I would like to know if you come across a lot of field filtering of polycyclic aromatic hydrocarbon (PAH) groundwater samples?

Should you filter groundwater samples for PAH analysis? The short answer is: typically this is not done and it’s generally not an accepted practice. Often it is prohibited under many groundwater regulatory programs. Below is an example of this from a Wisconsin DNR document from 2004:

DO NOT FILTER PAH SAMPLES

Questions still arise regarding filtering groundwater samples that may contain polycyclic aromatic hydrocarbons (PAHs). It is true that particulates that result from well construction may carry undissolved PAHs, artificially raising groundwater PAH levels. However, it is also true that filtering removes natural colloids and other particles from the groundwater flow, and artificially reduces PAH levels in the sample. To avoid collecting groundwater samples with PAHs that are attached to artificially mobilized colloids and particulates, consultants may use low-flow purging and sampling techniques rather than a bailer. Benzo(a)pyrene (B(a)P) is one of the PAHs of greatest concern, and is also one of the least soluble PAHs. Its high retardation factor means that it is more likely to stick to a filter. If filtered, the analytical results would be biased low on B(a)P compared to other compounds with lower retardation factors. In other words, B(a)P doesn't go through filters, so any analysis of a filtered sample for PAHs is suspect. If consultants have questions, they should consult with their DNR project manager. Problems with filtering groundwater when sampling for organic contaminants like PAHs are discussed in DNR’s Groundwater Water Sampling Desk Reference (Sept 1996, PUBL-DG-037-96) beginning on page 85. This publication is available at http://www.dnr.state.wi.us/org/aw/rr/archives/pubs/DG037b.pdf. DNR’s companion Groundwater Sampling Field Manual is also available on line at http://dnr.wi.gov/org/aw/rr/archives/pubs/DG038.pdf.

The issues surrounding filtration of samples for PAHs, as well as other organics, reinforce the value of low-flow sampling. If turbidity is controlled at the source, the best estimate of the true mobile contaminant load should be obtained. Low-flow sampling is very effective at controlling artificial turbidity from purging wells with less than optimal well design, insufficient well development, or difficult geology (e.g., clay, fine silts, etc.).