<p>This study investigated the occurrence, environmental fate, and risk implications of atrazine, glyphosate, and its primary metabolite aminomethyl phosphonic acid (AMPA) in a maize-based highland agriculture in northern Thailand. Soil samples were collected from five slope transects during wet and dry seasons and analyzed using LC-MS and HPLC-FLD. An integrated framework combining mass balance analysis, multi-year accumulation modeling, slope-concentration regression, and probabilistic Monte Carlo simulations was applied. AMPA was detected in 92% of samples and dominated soil residue profiles, consistently exceeding parent glyphosate concentrations. Herbicide concentrations were significantly higher during the wet season and increased with slope gradient; atrazine concentrations rose by 1.78-fold per 5% slope increment. Mass balance analysis revealed that &gt; 99% of applied atrazine and glyphosate dissipated within a single growing season, whereas AMPA accumulated over time, approaching pseudo-steady state within five years. Scenario analysis indicated that moderate intensification of glyphosate application (1.5-2X current rates) could increase long-term AMPA concentrations by 1.5-2.0-fold within 15–20 years. Probabilistic risk assessment demonstrated negligible ecological and non-carcinogenic human health risks under current conditions. Atrazine dominated ecological risk due to its lower toxicity threshold, while AMPA was the primary contributor to uncertainty due to its persistence and limited compound-specific toxicological data. These findings highlight the critical role of slope and seasonal rainfall in controlling herbicide transport and dissipation in tropical highland systems. They further emphasize the need for AMPA-specific toxicological characterization, slope-targeted mitigation measures, and adaptive risk governance frameworks capable of addressing long-term accumulation under potential agricultural intensification.</p> Graphical Abstract <p></p>