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Fig. 10.6. Comparison of aqueous concentrations of PAHs at the sampling site in the river Weisse Elster (August 2002), estimated using two passive sampling approaches.

Fig. 10.6. Comparison of aqueous concentrations of PAHs at the sampling site in the river Weisse Elster (August 2002), estimated using two passive sampling approaches.

calculated for these compounds reflect also short-term (from the last week of exposure or so) fluctuations in concentrations, rather than the representative time-weighted mean. Further, SPMD-derived concentrations of heavy PAHs provide a qualitatively better estimate of TWA than those derived from MESCO (see Section 10.5.1.9). Finally, calculations of MESCO-derived concentrations relied on accuracy of KSW values reported in literature. There is only a limited number of studies that provide these values and relatively high risk that some of them may be biased.

Nevertheless, we believe that the two passive sampling methods provide a more representative picture of the water quality than occasional spot sampling. Moreover, with the use of MESCO II devices some disadvantages of the foregoing MESCO format (underestimation of very hydrophobic compounds, possible disintegration of sampler due to membrane degradation during longer exposure) can be prevented although this, in turn, sets other restrictions, especially with respect to more polar target substances which will not be accumulated due to their low permeability in the non-porous and hydrophobic LDPE membrane envelope.

10.5.2 Field trials with MESCO II—first results

Since 2004, water monitoring using MESCO II strips (SR+air) alongside other passive samplers (SPMDs, bare silicone rods and Chemcatcher)

was carried out at several sites in three rivers in Germany (Elbe, Saale, Mulde) and additionally in the Spittelwasser creek, a tributary to the Mulde river near Bitterfeld, an area heavily polluted by chemical industry during the last 100 years. The major goal of these trials was to test the field performance of MESCO II devices under different ambient conditions (regarding water flow and temperature, hydrochemistry and biological activity). Similar to the Twister bars, the silicone rod pieces were spiked with PRCs before sampler assembly in order to adjust the laboratory-derived sampling rates to in situ conditions. The data evaluation for these field campaigns is still under way.

As an example we present results obtained for HCB in the Spittel-wasser near Jessnitz (at the site 51°41'28"N; 12°17'25"E, estimated using Google Earth) during Summer 2005. MESCO II strips with 50 and 100 mm LPDE membrane thickness, respectively, were tested in two different deployment devices, i.e. in a wide-mesh protective grid and in a long narrow perforated cage as used for SPMDs. The samplers were exposed for 28 days and TWA concentrations were estimated from the amounts accumulated using the sampling rates listed in Table 10.2. Figure 10.7 shows the TWA concentrations against snapshot results obtained every two weeks from grab samples pre-concentrated by SPME and analysed using GC-MS (for analytical details see Ref. [14]). The correspondence between the results of the different sampling strategies is remarkable, especially if one bears in mind that the evaluation of MESCO data is based on preliminary sampling rates from a rapid semi-continuous laboratory calibration test. Figure 10.7 also shows a slight influence of hydrodynamics on the HCB accumulation. Reduced flow in the cage seemed to have lowered the substance uptake into MESCO samplers. This aspect is currently under investigation in flow-through experiments.

Also from another monitoring exercise, the field trial in the Meuse river in Eijsden (The Netherlands) which was organised in April-May 2005 within the framework of the EU project SWIFT-WFD [19], interesting results are expected on the field performance ofMESCO I and II devices in comparison to other passive sampler formats that were applied [20].

Currently, field trials are in progress in the region of Bitterfeld (Saxony-Anhalt, Germany) with miniaturised MESCO II strips for time-integrative and depth-oriented monitoring of groundwater wells. The first results show that silicone rods enclosed in LDPE membrane are even able to accumulate volatile organic compounds such as 1,4-dichlorobenzene over several weeks. A comparison of substance amounts accumulated with the water concentrations obtained from parallel exposed passive diffusion

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