Health Risk Assessment, Pore Water Chemistry, and Assessment of Trace Metals Transfer from Two Untreated Sewage Sludge Types to Tomato Crop (Lycopersicon esculentum) at Different Application Levels

The application of untreated sewage sludge to cropland in water-deficient areas is common practice. A study was conducted to investigate the transfer of trace elements from sewage sludge to tomato crop and the potential health risk to humans. Two types of sewage sludge, ISS (I-9 Sector wastewater treatment plant, Islamabad) and WSS (Water and Sanitation Authority wastewater treatment plant, Faisalabad), were applied at 0.5%, 1.0%, and 1.5% w/w and compared with control (without any amendment). The test crop was tomato (Lycopersicon esculentum). Results revealed that the ECe of soil was increased by these treatments with respect to all application levels. The levels of Pb (lead) and Zn (Zinc) solubility in soil were increased about 46- and 28-fold by the application of ISS at 1.5% and 16- and 22-fold by the application of WSS at 1.5%, respectively. The highest shoot biomass was recorded with 0.5% level of WSS, while higher rates (1.0% and 1.5%) of both ISS and WSS showed significant (p < 0.001) decline in shoot biomass production, and the lowest SFW was recorded with the addition of ISS at 1.5% application rate (a significant reduction of 44%) compared to control. The pore water analysis and correlation depicted that dissolved organic carbon (DOC) controlled the release of Zn and Pb. The PLI (value < 1) indicated that the overall pollution of trace metals in the investigated samples was absent, but the Igeo and CF showed the contamination potential for Cd, Cu, and Ni was moderate to strong. DIM and HRI analysis suggested that the tomato was safe for human consumption. The HRI values for all trace metals were below the permissible limit (HRI = 1) described by USEPA-IRIS. It was concluded that a lower rate of both sewage sludge types produces more biomass and less accumulation of trace metals in the test crop.