Effect of Wastewater Irrigation on Heavy Metal Accumulation, Growth and Yield of Vegetables

Use of wastewater for irrigation is on the rise in India and other developing countries. Wastewater contains plant nutrients that favour crop growth but leave a burden of heavy metals which can enter the food chain and is a cause of great concern. This study was conducted to explore the possibility of using wastewater to grow four vegetables fenugreek (Trigonella foenum-graecum L.), spinach (Spinacia oleracea L.), radish (Raphanus sativus L.) and carrot (Daucus carotaL.). Two aspects namely (1) effect on plant growth and yield (2) accumulation of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) in leaves and roots of the plant have been presented in this paper. The physico-chemical analysis of the wastewater showed that it was rich in total suspended and dissolved solids with large amount of BOD and COD. The higher amount of Cl-, Ca++, Mg++ and K+ were also present in the effluent. The heavy metal (Cd, Cr, Cu, Ni, Pb and Zn) content in wastewater is comparatively more than groundwater (GW). The values of these heavy metals were slightly higher in the soil irrigated with wastewater. The effluent severely affects crop plants and soil properties when used for irrigation. The growth characteristics (plant length, plant fresh and dry weight, leaf number and leaf area) as well as yield characteristics (seed number, 1000 seed weight and seed yield) of all the plants, irrigated with 100% wastewater, were more than that with groundwater. The pattern of increase for the plants was fenugreek> radish> spinach> carrot. Though the wastewater contains low levels of the heavy metals, the soil and plant samples show higher values due to accumulation, but their level was under permissible limits in plants. The trend of metal accumulation in wastewater-irrigated soil is in the order: Pb> Ni> Zn> Cu> Cr> Cd. Of the four plants that are grown, the order of total heavy metal accumulation in roots is Carrot> Radish, while in leaves the order is Spinach> Fenugreek. The enrichment factor (EF) of the heavy metals in contaminated soil is in the sequence of Ni (3.1)>Pb (2.6)> Cd (2.35)> Zn (2.18)> Cu (1.66)> Cr (1.05), while in plants EF varies depending upon the species and plant part. Radish and carrot show a high transfer factor (TF >1) for Cd signifying a high mobility of Cd from soil to plant whereas the TF values for Pb are very low as it is not bioavailable. Thus, it may be concluded that wastewater may be used profitably for the cultivation of these vegetables and could effectively supplement not only the nutrient requirement of the crop but may also act as the source of water.