Impact of recurring irrigation with treated domestic wastewater on heavy metal accumulation in the soil

Water reuse for irrigation is a common practice globally, serving as an alternative to clean water. Wastewater contains traces of heavy metals that accumulate in the soil and food chain, posing an environmental and public risk. This study investigated the effect of irrigation with treated domestic wastewater (TDW) emanating from a homogenous community on heavy metal accumulation in the soil irrigated for three recurring farming cycles, namely R1, R2, and R3. A greenhouse experiment was conducted involving plots irrigated with TDW (PTw) and plots irrigated with fresh water (PFw) in triplicate. Soil samples were collected at depths of 0–20, 20–40, and 40–60 cm. Results revealed that heavy metals Pb, Cu, Cr, Zn, Cd, and Ni remained within the WHO permissible limits in the PTw and PFw, except Cd at 40–60 cm after the R3 in the TPw. Irrigation with TDW caused a decrease in Pb and Cd with respect to the baseline levels by 86–91% and 88–96% respectively. Other heavy metals increased in the soil in the order of Cr>Cu>Ni>Zn at 0–20 cm, Cr>Cu>Zn>Ni at 20–40 cm, and Cr>Cu>Ni>Zn at 40–60 cm. Variations in heavy metal concentrations were inconsistent with farming cycles except for Cu, which increased in the order of 3.2±0.75, 4.4±0.66, and 4.8±1.23 mg/kg at 0–20 cm for R1, R2, and R3, respectively, and 3.4±0.66, 3.7±0.62, and 6.0±0.44 mg/kg at 20–40 cm in the same order. Pollution index values revealed that irrigation with TDW for three recurring farming cycles caused slight soil contamination (1≤PI<2) with Zn and Ni, moderate soil contamination (2≤PI<3) with Cu, and high soil contamination (3<PI) with Cr at depths of 0–20 and 40–60 cm. The pollution load index indicated moderate soil pollution (1<PLI≤2) with heavy metals after the R3 at 40–60 cm. The findings suggest that irrigation with TDW proceed for at least three recurring farming cycles without causing significant soil heavy metal accumulation.