Strontium isotopic signatures and source apportionment in urban soils of Salgótarján: unraveling the impact of industrial emissions.

Urban soil contamination is primarily associated with human activities, especially industrial by-products, which have introduced high concentrations of heavy metal(loid)s from diverse sources over centuries of city development. This study investigated the distribution of strontium and its stable isotopic ratios (87Sr/86Sr) in urban soils of Salgótarján, a former industrial city in northern Hungary, with a focus on anthropogenic contamination, particularly from a former coal-fired power plant (CFPP). Fifteen urban soil samples from kindergartens, playgrounds, parks, and roadsides, in the northeastern part of the city were analyzed for Sr concentration, isotopic composition, and physicochemical properties. The analytical results revealed considerable variations in Sr concentrations (17.5-51.3 mg kg⁻1), and 87Sr/86Sr ratios (0.710303-0.719559), indicating mixed natural and anthropogenic inputs. A two-end-member 87Sr/86Sr isotopic model was applied to semi-quantitatively estimate source contribution, identifying coal ash as a major contamination source in soils near the former CFPP. However, certain samples gathered far from the CFPP exhibited even stronger anthropogenic influence, suggesting an additional source, likely from smelter slag (known additional industrial source in the environment) and/or unknown technogenic soils (human-transformed soils). In contrast, other samples at greater distances from the CFPP showed minimal or no coal ash contribution, instead reflecting local soil inputs (i.e., natural/artificially made quartz-rich soils), introduced during land use changes. Our study highlights the effectiveness of 87Sr/86Sr isotopic tracing in urban contamination assessment, offering a robust tool for understanding and managing urban environment quality.