<p>This study investigated the occurrence of pesticides, their ecological risks, and potential human health implications in irrigation–drainage canals of Chiang Mai’s longan orchards during two phenological stages: flowering and easrly fruit development. In total, 25 pesticides were detected, including four fungicides, seven insecticides, and fourteen herbicides, across three canal sites (upstream, midstream, and downstream). Concentration dynamics varied strongly by phenological window and sampling sites along the canal. Among fungicides, carbendazim and hexaconazole increased during fruit set, whereas kresoxim-methyl and metalaxyl declined. For insecticides, carbosulfan peaked during flowering (&gt; 4,200&#xa0;ng L⁻¹) but declined sharply at fruit set, while formothion peaked at fruit set. Among herbicides, 2,4-D, alachlor, and atrazine generally declined, whereas isoproturon and metobromuron increased markedly during fruit set, indicating a shift toward phenylurea use. Spatially, carbosulfan peaked upstream, fungicide and insecticide spikes were greatest midstream, and herbicide increases were most pronounced downstream during fruit set. Carbendazim and carbosulfan consistently dominated and produced high ecological risk quotients (RQ ≥ 1) at all sites, while ethion and formothion contributed episodically to elevated risks. From a human-health perspective, all single-compound pesticide daily intakes (PDIs) were below their reference doses; however, when summed across compounds, the site-specific drinking-water hazard index (HI) exceeded 1, ranging from 7.8 to 38.0 at flowering and from 1.6 to 8.1 at fruit set, with the downstream sampling station and infants showing the largest HIs. Despite being a one-time assessment, these patterns likely mirror common pesticide practices in longan orchards and may emerge in other water-adjacent production areas worldwide.</p>