Post-ban Assessment of Residual Paraquat in Surface Water and Self-Reported Health Symptoms in An Intensive Agricultural Watershed, Thailand

Abstract

Despite Thailand’s nationwide ban on paraquat, its strong adsorption to soil particles contributes to its long-term persistence in agricultural landscapes. While the ban has curtailed new applications, evidence evaluating environmental residues in postban settings remains limited. In this study, surface water quality indicators and paraquat residue across three districts in Phitsanulok Province were assessed using integrated environmental monitoring, ecological risk characterization, chronic and upper-bound screening hazard quotient (HQ) analysis, and a household questionnaire of 231 residents in the district with the highest paraquat detection frequency and screening-level HQs. The results indicated that paraquat was detected at 75% of the sampling sites in Phrom Phiram, whose mean concentration was the highest (0.39 ± 0.06 mg L^-1). Phosphate levels were moderately positively correlated with paraquat (r = 0.426, p = 0.030), indicating an association between the two variables. The screening-level HQs exceeded the ecological benchmark in all the districts under the evaluated exposure scenarios, including the upper-bound screening scenario. Among the surveyed farmers, 69% reported acute symptoms following herbicide exposure, primarily ocular and respiratory irritation. The persistence of paraquat residues indicates potential ecological concern, whereas the high prevalence of self-reported symptoms highlights broader occupational health concerns among farmers. These findings underscore that regulatory bans should be complemented by ongoing environmental monitoring and management to mitigate residual exposure risks.