Impact of wastewater irrigation on the dynamics of metal concentrations in the vadose zone: monitoring: part I

Agricultural lands in the peri-urban area of New Delhi have been irrigated with treated wastewater from the Keshopur Effluent Irrigation Scheme (KEIS) since 1979. An attempt has been made to study the influence of wastewater irrigation on pH, electrical conductivity, organic carbon, and dynamics of heavy metal concentrations in vadose zone under KEIS. For this study, agricultural lands which have been receiving the sewage irrigation for 20, 10, and 5 years were selected. Adjacent tube well water-irrigated fields were selected and used as reference. Results indicate that there was a significant decrease in soil pH and electrical conductivity (EC) of sewage water-irrigated fields as compared to tube well water-irrigated fields. Organic carbon (OC) content in 20 years sewage-irrigated soil (0–15 cm) increased by 244 % over tube well water-irrigated soil. The increases in organic carbon content for 10 and 5 years sewage-irrigated surface soil (0–15 cm) were 138 and 60 % over tube well water-irrigated soils. The second-order polynomial model suggested that there is a possibility of accumulation of organic carbon in soil due to sewage irrigation at 1.55 t⁻¹ ha⁻¹ year. There was an increase in diethylene triamine pentaacetic acid (DTPA)-extractable Zn content in 20, 10, and 5 years sewage-irrigated soils (0–15 cm) to the extent of 86, 38, and 36 % over tube well water-irrigated soils, respectively, while only 20 years sewage-irrigated soils showed a significant increase in DTPA-extractable Cu (289 %) in surface layer (0–15 cm) over tube well-irrigated soils. In case of Fe, 127, 88, and 76.6 % increases in available Fe content (DTPA-Fe) were recorded under 20, 10, and 5 years sewage-irrigated soils, respectively, over controls. Like Cu, DTPA-extractable Ni also exhibited a significant increase (42.2 %) in 20 years sewage-irrigated soil over tube well-irrigated ones. It appears that due to sewage irrigation particularly in surface soil, DTPA-Mn content either remained same or showed a slight decline as compared to tube well-irrigated soils. The assessment of contamination factor (CF) in respect of total metal content in soil indicated the moderate level of metal contamination even after such long-term of sewage irrigation.