Re: simulation of degassing and subsequent gas-transport in PHREEQC

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

> I simulated methane degassing as well in another 1D vertical transport
model
of the rising plume fringe (chapter 6 in thesis). However, I do not know
how
to simulate detailed/real gas-transport towards the atmosphere in PHREEQC.
Methane gas could for example re-dissolve in aerobic water above the
leachate plume...This would be interesting to add to the model.

Unfortuately, for gas reactions, PHREEQC is more like a batch reactor than
a transport code. We have made some tentative efforts with a stagnant
liquid and a diffusive gas, but we have not attempted a fully coupled gas
and liquid transport model. The national labs programs (Lichtner, Steefel,
others), MIN3P from Waterloo (actually Mayer is at the Univ of British
Columbia now I think), and perhaps Hydrogeochem (I'm not sure about this
one) address this class of problems.

Therefore my question:

> Is it possible to transfer a gasphase from a lower cell to an upper cell
using BASIC statements in the RATES command? For example, a PHREEQC
vertical
column model has 100 cells, where cell 100 is the deepest cell and cell 1
is
at the groundwater table. A gas-phase is formed in cell 40 because the gas
pressure exceeds hydrostatic pressure. Is it possible to transfer the
gas-phase from cell 40 to the upper cell 39? On its turn a gas-phase may
form in cell 39 because of transfer of the gas-phase from lower cell 40
and/or by reactions in cell 39, so the gas-phase from cell 39 is then
transported to cell 38, etc. I think you need to formulate a RATES/KINETICS
statement for each cell where the gas-phase from the neighboring lower cell
is added to the the current cell where is may dissolve or not... This model
would then assume that gas-transport and degassing/re-dissolution is a fast
equilibrium process. Is this possible in PHREEQC (with PUT and GET
functions, CEL_NO?)?

I think in concept you could do this, but it would be messy. You would have
to generate kinetic reactions (one for each gas) that remove or add a net
number of moles for each cell. You would have to have some way of
determining how a plausible formulation to redistribute the gas. The put
and get functions are available to each cell, so you can pass information
through these functions. Ideally, you would solve the diffusion equation to
redistribute the gas, but that would be pretty unwieldy in Basic I think.
The time scale of gas transport is so much faster than the liquid transport
that time stepping gets to be a problem. You sound like you have some idea
of how you would like to transport the gas, so maybe you have a reasonable
simplification.

The other complication is that you would have to do all this through
kinetic reactions, so you would have to generate rates that transfer the
right number of moles. In this case the rate should be the net amount of a
gas to transfer to a cell divided by the time step.

Sorry I don't have a good answer for you at this time.

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