EFFECT OF SEWAGE SLUDGE ADDITION ON HEAVY METAL CONCENTRATIONS IN AGRICULTURAL SOILS

Land application of sewage sludge has been a worldwide agricultural practice for many years. Although sewage sludge provides essential plant nutrients and organic matter for plant growth, its continual use over extended periods can result in accumulation of heavy metals to levels detrimental to the environment. Such enhanced soil metal contents may result in reduced plant growth (alongside other symptoms of phytotoxicity); metals can enter the food chain via human consumption, and are therefore of interest in regard to their potential impact on human health. Recently, this has been identified as a prime problem in the agricultural lands of the areas surrounding New Delhi, the capital city of India. For the reported study, soil samples were collected from ten spatially variable locations from an agricultural field in New Delhi, which was historically (exact duration unknown) amended with domestic sewage sludge of unknown chemical composition. The practice ended in the later part of the last century. Uncontaminated control soils were collected from one location in the vicinity (a residential garden soil) for comparison purposes. The primary focus of the study was to evaluate the effect of long-term application of sewage sludge on the geochemical fate (and hence, phytoavailability) of heavy metals in agricultural soils. The sludge-impacted soil samples are currently being analyzed for total concentrations of heavy metals typical to sewage sludge, namely Cr, Cu, Cd, Ni, Mn, Zn, Pb, Fe, Co as well as for their plant available fractions (soluble and exchangeable forms) in comparison with the control soil. Because soil properties play an important role in the retention/release characteristics of heavy metals, the soil samples are also being characterized for selected physico-chemical properties, such as texture, pH, salinity, cation exchange capacity, organic matter content, and total and extractable concentrations of Ca, Mg, Fe, Al and P. The soil properties will be correlated with total and available heavy metal concentrations to quantify their relative effects on metal uptake potential by plants.