Compound mobility and soil retention capacity shape neonicotinoid leaching to recharge-sensitive groundwater

Recharge-sensitive aquifers are vulnerable to current-use pesticides, but linking field occurrence to compound-specific transport remains difficult. We combined groundwater monitoring at 93 sites across dry and wet seasons in tropical-island Hainan, China, with compound-level mobility analysis and sorption and saturated-column experiments on two end-member neonicotinoids. Neonicotinoids occurred in 98.9% of samples. Among eight compounds with available property data, detection frequency increased with the calculated Groundwater Ubiquity Score and typical soil half-life (Spearman ρ = 0.83 and 0.93); the persistence association was more stable across reporting thresholds, compound subsets and parameter-range scenarios. Dinotefuran was weakly sorbed and eluted earlier, whereas thiacloprid showed delayed, tailed breakthrough and increasing apparent retardation across the tested soil cation exchange capacity (CEC) gradient. A two-site model improved the description of thiacloprid in both selected comparison soils. At field scale, widespread detections across both campaigns contrasted with infrequent, high wet-season imidaclothiz concentrations that dominated the concentration sum. The wet-season sum was 4.5 times the dry-season sum, although concentrations declined at most individual wells. These complementary observations support a screening framework based on compound mobility-persistence, soil retention capacity and seasonal sampling, without treating laboratory parameters as calibrated predictions of field transport.