Effects and risk assessment of linear alkylbenzene sulfonates in agricultural soil. 5. Probabilistic risk assessment of linear alkylbenzene sulfonates in sludge-amended soils

Linear alkylbenzene sulfonates (LAS) can be found in high concentrations in sewage sludge and, hence, may enter the soil compartment as a result of sl udge application. Here, LAS may pose a risk for soil-dwelling organisms. In the present probabilistic risk assessment, statistical extrapolation has b een used to assess the risk of LAS to soil ecosystems. By use of a log-norm al distribution model, the predicted no-effect concentration (PNEC) was est imated for soil fauna, plants, and a combination of these. Due to the heter ogeneous endpoints for microorganisms, including functional as well as stru ctural parameters, the use of sensitivity distributions is not considered t o be applicable to this group of organisms, and a direct, expert evaluation of toxicity data was used instead. The soil concentration after sludge app lication was predicted for a number of scenarios and used as the predicted environmental concentration (PEC) in the risk characterization and calculat ion of risk quotients (RQ = PEC/PNEC). A LAS concentration of 3.6 mg/kg was used as the current best estimate of PNEC in all RQ calculations. Three le vels of LAS contamination (530, 2,600; and 16,100 mg/kg), three half-lives (10, 25, and 40 d), and five different sludge loads (2, 4, 6, 8, and 10 t/h a) were included in the risk scenarios. In Denmark, the initial risk ratio would reach 1.5 in a realistic worst-case consideration. For countries not having similar sludge regulations, the estimated risk ratio may initially b e considerably higher. However, even in the most extreme scenarios, the lev el of LAS is expected to be well beyond the estimated PNEC one year after a pplication. The present risk assessment, therefore, concludes that LAS does not pose a significant risk to fauna, plants, and essential functions of a gricultural soils as a result of normal sewage sludge amendment. However, r isks have been identified in worst-case scenarios.