MenuISSN (0970-2083)
ISSN (0970-2083)
K. Usha Rani*, K.L. Sharma, K. Nagasri, D. Suma Chandrika, V.L. Savithri and Munna Lal
Central Research Institute For Dryland Agriculture, Hyderabad, Andhra Pradesh, India
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The study was conducted to investigate the extent of contamination due to accumulation of trace and heavy metals in the soil and leafy vegetables by continuous use of sewage water for irrigation purpose under Musi River Basin of Hyderabad, in India. The sewage water contaminated with the effluents of industrial units was used for irrigating the leafy vegetables, which in turn contaminates the surface and ground water and food chain. Some of the analyzed values for heavy and trace metals were found to be higher than the safe limits suggested by Food and Agricultural Organization. The mean Zn contents of sewage and ground water treated soils were 1.95 and 1.74, 1.27 and 1.20 mg kg-1 in surface and subsurface soils respectively. Whereas, in control soils, the corresponding values were 1.18 and 1.09 mg kg-1 respectively. The present study also established that the mean contents of heavy and trace metals especially Pb, Ni and Cd in sewage and ground water treated soils were higher compared to the control (un irrigated soils). Similarly, their contents in leafy vegetables grown with sewage water were higher than those grown with ground water and no irrigation (control). The mean concentrations of Cadmium (Cd) in plant samples of Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass under sewage irrigated condition were 2.08, 2.61, 2.28, 0.93, 2.11 and 1.80 mg kg-1 respectively. However, with ground water irrigation, the corresponding values were 1.31, 1.53, 1.06, 0.53, 1.34 and 1.22 mg kg-1 which were relatively lower. Under no irrigation (control), the corresponding values were 0.31, 0.15, 0.15, 0.17, 0.11 and 0.20 mg kg-1 which were quite lower. The plant concentration of heavy metals correlated well with their respective concentration in sewage water and soil. Thus, the information presented in this paper will be useful to future researchers, periurban consumers of leafy vegetable products, environmentalists and others stake holders.
Sewage irrigation, Micronutrients and Heavy metals contents in soil, Leafy vegetables and grasses
With the increase in civilization, the industrialization, population and urbanization have also increased rapidly.
Though industrialization and development in agriculture are necessary to meet the basic requirement of people, at the same time, it is necessary to protect the health of the people and to preserve the environment. Like many other countries in the world, in India, too, the environmental pollution has become a cause of concern at various levels. Due to lack of sewerage treatment plants, in most of the developing countries including India, untreated sewage effluents are released either on agricultural land for irrigation or disposed off in nearby water bodies (Singh & Chandel, 2006). Hyderabad, city in India, which is a dwelling place of about more than 6.5 million population and is a State capital of Andhra Pradesh, a potential agrarian States of India, is one of the fastest growing cities. Beside being a potential city to become the cyber capital of India, this city is witnessing a rapid expansion in Industries, coupled with high population growth rate (decadal rate 40%) accelerated due to migration of people from rural areas.
The city is facing increased urban environmental problems like inadequate provision of basic infrastructure such as water supply, inappropriate sewerage system etc.
The most important issues of concern include contamination of water bodies and lakes due to improper management of domestic and industrial effluents disposed off to the Musi River passing through this city, which now has become non functional slow flowing stagnated water body. The untreated sewage water is used for irrigating the periurban agricultural lands for growing the leafy vegetables and grasses. The vegetables thus, grown are being consumed by the farming community and also being supplied to the city based population. The grasses grown in the river basin are sold to the periurban dairy owners. Thus, the farmers in the river basin are earning good amount of money from these lands. As the effluents originate from various industries such as electroplating, cooking oil mills, lead extraction / battery units, pharmaceutical, leather, textile, soap and Jewelry industries, these are rich in trace elements or micronutrients like Fe , Mn , Zn, Cu etc and heavy metals such as Pb. Ni, Cr, Cd and other toxic pollutants.
Trace elements are elements found in minute quantities in the earth’s crust. The soil is the major source of trace elements for plants, and it is through food and feed crops that these elements are transferred to animals and humans (Welch et al., 1991). Soil parent material and soil physical conditions of water, aeration, drainage and other factors affect the chemical nature of the elements and their uptake by plants. Waste applications, fertilizers and other chemicals applied to soils and deposition by water and air add trace elements to the soil. Several of these elements are essential to plants, animals, and humans. Others may be toxic or have no effect. Further, many elements are essential at low levels but toxic or have no effect. Further, many elements are essential at low levels but toxic at high levels. The literature often refers to several of the trace elements as “heavy metals.” This term is derived from their position in the periodic table. Heavy metals are elements with a relatively high molecular weight (density > 5 g/cu.cm) and are comprised of 40 elements. When several of these elements are taken into the body, they can accumulate in specific body organs. The term includes the metals cadmium (Cd), chromium (Cr), copper (Cu), lead Pb), mercury (Hg), nickel (Ni), and zinc (Zn). Several of these such as Cd, Hg, and Pb are toxic to humans and animals; others such as barium and antimony are not. Yet others such as Cu, Ni and Zn can be phytotoxic.
When sewage water rich in effluents is added to agricultural lands, initially soil acts as filters for toxic chemicals and may adsorb and retain heavy metals from wastewater. But when the capacity of soils to retain toxic metals is reduced due to continuous loading of pollutants or changes in pH, soils can release heavy metals into groundwater or soil solution available for plant uptake. Kimberly and William, (1999) reported that the amount of heavy metals mobilized in a soil environment is a function of pH, clay content, organic matter content, cation exchange capacity and other soil properties making each soil unique in terms of pollution management with the exception of Mo, Se and As, heavy metal mobility decreases with increasing soil pH due to precipitation of hydroxides, carbonates or formation of insoluble organic complexes (Smith, 1996). Heavy metals are capable of forming insoluble complex compounds with soil organic matter and according to Sauve et al., (2000) solid-solution partitioning of Cd, Cu, Ni, Pb and Zn is dependent on soil solution pH, total metal content and soil organic matter.
Heavy metals contribute to environmental pollution because of their unique properties, mainly that they are non-biodegradable, non-thermodegradable and generally do not leach from the topsoil. Unlike petroleum hydrocarbons and litter that visibly build-up on soils, heavy metals can accumulate unnoticed to toxic concentrations (Bohn et al., 1985) that affect plant and animal life. The duration of contamination by heavy metals may be for hundreds or thousands of years, even after their addition to soils had been stopped. The time taken for Cd, Cu and Pb to reach half of their concentrations (half lives) in soil were found to be 15-1100, 310-1500 and 740-5900 years, respectively, depending on soil type and physiochemical parameters (Alloway and Ayres, 1993).
Metals added in small concentrations find specific adsorption sites in soil where they are retained very strongly, either on inorganic or organic colloids (Sauve et. al, 2000). Following addition to soil, organic loading of wastewater undergoes decomposition of CO2, low molecular weight soluble organic acids, residual organic matter and inorganic constituents (Boyd et al., 1980). Decomposition can also release heavy metals into soil solution. But, because of their low solubility and limited uptake by plants, heavy metals tend to accumulate in surface soil and become part of the soil matrix (McGrath et al., 1994). With repeated wastewater applications, heavy metals can accumulate in soil to toxic concentrations for plant growth (Chang et al., 1992).
Not all heavy metals in soil are results of human activity. Trace metals in soil originally arose from the net effects of geological and soil-forming processes of the elements (Kabata-Pendias and Adriano, 1995) and the concentration in soil is governed by the parent material, climate, topography and human activities, factors which are responsible for soil formation. Sandy soils from granite rocks normally contain lower concentrations of heavy metals than clay soils derived from mafic rocks (Ross, 1994). According to Alloway and Ayres (1993), heavy metals may enter the soil from agricultural related sources such as pesticides. Fertilizers, composts and manure, and sewage sludge. Before, it is used for irrigation in agricultural lands, it is absolutely necessary to give due consideration towards its environmentally negative and positive aspects. Thus, the wastewater may be an important source of water to mitigate the increased water demand and to increase the agricultural produce close to the cities. But the untreated waste water either directly from the sewer, or indirectly from the polluted river also poses serious health risks of which the hazard of intestinal nematode infections have been classified as the higher one (Shuval et. al, 1986).
Long-term irrigation with untreated industrial sewage effluents causes accumulation of high concentration of heavy metals in soil and subsequently in crop plants (especially leafy vegetables), which can be phytotoxic to plants and / or a health hazard to animals and humans. These heavy metals create several defects in human system. Lead (Pb) is neurotoxic and children are particularly vulnerable because of the rapidly developing nervous system. Lead (Pb) is know to inhibit the activity of three critical enzymes (5-amino laevulinate dehydratase (ALA-D), coproporphyrinogen oxidase (COPRO-O) and Ferro chelatase (Ferro-C) critical in haem synthesis, causing abnormal concentrations of haem precursors in blood and urine Small amount of Nickel is needed by the human body to produce red blood cells, however, in excess, it become mildly toxic. Short term over-exposure to nickel is not known to cause any health problem, can cause decreased body weight, heart and liver damage, and skin irritation. In human, long-term exposure to Cd is associated with renal disfunction. High exposure has been linked to lung cancer. Cadmium may also produce bone defects (Osteomalasia, Osteoporosis) in humans and animals. High concentration linked to increased blood pressure and effects on the myocardium in animals, although confirmed in human findings too.
Dubey et al., 2006 reported that sewage sludge and effluents are frequently disposed off on agricultural lands for irrigation / manure purposes and both of them aggravate the problems, because sewage sludge and effluents may contain high amount of heavy metals . Soils normally contain low background levels of heavy metals. However, in areas where agricultural, industrial or municipal wastes are land applied as fertilizer, concentrations may be much higher.
The area under the Musi river basin is severly effected by the heavy metal contamination. This River originates in Ranga Reddy district and contributes to the flow of drinking water reservoirs viz. Osman Sagar and Himayat Sagar, the latter through its tributary, the ‘Musa’ for the city of Hyderabad. Except during heavy rains, the river acts primarily as drainage for the urban domestic and industrial effluents of Hyderabad and its surrounding region. The sewage water contaminated with these effluents is used for irrigating the leafy vegetable crops, which in turn pollutes the surface, ground water and food chain. The information on the influence of sewage water on soil, water and food chain for this region is restricted to research stations only. Therefore, with this background, the present research work was undertaken in the farmers field with the objectives : (i) to study the extent and magnitude of micronutrients and heavy metal contamination in soil due to irrigation with sewage water over years (ii) to study the content of heavy metals in leafy vegetables and fodder crops raised under sewage irrigated conditions with reference to standard safe limits and (iii) to establish the relationship of soil content of micronutrients and heavy metals with their respective plant concentration.
Experimental site
The study area geographically falls between170 -19’ to 17°-30’ N latitude and 78° – 23’ to 78° –30’ E longitude. It has a typical semi-arid climatic conditions characterized by mild winters and moderate summers, with occasionally severe (summers) shows moderate relative humidity during the months of June to September. The average rainfall of Hyderabad region is 518 mm. This area represents semi-arid tropical agroclimatic region. Soil samples were collected from ten randomly selected villages outside the city located in Ghatkesar Mandal of Ranga Reddy district, which forms the eastern part of the peri-urban area of Hyderabad. Out of these villages , five villages viz. Peerjadiguda (PJG), Parvathapur (PP), Kachavanisingaram (KS), Pratapsingaram (PS) and Sadat Ali Guda (SAG) were sewage irrigated, four villages Muthawaliguda (MWG), Korremula (KM), Chowdarguda (CHG), Narapally (NP) were ground water irrigated and one village viz. Annojiguda ANG) was control which was unirrigated and was rainfed.
Collection of sewage and ground water samples for micronutrients and heavy metals analysis
Five sewage effluent samples (SW) were collected from sewage canal passing beside the villages. These samples were analyzed for various chemical characteristics including pollution causing parameters by following the standard methods prescribed for examination of water and wastewater. (American Public Health Association, 1985). However, in this communication, we have considered only trace elements and heavy metals. Sixteen ground water (GW) samples (one from each village) were collected after 15 minutes of running of the hand pump from each tube well in two separate well-rinsed, polypropylene bottles of 500 mL and 1000 mL capacity. Water samples were brought to the laboratory for further processing and analysis.
Analysis of sewage and ground water samples
The sewage and ground water samples were analyzed, for micronutrients and heavy metals using Inductively Coupled Plasma Spectrophotometer (ICP-XP, GBC, Australian model)
Collection of soil samples
A total of 200 samples (100 surface samples and 100 sub surface samples ) were collected from the above mentioned ten villages at two depth i.e. at 0-15 cm and 15-30 cms, during June, 2006, the season represents the end of summer and start of monsoon season (1st - 10th June). Out of the 100 soil samples collected, 50 samples were from sewage irrigated (SW) land, 40 samples were from groundwater-irrigated (GW) and 10 were from unirrigated land as control (Control).
Procedure for soil sample collection
Each sample was composite of 2 sub samples. Thick and good quality polythene bags of 1 kg capacity were used to collect the soil samples. For making composite sample, small portion of soil upto the desired depth (i.e., 0-15 cm and 15-30 cm) using suitable sampling tools i.e., crowbar and kurfi was collected after scrapping off the surface litter, if any, without removing soil. The ‘V’ shaped cut were first made upto the plough layer and a uniformly 2 cm thick slice was taken out from one clean side. From fields having standing crop, samples were drawn in between the rows. Mixed the soil collected from two spots by hand on a clean piece of cloth or polythene sheet. The bulk was reduced to about 500 g by quartering process in which the entire soil mass was spread, divided into four quarters, two opposite ones were discarded and the remaining two were remixed. This process was repeated until about 500 g soil was left. These steps were repeated for all the samples. Samples were labeled for identification. Details were noted and kept inside the sample bag, another label carrying same details of field etc was pasted outside the bag.
Samples were brought to the laboratory and dried under shade. By using pestle, and mortar, the samples were pounded and passed through a 2 mm sieve for estimation of various parameters. For analysis of organic carbon, 0.5 mm sieve was used, and samples were preserved for physico -chemical and chemical analysis. Later soil samples were analyzed for trace elements and heavy metals.
Methods used for chemical analysis of trace and heavy metals in soil samples
DTPA extractable trace (micronutrients) and heavy metals
The micronutrients (trace) and heavy metals were extracted (Lindsay and Norvell, 1978) with DTPA reagent. The reagent was prepared by dissolving 1.967 gm of AR grade DTPA, 1.470 g of CaCl2 2H2O (AR grade) in about 25 mL of double distilled water (DDW), 13.3 mL of TEA was added, diluted and pH was adjusted to 7.3, the final volume was made to 1 liter. To estimate the extractable trace (micronutrients) and heavy metals, 10 g of soil was taken in 100 mL conical flask. To this, 20 mL of the DTPA reagent (0.005 M DTPA, diethylene triamine pentacetic acid +0.1M TEA, triethanolamine + 0.01 M CaCl 2, pH 7.3) was added and shaken for 2 hrs on a mechanical shaker. The contents were filtered using Whatman No. 42 filter paper and extractant was used for estimation of the micronutrients (trace) and heavy metals by Inductively Coupled Plasma Spectrophotometer (ICP-XP, GBC, Australian model).
Collection of Plant Samples
Plant Samples of Palak (Beta valgaris var bengalensis), Amaranthus sps., Spinach (Spinacea oleracea), Coriander (Coriandrum sativum), fodder grasses such as Paragrass (Brachiaria mutica) were collected from Farmers fields at the time of harvest where as field crops like green chillies (Capsicum annum) were collected at 60 days after planting. In order to study the influence of sewage irrigation on plant parameters, samples of leafy vegetables viz., Palak, Amaranth, Spinach, Coriander, Paragrass and green chillies which are cultivated on soils receiving continuous irrigation with raw sewage water were collected from the farmers fields. Plant samples, were washed with dilute HCl. After drying, these were powdered and passed through 0.2 mm sieve and stored fro further analyses.
Methods for plant analysis for estimating micronutrients and heavy metals
About 0.5 g finely powdered plant sample was taken in a 100 mL conical flask. To this 10 mL of di-acid mixture containing HNO-3+ HClO4 in the ratio of 9:4 was added. The contents were kept for about 6-8 hrs over night at a covered place in a chamber for predigestion. After predigestion, the samples were put on a hot plate in acid proof digestion chamber at about 100°C for first one hour and then raised the temperature to about 200°C. The digestion was continued until the contents became colourless with white dense fumes. The flasks from hot plate were taken out and allowed for cooling. The contents were washed with 15-20 mL portion of distilled water for 3-4 times and the volume was made up to 100mL.
Statistical analysis
Statistical methods recommended by Gomez and Gomez, (1984) were employed to process the data generated from laboratory and field studies. The data collected on various nutrient parameters were computerized and subjected to the statistical analysis. Statistical analysis involved computation of correlations and testing of their level of significance (P=0.05).
Distribution of micronutrients (trace metals) and heavy metals in sewage and ground water
While studying the micronutrient and heavy metal contents in sewage effluents, the mean concentration of Iron in sewage water was found 0.24 mg L-1. Highest value was observed in the villages of Peerjadiguda and Kachavanisingaram village. In general, the concentration of Iron in sewage waters was relatively lower than maximum permissible limit i.e., 5 mg L-1 suggested by many agencies (Environmental Protection Agency, 1973; Food and Agricultural Organization (FAO), 1985.
Mishra et al., (1993) reported the range of 8-12 mg L-1 of Iron in the sewage water collected at SDI experimental Farm, Allahabad, and Uttar Pradesh in India The content of iron in the sewage water in some of the studies made in Pakistan was as low as 1.75 mg L-1 (Wajahat Nazif et al., 2006).
In the sewage water, the mean concentration of copper was 0.01 mg L-1. Highest copper concentration was observed in the villages’ viz. Parvathapur (0.02 mg L-1) and Kachavanisingaram (0.02 mg L-1) which were quite lower than the safe limits prescribed for irrigation water (0.2 mg L-1). Kale et al., (1992) reported that the copper content in the waste waters of Hyderabad (India) city ranged from 0.15-0.19 mg L-1. Aziz et al., (1995) reported a higher copper content in sewage water. Similar observations were earlier made by Patel et al., (2004) for Ankleshwar region for Fe, Cu, Mn, Cd, Ni, Co and Cr.
In the sewage water samples collected from different villages, the mean manganese content was 0.26 mg L-1. The high concentration of Mn was observed in the Peerjadiguda 0.29 mg L-1 village. Aziz et al., (1995) reported that the Mn content was high in sewage water.
The contents of Mn in the sewage water in the present study slightly exceeded the safe limits earlier suggested (Pratt, 1972; Ayers and Westcot, 1994). These results confirm the results of earlier studies made for different regions (Mehdi et al., 2003 and Patel et al., 2004).
In the sewage waters collected, the mean Zinc content was 0.03 mg L-1, which was well below the maximum prescribed limit (15 mg L-1) for land application. Kale et al., (1992) reported that zinc content varied from 0.067 to 0.50 mg L-1 in the wastewaters of Hyderabad. Aziz et al., (1995) reported that the Zn content was considerably higher in the sewage water. Dubey et al., (2006) reported that the sewage water samples had very high amount of micronutrients. The effluents from Ankleshwar site were most polluted with respect to different elements such as Fe, Cu, Mn, Cd Ni, Co and Cr (Patel et al. 2004). Wajahat et al., (2006) stated that the Mn content of canal water and Bara River water used for irrigation in the villages of Akbarpura, Kurvi and Banda in Pakistan were 0.71, 0.67, 0.61 and 0.77, 0.82, 0.85 μg mL-1 respectively. Similarly, the Zn contents of Akbarpura, Kurvi and Banda villages were 0.04, 0.04 and 0.06 μg mL-1 respectively in canal water which was used for irrigation water. However, the corresponding Zn contents in Bara River waters were 0.02, 0.03 and 0.06 μg mL-1 respectively (Wajahat Nazif et al., 2006). Sekhar et al., (2005) reported the high level of Zinc and Cu in the sewage effluents of Musi River, Hyderabad. The irrigation source of Bara river water in the villages of Akbarpura, Kurvi and Banda of Pakistan were analyzed and the average value of Cu content were 0.90, 1.03 and 1.20 μg mL-1 respectively (Wajahat Nazif et al., 2006). Copper content of the Bara River water used for irrigation in Banda village of Pakistan approached to toxic level (>0.05 mg L-1) (Wajahat Nazif et al., 2006). Ahmed Usman and Ahmed Ghallab (2006) concluded that the use of sewage wastewater led to salt accumulation and an increase in the readily labile fraction of Zn, Cu and Cd in the surface layer. According to Chiroma et al., (2007), water samples from seven hand dug wells in Vinikilang, Shinko, Demsawo and Girei contained higher levels of micronutrients and heavy metals such as Fe, Zn, Cu and Pb, which were above the permissible limits of 0.1, 5, 0.5 and 0.05 mg L-1 respectively suggested by WHO. Rattan et.al, (2005) observed that the sewage irrigation for 20 years resulted into significant buildup of DTPA – extractable Zn (208%), Cu (170%), Fe (170%), Ni (63%) and Pb (29%) in sewage irrigated soils over adjacent tube well water irrigated soils, whereas Mn was depleted by 31% in peri-urban agricultural lands under Keshopur effluent irrigation scheme (KEIS) of Delhi.
Further, in the present study, among the heavy metal contents in sewage water used for irrigation, the mean Pb, Ni and Cd contents were 29.50, 3.66 and 2.14 mg L-1 respectively. These values were beyond the safe levels of 5.0 and 0.2 mg L-1 respectively for Pb and Ni (FAO, 1985 and Tandon, 1995). For Cd, the level was beyond the tolerable limits (0.01 mg L-1) as prescribed by FAO (1985). The relatively higher concentration of heavy metals especially Pb and Ni in sewage water could be attributed to the contaminated wastes and effluents released by various industries pertaining to electroplating, lead extraction /battery units, pharmaceutical, leather, textile, soap, Jewellery etc. Kopp and Kroner (1970) reported that general sources of lead include metal processing plants, petrochemical establishments, incinerator activities, agricultural operations, and mining effluents.
Mishra et al., (1993) and Shrabani et al., (1994) and Reddy (1998) also reported that the Cd concentration in sewage waters under their study were beyond the maximum permissible limit. The values of heavy metals in sewage water in the present study were higher than reported earlier for different regions (Patel et al., 2004; Wajahat Nazif et al., 2006). Rattan et al., (2005) stated that the sewage effluents contained much higher amount of Ni compared to ground water. Mukund and Gurmeet, (2006) stated that the concentrations of Pb, Cd and Ni in effluent contaminated sewage water were 20, 13 and 186 times higher than in shallow hand pump water. Similar observations were earlier reported by Ramesh et al., (2006). The Information regarding the distribution of micronutrients and heavy metals in secondary and raw effluents has also been earlier documented by Feigin et al., (1979). They reported that the level of trace elements (As, B, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, V and Zn) in treated sewage effluents (Secondary effluent included) is determined by the chemical properties of the raw sewage from which these effluents were derived and the treatment method used. Trace elements are universally present in water (Ayers and Westcot, 1985,) but their level in fresh water is negligible. Domestic use rarely yields large quantities of trace elements in the wastewater, whereas industrial processes usually result in enhanced levels. Secondary sewage treatment reduces the trace element content through the settling of suspended solids. The concentration of trace elements can be lowered by 70 - 90% through secondary treatment (Bouwer and Chaney, 1974) and to an even greater extent by tertiary treatment and certain advanced technologies decrease the trace-element levels to those found in the original freshwater. Since in the present study, sewage water being used by the farmers for last many years was mixed (treated + untreated) one, hence, it contained relatively higher quantities of metal ion contaminants. In the present study, spatially from the point of MSDP (mixed sewage disposal point), no specific trend in the concentration of micronutrients and heavy metals was observed in the sewage water samples collected from the five villages.
From the foregoing discussion, it can be inferred that raw sewage water generated in Hyderabad city which is subjected to basic sedimentation treatment is not fit for irrigation purpose, because of the hazards associated with toxic levels of Mn and heavy metals like Pb, Ni, Cd. Further, no specific trend in spatial variability in distribution of micronutrient and heavy metal cations in sewage water samples collected from different villages was observed. Further, it is essential that the sewage water should be treated properly before irrigating the fodder crops, and leafy vegetables.
From the data on general characteristics, micronutrients and heavy metal contents of ground water samples collected and analyzed from 16 villages, it was inferred there was no specific trend in spatial distribution of the characteristics, micronutrient and heavy metal cations across the villages. This implies that the ground water of hand pumps situated in the villages near the river stream and those which were slightly away did not differ much in relation to general water quality characteristics and contents of micronutrient and heavy metal cations. Surprisingly, when average values were considered, except Mn, ground water samples showed relatively higher amounts of other micronutrients (Cu, Zn and Fe) and heavy metal (Pb, Ni and Cd) contents compared to sewage water samples. This could be attributed to the accumulation of micronutrients and heavy metal cations in ground water due to lateral seepage and leaching of cations through soil near and even slightly away from the river stream. The river has been flowing for last many years and the farmers of the region have been using the sewage water for irrigation of their field crops such as Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass. This recurring process over years has resulted in the contamination of even ground water with micronutrients and heavy metals. However, when values were compared with the safe limits suggested by FAO for ground water, only Cu, Cd and Ni were found to be above the safe limits. Where as, in sewage water, Mn, Ni and Cd were found to be above the safe limits (Table 1). Based on the mean values calculated by grouping the hand pumps distance-wise from sewage canal, it was observed that Mn and Cu tended to decline in their concentration in ground water as the distance of the hands pumps increased from the sewage canal. However, the contents of Zn, Pb, Ni and Cd tended to increase with distance. This may be attributed to the more lateral diffusion of river water through the soil in to the ground water and more contribution of these cations to the ground water. Further, irrespective of the distance from river, farmers have been taking the river flow through small channels to their fields for irrigation. This sewage irrigation also played important negative role in polluting the ground water through deep percolation.
Table 1. Average contents of micronutrients and heavy metals (mg L-1) in Sewage and ground water samples versus safety limits.
Effect of sewage and ground water irrigation on content of micronutrients and heavy metals in soil
Among the micronutrients i.e trace metals, the mean content of Cu in sewage and ground water treated soils in surface and subsurface soil horizons were 1.86 and 1.64 and 1.74 and 1.98 mg kg-1. Whereas, in control soils, these values were 1.24 and 1.10 mg kg-1. Relatively higher accumulation of Cu was observed in surface and subsurface soil horizons in sewage and ground water irrigated soils compared to control soils. In surface and sub surface soils, the mean Fe contents under sewage and ground water irrigation were 33.03 and 32.36 mg kg-1 and 21.03 and 23.46 mg kg-1 respectively, whereas in control soils, these values were 6.40 and 6.10 mg kg-1. The maximum accumulation of Fe was observed in sewage treated soils. The mean manganese (Mn) contents of surface and subsurface soils under sewage and ground water irrigation were 9.95 and 8.48 mg kg-1 and 7.26 and 8.33 mg kg-1 respectively. Whereas, in control soils, the corresponding values in surface and subsurface soil horizons were 4.14 and 4.29 mg kg-1 respectively. Under sewage and ground water irrigated conditions, both surface and subsurface soils were higher in Mn content over control. The mean Zn contents of sewage and ground water treated soils were 1.95 and 1.74, 1.27 and 1.20 mg kg-1 in surface and subsurface soils respectively. Whereas, in control soils, the corresponding values were 1.18 and 1.09 mg kg-1 respectively (Table 2).
Table 2. Micronutrients (trace-elements) (mg kg-1) in Sewage and Ground water treated soils at surface and subsurface layer
Micronutrients such as Fe, Mn, Cu and Zn are essentially required in plant growth. Their contents are required in small quantities. Their build up in soils is considered as desirable feature unless or until they do not reach beyond the toxic range and affect the plant growth. Additions of organics regulate the availability of micronutrients by way of chelating them and releasing them slowly in synchronization with plant growth.
The higher contents of micronutrients in surface and subsurface soil layers under sewage-irrigated conditions were probably due to higher content of micronutrients in sewage water. Azad et al., (1987); Singh and Singh (1994) and Bhupal Raj et al., (1997) reported similar trends in micronutrient contents in the sewage-irrigated soils of Ludhiana, Varanasi and Hyderabad respectively. Krishna and Govil (2004) reported that the level of the metals in soils around the industrial area of Pali, Rajasthan, India were found to be significantly higher than their normal distribution in soil. Patel et al. (2004) observed that the soils continuously irrigated with effluents showed the highest Cu availability. Rattan et al., (2005) concluded that the sewage irrigation for 20 years resulted into significant build up of DTPA- extractable Zn (208%), Cu (170%), Iron (170%), Ni (63%) and Pb (29%) in sewage irrigated soils over adjacent tube well waterirrigated soils. Perusal of village wise data on distribution of micronutrients under sewage and ground water irrigated conditions and under control (unirrigated) conditions indicated no specific trend in spatial distribution of micronutrient cations with respect to distance from the main river.
Heavy metals in soils
In the present study, the DTPA extractable heavy metals (Pb, Ni, Cd) contents were considerably higher in sewage and ground water treated soil profiles than in control soils. The mean Cd contents of sewage and ground water irrigated soils in surface and subsurface layers were 2.39 and 2.60 mg kg1 and 2.13 and 2.10 mg kg-1 respectively. Whereas, in control conditions, the corresponding values were 0.64 and 0.97 mg kg-1 respectively. These values of Cd in soil were found to be slightly lower than the critical levels suggested by FAO (1985) (3.0 mg kg-1) and German standards (Pescod et al., 1985) (3.0 mg kg-1). However, the order of contamination of soils with respect to Cd was: Sewage treated soil > ground water treated soils > control. In the ground water treated soils, the detection of Cd is attributed to more concentration of Cd in ground water. This presence of Cd in the ground water is possibly due to contamination occurred on account of lateral flow and seepage from the contaminated river.
The mean nickel contents in sewage and ground water irrigated surface and subsurface soils were 4.03 and 4.88 mg kg-1 and 3.98 and 3.86 mg kg-1 respectively, whereas, in control soils, the corresponding values were 1.59 and 2.39 mg kg-1. These values were quite lower than the critical limits of Ni suggested for soil by FAO (1985) (50 mg kg-1) and German standards (Pescod et al., 1985) (50 mg kg-1).
The mean Pb contents of sewage and ground water irrigated soils in surface and subsurface layers were 68.08 and 72.47 mg kg-1 and 65.08 and 62.71 mg kg-1, whereas in control soils, these values were 14.38 and 26.96 mg kg-1 respectively. Some of these values were found to be higher than the safe limits suggested for soil by FAO (1985) (50 mg kg-1) and lower than the German standards (Pescode et al. (1985) (100 mg kg-1). The relatively higher content of accumulation of Pb in sewage water and main sources are lead acid storage batteries, pigments and chemicals, solder, other alloys and cables. It therefore becomes part of industrial waste from these industrial activities WHO (1993) stated that Pb is present in tap water primarily from household plumping systems containing Pb in pipes, solder, fittings or service connections to homes. This makes domestic waste a major source of Pb (Table 3).
Table 3. Heavy Metals in Sewage and Ground Water at Surface and Subsurface soil
Krishna et al., (2004) stated that the level of the Pb in soils around the industrial area of Pali, Rajasthan, India was found to be significantly higher than its normal distribution in soil. Concentrations of DTPA extractable Pb in soils irrigated with effluents contaminated water was 1.9 times higher than that in soils irrigated with deep underground water (Mukand Singh and Gurmeet, 2006). In the present study, it was clearly observed that the mean Pb contents of sewage and ground water treated soils were higher compared to the control soils. However, among the heavy metals studied, the Pb content was higher followed by Ni and Cd.
Earlier studies revealed that the relative availability of Pb was highest near Ahmedabad and Ankleshwar soils irrigated with sewage mixed with industrial effluent (Patel et al., 2004). On the critical appraisal of the data on heavy metal distribution in sewage and ground water irrigated soils, no specific spatial trend was observed with respect to the distance of the villages from the source river.
Micronutrient and heavy metal contents in leafy vegetables, fodder grass and other field crops grown with sewage and ground water treated soils
The mean iron content of plants varied considerably and contents were 1062.08, 1447.33, 895.71, 628.73, 491.25 and 269.74 mg kg-1 in palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass respectively in sewage-irrigated soils.
In case of ground water irrigated soils, the corresponding values were 850.23, 1234.33, 935.44, 547.05, 461.31 and 244.01 mg kg-1. However, the corresponding contents in control soils were 659.84, 1247.00, 946.60, 479.83, 410.11 and 251.83 mg kg-1. In general, the leafy vegetables had higher contents of iron. However, neither iron deficiency nor accumulation of iron in toxic proportions was recorded in all the plant species studied. Adhikari et al., (1993) reported high concentrations of iron in different vegetable crops grown under sewage water irrigation.
The mean Cu contents in sewage irrigated Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass were 2.42, 5.19, 4.61, 1.36, 15.40 and 4.35 mg kg-1 respectively (Figures 2, 3,4) . Whereas, under ground water irrigated conditions, the corresponding values of Cu were 1.47, 3.38, 3.55, 1.00, 13.28 and 2.56 mg kg-1. In case of control soils, these corresponding Cu concentrations were 1.72, 1.55, 1.43, 0.59, 10.97 and 2.27 mg kg-1. The concentration of copper in Palak, Amaranthus Sps., Spinach, Coriander, Green chillies and Paragrass were lower than the range of safe limits (4 to 15 mg kg-1) suggested by Bowen (1966) and Allaway (1968).
Fig. 1 Micronutrients (Fe, Cu, Zn, Mn) and heavy metals content (Pb, Ni, Cd) in Palak, Amaranthus, with sewage and groundwater irrigation
Fig. 2 Micronutrients (Fe, Cu, Zn, Mn) and heavy metals content (Pb, Ni, Cd) in Coriander and Paragrass with sewage and groundwater irrigation.
Fig. 3 Micronutrients (Fe, Cu, Zn, Mn) and heavy metals content (Pb, Ni, Cd) in Green chillies and Spinach with sewage and groundwater irrigation.
The mean concentrations of Zinc in sewageirrigated soils were 56.35, 25.78, 53.35, 21.01, 17.63 and 38.23 mg kg-1 in Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass and in ground water irrigated soils, the corresponding values were 49.51, 31.61, 49.72, 19.05, 16.72 and 32.77 mg kg- 1. Whereas, in control soils, these corresponding contents of Zn were 42.50, 27.39, 35.40, 19.42, 15.51 and 32.84 mg kg-1. The mean Zn uptake in plant samples under sewage and ground water irrigated conditions was within the safe limit of 15-200 mg kg- 1 as suggested by Bowen (1966) and Allway (1968).
The mean manganese (Mn) content of plant samples collected from sewage irrigated soils was 74.21, 24.49, 62.55, 32.10, 24.73 and 22.99 mg kg-1 in Palak, Amaranths, Spinach, Coriander, Green Chillies and Paragrass respectively. However, the corresponding values in ground water irrigated conditions were 48.82, 23.31, 62.07, 30.49, 22.72 and 22.73 mg kg-1. Under control soils, the corresponding Mn contents were 32.10, 24.47, 50.82, 26.23, 20.44 and 22.29 mg kg-1. The concentration of manganese (Mn) in Palak, Amaranthus, Spinach, coriander, green chillies and Paragrass was within safer limit (15-100 ppm) as reported by Bowen (1966) and Allaway (1968).
Similarly, the concentrations of lead (Pb) in Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass samples were 15.84, 13.54, 14.97, 6.81, 13.81 and 17.31 mg kg-1 respectively under sewage irrigated conditions and in case of ground water irrigation, the corresponding values were 7.26, 6.38, 6.58, 4.81, 8.09 and 11.74 mg kg-1. However, in control, the corresponding values were 1.50, 2.23, 1.88, 2.01, 1.79 and 2.47 mg kg-1 respectively. These concentrations of lead (Pb) in Palak, Amaranthus sps. Spinach, Green chillies and Paragrass samples analyzed were more than permissible range of 1.0 to 10 mg kg-1 (Bowen, 1966 and Allaway, 1968). In case of coriander, the values were within the permissible range as suggested by Bowen (1966) and Allaway (1968). In case of ground water irrigation, the Pb content in Palak, Amaranthus, Spinach, Coriander and Green chillies was within the permissible range but in Paragrass, the content was slightly higher (11.74 mg kg-1). The cultivation of plant samples with sewage irrigation showed the increased content of Pb. This may be due to continuous cultivation of plants with sewage irrigation for more than 20 to 25 years. Bhupal Raj et al., (1997) reported the similar findings in their earlier studies conducted along the course of Musi River in Hyderabad, India. The mean concentrations of Nickel (Ni) in Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass under sewage irrigation were 3.52, 4.08, 3.15, 1.45, 3.46 and 2.95 mg kg-1 respectively. Whereas, with ground water irrigation, the corresponding concentrations were 1.55, 2.53, 1.71, 1.10, 1.48 and 2.95 mg kg-1. However, in control soils, the corresponding values were 1.10, 1.27, 0.89, and 0.42. 0.66 and 0.50 mg kg-1. The concentrations of Ni in all the plant samples analyzed were more than safer limit (0.2 to 1 mg kg-1) under sewage irrigation and ground water treated plant samples. In control soils, Palak and Amaranthus had higher concentration of Ni than the safer limit (0.2 to 1.0 mg kg-1) suggested by Bowen (1966) and Allaway (1968). In the present study, the mean concentrations of Cadmium (Cd) in plant samples of Palak, Amaranthus, Spinach, Coriander, Green chillies and Paragrass under sewage irrigation were 2.08, 2.61, 2.28, 0.93, 2.11 and 1.80 mg kg-1 respectively, and with ground water irrigation, the corresponding values were 1.31, 1.53, 1.06, 0.53, 1.34 and 1.22 mg kg-1. However, in control, the corresponding values were 0.31, 0.15, 0.15, 0.17, 0.11 and 0.20 mg kg-1 (Table 4).
Table 4. Micronutrients and heavy metal content (mg kg-1) in crops/leafy vegetables/fodder crops grown in sewage and ground water irrigated areas of Musi River Basin
Correlation between micro and heavy metal contents in soil and plant
Correlation (r) of micro and heavy metal contents in soil and their corresponding contents in plant
Based on correlation studies, it was observed that irrespective of soil depth, significantly positive correlations were observed in case of Mn in Palak (r = 0.677) in Peerjadiguda village, Mn and Zn in Paragrass (r =0.670, 0.752) in Kachavanisingaram village, Mn in Chillies (r = 0.634) in Muthawaliguda village. However, significantly negative correlations were recorded in case of Mn in Palak (r =-0.730) in Parvathapur village, Mn in Palak (r=-0.753) and Fe in Green chillies -0.660 in Kachavanisingaram village. Further, significantly negative correlations were recorded in case of Zn in Coriander (r = -0.634) and Paragrass (r = -0.672) in Pratapsingaram village. Among the heavy metals, Cd in soil was significantly and positively correlated with its corresponding content in Spinach in Parvathapur (r = 0.654) and Pratapsingaram (r =0.812) villages and Palak (r = 0.655) only in Pratapsingaram village. Similarly, increasing Pb concentration in soil under sewage treated conditions, significantly and positively (r = 0.685) influenced the Pb content only in Coriander crop in Parvathapur village. The heavy metals whose concentrations in soil and plant negatively correlated were Pb (r =-0.809) in Peerjadiguda and Ni (r = - 0.811) in Spinach in Muthawaliguda village.
The correlation studies between micronutrients and heavy metal contents in ground water treated soils and their corresponding contents in plant indicated that irrespective of soil depth, significantly positive correlations were observed in case of Mn in Amaranthus (r=0.737) in Sadat Aliguda village. Zinc content in soil was significantly correlated with its content in Amaranthus (r= 0.702). Similarly, Cd content in soil (r= 0.641) was significantly correlated with Palak and Paragrass in Korremula village, here as in chowdarguda village soil Cu and Fe (r=0.728 and r=0.814) had significant correlations with their respective plant content in Coriander crop. While in case of Pb, significant correlations were observed between soil and plant contents in Paragrass (r=0.667) and Palak (r=0.820). In Narapally village also Pb content in ground water treated soils significantly correlated with its uptake by Palak (r=0.681).
In case of Annojiguda village, which did not receive any irrigation, only soil Mn content was found correlated with its corresponding content in Palak crop in control soils. From the correlation studies, it was inferred that in sewage treated soils only Cd content of soil was found significantly and positively correlated in Palak and Spinach on consistent basis. This indicated that as the concentration of Cd in soil increased, its absorption in these leafy vegetables also increased. Other elements did not reflect their consistent correlation trend. Interestingly, in case of ground water treated soils, among the heavy metals and micronutrients, only Zn (Amarnathus) and Cd and Pb (Palak and Paragrass) had their significant and positive association with their respective contents in plant. In the soils, which did not receive any irrigation (Control), only Mn content of soil had significant positive association with its corresponding plant content in Palak.
Whilst it is clear that urban and peri-urban agriculture has a vital and increasing role to play for the subsistence of the poor and for local and national economies in India, there is little awareness of the increasing environmental threats associated with food production in these areas, and very limited structured systems in place through which these issues can be monitored and addressed. Clearly, toxic pollutants in urban and peri-urban areas, which can impact on the safety of crops grown here, can have a dramatic and widespread impact on urban inhabitants. One of the threats to food quality and safety in these areas are heavy metals in industrial effluents, and from sewage plants. Dietary intake of heavy metals is a substantial risk to the health of families who depend upon the use of contaminated irrigation water to irrigate their crops to meet their food requirements. Plants frequently act as bio accumulators of heavy metals, with concentrations in crops such as spinach, cauliflower and wheat reported to exceed international food standards in several earlier studies made across the country. The extent of contamination to food crops is likely to increase with intensification of production systems, urbanization and industrialization, but levels of food contamination are not regularly monitored (other than for pesticide residues in India) or controlled. Hence, the present study was undertaken to assess the quality of sewage water flowing in the Musi River and its impact on soil, groundwater quality and agricultural produce especially leafy vegetables in relation to soil characteristics.
On an average, except Mn, ground water samples showed relatively higher amounts of other micronutrients (Cu, Zn and Fe) and heavy metal (Pb, Ni and Cd) contents compared to sewage water samples. This could be attributed to the accumulation of micronutrients and heavy metal cations in ground water due to lateral seepage and leaching of cations through soil near and even slightly away from the river stream. In the present study, contents of Pb and Cd were higher in most of the vegetable crops (Palak, Spinach and Paragrass) studied than their legislated levels suggested for foodstuffs in India. Hence, the consumption of these vegetables grown in this region need to be restricted. Continuous application of sewage effluents to arable lands will go on increasing the concentration of these heavy metals in the feeding zone of plant roots, which may not only become toxic to plants but would also create critical problems in animals and human beings because of entry of micronutrients and heavy metals into the food chain. The sewage-irrigated soils have the potential to act as storehouses of micronutrients, heavy metals, toxicants, biologically dangerous microorganisms, pathogens and parasites. There is a threat of further increase in the concentration of heavy metals in the ground water in the years to come. Hence, remedies need to be worked out at priority. Such studies will be eye opening and highly useful to food consumers in periurban areas, growers, researchers, voluntary agencies and Governmental organizations, environmentalists etc who are involved in environmental protection and human health safety.
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