Occurrence and spatial distribution of selected heavy metals and plastic fractions in surface soils irrigated with urban wastewater of Okara City, Pakistan

Abstract

Background: The use of wastewater for irrigation has been suggested as an alternative to the use of freshwater. Several studies have reported heavy metals (HMs) and microplastics (MPs) accumulation in wastewater irrigated surface soils and their related health outcomes. In Okara city, agricultural soils have been irrigated with wastewater. Objectives: The present study was aimed to investigate the HMs and MPs concentration level and their spatial distribution in wastewater irrigated surface soils of Okara, Pakistan. Furthermore, ecological, and human health risks associated with wastewater irrigated surface soils contaminated with HMs were evaluated. Methodology: Three wastewater disposal facilities North disposal station 1 (NDS1), North disposal station 2 (NDS2), South sewage line (SSL) were selected in Okara city. A total of 27 surface soil samples irrigated with three wastewater disposal facilities from nine locations on the basis of distance (Start, Middle, End) were collected and further examined the soil for physicochemical properties such as pH, EC, salinity, OM, WHC and HMs (Cd, Cu, Zn, Pb, Fe, Mn) via Atomic Absorption Spectrophotometer AAS (Agilent 55AA). Furthermore, MPs in soil were characterized by binocular microscopy. Pollution indices for ecological risk and non-carcinogenic health risks were calculated for HMs in contaminated soil. Results: The results revealed that wastewater irrigated surface soils were contaminated with HMs having mean concentrations (Cd: 0.48 mg kg-1 , Cu: 17.99 mg kg-1 , Zn: 29.73 mg kg-1 , Pb: 19.65 mg kg-1 , Fe: 8559.5 mg kg-1 , Mn: 182.94 mg kg-1 ) were in permissible limits recommended by WHO/FAO and EU, except Mn. Moreover, high HMs contents (Cd, Cu, Pb) were recorded in soil samples of SSL attributed to statistically significant variations from soil samples of NDS1 and NDS2 while Zn, Fe, Mn showed non-significant difference. The results depicted MPs abundance in wastewater irrigated surface soils was ranged between 1850-5300 particles kg-1 and depicted non-significant variations among soil samples of three disposal facilities. The small sized MPs <500µm showed greater portion (31%) in wastewater irrigated surface soil samples. Soil physicochemical properties such as pH affected HMs mobility. EC IX and salinity showed negative correlation with HMs and MPs while OM and WHC showed positive correlation. On the basis of pollution indices CF, IPOLL, Igeo, EF HMs pollution in soil was in the range of low pollution to considerable pollution and Risk index (RI) for all HMs was <150 attributed to low ecological risk. In the present study, negligible non-carcinogenic human health risk via three pathways (oral, inhalation, dermal) to both adults and children was detected. However, high vulnerability and susceptibility for children was investigated via HMs contamination in soil. Conclusion: The study revealed high level of Mn and <500µm MPs in wastewater irrigated surface soils. However, HMs contamination presented low ecological risks and negligible non-carcinogenic human health risks. But, monitoring of HMs and MPs level in the soil of particular area and their remediation plans are suggested for future management approaches