HEAVY METAL IN THE FOOD CHAIN-CONSEQUENCES OF POLLUTING WATER BODIES

Purpose of the study: Heavy metals in food (vegetables, etc.) are harmful to humans due to their non-biodegradable nature, long biological half-lives, and their potential to accumulate in different body parts. Prolonged consumption of such heavy metal contaminated vegetables through foodstuffs may lead to chronic accumulation of heavy metals in human beings' kidneys and liver, disrupting numerous biochemical processes, leading to cardiovascular, neural, kidney and bone diseases. Method: The study on heavy metal concentrations in vegetables grown in the command areas of Varthur lake, Bangalore. The collected vegetable samples were analyzed using ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy) technique to assess the level of heavy metal in acid digested samples. Main Findings: The study has shown a significant accumulation of heavy metals in vegetables that correlated well with its concentrations in soil and lake water. The prolonged irrigation of vegetables using contaminated lake water has led to soil contamination, which ultimately resulted in contamination of vegetables due to the uptake and accumulation of heavy metals in edible portions of vegetables. Application of the Study: Urgent attention is needed to devise and implement appropriate means of regular monitoring of the toxic heavy metals from domestic sewage and industrial effluent and provide proper advice and support for the safe and productive use of wastewater for irrigation purposes to prevent excessive buildup of heavy metals in the food chain.


INTRODUCTION
Wetlands are transitional zones between land and water resources and play a significant role in the ecological sustainability of the region, while constituting the most productive ecosystems. A wetland is referred to an area that is periodically or permanently inundated with the surface or groundwater, which supports the growth of diverse aquatic vegetation Mitsch and Gosselink, 2000). Wetlands play a prominent role in meeting the domestic and irrigation needs of the region apart from being habitats for a wide variety of flora and fauna. The quality and hydrologic regime of the wetlands is directly dependent on the integrity of its watershed (catchment or basin). Wetlands function as kidneys of the landscape and help as a sink to myriads of toxic compounds and pollutants. However, an excess inflow of nutrients and other contaminants beyond the treatment capability results in changes in the water quality, impairing the ecological functions and hampering the ecological integrity of aquatic ecosystems. The sustained inflow of untreated or partially treated sewage to water bodies leads to the enrichment of nutrients such as nitrogen (N) and phosphorus (P), which is evident from the algal blooms and profuse growth of macrophytes. This has led to eutrophication of the surface water. This has also contaminated nearby groundwater sources via leaching affecting human health.
Unplanned rapid urbanization in recent years has led to the deterioration of wetlands. The sustained inflow of untreated or partially treated sewage to wetlands leads to the enrichment of nutrients such as nitrogen (N) and phosphorus (P), which is evident from the algal blooms and profuse growth of macrophytes. This has led to eutrophication of the surface water. This has also contaminated nearby groundwater sources via leaching, which affects human health. Nitrogen as nitrate-N pollution leads to physiological disorders including blue baby syndrome (methemoglobinemia) and the persistent assimilation of nitrate rich water leads to carcinogenic symptoms (as nitrosamines, which are carcinogens) Mahapatra et al., 2011;. The movement of chemicals through biological systems due to toxicokinetic behavior. The heavy metal content in vegetables exceeded the safe limits in India. The continued consumption of heavy metal contaminated food can seriously deplete some essential nutrients in the body causing a decrease in immunological defenses, intra-uterine growth retardation, impaired physico-social behavior, malfunctioning of kidneys, disabilities associated with malnutrition and a high prevalence of upper gastrointestinal cancer.
The prevalence of chronic kidney disease (CKD) is increasing, evident from the number of kidney patients in the city. The number of kidney failures has increased from 1 in 1 lakh population (in early 2000) to 1 in 5000 people. In this regard, the diverse and emerging issues of food security and healthy youth would be of serious concern considering the changes in the climate with global warming. Hence, it needs to stop the contamination of food by regulating the polluters by implementing the 'Polluter pays' principle. This entails stringent action against polluters (erring industries and inefficient regulatory agencies) for indiscriminately discharging pollutants in the environment.
Heavy metals are one of the most toxic chemicals that have been posing challenges to decision-makers. Heavy metals are elements with atomic number >20 that possess higher density (>5 g/cm³) and metallic properties and mainly include cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), cobalt (Co), iron (Fe), nickel (Ni), manganese (Mn), zinc (Zn) and arsenic (As). These heavy metals are mainly released from industrial processes; industrial discharges, mining operations, and acid mine drainage (Rungwa et al., 2013). These heavy metals move through various biogeochemical cycles and enter the food chain resulting in bioaccumulation and consequent bio-magnifications. The non-biodegradable nature and long biological halflife of heavy metals, when compared to other xenobiotics, are the reasons for various environmental problems. The primary sources of heavy metals into the environment are; fertilizers, pesticides, untreated or partially treated wastewater, leachates from mining sites, industrial wastes (e-wastes), wastes from smelting ores, and sewage sludge.

LITERATURE REVIEWS
Cultivation of vegetables and other food crops in heavy metal contaminated sites or irrigation with heavy metal contaminated water may result in uptake of such contaminants from soil, ultimately resulting in its accumulation in edible portions of vegetables (Sun et al., 2016). Heavy metal analysis in vegetables grown near an e-waste recycling facility in China (Luo et al., 2011) showed that the soils of former incineration sites had high concentrations of Cd, Cu, Pb, and Zn with mean values of 17.1, 11,140, 4500, and 3690mg/kg, respectively. In the edible tissues of vegetables, the concentrations of Cd and Pb in most samples exceeded the maximum level permitted for food in China. Sequential leaching tests revealed that the Cu, Pb, and Zn were predominantly associated with the residual fraction, followed by the carbonate/specifically adsorbed phases except for Cd, which was mainly in the extractable form in paddy fields and vegetable soils. Uptake of heavy metals by some edible vegetables irrigated using wastewater revealed a mean concentration in the order of Fe (183.11±161.2)>Zn (5.38±3.50)>Ni (3.52±1.27)>Pb (2.49±1.81)>Cr (1.46±0.51)>Co (0.66±0.25)>Cd (0.36±0.15). The water samples used for irrigation were found to be unsuitable for irrigation due to high SAR values (Ackah et al., 2014). Transportation and marketing systems of vegetables play a significant role in elevating the contaminant levels of heavy metals pose a threat to the quality of the vegetables with health consequences for the consumers of locally produced foodstuffs (Sharma et al., (2009). Heavy metal analysis in vegetables grown in an industrial area in Dhaka, Bangladesh the contaminations in the order of metal contents Fe > Cu > Zn > Cr > Pb > Ni >Cd in contaminated irrigation water, and a similar pattern Fe > Zn > Ni > Cr > Pb > Cu > Cd was also observed in arable soils. Metal levels observed in different sources were compared with WHO, SEPA, and established permissible levels. The mean concentration of Cu, Fe, and Cd in irrigation water and Cd content in soil were much above the recommended level. Accumulation of the heavy metals in vegetables studied was lower than the recommended maximum tolerable levels proposed by the Joint FAO/WHO Expert Committee on Food Additives (1999), except Cd, which exhibited elevated content.
Vegetables with protein, vitamins, iron, calcium, and other essential micro-nutrients constitute essential diet components. Contamination of vegetables grown with the city effluents has been posing a serious threat to humans evident from the enhanced episodes of various health disorders in the catchment. Heavy metals in vegetables are harmful to human beings due to their non-biodegradable nature, long biological half-lives, and their potential to accumulate in different body parts (Monu et al., The heavy metal removal mechanisms in plants include (i) uptake, (ii) translocation, and (iii) storage. The absorption of metals (for e.g., Pb) by roots occurs via the apoplastic pathway or via Ca ²+ permeable channels. The behaviour of metals in soil and uptake by plants is controlled by its speciation and soil pH, soil particle size, cation-exchange capacity, root surface area, root exudation, and degree of mycorrhizal transpiration ( Special plant membrane proteins bind with specific metals and become ready for uptake and transport. Root to shoot transport -elements are transported via the vascular system to the above-soil biomass (shoots). Different chelators are involved in the translocation of metal cations through the xylem, such as organic acid chelators -malate, citrate, histidine, or nicotianamine (Ali et al., 2013). Since the metal is complexed within a chelate, it can be translocated upwards in the xylem without being adsorbed by the high cation exchange capacity of the xylem. The transported metals are stored in the plant cell's vacuoles (Taiz and Zeiger, 2003). Heavy metals like arsenic (as arsenate) might be taken up by plants due to the similarities to the plant nutrient phosphate, while Selenium (Se) replaces the nutrient sulphur in compounds taken up by a plant (Brooks, 1998).

AIM OF THE STUDY
The Objective of the current study is to assess the heavy metal accumulation in vegetables.

MATERIAL AND METHODS
Bangalore is the principal administrative, cultural, commercial, industrial and knowledge capital of the state of Karnataka. Greater Bangalore spatial extent was expanded to 741 km² including the city, neighboring municipal councils, and outgrowths, in December 2006. The field sampling was done in the vicinity of Varthur Lake.
Varthur Lake: Varthur Lake is situated in the eastern periphery of Bangalore City and a part of the internationally famous Whitefield Township. Varthur is a Hobli and part of Bruhat Bangalore Mahanagara Palike (BBMP). The lake ecosystem is an integral part of Bangalore, although unplanned urbanization and industrialization have led to the contamination of these water bodies. Varthur Lake, with a spatial extent of 216 ha, is the second largest lake in Bangalore city. Recently, siltation and encroachment, the spatial extent of the lake has been reduced to 165.75 ha and also one of the most polluted lakes in Bangalore due to (i) the sustained inflow of untreated and partially treated domestic sewage, (ii) discharge of untreated industrial effluents (through stormwater drains and through trucks (who discharge untreated effluents), (iii) dumping of solid waste in the lake, drains and in the buffer zone. The wetland water accounts to irrigate 625 ha of agricultural fields in the command area for growing crops like rice, ragi millet, coconut, flowers, and various fruits and vegetables. Earlier reports have indicated the possible uptake of trace elements by plants grown in the command area of the lake (Ramachandra et al., 2011; Jumbe and Nandini, 2009). Field samples of vegetables grown using the lake water in the command area of Varthur Lake (Varthur) were collected to assess the level of heavy metal uptake (figures 1 -3).

Heavy metal analysis:
The collected vegetable samples were analyzed using ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy) technique to assess the level of heavy metal in acid digested samples.
Digestion procedure: For the water sample, the digestion was carried out with an acid combination of nitric and sulfuric acids at a ratio of 10:1. The plant samples were digested using nitric, sulfuric, and perchloric acids combinations of 5:1:1. Nitric and  Ni is an essential metal in the diet of humans but at low concentrations. Excess levels of Ni in humans can lead to various forms of cancer. Ni is mainly found in the soil as an ore together with iron. Other anthropogenic sources are dust/gases released by power plants which settle on the soil or precipitate with raindrops (table 2). When the soil pH is acidic, Ni becomes immobile and is taken up by plants.
In a similar study in West Bengal, the heavy metal content in vegetables like Pudina, Spinach, and Coriander were found to exceed their Indian safe limits (Gupta et al., 2012). The average Cd content in vegetables ranged from 9.4 to 13.2, Cu content from 25 to 32.1, Pb from 21 to 47.7, Cr from 44.1 to 95.8and Ni from 51 to 68.6 mg/Kg. The heavy metal content in Amaranthus sp. grown near Vrishabhavathi River, Bangalore, were observed to be 6.1 mg/Kg for Pb, 16 mg/Kg for Cu, 11.9 mg/Kg for Ni, and 10 mg/Kg dry weight of Cr (Jayadev and Puttaih, 2013).

Heavy metals in lake water and soil from the vegetable farm at Varthur
Among the metals analyzed, Ni was not detectable in the lake water sample (table 3). The concentration of Cd was found to be 0.6±0.07 mg/L exceeding the safe limits for irrigation, i.e., 0.01 mg/L. Cr level in water was observed to be 0.5±0.14 mg/L and In the case of soil 20.5±3.4, mg/Kg and 189±71 mg/Kg of Cd and Ni were observed, respectively. They were found to exceed the safe limits, i.e., 3-6 mg/Kg and 135-270 mg/Kg (Pescod, 1992  Sodium adsorption ratio (SAR) of Varthur lake water: The wastewater quality for irrigation was determined using the sodium absorption ratio (SAR). The SAR is commonly used as an index for evaluating the sodium hazard associated with an irrigation water supply (Rajendra et al., 2009). The SAR is defined as the square root of the ratio of the sodium to calcium & magnesium ions (Ca+Mg).
The SAR recorded for Varthur lake water of the two sampling periods was found to be 28.6 (table 3), which poses a very severe degree of restriction for irrigation purposes (FAO, 1985). A SAR of 14.9 was observed in a study conducted by Ackah et al. (2013) in Accra, Ghana. Irrigation waters having higher SAR could lead to build-up of high soil Na levels over time, which in turn can adversely affect soil infiltration and percolation rates (due to soil dispersion). Additionally, excessive SAR levels can lead to soil crusting, poor seedling, emergence, and poor aeration. This problem is also related to factors such as the salinity rate and the type of soil. For example, sandy soils may not get damaged as easily as other heavier soils when it is irrigated with high SAR water. It means excess sodium in irrigation water, relative to calcium and magnesium or total salt content, can affect soil structure, soil aeration, flow rate, permeability, infiltration, etc.

CONCLUSION
The study on heavy metal concentrations in vegetables grown near Varthur lake, Bangalore, has shown significant accumulation of heavy metals in vegetables that correlated well with its soil and lake water concentrations. The prolonged irrigation of vegetables using such contaminated lake water has led to soil contamination, which ultimately resulted in contamination of vegetables due to the uptake and accumulation of heavy metals in edible portions of vegetables. The heavy metal content in vegetables exceeded the safe limits in India. Thus, urgent attention is needed to devise and implement appropriate means of regular monitoring of these toxic heavy metals from domestic sewage and industrial effluent and provide proper advice and support for the safe and productive use of wastewater for irrigation purposes in order to prevent excessive build-up of heavy metals in the food chain. The higher SAR is another important factor that highlights the lake water is unsuitable for irrigation purposes.

Green Chemistry & Technology Letters
• Development of a self-sustaining bioremediation-based wetland systems are; i. Nutrient removal through selected wetland plants, preferably native species (such as Alternanthera sp., Ludwigia sp.) for efficient wastewater treatment and resource recovery.
ii. Removal of heavy metals dual-mode cation/anion-based natural resins at the treatment locations or through natural porous beds with high cation exchange capacity.
iii. Further screening of macrophytes to check their efficiencies in nutrient and heavy metal removal. iv.
Design of a suitable treatment to treat municipal and agro-industrial wastewater.
v. Regular de-sludging of algal ponds for maintaining critical depths for optimal algal photosynthesis.