The challenge of establishing natural geochemical backgrounds in human-impacted mangrove soils of Northeastern Brazil.

Mangroves are delicate ecosystems constantly pressured by urbanization, pollution, and climate change. Establishing natural geochemical backgrounds (GB) or geochemical baseline levels (GBL) for metals in these soils is challenging due to the dynamic coastal conditions and the combined influence of anthropogenic and natural geological factors. This study aims to establish the natural geochemical background of trace elements in mangrove soils, a more complex task than establishing GBL. A total of 360 soil samples were collected from the mangrove environments of Todos os Santos Bay, Northeast Brazil, specifically from the estuaries of the Jaguaripe, Paraguaçu, and Subaé rivers. The samples were analyzed using handheld X-ray fluorescence spectrometry (XRF), and the data were processed using the Tukey-Inner Fence (TIF) and Cumulative Distribution Function (CDF) methods, with results compared to the multiple regression method based on geochemical data. The Subaé estuary has the highest trace element concentrations, primarily due to its unique geological context, independent of diffuse metal contamination sources. CDF effectively proposed GBL values, providing a comprehensive view of trace element distribution. However, multiple regression outperformed both TIF and CDF methods in managing environmental complexity and modelled precise estimated individual background values, independently of anthropic contamination or variation of soil texture. The predicted background values were 19% and 43% lower than CDF and TIF values, respectively. Some samples showed particularly low modelled background values, up to 75, 25 or 11 times lower than CDF values for V, Mo, Pb respectively. This study enhances the understanding of geochemical dynamics in mangrove estuaries, demonstrating the potential to estimate reliable natural soil backgrounds using advanced statistical methods, even in contaminated and highly changing coastal environments. The findings provide valuable guidelines for assessing and preserving environmental quality in mangrove soils under anthropic pressures.