Geoscience Reference
In-Depth Information
reactions most relevant to arsenic transport in groundwater and approaches for representing these
within a numerical modeling framework are described in the following sections.
The coupled hydrologic transport and geochemical reaction equations can only be solved ana-
lytically for some very simple cases. For more complicated cases, e.g., involving heterogeneous
aquifers, transient boundary conditions, etc., most solution procedures are based upon numerical
techniques such as the finite difference and finite element methods and the reader is referred to
modeling-specific texts such as Anderson and Woessner (1992), Chiang and Kinzelbach (2000)
and Zheng and Bennett (2005) for a detailed description of these techniques.
2.3.2 Processes controlling the geochemical environment
To a large extent, the geochemical changes in a hydrogeological system are controlled by micro-
bially mediated redox reactions (e.g., Champ 1979; Christensen et al ., 2000; Eckert and Appelo,
2002; Massmann et al ., 2004). In many groundwater systems, these redox reactions are often
driven by the mineralization of naturally occurring sediment-bound and/or dissolved organic
matter (e.g., Hunter et al ., 2008; Kirk et al ., 2004; Park et al ., 2009). However, in contaminated
aquifers, these biogeochemical redox reactions may also be driven by mineralization of xenobiotic
substances such as petroleum hydrocarbons or pesticides that may co-occur with arsenic (e.g.,
Baedecker et al ., 1993; Levine et al ., 1997). Mineralization of organic substances involves the
consumption of electron acceptors such as oxygen, nitrate, manganese- and iron-(hydro)oxides
and sulfate (e.g., Chapelle and Lovley, 1992; McMahon and Chapelle, 1991). The consumption of
the electron-acceptors typically occurs in a sequential order constrained by thermodynamic prin-
ciples (e.g., Christensen et al ., 2000; Postma and Jakobsen, 1996; Stumm and Morgan, 1996).
This usually results in the formation of spatially distinct redox zones along the groundwater
flow direction which range typically from aerobic, to denitrifying, Mn- and Fe-reducing, sulfate
reducing and methane producing conditions.
Due to its variable oxidation states the mobility of arsenic is strongly influenced by the redox
environment, which determines sorption characteristics and therefore the transport behavior of
As (e.g., Goldberg, 2002). For example, arsenate (As(V)), the predominant chemical species
under aerobic conditions, tends to be more strongly adsorbed onto oxide mineral surfaces such as
ferrihydrite and thus is typically largely immobile under near-neutral pH conditions. Conversely,
arsenite (As(III)), the prevalent chemical species under reducing conditions, adsorbs less strongly
and is generally more mobile.
Biogeochemical redox reactions often induce additional secondary reactions such as precip-
itation and dissolution of minerals, ion exchange or surface complexation reactions, which can
also greatly influence the mobility of arsenic (e.g., Appelo and de Vet, 2002). Therefore, efforts
to simulate As behavior in the subsurface should incorporate the processes accounting for the
spatial and temporal variations of the redox chemistry of the aquifer.
Within reactive transport models, different approaches can be applied to simulate organic
substrate oxidation. The most commonly applied reaction rate formulations are based on the
application of Monod kinetics (Barry et al ., 2002). A relatively simple variant of Monod-type
rate expressions for the kinetically controlled oxidation of organic matter by oxygen, nitrate or
sulfate is (Parkhurst and Appelo, 1999; Prommer et al ., 2006, Sharma et al ., 2012):
k O 2
C NO 3
1 . 55 × 10 4
C SO 2 4
1 . 0 × 10 4
C O 2
2 . 94 × 10 4
r DOC
=
+ C O 2 +
k NO 3
+ C NO 3 +
k SO 2 4
+ C SO 2 4
(2.2)
where r DOC is the overall degradation rate of organic matter, k O 2 , k NO 3 , k SO 2 4 are the rate constants
of carbon mineralization under aerobic, denitrifying and sulfate-reducing conditions and C O 2 ,
C NO 3 , C SO 2 4 are the concentrations of oxygen, nitrate and sulfate in the groundwater.
The geochemical response of the organic substrate mineralization, i.e., the ensuing redox
reactions, is typically simulated either via a single-step or by a two-step approach (Brun and
 
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