Uptake of dioxin-like compounds from sewage sludge into various plant species–assessment of levels using a sensitive bioassay

A bioassay for the detection of dioxin-like compounds was used to estimate uptake of dioxin-like compounds in carrots, oil seed rape seeds, zucchinis and cucumbers grown in soil amended with sewage sludge from Swedish sewage treatment plants (STP). This sensitive bioassay is based on 7-ethoxyresorufin O-deethylase (EROD)-induction in cultured chicken embryo livers and reflects the combined biological effect of all dioxin-like compounds in a sample, including ones that seldom are analyzed. The bioassay detected low concentrations of dioxin-like compounds in all carrot, zucchini and cucumber samples, but did not detect any dioxin-like compounds in the rape seeds. In carrots the concentrations were increased up to seven times when grown in soil amended with high applications of some of the sludge samples, while others did not increase the concentrations compared to control. More realistic sludge applications only increased the concentrations slightly. The sludge-fertilized carrots contained the highest concentrations of the investigated plants (up to 14 pg bioassay-derived TCDD equivalents (bio-TEQs)/g d.w.). In the carrots, differences in uptake of dioxin-like compounds depended on the sludge origin, which may be due to more easily bioaccumulated dioxin-like compounds in some sludge samples, or other components that facilitated uptake into the carrots. In the cucumbers, a more than two-fold increase (from 0.2 to 0.5 pg bio-TEQs/g d.w.) was observed in specimens grown in sludge-amended soil when compared to controls, suggesting a small uptake from the roots to the shoots. No sludge-dependent increase in uptake was seen in the zucchini fruits. The bio-TEQ levels were generally low in the consumable above ground plant parts of the investigated species. However, the question if repeated sludge application results in a soil accumulation of dioxin-like compounds, thereby increasing the risk of plant uptake, remains to be investigated.