Groundwater Sampling Bubble Trouble: Does Headspace Matter?
Several years ago, I performed quarterly groundwater sampling at a former landfill site in central Florida. Once I finished purging each monitoring well, I’d fill up three volatile bottles, inverting each one and lightly tapping the glass to ensure no air bubbles were present. There were several monitoring wells that, for whatever reason, always gave me problems. No matter how hard I tried, there were always bubbles in the bottles, and I’d have to start the whole process over again: slide the tubing back down into the well, turn on the pump, turn off the pump, pull up the tubing, reverse the pump, fill the bottles, replace the caps, invert the bottles, tap, curse, curse some more, and repeat. 
Sound familiar? Have you ever asked yourself, does it really matter if there are air bubbles in these 40 mL VOA bottles?
An article about this topic was published in the Winter edition of the Groundwater Monitoring & Remediation magazine, a publication of the National Groundwater Association. The study, entitled "The Effect of Air Bubbles and Headspace on the Aqueous Concentrations of Volatile Organic Compounds in Sampling Vials," determined that headspace in the bottles had to be at least 10% of the volume of the vial to affect the total VOC concentrations of the sample.
Although the results of this study may be true, there are still regulations that stipulate that laboratories are required to document the presence of any gaseous void space, or “air bubbles," and invalidate those samples.
So, what should you do?
Make sure to talk about your sampling methodology with both your laboratory project manager and your state/federal regulator, as applicable, and determine exactly what process you should follow. You may find out that you’ve just saved yourself a bunch of time and never have to worry about those pea-sized or even marble-sized bubbles again. On the other hand, you may need to consider deep breathing techniques and just make sure that no one is within ear shot of you at those pesky monitoring wells.
Good luck!!