POCIS calibration for organic compound sampling in small headwater streams.

: Field-based atrazine sampling rates (Rs ) by the polar organic chemical integrative sampler (POCIS) were measured in nine headwater streams in three years covering 5 to 6 exposure periods of 2 to 3 weeks per site per year. Rates were best in line with the model Rs = 148 mL/d, with a standard deviation of 0.17 log units (factor 1.5). POCIS canisters reduced mass transfer coefficients of the water boundary layer by a factor of 2 as measured by alabaster dissolution rates. A mechanistic model that accounts for flow and temperature effects yielded a fair estimate of the effective exchange surface area (12.5 ± 0.8 cm2 ). This model could only be tested for higher flow velocities because of uncertainties associated with the measurement of flow velocities < 1 cm/s. Pictures of sorbent distributions in POCIS showed that the effective exchange surface area varied with time during the exposures. Error analysis indicated that sorbent distributions and chemical analysis were minor error sources. The main conclusion of this study is that an atrazine sampling rate of 148 mL/d yields consistent results for all three years across nine headwater streams. This article is protected by copyright. All rights reserved.