Geochemical Behavior of Uranium and Arsenic in Watercourse Sediments of the Los Planes Watershed, Baja California Sur, Mexico: Assessment of Anthropogenic and Natural Factors

Abstract

In Baja California Sur (BCS), Mexico, the municipality of La Paz has reported higher cancer rates compared to nearby areas, linked to arsenic contamination from abandoned gold mines and naturally high uranium (U) and arsenic (As) levels in sediments. This study evaluates the impact of human activities on natural U and As anomalies in watercourse sediments of the Los Planes watershed and adjacent areas. The geochemical database included 229 analyses from the Mexican Geological Service (SGM 2017) and nine samples analyzed via Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Multivariate statistical and geostatistical methods were used to interpret the data. Using the kriging method for U and the nearest neighbor algorithm for As, spatial models were developed to define the anomalies’ positions and extents. Hierarchical cluster analysis on 85 analyses and 28 parameters identified six clusters representing different influence areas. The study found As concentrations exceeding the Mexican limit of 22 mg/kg for soils in 13 cases, with a maximum of 1520 mg/kg, primarily due to historic gold mine contamination. U concentrations ranged from 0.53 mg/kg to 7.35 mg/kg, within international protection limits, originating from Sierra la Gata’s granites and granodiorites, with potential secondary enrichment in topsoil. The possibility of anthropogenic U impact from phosphatic fertilizers is noted, warranting further investigation.