Hidden below the surface: Depth- and season-resolved pesticide mixtures in agricultural soils

Soil degradation and contamination pose significant risks to environmental quality, food security, and human health, particularly in intensively managed agricultural systems. Agricultural soils act both as sinks and secondary sources of chemical contaminants, regulating their mobility, persistence, and transfer to other environmental compartments such as groundwater and surface water. Among these contaminants, pesticides are of particular concern due to their widespread use, frequent co-occurrence as complex mixtures, and the formation of transformation products with uncertain environmental and toxicological profiles. Yet pesticide contamination is still predominantly assessed in surface soils, implicitly assuming rapid dissipation and limited relevance of subsurface layers.

In reality, soils are vertically structured and dynamic systems in which hydrological processes, soil properties, and climatic drivers, such as monsoons or heavy rainfall, can facilitate the downward transport, accumulation, and long-term persistence of pesticide residues beyond the surface layer. Empirical evidence on how pesticide mixtures redistribute across soil depths and seasons under field conditions remains scarce, especially in subtropical agroecosystems subject to intense rainfall pulses. This knowledge gap limits accurate exposure assessment and weakens soil quality indicators used to protect environmental and human health.

Here, we investigated depth- and season-resolved pesticide contamination in a subtropical agricultural landscape by analyzing 181 pesticides and metabolites in 246 soil samples collected from 41 agricultural sites. Samples were obtained from three depths: 0–5 cm (surface soil), 15–20 cm (plough layer), and 55–60 cm (deep soil) during pre-monsoon (June 2024) and post-monsoon (February 2025) periods. Sixty-one compounds were detected above quantification limits, with pesticide mixtures present in nearly all surface and plough-layer soils and in 71% of deep soils. Metabolites were particularly prominent at depth and frequently exceeded parent compounds in detection frequency, indicating transformation-driven persistence in subsurface horizons.

Comparisons with commonly used property-based mobility and persistence indicators (Koc, DT50, and GUS) revealed systematic mismatches between predicted and observed field behavior, including the deep occurrence of compounds classified as non-leaching or low-risk. These discrepancies highlight the limitations of equilibrium-based screening approaches under hydrologically dynamic conditions. To address this, we developed field-derived indices integrating seasonal occurrence, vertical mobility, and inter-seasonal carry-over with hazard classification. This approach identified several current-use pesticides, including clothianidin, carbendazim, bifenthrin, and difenoconazole, as high-priority compounds for routine monitoring due to their persistence and high toxicity to non-target organisms, including humans.

Our findings demonstrate that subsurface pesticide mixtures represent an overlooked exposure compartment in agricultural soils. Incorporating depth-resolved, metabolite-inclusive monitoring and field-based behavioral indicators into soil quality assessment and regulatory frameworks is essential for improving environmental risk evaluation, protecting groundwater resources, and safeguarding environmental and human health.