Trace-element enrichment in agricultural soils under wastewater-influenced surface-water irrigation: a plot-scale study in an Andean micro-watershed

Abstract

Abstract The use of wastewater-influenced surface waters for agricultural irrigation is common in many regions, yet its implications for soil geochemistry remain insufficiently characterized, particularly in Andean micro-watersheds. This study evaluates the occurrence and distribution of potentially toxic elements (PTEs) in agricultural soils irrigated with wastewater-influenced surface waters within an Andean micro-watershed in Peru. Seven composite soil samples representing individual cultivated plots were analyzed for trace element concentrations using inductively coupled plasma mass spectrometry, together with physicochemical and mineralogical characterization. Hydrochemical data were used to characterize irrigation water quality within the micro-watershed context. Contamination factor (CF) and geoaccumulation index (Igeo) were applied to assess relative enrichment with respect to reference background values. All measured PTE concentrations were below the Peruvian Environmental Quality Standards and the World Health Organization and Food and Agriculture Organization guideline values. However, arsenic and cadmium exhibited the highest relative enrichment among the analyzed elements, with CF values of 2.89 and 1.89, respectively, and positive Igeo values indicative of unpolluted to moderately polluted conditions. In contrast, hydrochemical analyses showed stable Ca–HCO₃ water facies and low trace element concentrations at the time of sampling. The results indicate that regulatory compliance does not necessarily imply uniform geochemical behavior among elements at the plot scale. These findings highlight the value of enrichment-based indicators for identifying subtle deviations from background conditions that may not be evident from regulatory threshold assessments alone.