Microseeps: Technical Articles

Dissolved Gas Analysis for Natural Attenuation-
The Choices

Microseeps refers to dissolved gas analytes to include three suites, the permanent gasses, the light hydrocarbons, and hydrogen:

Permanent Gasses	Light Hydrocarbons	Hydrogen
Methane (high level) Oxygen Nitrogen Carbon Dioxide	Methane (all levels) Ethane Ethene Propane iso-butane n-butane	Hydrogen

Several questions are frequently asked about choosing sampling and sample procedures. These questions, and our responses to them, are given below:

How should I sample?

If hydrogen is not a target analyte, then sampling simply means filling two VOA vials at each well. There is no need for a preservative, but the vials must have been obtained from Microseeps, since the vials that Microseeps uses insure the stability of the analyte concentrations. Simply fill the vials as you would for any other volatile compound sampling event:

1. Purge the well slowly until the pH and DO (as measured from a calibrated probe placed in a flow through cell) are stable (no more than 10% variation among three consecutive samplings taken at five minute intervals - this requires pumping approximately one well volume.)

2. Label the sample containers and fill out the chain of custody form.

3. Holding the hose in the vial, run the water down the side of the vial to fill it up. As the vial fills, remove the hose slowly and over-fill the vial.

4. Cap the vial so that there are no air bubbles trapped inside.

5. Turn the vial over and tap it, looking for air bubbles. If there are any, remove the cap (replace the cap with a new one, if available) and repeat steps 3-5. The samples are good for two weeks. They should be stored on ice and returned to the Microseeps laboratory.

Do I need to do the bubble strip procedure?

If, and only if, hydrogen is a target analyte at a specific site, bubble stripping must be done. Microseeps has developed a sampling cell, a set of instructions and an illustrated web-site for bubble strip sampling. Additionally, the procedure we follow in the field is detailed in the Microseeps sampling SOP SM9. Every effort was made to include detail in the instructions and the web site, but those who seek further information should obtain a copy of that procedure.

I’m going to do the bubble strip procedure. What should I use for the bubble?

If bubble stripping is done, a seed gas must be chosen to create the bubble. The simplest choice is ambient air. The only negative consequence of this comes from a practical standpoint - while theory says that an accurate hydrogen concentration measurement should also produce an accurate measurement of each of the other components of air, we find that, in practice, this is not always true for nitrogen and oxygen. Whether the bubble strip procedure is used or liquid water is sampled from a vial, equivalent results were obtained for all other dissolved gasses. The concentration of nitrogen in the groundwater is not of biogeochemical interest, and the concentration of oxygen can be obtained either from the field probe or from field kits. (The later are much more reliable for dissolved oxygen concentrations less than 1.0 mg/l.) However, a dissolved oxygen concentration reflective of the groundwater can be made by using a seed gas of pure nitrogen and such a seed gas will be provided if requested.