Re: Editing Netpath's databases

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

> I have been using Netpath to do some weathering calculations in a low
TDS, low alkalinity, low pH coastal groundwater. I successfully added
some minerals to netpath.dat. My aquifer is primarily a quartz sand and
I wanted to use the 1997 Rimstidt quartz Ksp of 10^-3.72, rather than
the constant in Netpath's DB.dat. I think that quartz is being dissolved
as the water travels down the flowpath, but the program still thinks
quartz is oversaturated in most of the waters.

Quartz is pretty insoluble, probably about 10 mg/L of dissolved silica.
Most waters exceed this concentration. I think it means that there is
another source of silica, either alumino silicate minerals (probably
feldspar) or a less stable form of silica (chalcedony or microcrystalline
quartz, for example).

> I changed the constant in that database, but the program is still using
the old constant (10^-3.9). Do I need to recompile netpath?

I don't think you have to recompile netpath. Db is making the saturation
index calculations and changing db.dat should be sufficient (there may be
more than one copy of db.dat in different directories, so make sure you
change the one that db is actually using. Change a mineral name to quickly
test if the db.dat is being used). At a more fundamental level, NETPATH
doesn't really care about saturation indices, it only uses mole balance, so
it will dissolve quartz if that's what is needed, even if quartz is
supersaturated.

> The Netpath version I am using is an old one, from 1994, and it's the
Mac Netpath.

It should be ok. Netpath has not changed much.

You may want to consider using PHREEQC. It should be able to handle the
same type of inverse problems and NETPATH, however, it does not have the
isotope capabilities of NETPATH.

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