I ran a column experiment with a tritium tracer and am trying to find out the porosity of the soil. Unfortunately, I discarded the soil inside the column already without measuring the mass used. The tritium arrived at 50% of injection concentration after 98 hours while the flow rate was 5 ml/h with a total column volume of 294 mL. The breakthrough curve itself was very sharp so there was little diffusion or dispersion.
Nope, sorry. You are measuring two different things. You have the retentivity for the soil for tritium, which is really cool. Unfortunately, while it is indirectly related to the soil permeability it is not a measure of either the permeability nor the porosity.
Dear Mark,
By assuming a fully saturated soil, the classical breakthrough curves model for miscible displacement, show mathematicaly at 50% concentration of the injected fluid, the volume of injected fluid at this concetration is equal to one pore volume. at my knowledge , this is your pore volume.
i hope it will help
Kind regards
Louadj Yacine
I would prefer taking samples after finishing the experiments, to measure both bulk and particle Densities, and then calculate the porosity. However, logical prediction is described by Louadj.
Why you do not try both ways and see, (If you lost the columns try in new once?
Good luck
Assuming the tritium is bound as water, and that you do not have dual porosity, the C/Co = 0.5 volume is one pore volume (porosity * soil volume) of the soil. However, you need to:
1. Adjust the volume (reduce) for the inlet and outlet space of the column. Measure the weight of the empty and water filled column, lines and fittings. Subtract from the difference for the volume of the packed porous media.
2. Verify you do not have dual porosity. Was the breakthrough curve symmetric and reach C/Co =1? If so, you are fine.
Regards,
Thank you Glenn and Louadj. One thing I was confused about was using the pump flow rate of 5 ml/h. I get nonsense porosities when trying to adjust for that.
There was no dual porosity in the columns, though. The tracer curves look almost like perfect squares and reach ~0.98-1.02 C/Co. Unfortunately the columns and soils were already discarded, so tracer breakthrough is all I have to go with, aside from using a default 0.38 porosity.
Was the tritium bound as water? If so, something is wrong with either the flow rate, time, or column volume stated. If the tritium was bound as something else, you have measured the retardation of that compound. If the tritium was molecular hydrogen gas, you measured the solubility of the hydrogen in the aqueous phase, since the exchange of the hydrogen with mineral cations should be pretty low.
Yes the tritium was bound as water. I did use a peristaltic pump and had to manually calibrate and maintain the flow rate, so there might have been an issue there. If you get nonsense results as well, then at least I know not to use that value. Thanks!
Yes, the flow rate must be off. I have done several tests with tritium bound water. It should have given you the effective porosity.