In planning remediation strategies, assessing the progress and/or potential of bioremediation and developing the necessary support for regulatory acceptance of remediation work it is often desirable to characterize the dominant reduction processes for hydrocarbon degradation throughout a particular site. Molecular hygrogen (H₂) has been identified as a simple parameter with which to do this.

H₂ is generated during anaerobic fermentation of hydrocarbons. Microorganisms that use alternate terminal electron acceptors (TEAP’s) consume H₂. These alternate TEAP’s are: denitrification through nitrate(NO₃^-) consumption, iron reduction through ferric iron (Fe⁺³) consumption and ferrous iron (Fe⁺²) production, sulfate reduction through sulfate (SO₄^2-) consumption and sulfide (S and methanogenesis through carbon dioxide (CO₂) consumption. Because H₂ is consumed at different rates during each of these processes it is indicative of the dominant local TEAP. A measurement of the steady state concentration of dissolved hydrogen can indicate the dominant TEAP much like measurement of the steady state level of water in a bath tub which is simultaneously being filled and drained tells you about the effectiveness of the drain. The following table gives the levels of H₂ indicative of each of the aforementioned TEAP’s:

Hydrogen Concentration (in nanomoles/L or nM)	Indicated TEAP
30 - 5	Methanogenesis
4 - 1	Sulfate Reduction
0.8 - 0.2	Iron reduction
<0.1	Denitrification

 

Further, H₂ measurement is equally sensitive to all TEAP’s, which E_h/pH measurements (also known as oxidative – reductive potential or ORP measurements) are not. For these reasons Frank Chapelle writes "concentrations of H₂ are a useful indicator of TEAP’s in groundwater systems, particularly when interpreted in the context of electron acceptor (nitrate, Fe(III), and sulfate) availability and the presence of final products (Fe(II), sulfide, and methane) of microbial metabolism" (F.H. Chapelle et al, "Practical Considerations for Measuring Hydrogen Concentrations in Water", Env. Sci. Technol. in press). He supports this statement with four (4) references to published scientific literature.

Unfortunately the measurement of dissolved H₂ is not straightforward. There are many important considerations one must take into account when sampling groundwater for dissolved hydrogen. Care must be taken during analysis as well and this analysis must be able to achieve an MDL <0.1 nM to identify all TEAP’s. Commercial availability of these services is extremely limited, but Microseeps’ sampling methods and analysis utilizing the bioremediation GC can achieve these things. We have developed ways to make the sampling easier and allowing our clients to sample on their site and utilize our "in-house" services.

Meanwhile, we present a quick summary and analysis of the various options people seek in acquiring data regarding dissolved H₂ information.

If:

1. sample is acquired by Chapelle’s "bubble strip" method
2. sample is analyzed in the field
3. sample is analyzed using the Microseeps bioremediation GC as in our method AM 19GA

the interpretation of the results as representative of the groundwater

• is scientifically justified
• can be referenced to refereed publications
• is acceptable for regulatory purposes

Microseeps bioremediation mobile laboratory can provide this service, with an MDL for dissolved H₂ of 0.08 nM.

If:

1. sample is acquired by Chapelle’s "bubble strip" method
2. sample is stored in a properly prepared container (in these containers the holdtime is as long as 14 days)
3. sample is shipped to an appropriately equipped laboratory
4. sample is analyzed using the Microseeps bioremediation GC as in our method AM 19GA

the MDL of the analysis depends on the style in which the sample container was prepared. If the MDL is sufficiently low, interpretation of the results as representative of the groundwater

• is scientifically justified
• can be referenced to refereed publications
• is acceptable for regulatory purposes

Microseeps can provide equipment and instructions for sampling, as well as the "in-house" analysis of the samples. The MDL that Microseeps currently provides is 0.3 nM. Development efforts are underway to improve this MDL and to simplify the recommended sampling procedure.

If:

1. sample is acquired as a typical water sample for dissolved gas analysis
2. sample is stored in a properly prepared container
3. sample is shipped to an appropriately equipped laboratory
4. sample is analyzed using the Microseeps bioremediation GC as in our method AM 15.01

there are two potential problems.

1. Because of the unavoidable difference between the analysis methods our MDL increases from less than 0.08 nM to 1.5 nM. This eliminates the utility of dissolved hydrogen measurements in differentiating between sulfate reduction, iron reduction and denitrification when trying to determine the dominant TEAP.
2. An exact determination of appropriate holdtimes has not been undertaken, but currently available evidence suggests that the holdtimes appropriate for this analysis may be quite short – on the order of 24 hours.

Due to these problems the justifiability of using these results to determine the TEAP’s is questionable. Microseeps does not currently provide analysis of water samples for dissolved hydrogen content in house.

Pat McLoughlin, October 30, 1997