Draft Internal Memorandum
D R A F T
Internal Memorandum
21 January 2004
Page 5
GeoSyntec Consultants
Where present we can look at VOC data to see if the indigenous microorganisms
have a steady diet of a carbon source (drives groundwater reducing, which may
solubilize arsenic).
Ammonia – nutrients for indigenous microorganisms.
MODELING APPROACHES TO EVALUATE ARSENIC IMPACTS TO
GROUNDWATER
There are several modeling strategies that may assist in evaluating arsenic impacts to
groundwater. The key to proper application of modeling techniques is to make sure that
the modeling approach is appropriate to address the specific concern that is being
raised. In this case, there are at least three questions that the committee may be
interested in addressing:
• Is arsenic leaching from C&D landfills into groundwater?
• If so, are there significant groundwater impacts from arsenic leaching?
• Will any arsenic that leaches be sufficiently mobile to impact groundwater at a
compliance boundary?
It is likely that each of these questions would be addressed with a different type of
model. It is our understanding that the issue of arsenic leaching from landfills has been
studied in some detail by Dr. Townsend and Dr. Solo-Gabrielle of the Florida Center for
Solid and Hazardous Waste Research, and that much of this research is on-going. The
data collected in these studies are likely to be superior to any model-generated
predictions on the quantity and quality of landfill leachate emanating from C&D
landfills in Florida. Nonetheless, models for leachate quantity (HELP, SMLRM,
DMLRM and others) and quality (HELPQ, VLEACH and others) are available if
needed.
The second question would need to be addressed by using a combination of
geochemical equilibrium models (MINTEQ, PHREEQEC and others) and calculations
to evaluate the amount of arsenic that could be sorbed (primarily) by the aquifer matrix.
Arsenate (V) and arsenite (III) can both exist under normal groundwater conditions,
however, arsenate is more common. Arsenate (V) species tend to form complexes with
ferri-hydrites, as well as sorb to mineral complexes within the aquifer matrix. Arsenite
(III) will dominate in natural waters having lower Eh and pH, and will tend to be
substantially more mobile. A combination of groundwater sampling, aquifer matrix
sampling, and geochemical modeling can estimate the capacity of the aquifer to
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