Effects of sewage water irrigation of cabbage to soil geochemical properties and products safety in peri-urban Peshawar, Pakistan

Irrigation of agricultural land with municipal wastewater has become a serious environmental issue due to soil contamination. The objective of the present study was to investigate the extent of contamination of vegetables grown on soil irrigated with untreated sewage water for the last four decades in suburban Peshawar. Samples of sewage water, soil, and vegetables were collected from three different sectors selected for the study. Heavy metals like Pb, Cr, Cd, Cu, Zn, and Ni in the three media were determined with atomic absorption spectrometer (AAS). Statistical analysis of data revealed that the distribution order of these metals is quite different in the analyzed samples. Some of the heavy metals, viz., Cr, Cd, Zn, and Ni, have high skewness with non-normal frequency distribution. The soil of polluted areas showed highest mean concentration of Zn (51.25 mg kg(-1)), followed by Pb (43.51 mg kg(-1)), Cu (43.3 mg kg(-1)), Ni (37.05 mg kg(-1)), Cr (28.18 mg kg(-1)), and Cd (8.51 mg kg(-1)), which were 43, 22, 33, 26, 20, and 6 times higher than the control area, respectively. The ammonium bicarbonate diethylenetriaminepenta acetic acid (AB-DTPA) extractable concentrations of Ni, Cr, and Cu in soil of polluted sectors were 12, 10, and 10 times greater than the those in control sector, respectively. Linear regression analysis revealed that enhanced levels of these toxic heavy metals in cabbage (Brassica oleracea) were strongly correlated with extractable and total heavy metal concentration in the soil. Significant difference (at P<0.012) was observed in the Pb concentration in cabbage of polluted sectors. Zn showed the highest transfer factor (TF) followed by Ni and Cu. Heavy metal concentration in sewage water and soil has exceeded the maximum permissible limits of World Health Organization (WHO 1997).