The use of passive sampling in WFD monitoring : the possibilities of silicon rubber as a passive sampler

This desk study examined the feasibility of passive sampling as an alternative monitoring method for the organic WFD-relevant substances in surface water (the priority substances and the specific pollutants). It specifically considered the possibilities of passive sampling with\r\nsilicon rubber.\r\nIt emerged from the study that \'Brussels\' will accept passive sampling as a supplementary\r\nmethod for WFD surveillance monitoring, on condition that the method is officially validated\r\nand documented. Although this is not yet the case for any of the existing sampling methods, it is nevertheless possible to deploy passive sampling as the \'best available technique\'. An ongoing issue here is that the compliance checking of the water quality under the WFD with respect to organic compounds considers the total concentration in water and that passive\r\nsampling measures the freely dissolved concentration. However, this problem can be\r\naddressed by converting this freely dissolved concentration into a total concentration.\r\nPassive sampling with silicon rubber appears to be an excellent approach to WFD monitoring and the time would appear to be ripe for the more extensive use of silicon rubber for this purpose. Silicon rubber can be potentially used for the measurement of 74% of the nonionogenic organic priority substances. This is 31% for the specific pollutants and 81% for the possible future priority substances that were studied.\r\nPassive sampling with silicon rubber is also suitable as a replacement for most bio-monitoring for water quality purposes. A major benefit of passive sampling compared with bio-monitoring is that no separate standards are required. It is possible to draw on the WFD standards in place for surface water (after conversion into freely dissolved concentrations).\r\nIt is difficult to say whether passive sampling increases or reduces costs. On the one hand, laboratory costs are higher because of the additional analysis of performance reference compounds required for the sampling rate. On the other hand, the sampling frequency for highly hydrophobic compounds can be reduced because of the time-integrated nature of passive sampling. The price-quality ratio is better with passive sampling.\r\nThe recommendation is to initiate passive sampling first at ten locations in the Netherlands and to start monitoring for those compounds that are difficult or impossible to measure using classical sampling methods because of the low concentrations in which they occur. On the basis of this first test, it will be possible to optimise the monitoring frequency and the number of samplers that have to be deployed in parallel.