Geological knowledge and land use. Key drivers in determining natural background levels of groundwater contaminants arsenic, radon, and fluoride in the volcanic area of Viterbo (Central Italy)

Study region: The volcanic–sedimentary domain of Viterbo (Central Italy), a region characterized by complex hydrogeology, naturally elevated arsenic, fluoride, and radon concentrations, and intense industrial land-use pressures. Study focus: Understanding the combined influence of geological and anthropogenic factors on groundwater quality is essential for sustainable water resource management. This study develops an integrated regional screening framework to distinguish geogenic from anthropogenic contributions to groundwater contamination in complex hydrogeological settings, even when data availability is limited. A dataset of 659 groundwater samples was integrated with geological, structural, and land-use information within a geographic information system. Multivariate statistical analysis based on principal component analysis was combined with spatial analysis to identify dominant environmental controls and screen samples potentially influenced by human activities prior to natural background level estimation. New hydrological insights for the region: Radon concentrations are predominantly controlled by geological factors, particularly ignimbrite formations and fault systems, confirming a geogenic origin. In contrast, arsenic and fluoride exhibit mixed controls, reflecting natural background conditions locally amplified by industrial and agricultural land use. The screening framework enabled the identification and exclusion of anthropogenically influenced samples, as well as the estimation of representative regional natural background levels: 57 µg/L for arsenic, 2.80 mg/L for fluoride, and 187 Bq/L for radon. The proposed workflow provides a robust and reproducible tool for groundwater quality screening and supports informed assessment of natural background levels in hydrogeologically complex and anthropized regions.