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The samplers were exposed 28 days in August 2002 at a site in the river Weisse Elster in Saxony-Anhalt, Germany. aCV, coefficient of variation or relative standard deviation of multiple samples.

The samplers were exposed 28 days in August 2002 at a site in the river Weisse Elster in Saxony-Anhalt, Germany. aCV, coefficient of variation or relative standard deviation of multiple samples.

contained quantifiable amounts of lindane, PCB 28 and PAHs with up to four aromatic rings. Nevertheless, analyte levels found in field exposed samplers were in all cases significantly higher than those in control blanks. The variation of the masses recovered from three replicate field exposed devices ranged from 1% (benzo[b]fluoranthene) to 27% (lindane). This is an excellent precision despite the degradation of the protective cellulose membranes of the MESCOs during exposure.

10.5.1.7 In situ exchange kinetics from PRC offload Our previous investigations have shown that both uptake and elimination of a particular compound in MESCO I are characterised by the same exchange rate constant ke, according to Eq. (10.1) [13]. The use of PRCs allowed a two-point estimation of the first-order exchange rate constants ke. These were calculated from the rearranged Eq. (10.3) using mean values (from replicate samples) of the PRC amounts found in field exposed samplers (mS) and in the controls (m0) and exposure time of 28 days:

The calculated ke values ranged from 0.072 day—1 (D10-PYR) to 0.126 day—1 (D10-BIP). Student's t-test (a — 0.05) was performed to ensure that changes in PRC residue concentrations were statistically significant, according to the law of error propagation. This was the case for all PRCs excepting D12-BaA with no significant offload during exposure.

The field-derived ke values were two to three times higher than those reported in a laboratory calibration study [13]. This indicates faster exchange kinetics at the sampling site than those observed under laboratory conditions. The temperature at the sampling site during the field study was similar to that in the calibration study. Although this investigation [13] indicated that the flow velocity had no significant effect on the exchange kinetics, this was tested only at low velocities. The flow around the cage with samplers in the field was much faster than the simulated flow in the calibration apparatus, and the increased water turbulence might have affected the analyte mass transfer between water body and samplers, despite the buffering effect of the protective cage. The elevated exchange kinetics can also be explained by degradation of cellulose membranes during the field exposure, resulting in a significant loss of resistance to analyte exchange between Twister and water.

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