Probabilistic risk assessment for linear alkylbenzene sulfonate (LAS) in sewage sludge used on agricultural soil

Abstract

Deterministic and probabilistic risk assessments were developed for commercial LAS in agricultural soil amended with sewage sludge. The procedure done according to ILSI Europe's Conceptual Framework [Schowanek, D., Carr, R., David, H., Douben, P., Hall, J., Kirchmann, H., Patria, L., Sequi, P., Smith, S., Webb, S.F., 2004. A risk-based methodology for deriving quality standards for organic contaminants in sewage sludge for use in agriculture-conceptual Framework. Regul. Toxicol. Pharmacol. 40 (3), 227-251], consists of three main steps. First, the most sensitive endpoint was determined. This was found to be the chronic ecotoxicity of LAS to soil invertebrates and plants. Additional endpoints, such as potential for plant uptake and transfer in the food chain, leaching to groundwater, surface erosion run-off, human health risk via drinking water, plant consumption and soil ingestion were also systematically evaluated but were all assessed to be of little toxicological significance. In the second step, a back-calculation was conducted from the Predicted No-Effect Concentration in soil (PNECsoil) to a safe level of LAS in sludge (here called 'Sludge Quality Standard'; SQS). The deterministic approach followed the default agricultural soil exposure scenario in the EU-Technical Guidance Document (TGD). The SQS for LAS was calculated as 49 g/kg sludge Dry Matter (DM). In order to assess the potential variability as a result of varying agricultural practices and local environmental conditions, two probabilistic exposure assessment scenarios were also developed. The mean SQS was estimated at 55 and 27.5 g/kg DM for the homogeneous soil mixing and soil injection scenarios, respectively. In the final step, the resulting SQS values were evaluated for consistency and relevance versus available information from agricultural experience and field tests. No build-up, adverse impact on soil fertility, agronomic performance, or animal/human health have been reported for agricultural fields which have received sludge with high LAS levels for up to 30 years. Distribution statistics of LAS concentrations in anaerobically digested sewage sludge measured across Europe were created (mean value: 5.56 g LAS/kg sludge DM). When compared to the above mean SQS values, adequate risk characterisation ratios of 0.08-0.2 were found. The 'ecological risk' parameter calculated for anaerobic sludge from the probabilistic approaches was below 3%. A regulatory Limit Value for LAS of 2.60 g/kg sludge DM was originally proposed in the 3rd Draft of the Working Document on Sludge [CEC, 2000b. Working Document on Sludge. Third Draft, Brussels 27 April 2000, DG. Environment, 18 p.]. The current assessment, based on an updated dataset and a refined assessment procedure, suggests that the need for a limit value for LAS in sewage sludge cannot be substantiated on a risk basis.