Quantitative Assessment Of Heavy Metals In Soils And Vegetables Irrigated With Sewage In Rawalpindi Area

Pakistan is suffering from shortage of surface supplies, which is being overcome by the conjunctive use of good quality ground water for growing cereals in irrigated areas anduse of sewage/ industrial effluents for raising vegetables in city conurbations. The farmers get good vegetable yields with sewage water as it contains large amount of organic matter and plant nutrients. However, sewage also contains some metals which could be potentially toxic for biological life. The amount of each metal accumulated in plant depends on the quality of sewage used for irrigation. The metals which have been identified in sewage include chromium, cadmium, copper, lead, nickel, zinc, cobalt, magnesium, iron and arsenic.This study was carried out in Rawalpindi area with the objective to quantify the heavy metal accumulation in different vegetables and soils irrigated with sewage. Study was carried out in two different stages. In the first stage, field survey was carried out during 2003 and2004. During this survey, water samples from different irrigation sources such as tubewells, wells, minidams and sewage were collected. Vegetable leaf samples and soil samples (0-30 cm depth) were obtained from the fields from where water samples were collected. The collected samples were processed following standard procedures and were analysed for lead, cadmium, chromium and nickel.After the survey work, three experimental sites having good quality tubewell water and sewage were selected in Taxila, Rawalpindi and Gujar Khan area. In the second stage,field experiments on the selected sites were conducted in Rabi, 2004 and Kharif, 2005. At each site, three treatments were tested viz, T1 = sewage irrigation, T2 = sewage and tubewell irrigation alternativeely, T3 = tubewell irrigation. Each treatment was replicated thrice following two factor factorial design. The plot size was 10 x 5 meter square. Cauliflower was sown in august, 2004 and spinach was sown in November, 2004. Okra and squash were sown in march, 2005. Each field was irrigated with measured quantity of water depending upon their crop water requirements. Water samples were collected before irrigation. Soil samples were collected from 0-30, 30-60 and 60-90 cm depth before and after each season. Leaves samples were collected after 30, 60 and 90 days and fruit samples were collected after 90 days of crop germination. The collected soil, water and plant samples were analyzed for lead, cadmium, chromium and nickel along with zinc, iron, copper and manganese.The survey data showed variation in heavy metals concentration of different water sources. All water samples for lead were within safe limits. Heavy metals in tubewell irrigated vegetables were below the safe limits and in sewage and industrial effluents irrigated vegetables were above the safe limits. Heavy metals in soil samples irrigated with different water sources were below the critical levels.The field experimental data showed that due to sewage application, lead and copper contents were much higher in leaves of spinach, leaves and fruits of cauliflower, okra andsquash as compared to tubewell irrigation at Taxila, Gujjar Khan and Rawalpindi sites.Cadmium accumulation in all the vegetables irrigated with sewage at all the three sites were also much higher as compared to tubewell irrigation, but found less than lead and higher than chromium. Like all the heavy metals, nickel showed the similar trend for its accumulation in all the vegetables. The accumulations of all metals in vegetables irrigated with alternate treatment were even lower than those irrigated with sewage and higher than tubewell irrigation. Similarly, heavy metals accumulation in soil and vegetables were maximum at Taxila site, as the sewage used for irrigation was more contaminated as compared to other sites.