Pesticides are essential tools for agricultural productivity, yet their inherent toxicity and high mobility pose significant environmental and health risks. This study investigates the mechanisms of hydraulic transport for soluble pesticides within Cambodia's major river systems: the Bassac (BSR), Tonle Sap (TSR), and Mekong (MKR). Our analysis integrates a re-analysis of published data from 48 surface-water samples (May-June 2020, covering BSR and MKR dry and wet seasons) with new, original data (March 2025 dry season, 16 locations, 48 samples) that extends coverage to the TSR. The reinterpreted 2020 data suggested potential groundwater influence or accumulation in low-flow areas, supporting localized elevated concentrations of highly soluble pesticides, including Cypermethrin. The new 2025 data established critical hydraulic propagation pathways, confirming BSR's primary connection to MKR and its disconnection from TSR. Concurrently, a localized peak in electrical conductivity (EC) at S11-BC suggests a significant influence from sewage discharge on BSR's contaminant profile. The 2025 campaign's contaminant profile was dominated by Cypermethrin, found at an extreme localized concentration of at site 10-mk. This peak concentration results in an ecological risk of RQ ,placing it at the threshold of minimal ecological risk to aquatic life. The non-detection of other monitored pesticides, resulting in minimal to low ecological risk (RQ threshold), suggests minimal use, rapid degradation, or analytical limitations. These findings highlight the continued, risky use of banned or restricted pesticides and emphasize the critical role of localized inputs and hydraulic context in pesticide distribution. Targeted, spatio-temporal monitoring is critically needed to safeguard aquatic ecosystems in identified high-risk zones.