Biological activity and soil gas composition

The interaction between biological activity soil gas may also affect inert gases such as He, even though they are not involved in the reactions. For example, if O2 is consumed and not replaced with an equal volume of COj (due to solution of CO2 in soil moisture, or the formation of carbohydrates) then the concentration of the residual gases will be enhanced. A 2% enhancement will increase the He concentration from a nominal 5.24 ppm to 5.34 ppm, a level considered to have exploration significance by some workers. 

In order to determine whether residual enhancement has occurred. He must be normalised, most usefully to the non-radiogenic atmospheric components Ne or Ar residual enhancement will change the absolute contents of He, Ne and Ar but not the ratio of their concentrations. Gole and Butt (1985) document several sample sites at which soil-gas He concentrations are residually enhanced, the maximum being 6.4 ppm He which is normalised to 5.25 ppm when ratioed with Ne. This particular He enhancement occurred over a natural-gas microseep, but presumably residual enhancement of He occurs in many other environments which do not have any exploration significance. 

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