Re: PHAST/PHREEQC question

["David L Parkhurst" <dlpark@xxxxxxxx>]

1- When I vary the kinetics with PHAST (ie vary qm and log_k in the '.chem'
file) by several orders of magnitude, there is no change in benzene
concentrations.

The log K for the reaction Benzene = Benzene should be 0.

2- I am comparing the results with MIN3P, and the concentrations for PHAST
are a little bit higher, even with kinetics shut off (k = 0).  The only way
I have been able to get closer to a match is by decreasing the input
concentrations of benzene (I have included a graph of this problem where
for all simulations k = 0). Any idea why this might be assuming i'm
entering the same concentrations for both programs?

If conservative chloride matches then so should conservative Benzene (qm =
0). I ran this case and benzene and chloride are identical. I assume MIN3P
also gives the same result for conservative Benzene.

So the difference you are plotting must be related to kinetics and I don't
know the explanation. Could the boundary conditions be different? Perhaps
MIN3P fixes the oxygen concentration at the boundary, whereas PHAST does
not. That would allow MIN3P to degrade more Benzene. I think PHAST is
giving the correct result for the problem definition, but it is hard to
check.

One thing you can do is to refine the grid on both programs to determine
whether the results change. I reran phast with twice as many nodes in each
direction and half the time step. The results were nearly identical. Peak
chloride concentration increased from .000925 to .000935. Peak benzene
concentration increased from .000858 to .000869.

The simulation looks ok to me. Benzene is slightly less than chloride
because of degradation. Things may look similar with different rates
because you run out of oxygen. Oxygen is completely depleted within the
benzene plume.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled