RBF_Cover (for eps) - National Water Research Institute

intuitive system behavior. In the case of RBF, such examples might be:
• The reductive dissolution of iron oxides by DOC and the resulting release of sorbed
heavy metals.
• The assessment of pesticide mobility during flood events.
Denitrification, degradation of pesticides, and dissolution of minerals are closely linked to DOC,
a key component of river water whose concentration can vary depending on the season and flow
events. Because of the reaeration process, river water is oxygenated compared to groundwater.
During the induced infiltration process, oxygen-rich river water enters the aquifer, as does DOC.
On its travel path, DOC from river water is oxidized and either mineralizes completely or is
transformed to intermediates through bacterial catalysis, together with the organic carbon that is
perhaps naturally abundant in the aquifer as sediment-bound organic matter. Oxygen in
the invading water is used as an electron acceptor in the process. Normally, there is sufficient
carbon available for microbial use; however, oxygen can become in short supply along the flow
path. Once microbes consume the oxygen, an anoxic zone develops where the nitrate of the
infiltrating river water and groundwater is used as a substitute electron acceptor. This leads to the
reduction of nitrate along the flow path. Once nitrate is also depleted, thermodynamically less
favorable oxidized iron and manganese minerals and/or sulfate might act as alternative electron
acceptors. Note that the simulation of the oxidation of one or multiple organic substrates using a
sequence of electron acceptors is routinely applied in the field of bioremediation modeling, mainly
where the transport and natural attenuation of oxidizable organic contaminants is simulated
(Barry et al., 2002).
The simultaneous simulation of RBF-typical denitrification and mineral dissolution reactions can
be handled by a number of existing codes (for examples, see Table 1). One such code is EASY-
LEACHER (Stuyfzand and Lüers, 2000), which was used to simulate reactions along a transect of
the Torgau RBF site on the Elbe River in Germany. EASY-LEACHER is a two-dimensional
reactive transport code in EXCEL spreadsheet, combining chemical principles with empirical
rules in an expert system. The code was found useful to attain a first estimate of water quality in
the production well for a RBF site where the operation of wells is started and for an easy
calculation of different boundary conditions. In contrast to EASY LEACHER, which uses a
collection of (one-dimensional) flow tubes to account for the transport of chemicals, the
FEREACT model (see Tebes-Stevens et al., 1998; Tebes-Stevens and Valocchi, 2000) is a
Table 1. Examples of Reactive Multi-Component Transport Models
Model Reference
CRUNCH Steefel (2001)
EASY-LEACHER Stuyfzand and Lüers (2000)
FEREACT Tebes-Stevens et al. (1998)
PHT3D Prommer et al. (2003a)
PHAST Parkhurst et al. (1995)
MIN3P Mayer (1999)
TBC Schäfer et al. (1998)
HBGC123D Salvage and Yeh (1998)
75