Residues and source identification of persistent organic pollutants in farmland soils irrigated by effluents from biological treatment plants.

Sewage and industrial effluents from biological treatment plant have been widely used for agricultural irrigation in north part of China. However, effluents after biological treatment still contain heavy metals and persistent organic contaminants. The persistent organic contaminants accumulated in soil may transfer through the food chains and cause adverse health effects on human or biological effects on soil fauna and flora after long-term application. In present study, field surveys were carried out in the farmlands irrigated by effluents from biological treatment plants that receive sewage wastewater and industrial discharges. Residues of polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs) in the soils irrigated using both ground water and effluents were compared. The origins of PAHs in the soils were discussed. The results showed that wastewater irrigation could cause accumulation of PAHs in soils close to the pollution discharge. Significantly higher concentrations of PAHs were observed in the sampling sites close to the entrance of main channel in contrast to those along branches and the reference sites. There was no significant relationship between the accumulation of persistent organic pollutants and organic matter content in soil (TOC). Soil contamination of these persistent organic pollutants as affected by effluent irrigation was characterized by the dominant accumulation of high-molecular-weight PAHs (HMW-PAHs). In the case study, concentration of benzo[a]pyrane (BaP, 45.6 ng/g), indeno[1,2,3-cd]pyrene (IcP, 86.3 ng/g), benzo[g,h,i]perlene (BgP, 66.9 ng/g) could exceed the limits of the soil quality standard for biodegraded soils. In identification of the sources, the IcP/BgP values of PAHs in soils were more close to that in air particulates from coal/coke source (1.09+/-0.03 ng/g) [Dickhut RM, Canuel EA, Gustafson KE, Liu K, Arzayus KM, Walkers E, et al. Automotive sources of carcinogenic polycyclic aromatic hydrocarbons associated with particulate matter in the chesapeake bay region. Environ Sci Technol 2000;34:4635-40]. Therefore, both of the PAHs residues in effluents and emission from a nearby coal/coke plant were responsible. Also in this case study, low levels of the OCPs were observed and were not of significant concern in this wastewater irrigation area. Among the different OCPs analyzed, DDTs (mean 8.41 ng/g) and HCHs (mean 2.91 ng/g) were the major components. From the ratios of DDT/DDTs and beta-HCH/HCHs, it indicated that OCPs residues should be from historical usage.