Field deployed passive samplers including semi-permeable membrane devices (SPMDs) provide an informative and cost-effective approach in the monitoring of contaminants over time. However, sequestration of contaminants in passive samplers can vary based on a number of environmental factors including water chemistry and flow, biofouling and deployment time. In three one-month long trials, SPMDs were concurrently deployed for 4, 15 and 28 days in three different stream sites in an extensive agricultural area in south-eastern Costa Rica. Water, bottom sediment and suspended sediment grab samples were also collected at corresponding time intervals over each month and several environmental variables were monitored daily. In all three sites, SPMD concentration of the widely-used insecticide chlorpyrifos increased with deployment time, with no significant relationship between membrane biofouling and pesticide uptake. Higher suspended solids concentrations combined with lower water flow conditions were positively correlated to increased chlorpyrifos concentration among replicate sites. The longer exposure period of passive samplers also allowed for the detection of lower concentrations of chlorpyrifos, terbufos and difenoconazole compared to grab samples. In addition to flow and deployment time, several other environmental variables including stream turbidity and morphology may also be important for optimum SPMD deployment and data interpretation.