Scenario-Based Semi-Quantitative Assessment of Potential Contaminant Loading at a Legacy Waste Site in the Baltic Sea...

Legacy waste sites may exhibit substantial spatial heterogeneity and incomplete information on waste composition and distribution, which can constrain statistically representative assessment. This study applied a scenario-based framework for semi-quantitative hazard assessment of the Manushkino legacy waste site in the Baltic Sea drainage basin using limited state environmental inspection data. The framework integrated six lines of evidence within a Multiple Lines of Evidence/Weight of Evidence approach, including state inspection records, chemical analysis, bioassays, spatial and remote-sensing data, sensitivity analysis of potential sampling bias, and scenario-based calculations. Arsenic, a persistent and carcinogenic contaminant, exceeded the applicable soil maximum permissible concentration in all four investigated waste samples, and all four waste piles met the criterion for a technological emergency situation based on arsenic concentrations alone. Bioassays indicated biological effects across all investigated samples. Scenario-based calculations indicated a potential arsenic mass of up to 72,490 kg under the highest assumed proportion of criterion-positive material (10%), corresponding to a theoretical dilution-equivalent water volume of 7.249 × 10⁹ m³ under assumptions of complete solubility and instantaneous mixing. Under unbiased sampling (b = 1), the calculated probability of observing four criterion-positive waste piles ranged from 1.0 × 10⁻¹² to 1.0 × 10⁻⁴ across the assumed proportions of 0.1-10%. Increasing the preferential sampling coefficient increased the calculated probability of observing the 4/4 pattern. These results demonstrate the sensitivity of interpretation to assumptions regarding the spatial proportion of criterion-positive material and potential sampling bias. The calculated values represent conditional scenario-based estimates rather than statistically representative estimates of contamination across the entire waste body and do not predict actual contaminant concentrations or migration. Given the site's location within the Baltic Sea drainage basin, the findings are relevant to the precautionary approach established under the Helsinki Convention and support further site investigation and long-term monitoring under conditions of substantial uncertainty.