kg1) Silt (%) Clay (%)
0–170 402–2485 11–71 7.7–8.6 0.15–0.28 0.41–0.65 0.50–0.87 35.8–102.0 5.1–20.3 44.2–65.8 11.2–36.0
0–195 1044–3247 9–98 8.2–8.7 0.14–0.25 0.34–0.63 0.50–9.92 39.0–84.3 11.8–18.9 45.7–53.9 23.6–41.0
0–155 294–1729 8–36 8.1–8.8 0.23–0.32 0.39–0.58 0.32–4.75 19.6–45.7 13.2–34.3 41.2–51.7 22.9–81.2
0–166 698–1432 10–31 8.3–8.8 0.16–0.23 0.33–0.53 0.30–2.65 30.4–40.3 14.6–23.3 48.6–52.0 26.2–30.4
0–163 507–2322 6–27 7.8–8.6 0.23–0.34 0.32–0.53 0.35–4.87 20.6–94.8 9.1–16.0 44.0–49.1 35.1–42.8
The highest values belong to surface layer in all the soil profiles.
and pathogens ranging from prairie dogs to a variety of arthropods and fungi (Fusarium and Alternaria) (Barillas et al., 2011). Hladun et al. (2012) observed significant reduction in productivity, longevity and survival rate of honey bees (Apis mellifera L.) foraging Brassicas growing on seleniferous soils. In Brassica juncea Se accumulation >0.05–0.1% leads to decrease in biomass, pollen
germination, individual seed and total seed weight, number of seeds produced, and seed germination (Prins et al., 2011). Soils with as high as 10 mg Se kg1 exist in the state of Haryana, India (Singh and Kumar, 1976); but no relationship has been reported between high Se levels and health of animal and humans. Only at one location near Karnal at village Chamar Khera,
242 ± 190 247 ± 211 11–847 46–644 25 9
a The seasonal rivulets locally known as choe emanating from nearby hills of Shiwalik range during rainy season helped in transporting flood-water loaded with Se-rich sediments and depositing in the plain area at their dead ends. b First four locations represent highly toxic soils located at the seasonal rivulet endings and the rest represent the soils located near the seasonal rivulet banks.
2341 ± 336
Mean ± SD
5
Cluster of rocks of lower Shiwaliks near Pojewal along the road from Grahshanker to Nurpur Bedi and at Basu Khad Upper Shiwalik hills along the road from Hoshiarpur to Gagret, Nurpur Bedi and Una Upper Shiwaliks along the Chandigarh–Basathu–Solan road
Range
Se content (lg Se kg1) Type of rocks No. of samples Location
Sedimentary rock samples collected from nearby hills of Shiwalik range
1864–2754
44.1 23.5 53.9 30.9 2.74 – – Brassica tops Triticum aestivum shoots Triticum aestivum shoots Triticum aestivum shoots Trifolium alexandrinum and Brassica tops mixture – – 4.38 2.75 4.21 3.63 0.86 1.83 1.94 Simbly Jainpur Barwa Mehindpur Rurki Khurd Achalpur Jhugian 0.59 0.76 0.57 0.76 – – – donax donax donax donax Arundo Arundo Arundo Arundo – – – 0.73 0.95 0.78 1.62 0.57 2.81 2.89 Simbly choe Jainpur choe Dhamai drain Garhi choe Malewal choe Mujhot choe Jhandupur choe
Claystone, Salt and pepper sandstone, yellowish brown sandstone, light grey sandstone Sandstone, shale, clay-stone, conglomerates, siltstone, limestone Sandstones, claystone, siltstones, wood coal
Se conc (mg kg1) Plant spp.
Plants
Se conc (mg kg1) Location Se conc (mg kg1) Plant spp. Se conc (mg kg1) Location
Samples from cultivated land near the seasonal rivulet
Soilb Plants Sediment
Samples from the seasonal rivulet bed
Table 3 Selenium content of seasonal rivuleta bed sediments, soil, plant and sedimentary rock samples collected from different locations in the seleniferous region.
K.S. Dhillon, S.K. Dhillon / Chemosphere 99 (2014) 56–63
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symptoms resembling Se toxicity were observed on some buffaloes (Bubalus bubalis) feeding on fodders containing 0.9–6.7 mg Se kg1 (Arora et al., 1975). In the sub-Himalayan region of West Bengal, Se content of soil from the contaminated pastures ranged from 1.45 to 2.25 mg kg1 (Ghosh et al., 1993). Selenium-rich soils have been identified in many parts of the world. In the contaminated region of Western United States, Se content of surface soils ranged from 1.5 to 20 mg kg1 (Anderson et al., 1961). In China, soils containing total Se > 3.0 mg kg1 and water-soluble Se > 0.02 mg kg1 are associated with Se poisoning and are located in Sangliao, Weihe and Hua Bei plains (Tan et al., 1994). Acute poisoning and chronic selenosis has been reported from the regions where Se content in soil ranged from 0.3 to 0.7 mg kg1 in Canada, 0.3 to 20 mg kg1 in Mexico, 1 to 14 mg kg1 in Columbia, 1.2 to 324.0 mg kg1 in Ireland and up to 6.0 mg kg1 in Israel (Rosenfeld and Beath, 1964). 3.2. Selenium in soil profiles At the contaminated sites (Fig. 2), Se was present in the soil profile up to 2 m depth, but its distribution in different layers of the soil profile did not follow any specific pattern (Figs. 3a and 3b). At all of the contaminated sites, surface soil layer was found to be rich in Se as it contained 1.5 to 6.0 times more Se in comparison to the lower layers. Among the surface soil layers at different sites, total Se content was the greatest at Simbly followed by that at Nazarpur, Barwa II, Jainpur and Barwa I sites. However the trend in hot water soluble Se was slightly different and was: Simbly > Nazarpur > Jainpur > Barwa I > Barwa II. Wheat plants exhibited toxicity symptoms of Se at all the contaminated sites except Barwa I. Lower total and water soluble Se content at Barwa I may be responsible for differential response. In twenty soil profiles examined in the seleniferous area of eastern Colorado, no apparent uniformity in Se distribution was observed with origin of soils, location and depths of soil profiles (Rosenfeld and Beath, 1964). As in case of surface soils (Dhillon et al., 1992), total Se content in the soil profile also exhibited significant coefficients of correlation with hot water soluble (available) Se (r = 0.80), organic carbon (r = 0.72), available P (r = 0.57) and available S (r = 0.24) content of the soil. The main factors influencing the biological availability of soil Se, in order of their importance are CaCO3, silt particles, organic matter and clay particles (Zhao et al., 2005). The ranges in physical and chemical characteristics of different soil layers are reported in Table 2. Among the various soil characteristics, the values of pH, organic carbon and available P was always the highest in surface layer, whereas the CaCO3 content was the highest in last layer of the soil profile except in Barwa II and Jainpur. A few Fe–Mn concretions of black colour were visible in the 2nd and 3rd layers of different soil profiles except at Barwa I and Barwa II sites. On the basis of physico-chemical characteristics of surface and profile samples, it may be concluded that the seleniferous soils identified in northwestern India belong to Order – Inceptisol, Great group – Haplustept and Subgroup – Fluventic. 3.3. Sources of Se contamination 3.3.1. Deposition of Se-containing sediments Location of Se contaminated sites at the dead ends of seasonal rivulets provided a valid reason to believe that Se-rich sediments could have been transported through these rivulets along with flood-waters. During rainy season, these rivulets emanate from the nearby hills of Shiwalik range and become extinct in the plane area. Although majority of these are not visible now, some portions were relocated with the help of toposheets as well as discussion with local revenue officials and farmers, The data presented in Table 3 indicate that all the sediment samples contained high levels of Se and it was 1.2 to 5.8 times more than the minimum
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K.S. Dhillon, S.K. Dhillon / Chemosphere 99 (2014) 56–63
Table 4 Net Se balance in the soil under different cropping sequences irrigated with underground water in the seleniferous region in northwestern India. Parameters 1
Amount of underground water required for irrigation up to maturity (cm ha ) Range in Se concentration of underground water at the seleniferous sites (lg Se L1) Total Se addition through irrigation water (g ha1 y1) Total Se removal through harvested biomass (g ha1 y1) Net Se balance in soil (g ha1 y1)
permissible level (0.5 mg Se kg1) for the normal soils (Table 1). Presence of Se in sediments suggests that Se-rich sediments were being transported through these channels. Moreover, the soils located at the dead ends of channels contained significantly more amount of Se (1.7–7.7 times) than recorded in the bed sediments. Deposition of Se containing sediments repeatedly at the same site may have resulted in building up Se level in soil. Selenium content of surface soils is several times more than lower layers of soil profile (Fig 3a). The area remained uncultivated for a long time and thus Se absorbed by natural vegetation from lower layers of soil got deposited in the surface layer following countless cycles of growth and death. This fact may be partly responsible for higher concentration of Se in the surface soil. The nature of the parent material of seleniferous soils is not adequately known. It could be related to rocks in upper Shiwalik range (Karunakaran and Rao, 1979). Majority of the rock samples collected from upper and lower Shiwaliks in the present investigation comprised of sandstones and a few shales (Table 3). Both the rocks are highly permeable and may contain more than 80–90% of the total Se in water soluble form (Rosenfeld and Beath, 1964). In comparison to the rock samples from upper Shiwaliks, the average Se content of sedimentary rocks from the lower Shiwaliks (i.e. near to the starting point of rivulets) was about 10 times higher (Table 3). Selenium being highly soluble, the rain-water helped in depleting Se from rocks and the solubilized Se was transported along with alluvial sediments and deposited at the channel endings. This process of transportation and deposition of Se-rich sediments continued for a long period before the soils were brought under cultivation. Obviously the sedimentary rocks were even richer in Se initially than at the present time. The process of decomposition of parent rocks by water and its subsequent transport by ground or surface water plays an important role in the development of seleniferous soils (Rosenfeld and Beath, 1964). While reviewing the distribution of rare metals like Se in sedimentary rocks, Krauskopf (1955) has observed that the processes responsible for enrichment of Se in geological materials are: mechanical enrichment, precipitation, adsorption, substitution and presence of organic material in the deposits. The Sakesar Limestone formation of Eocene age rich in shales and fossils (median Se concentration of 7.2 mg kg1) has been identified as the source of Se in saline lakes of Soan–Sakesar Valley, Salt Range, Pakistan (Afzal, 1999). Significant Se losses through drainage and run-off (water + sediment) from seleniferous soils have also been observed in experiments conducted under simulated rainfall (250–260 mm) with intensities ranging from 56 to 120 mm h1 (Dhillon et al., 2008). The results suggest that due to heavy irrigation or rainfall, Se lost through drainage and run-off from seleniferous soils (materials) on higher slopes may reach sub-surface soil as well as adjoining nonseleniferous areas. Vertical displacement of dissolved Se due to rainfall infiltration and evaporation has been reported by Zawislanski et al. (1992). In Se-contaminated regions of China, the leaching conditions controlled by microtopography features are mainly responsible for the distribution and redistribution of Se in the soil–plant system (Zhu and Zheng, 2001). 3.3.2. Underground water Selenium is also being continuously deposited in the soils through underground water – the only source of water available
Maize–wheat sequence
Rice–wheat sequence
60 2.5–69.5 19–525 138–520 119–+5
200 2.5–69.5 56–1417 203–536 147–+881
for irrigation and drinking purposes in the affected region. Selenium concentration ranged from 0.25 to 69.5 lg L1 (av. 4.7 lg L1) in underground water drawn from a depth varying from 24 to 36 m (Dhillon and Dhillon, 2003). However, the water samples collected specifically from the contaminated sites contained 2.5 to 69.5 lg Se L1 (av. 24.6 lg Se L1). As per water quality guidelines (NAS-NAE, 1973), 90% of water samples contained Se in the safe range, 11% were not fit for drinking purposes and only 5% of the samples were unfit for irrigation of crops. Addition of Se through underground water was computed for the copping sequences being practiced by the farmers in the affected region (Table 4). Selenium inputs were assessed by multiplying total amount of water applied for raising crops to maturity and the concentration of Se in underground water. Removal of Se by different crops was calculated by multiplying the average yield and concentration of Se in biomass. As a consequence of higher water requirement of rice, the addition of Se through irrigation was 2–3 times more in case of rice–wheat as compared to maize-wheat system (Table 4). By raising rice–wheat system with water having higher Se concentration, Se addition into the soil through irrigation far outweighed the Se removal by harvested plant biomass. Positive Se balance in the soil observed under rice–wheat system implies that cultivation of rice in the seleniferous region is further aggravating the Se toxicity problem. If cultivation of rice–wheat sequence is continued for a decade at the same site using irrigation water containing 69.5 lg Se L1, net addition of 8810 g Se ha1 may obviously lead to the development of a highly contaminated soil. A similar situation at Kesterson reservoir has been described by Ohlendorf and Santolo (1994). The reservoir spread over 500 ha was constructed in the San Joaquin Valley for storage of agricultural drainage water and further redistribution for irrigation purposes. Unfortunately, the storage of drainage water resulted in the deposition of 9000 kg Se just within a period of five years and it proved highly toxic for aquatic wildlife species. Human activities have played an important role in the distribution, transport and bioavailability of Se in the seleniferous soils developed in Enshi county of Hubei province in China (Zhu et al., 2008). References Afzal, S., 1999. Determination of selenium speciation of saline lakes from Soan– Sakesar Valley Salt Range, Pakistan. Ph. D. Thesis, Institute of Chemistry, University of Punjab, Lahore, Pakistan. Anderson, M.S., Lakin, H.W., Beeson, K.C., Smith, F.F., Thacker, E., 1961. Selenium in Agriculture, USDA Handbook 200. U.S. Government Printing Office, Washington, DC. Arora, S.P., Kaur, P., Khirwar, S.S., Chopra, R.C., Ludri, R.C., 1975. Selenium levels in fodders and its relationship with Degnala disease. Indian J. Dairy Sci. 28, 246– 253. Barillas, J.R.V., Quinn, C.F., Pilon-Smits, E.A.H., 2011. Selenium accumulation in plants – Phytotechnological applications and ecological implications. Int. J. Phytorem. 13 (Suppl 1), 166–178. Chesnin, L., Yien, C.H., 1950. Turbidimetric determination of available sulphates. Soil Sci. Soc. Amer. Proc. 15, 149–151. Cummins, L.M., Martin, J.L., Maag, G.W., Maag, D.D., 1964. A rapid method for the determination of selenium in biological material. Anal. Chem. 36, 382–384. Day, P.R., 1965. Particle fractionation and particle size analysis. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 1. American Society of Agronomy, Madison, WI. Dhillon, K.S., Dhillon, S.K., 1997. Distribution of seleniferous soils in north–west India and associated toxicity problems in the soil–plant–animal–human continuum. Land Contam. Reclam. 5, 313–322.
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Development and mapping of seleniferous soils in northwestern India Karaj S. Dhillon, Surjit K. Dhillon ⇑ Department of Soil Science, Punjab Agricultural University, Ludhiana 141 004, Punjab, India
h i g h l i g h t s Periodic surveys to identify and characterize Se sources and contaminated soils. Se in soil, plant varied from 0.02-4.9, 0.6-515 mg/kg; present in profile 2 m depth. Developed map showing different categories of seleniferous soils in northwest India. Shiwalik rocks and underground water are the main sources of Se contamination. Se-rich sediments deposited by rain water lead to development of seleniferous soils.
a r t i c l e
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Article history: Received 7 May 2013 Received in revised form 30 August 2013 Accepted 19 September 2013 Available online 7 November 2013 Keywords: Seleniferous soils Distribution map Rocks Sediments Seasonal rivulets Shiwaliks
a b s t r a c t Periodic surveys were undertaken to identify and characterize Se-contaminated soils in northwestern India. Total Se content varied from 0.023 to 4.91 mg kg1 in 0–15 cm surface soil and 0.64–515.0 mg kg1 in samples of vegetation. Selenium-contaminated land occupying an area of 865 ha was classified into different categories based on total Se content of soils as moderately contaminated (0.5–2.0 mg Se kg1) and highly contaminated (>2.0 mg Se kg1). The normal soils contained 0.5 mg Se kg1 are,
in fact, associated with plant samples containing >5 mg Se kg1 and hence were designated as seleniferous soils. This critical level should, however, be used cautiously in differentiating seleniferous from nonseleniferous soils. The plant species differ greatly in their capacity to absorb Se from soil (Dhillon and Dhillon, 2009). In general, plants belonging to the Cruciferae family absorbed the largest amounts of Se, followed by those of the Leguminoseae and Gramineae families. In the present investigation, 88% of the plant samples belonged to 30–40 day-old wheat (T. aestivum) shoots and the rest belonged to fresh fully developed leaves of sugarcane (Sachharum officinarum). Only about 8% of the plant samples from the soils containing 2.0 mg Se kg1 as ‘highly toxic’. The limits for moderately toxic and highly toxic levels of Se in soil were fixed arbitrarily. The fields showing snow-white chlorosis on more than 50% of wheat plants (Fig. 1) were also included in the ‘highly toxic’ category. All the values for soil-Se were plotted on the unified Cadastral map and different categories were demarcated (Fig. 2). The seleniferous soils comprising 865 ha are sporadically distributed in a number of villages located in Hoshiarpur (300 ha) and Nawanshahar (565 ha) districts of the state of Punjab (Table 1). No significant differences were observed among the physical and chemical characteristics of soils in the seleniferous region. The soils are alkaline in reaction (pH 7.9–8.8) with normal electrical conductivity (0.3–0.7 dS m1), calcium carbonate (0.1–4.1%), organic carbon (0.4–1.0%), cation exchange capacity (2.6–36.7 cmol kg1), silty loam to silty clay loam in texture and moderately well drained. There are no major difference in the productivity potential of seleniferous and normal soils except that the farm produce obtained from contaminated soils is always rich in Se and is thus not fit for animal and human consumption. Selenium contents in contaminated soil and plants (Table 1) are several times higher than the minimum safe levels permissible in normal soils and plants. This is also true for the Se content in cereal grains (5–66 mg kg1), vegetables (1–51 mg kg1) and forages (4–41 mg kg1) produced in the contaminated region (Dhillon
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K.S. Dhillon, S.K. Dhillon / Chemosphere 99 (2014) 56–63 Table 1 Variation in Se content of plant and soil samples from contaminated land demarcated into different categories. Se concentration (mg kg1)
Soil category
Plants
Normal soil (0.5–2.0 mg Se kg1 soil) Highly toxic soil (>2.0 mg Se kg1 soil)
Seleniferous area (ha)
a
Soil
No. of samples
Range
Mean ± SD
No. of samples
Range
Mean ± SD
66 51 25
0.64–16.7 2.8–351.0 43.8–515.0
3.31 ± 3.59 61.32 ± 76.13 259.1 ± 152.1
65 60 49
0.023–0.47 0.53–1.95 2.05–4.91
0.31 ± 0.11 1.22 ± 0.45 2.75 ± 0.78
–b 602 263
a 88% of the plant samples belonged to 30–40 day-old wheat (Triticum aestivum) shoots and the rest belonged to fresh fully developed leaves of sugarcane (Sachharum officinarum). b Only 865 ha were affected by selenium toxicity problem in this region. The remaining area was found to be normal.
Fig. 2. Map showing the distribution of highly and moderately Se-toxic soils in the Punjab state of northwestern India.
and Dhillon, 1997). The greatest risk of Se exposure to domestic animals and humans living in the affected region appears to be by the possible consumption of forages and grains containing Se above the maximum permissible level of 4–5 mg kg1 (Underwood and Suttle, 1999). In fact, typical symptoms of Se poisoning resembling chronic selenosis (Rosenfeld and Beath, 1964) have been observed in both animals and humans due to inadvertent consumption of Se-rich farm produce (Dhillon and Dhillon, 1997). The farmers are experiencing substantial losses in cattle wealth due to Se poisoning. Thus it is quite obvious that the animals and humans consuming farm produce from the fields belonging to the highly toxic category will always be at higher risk as compared to others with adjoining fields. In addition to Se content, redox potential and pH of the soils determine the extent of toxicological or environmental risks due to Se. In the well aerated alkaline seleniferous soils of this region, selenate-Se is the dominant species (Dhillon and Dhillon, 2004) which is the most mobile and readily available for plant uptake. In semiarid western USA, Se toxicity problems are usually associated with alkaline soils where Se is present in the selenate form (Jump and Sabey, 1989). When alkaline soils are subjected to reduced conditions, the available fraction of Se is drastically reduced due to conversion of selenate to selenite species (Dhillon and Dhillon, 2004). Due to strong adsorption on clays and hydrous
oxides of Fe, selenite-Se becomes fairly insoluble and thus, largely unavailable to plants (Mikkelsen et al., 1989). There is also risk of spreading Se contamination to the normal soils situated on the periphery of the seleniferous region through moving water as a consequence of heavy rainfall and irrigation events. Studies under simulated rainfall conditions revealed that significant amounts of Se could be lost from seleniferous to the adjoining soils through run-off and drainage processes The losses however were controlled by soil properties like total and water soluble Se content, calcium carbonate and texture of the soils (Dhillon et al., 2008). Highly significant and positive relationship of total Se have been recorded with hot water soluble (available) Se (r = 0.83), CaCO3 content (r = 0.42), electrical conductivity (r = 0.39) and silt content (r = 0.31) of seleniferous soils (Dhillon et al., 1992). Although Se levels in soil and plants (Table 1) indicate the potential risk of Se toxicity problem in animals and humans, but no incidence of selenosis have been reported in the recent years from northwestern India. This suggests that either the local population may have adapted to the high Se intake or have substituted their diet with foods from outside the high Se areas. Selenium accumulation in plant parts may also affect the ecological interactions of plants. Results of several laboratory and field studies indicate that elevated Se concentrations in Se accumulators and hyperaccumulators are toxic to a wide variety of herbivores
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K.S. Dhillon, S.K. Dhillon / Chemosphere 99 (2014) 56–63
Fig. 3a. Distribution of total selenium in soil profiles representing different seleniferous sites.
Fig. 3b. Distribution of hot water soluble selenium (HWS-Se) in soil profiles representing different seleniferous sites.
Table 2 Physical and chemical characteristics of soil profiles representing seleniferous sites in northwestern India.
a
Soil profile parameters
Nazarpur
Simbly
Jainpur
Barwa I
Barwa II
Depth (cm) Total Se content (lg kg1)a Water-soluble Se (lg kg1)a pH Electrical conductivity (dS m1) Organic carbon (%) Calcium carbonate (%) Available P (mg kg1) Available S (mg kg1) Silt (%) Clay (%)
0–170 402–2485 11–71 7.7–8.6 0.15–0.28 0.41–0.65 0.50–0.87 35.8–102.0 5.1–20.3 44.2–65.8 11.2–36.0
0–195 1044–3247 9–98 8.2–8.7 0.14–0.25 0.34–0.63 0.50–9.92 39.0–84.3 11.8–18.9 45.7–53.9 23.6–41.0
0–155 294–1729 8–36 8.1–8.8 0.23–0.32 0.39–0.58 0.32–4.75 19.6–45.7 13.2–34.3 41.2–51.7 22.9–81.2
0–166 698–1432 10–31 8.3–8.8 0.16–0.23 0.33–0.53 0.30–2.65 30.4–40.3 14.6–23.3 48.6–52.0 26.2–30.4
0–163 507–2322 6–27 7.8–8.6 0.23–0.34 0.32–0.53 0.35–4.87 20.6–94.8 9.1–16.0 44.0–49.1 35.1–42.8
The highest values belong to surface layer in all the soil profiles.
and pathogens ranging from prairie dogs to a variety of arthropods and fungi (Fusarium and Alternaria) (Barillas et al., 2011). Hladun et al. (2012) observed significant reduction in productivity, longevity and survival rate of honey bees (Apis mellifera L.) foraging Brassicas growing on seleniferous soils. In Brassica juncea Se accumulation >0.05–0.1% leads to decrease in biomass, pollen
germination, individual seed and total seed weight, number of seeds produced, and seed germination (Prins et al., 2011). Soils with as high as 10 mg Se kg1 exist in the state of Haryana, India (Singh and Kumar, 1976); but no relationship has been reported between high Se levels and health of animal and humans. Only at one location near Karnal at village Chamar Khera,
242 ± 190 247 ± 211 11–847 46–644 25 9
a The seasonal rivulets locally known as choe emanating from nearby hills of Shiwalik range during rainy season helped in transporting flood-water loaded with Se-rich sediments and depositing in the plain area at their dead ends. b First four locations represent highly toxic soils located at the seasonal rivulet endings and the rest represent the soils located near the seasonal rivulet banks.
2341 ± 336
Mean ± SD
5
Cluster of rocks of lower Shiwaliks near Pojewal along the road from Grahshanker to Nurpur Bedi and at Basu Khad Upper Shiwalik hills along the road from Hoshiarpur to Gagret, Nurpur Bedi and Una Upper Shiwaliks along the Chandigarh–Basathu–Solan road
Range
Se content (lg Se kg1) Type of rocks No. of samples Location
Sedimentary rock samples collected from nearby hills of Shiwalik range
1864–2754
44.1 23.5 53.9 30.9 2.74 – – Brassica tops Triticum aestivum shoots Triticum aestivum shoots Triticum aestivum shoots Trifolium alexandrinum and Brassica tops mixture – – 4.38 2.75 4.21 3.63 0.86 1.83 1.94 Simbly Jainpur Barwa Mehindpur Rurki Khurd Achalpur Jhugian 0.59 0.76 0.57 0.76 – – – donax donax donax donax Arundo Arundo Arundo Arundo – – – 0.73 0.95 0.78 1.62 0.57 2.81 2.89 Simbly choe Jainpur choe Dhamai drain Garhi choe Malewal choe Mujhot choe Jhandupur choe
Claystone, Salt and pepper sandstone, yellowish brown sandstone, light grey sandstone Sandstone, shale, clay-stone, conglomerates, siltstone, limestone Sandstones, claystone, siltstones, wood coal
Se conc (mg kg1) Plant spp.
Plants
Se conc (mg kg1) Location Se conc (mg kg1) Plant spp. Se conc (mg kg1) Location
Samples from cultivated land near the seasonal rivulet
Soilb Plants Sediment
Samples from the seasonal rivulet bed
Table 3 Selenium content of seasonal rivuleta bed sediments, soil, plant and sedimentary rock samples collected from different locations in the seleniferous region.
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symptoms resembling Se toxicity were observed on some buffaloes (Bubalus bubalis) feeding on fodders containing 0.9–6.7 mg Se kg1 (Arora et al., 1975). In the sub-Himalayan region of West Bengal, Se content of soil from the contaminated pastures ranged from 1.45 to 2.25 mg kg1 (Ghosh et al., 1993). Selenium-rich soils have been identified in many parts of the world. In the contaminated region of Western United States, Se content of surface soils ranged from 1.5 to 20 mg kg1 (Anderson et al., 1961). In China, soils containing total Se > 3.0 mg kg1 and water-soluble Se > 0.02 mg kg1 are associated with Se poisoning and are located in Sangliao, Weihe and Hua Bei plains (Tan et al., 1994). Acute poisoning and chronic selenosis has been reported from the regions where Se content in soil ranged from 0.3 to 0.7 mg kg1 in Canada, 0.3 to 20 mg kg1 in Mexico, 1 to 14 mg kg1 in Columbia, 1.2 to 324.0 mg kg1 in Ireland and up to 6.0 mg kg1 in Israel (Rosenfeld and Beath, 1964). 3.2. Selenium in soil profiles At the contaminated sites (Fig. 2), Se was present in the soil profile up to 2 m depth, but its distribution in different layers of the soil profile did not follow any specific pattern (Figs. 3a and 3b). At all of the contaminated sites, surface soil layer was found to be rich in Se as it contained 1.5 to 6.0 times more Se in comparison to the lower layers. Among the surface soil layers at different sites, total Se content was the greatest at Simbly followed by that at Nazarpur, Barwa II, Jainpur and Barwa I sites. However the trend in hot water soluble Se was slightly different and was: Simbly > Nazarpur > Jainpur > Barwa I > Barwa II. Wheat plants exhibited toxicity symptoms of Se at all the contaminated sites except Barwa I. Lower total and water soluble Se content at Barwa I may be responsible for differential response. In twenty soil profiles examined in the seleniferous area of eastern Colorado, no apparent uniformity in Se distribution was observed with origin of soils, location and depths of soil profiles (Rosenfeld and Beath, 1964). As in case of surface soils (Dhillon et al., 1992), total Se content in the soil profile also exhibited significant coefficients of correlation with hot water soluble (available) Se (r = 0.80), organic carbon (r = 0.72), available P (r = 0.57) and available S (r = 0.24) content of the soil. The main factors influencing the biological availability of soil Se, in order of their importance are CaCO3, silt particles, organic matter and clay particles (Zhao et al., 2005). The ranges in physical and chemical characteristics of different soil layers are reported in Table 2. Among the various soil characteristics, the values of pH, organic carbon and available P was always the highest in surface layer, whereas the CaCO3 content was the highest in last layer of the soil profile except in Barwa II and Jainpur. A few Fe–Mn concretions of black colour were visible in the 2nd and 3rd layers of different soil profiles except at Barwa I and Barwa II sites. On the basis of physico-chemical characteristics of surface and profile samples, it may be concluded that the seleniferous soils identified in northwestern India belong to Order – Inceptisol, Great group – Haplustept and Subgroup – Fluventic. 3.3. Sources of Se contamination 3.3.1. Deposition of Se-containing sediments Location of Se contaminated sites at the dead ends of seasonal rivulets provided a valid reason to believe that Se-rich sediments could have been transported through these rivulets along with flood-waters. During rainy season, these rivulets emanate from the nearby hills of Shiwalik range and become extinct in the plane area. Although majority of these are not visible now, some portions were relocated with the help of toposheets as well as discussion with local revenue officials and farmers, The data presented in Table 3 indicate that all the sediment samples contained high levels of Se and it was 1.2 to 5.8 times more than the minimum
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Table 4 Net Se balance in the soil under different cropping sequences irrigated with underground water in the seleniferous region in northwestern India. Parameters 1
Amount of underground water required for irrigation up to maturity (cm ha ) Range in Se concentration of underground water at the seleniferous sites (lg Se L1) Total Se addition through irrigation water (g ha1 y1) Total Se removal through harvested biomass (g ha1 y1) Net Se balance in soil (g ha1 y1)
permissible level (0.5 mg Se kg1) for the normal soils (Table 1). Presence of Se in sediments suggests that Se-rich sediments were being transported through these channels. Moreover, the soils located at the dead ends of channels contained significantly more amount of Se (1.7–7.7 times) than recorded in the bed sediments. Deposition of Se containing sediments repeatedly at the same site may have resulted in building up Se level in soil. Selenium content of surface soils is several times more than lower layers of soil profile (Fig 3a). The area remained uncultivated for a long time and thus Se absorbed by natural vegetation from lower layers of soil got deposited in the surface layer following countless cycles of growth and death. This fact may be partly responsible for higher concentration of Se in the surface soil. The nature of the parent material of seleniferous soils is not adequately known. It could be related to rocks in upper Shiwalik range (Karunakaran and Rao, 1979). Majority of the rock samples collected from upper and lower Shiwaliks in the present investigation comprised of sandstones and a few shales (Table 3). Both the rocks are highly permeable and may contain more than 80–90% of the total Se in water soluble form (Rosenfeld and Beath, 1964). In comparison to the rock samples from upper Shiwaliks, the average Se content of sedimentary rocks from the lower Shiwaliks (i.e. near to the starting point of rivulets) was about 10 times higher (Table 3). Selenium being highly soluble, the rain-water helped in depleting Se from rocks and the solubilized Se was transported along with alluvial sediments and deposited at the channel endings. This process of transportation and deposition of Se-rich sediments continued for a long period before the soils were brought under cultivation. Obviously the sedimentary rocks were even richer in Se initially than at the present time. The process of decomposition of parent rocks by water and its subsequent transport by ground or surface water plays an important role in the development of seleniferous soils (Rosenfeld and Beath, 1964). While reviewing the distribution of rare metals like Se in sedimentary rocks, Krauskopf (1955) has observed that the processes responsible for enrichment of Se in geological materials are: mechanical enrichment, precipitation, adsorption, substitution and presence of organic material in the deposits. The Sakesar Limestone formation of Eocene age rich in shales and fossils (median Se concentration of 7.2 mg kg1) has been identified as the source of Se in saline lakes of Soan–Sakesar Valley, Salt Range, Pakistan (Afzal, 1999). Significant Se losses through drainage and run-off (water + sediment) from seleniferous soils have also been observed in experiments conducted under simulated rainfall (250–260 mm) with intensities ranging from 56 to 120 mm h1 (Dhillon et al., 2008). The results suggest that due to heavy irrigation or rainfall, Se lost through drainage and run-off from seleniferous soils (materials) on higher slopes may reach sub-surface soil as well as adjoining nonseleniferous areas. Vertical displacement of dissolved Se due to rainfall infiltration and evaporation has been reported by Zawislanski et al. (1992). In Se-contaminated regions of China, the leaching conditions controlled by microtopography features are mainly responsible for the distribution and redistribution of Se in the soil–plant system (Zhu and Zheng, 2001). 3.3.2. Underground water Selenium is also being continuously deposited in the soils through underground water – the only source of water available
Maize–wheat sequence
Rice–wheat sequence
60 2.5–69.5 19–525 138–520 119–+5
200 2.5–69.5 56–1417 203–536 147–+881
for irrigation and drinking purposes in the affected region. Selenium concentration ranged from 0.25 to 69.5 lg L1 (av. 4.7 lg L1) in underground water drawn from a depth varying from 24 to 36 m (Dhillon and Dhillon, 2003). However, the water samples collected specifically from the contaminated sites contained 2.5 to 69.5 lg Se L1 (av. 24.6 lg Se L1). As per water quality guidelines (NAS-NAE, 1973), 90% of water samples contained Se in the safe range, 11% were not fit for drinking purposes and only 5% of the samples were unfit for irrigation of crops. Addition of Se through underground water was computed for the copping sequences being practiced by the farmers in the affected region (Table 4). Selenium inputs were assessed by multiplying total amount of water applied for raising crops to maturity and the concentration of Se in underground water. Removal of Se by different crops was calculated by multiplying the average yield and concentration of Se in biomass. As a consequence of higher water requirement of rice, the addition of Se through irrigation was 2–3 times more in case of rice–wheat as compared to maize-wheat system (Table 4). By raising rice–wheat system with water having higher Se concentration, Se addition into the soil through irrigation far outweighed the Se removal by harvested plant biomass. Positive Se balance in the soil observed under rice–wheat system implies that cultivation of rice in the seleniferous region is further aggravating the Se toxicity problem. If cultivation of rice–wheat sequence is continued for a decade at the same site using irrigation water containing 69.5 lg Se L1, net addition of 8810 g Se ha1 may obviously lead to the development of a highly contaminated soil. A similar situation at Kesterson reservoir has been described by Ohlendorf and Santolo (1994). The reservoir spread over 500 ha was constructed in the San Joaquin Valley for storage of agricultural drainage water and further redistribution for irrigation purposes. Unfortunately, the storage of drainage water resulted in the deposition of 9000 kg Se just within a period of five years and it proved highly toxic for aquatic wildlife species. Human activities have played an important role in the distribution, transport and bioavailability of Se in the seleniferous soils developed in Enshi county of Hubei province in China (Zhu et al., 2008). References Afzal, S., 1999. Determination of selenium speciation of saline lakes from Soan– Sakesar Valley Salt Range, Pakistan. Ph. D. Thesis, Institute of Chemistry, University of Punjab, Lahore, Pakistan. Anderson, M.S., Lakin, H.W., Beeson, K.C., Smith, F.F., Thacker, E., 1961. Selenium in Agriculture, USDA Handbook 200. U.S. Government Printing Office, Washington, DC. Arora, S.P., Kaur, P., Khirwar, S.S., Chopra, R.C., Ludri, R.C., 1975. Selenium levels in fodders and its relationship with Degnala disease. Indian J. Dairy Sci. 28, 246– 253. Barillas, J.R.V., Quinn, C.F., Pilon-Smits, E.A.H., 2011. Selenium accumulation in plants – Phytotechnological applications and ecological implications. Int. J. Phytorem. 13 (Suppl 1), 166–178. Chesnin, L., Yien, C.H., 1950. Turbidimetric determination of available sulphates. Soil Sci. Soc. Amer. Proc. 15, 149–151. Cummins, L.M., Martin, J.L., Maag, G.W., Maag, D.D., 1964. A rapid method for the determination of selenium in biological material. Anal. Chem. 36, 382–384. Day, P.R., 1965. Particle fractionation and particle size analysis. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 1. American Society of Agronomy, Madison, WI. Dhillon, K.S., Dhillon, S.K., 1997. Distribution of seleniferous soils in north–west India and associated toxicity problems in the soil–plant–animal–human continuum. Land Contam. Reclam. 5, 313–322.
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