Purpose

Heavy metal/metalloid pollution in sewage-irrigated soils (SIS) is one of the major environmental issues in China. Mobility of heavy metals/metalloids in the contaminated soils and their potential threats to groundwater are needed to be understood. This study investigated impacts of organic matter, solution pH, and water level on forms and migration of heavy metals/metalloids (As, Cr, Cu, Pb, and Cd) in the contaminated soils from a typical sewage-irrigated area in the North China Plain (NCP).

Materials and methods

Based on extensive field measurements, two sewage-irrigated sites and a non-sewage-irrigated site were selected in the study area. Soil samples were collected every 10 cm in a profile (no more than 1 m) from each site. The release and mobility of heavy metals/metalloids in the SIS were investigated using column experiments with two leaching stages (stage I: low water level; stage II: high water level). Deionized water, synthesized acid rain, and poultry manure extractant were used as influents for different columns to stimulate leaching of rainfall, acid rain, and compost leachate, respectively. Soil samples before and after leaching were subjected to sequential extraction for 7 target phases, evaluating the impacts of three leaching conditions on forms of heavy metals/metalloids in soils at different depths.

Results and discussion

Results revealed that more As and Cu were mobilized in the column with a compost leachate as the influent than the columns with organic matter-free influents, resulting from reductive dissolution, complexation, and desorption. Cr and Pb concentrations were higher in effluents from the column with low pH influent (acid rain) than those from near-neutral pH influents (rainfall and compost leachate). Proton-promoted desorption and dissolution of very amorphous Fe oxides was the major cause for mobilized Cr and Pb in porewater. Moreover, decrease in dissolved oxygen (DO) was observed with the increase in water level in the column, triggering microbial respiration of Fe/Mn oxide reductions in the presence of bioactive dissolved organic matter, which elevated concentrations of porewater heavy metals/metalloids.

Conclusions

This study suggested that sewage irrigation significantly increased heavy metal/metalloid contents associated with labile and bioavailable fractions in the SIS, and both the compost leachate and low pH solution should enhance the mobility of heavy metals/metalloids in the SIS and potentially contaminate groundwater.