Draft Internal Memorandum
D R A F T
Internal Memorandum
21 January 2004
Page 4
GeoSyntec Consultants
strongly to the aquifer matrix. It is anticipated that leachate coming from a landfill
would be anaerobic and facilitate chrome reduction and sorption. This is consistent
with what is indicated in the database (only 27 exceedances in the database of 1714
records).
One of the major immobilization processes for arsenic is sorption to iron minerals like
ferri-hydrite that maintain a positive charge that captures negatively charged arsenic
anions. If the arsenic is coming out of the landfill, reducing conditions caused by
plentiful nutrients and a carbon source within the landfill may result in ferri-hydrite
reduction and subsequent arsenic release. This process may increase the solubility of
both naturally-occurring and arsenic emanating from the landfill. Arsenic may also be
reduced to the more soluble As +3 form that is charge-less. Other hypothesis and/or
scenarios for elevated arsenic and chromium levels in groundwater may include:
Arsenic and chromium were never really bound in the landfill and the aquifer
system does not have sufficient attenuation capacity to sorb arsenic and
chromium out of solution.
Reducing landfill leachate is reducing the attenuation capacity of the aquifer
system as it dissolves out iron minerals as it leaves the facilities.
Arsenic may also form insoluble complexes with sulfide. For this arsenic to ever re-
solubilize, the landfill leachate would have to be aerobic, which is highly unlikely.
CHARACTERIZING ARSENIC MOBILITY WITH GEOCHEMICAL DATA
We may be able to use available pH data and some of the geochemical parameters to
evaluate, to the extent possible, geochemical conditions conducive for arsenic and/or
chromium to be soluble in groundwater. Examples include:
DO – where available, information provides insights to the redox state.
Iron – may not be ferrous analysis, but if it is present in groundwater it is likely
to have a significant ferrous component. The same is true for manganese.
Nitrate and sulfate – more information that may be used to target the
groundwater redox in the absence of ORP measurements. Sulfate may also be
used to tell us if naturally occurring arsenic is being solubilized (probably not
with landfill leachate – as discussed previously).
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