Long-term impact of irrigation with sewage effluents on heavy metal content in soils, crops and groundwater—a case study

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Abstract

There is a gradual decline in availability of fresh water to be used for irrigation in India. As a consequence, the use of sewage and other industrial effluents for irrigating agricultural lands is on the rise particularly in peri-urban areas of developing countries. On the other hand, there is increasing concern regarding the exceedance of statutory and advisory food standards for trace metals throughout the world. Hence, a case study was undertaken to assess the long-term effect of sewage irrigation on heavy metal content in soils, plants and groundwater. For this purpose, peri-urban agricultural lands under Keshopur Effluent Irrigation Scheme (KEIS) of Delhi, India were selected where various cereals, millets, vegetable and fodder crops have successfully been grown. Sewage effluents, ground water, soil and plant samples were collected and analysed mainly for metal contents. Results indicated that sewage effluents contained much higher amount of P, K, S, Zn, Cu, Fe, Mn and Ni compared to groundwater. While, there was no significant variation in Pb and Cd concentrations in these two sources of irrigation water and metal content were within the permissible limits for its use as irrigation water. There was an increase in organic carbon content ranging from 38 to 79% in sewage-irrigated soils as compared to tubewell water-irrigated ones. On an average, the soil pH dropped by 0.4 unit as a result of sewage irrigation. Sewage irrigation for 20 years resulted into significant build-up of DTPA-extractable Zn (208%), Cu (170%), Fe (170%), Ni (63%) and Pb (29%) in sewage-irrigated soils over adjacent tubewell water-irrigated soils, whereas Mn was depleted by 31%. Soils receiving sewage irrigation for 10 years exhibited significant increase in Zn, Fe, Ni and Pb, while only Fe in soils was positively affected by sewage irrigation for 5 years. Among these metals, only Zn in some samples exceeded the phytotoxicity limit. Fractionation study indicated relatively higher build-up of Zn, Cu, Fe and Mn in bioavailable pools of sewage-irrigated soils. By and large, tissue metal concentrations in all the crops were below the generalized critical levels of phytotoxicity. Based on the soil to plant transfer ratio (transfer factor) of metals, relative efficiency of some cereals, millet and vegetable crops to absorb metals from sewage and tubewell water-irrigated soils was worked out. Risk assessment in respect of metal contents in some vegetable crops grown on these sewage-irrigated soils indicated that these vegetables can be consumed safely by human.

Introduction

India supports more than 16% of the world's population with only 4% of the world's fresh water resources (Singh, 2003). Although agriculture sector in this country has been major user of water, share of water allocated to irrigation is likely to be decreased by 10–15% in next two decades (CWC, 2000). In this changing scenario, reuse of domestic and industrial waste water in agriculture for irrigating crops appears to be a lucrative option. Besides being source of irrigation water, these waste waters contain appreciable amounts of plant nutrients. In India, total waste water generated per annum from 200 cities is about 2600 Mm3 (Kaul et al., 1989) and also the use of sewage effluents for irrigating agricultural lands is on the rise especially in the peri-urban area. These waste waters carry appreciable amounts of trace toxic metals (Feign et al., 1991, Pescod, 1992, Som et al., 1994, Gupta et al., 1998, Brar et al., 2000, Yadav et al., 2002) and concentrations of trace metals in sewage effluents vary from city to city (Rattan et al., 2002). Although the concentration of heavy metals in sewage effluents are low, long-term use of these waste waters on agricultural lands often results in the build-up of the elevated levels of these metals in soils (Rattan et al., 2002). Extent of build-up of metals in waste water-irrigated soils depends on the period of its application (Bansal et al., 1992, Palaniswami and Sree Ramulu, 1994). Crops raised on the metal-contaminated soils accumulate metals in quantities excessive enough to cause clinical problems both to animals and human beings consuming these metal rich plants (Tiller, 1986). Since food chain contamination is one of the major routes for entry of metals into the animal system, monitoring the bioavailabile pools of metals in contaminated soils has generated a lot of interest (Datta et al., 2000, Yadav et al., 2002). Also information on the fractionation of trace metals in soils is potentially valuable in predicting bioavailability, metal leaching rates, and transformation between chemical forms in agricultural and polluted soils (Jenne and Louma, 1977, McBride, 1981, Miller et al., 1986). But such information on agricultural lands receiving sewage irrigation for quite a long time is meagre.

Crop species exercise differentiality in accumulating metals in their tissue (Lepp, 1981, Datta et al., 2000) and efficiency of different crops in absorption of metals is judged either by plant metal uptake or by transfer factor of metals from soil to plants. Uptake of metals by plants may be a good indicator of efficiency of metal absorption of different crop species grown on soils having uniform metal levels under controlled conditions. Whereas, transfer factor of metal from soil to plants indicates the efficiency of crop species better where crops are grown on soils having variable metal contents, e.g. farmers’ fields. Generally, soil to plant transfer factor of metals is computed based on total metal contents of soils (Hooda et al., 1997). However, total metal content in soils does not take into account the other soil factors that modify the bioavailability of metals. Hence, computation of soil to plant transfer factor of metals should be based on available soil metal pools.

We attempted to study some of the above-mentioned aspects in sewage-irrigated peri-urban agricultural lands under Keshopur Effluent Irrigation Scheme (KEIS) of the Delhi Government, India. Under this scheme, sewage irrigation has been provided to the farmers’ fields for more than two decades. Various cereals, millet, leafy vegetables and fodder crops have successfully been grown thereon. No investigation has been carried out in sewage-irrigated soils of this peri-urban area except one where we reported the build-up of some of major and secondary plant nutrients in soils as a result of sewage irrigation (Rattan et al., 2001). The objectives of this study were: (i) to study the chemical composition of sewage effluents emanating from KEIS and groundwater; (ii) to assess the effects of long-term irrigation with sewage effluents on metal contents in soils and plants; and (iii) to assess the risk of consuming leafy green vegetables in respect of their heavy metal contents grown on sewage-irrigated soils to human beings.

Section snippets

Study area and collection of samples

The study area is located in the western Delhi, India, where sewage effluents originating from Keshopur Sewage Treatment Plant have been used for irrigation purposes since 1979 (Fig. 1). Our study confined to six villages, viz. Nilothi, Mundka, Ranhola, Bakarwala, Hirankudna and Dichaonkalan covering 123, 233, 196, 775, 183 and 98 acres of agricultural lands, respectively. Agricultural lands of first four villages have been under sewage irrigation for 20 years, while in Hirankudna and

Physico-chemical properties of sewage effluents and groundwater

Plant nutrients and heavy metal contents in sewage effluents and groundwater samples of first sampling are presented in Table 1. Data for second, third and fourth sampling were not shown, since similar values of these elements were obtained except S. In first sampling, on an average, P, K, and S contents were 0.22, 3.58 and 14.3 mg L−1, respectively in groundwater samples, whereas, corresponding values for sewage effluents were 2.57, 11.7 and 15.9 mg L−1. Among these plant nutrients, sewage

Conclusions

From this study, it can be concluded that besides use as irrigation water, these sewage effluents are also a potential source of plant nutrients. Build-up of heavy metals, particularly Zn, Cu and Ni in sewage-irrigated soils needs to be monitored periodically in view of their significant accumulation in bioavailable pool associated with decline in pH. Appreciable depletion in available Mn under these intensively cultivated sewage-irrigated soils is likely to induce unsustainability in soil

Acknowledgement

Authors would like to place on record their sincere thanks to Indian Council of Agricultural Research, New Delhi, India for funding this study as a part of AP Cess Fund project.

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