Implemented Reaction and Boundary Conditions

The biogeochemical model represents wetland-typical, redox-sensitive processes, which are implemented using different kinetic reactions. In particular, the following redox-sensitive processes are being simulated: aerobic respiration, denitrification, iron(III) reduction, sulfate reduction, iron(II) oxidation, ammonium oxidation, aerobic and anaerobic sulfide oxidation. Kinetics for all reduction processes (aerobic respiration, denitrification, iron(III) reduction, sulfate reduction) where microorganisms use different electron acceptors (oxygen, nitrate, iron(III) and sulfate) for turnover of organic material are formulated based on Monod kinetics (Monod, 1949). For reactions following Monod kinetics, as shown in Eq. 5, the kinetic rate R_k [ML^-3T^-1] is calculated as a function of the solutes concentration ck [ML^-3] and the reaction specific constants μmax [ML^-3T^-1] and Ks,k [ML^-3].

MATERIALS AND METHODS

, Eq. 5

In the model, Monod kinetic constants for the different reduction processes are based on laboratory studies of biodegradation of organic chemicals (references are listed in Table 2 of study 2) and were adjusted as part of the calibration process. Finally, calibrated coefficients are listed in Table 2 of study 2. Oxidation processes (iron(II) oxidation, ammonium oxidation, anaerobic and aerobic sulfide oxidation) were formulated using higher order reaction kinetics as listed in Table 2 of study 2. In redox controlled systems like wetlands, reduction processes occur sequentially where microorganisms use oxygen as primary electron acceptor first, before nitrate, iron(III) and sulfate are being used. To represent this sequential behavior within the biogeochemical model, different conditions were formulated for which the different reduction processes are being initiated. In the approach presented here, these conditions are represented by critical concentrations for redox-sensitive solutes which control whether a redox process is initiated or not. For the different reduction processes, controlling critical concentrations are listed in Table 1. Table 1 must be red row-wise, where entries “>0” mean that the corresponding redox-sensitive reactant (column) must be available and “-“ means that this process does not depend on the presence of the redox-sensitive compound. For example iron(III) reduction in the biogeochemical simulation is only initiated if: (1) Dissolved oxygen concentrations fall below ; (2) Most of the nitrate is already depleted and actual concentrations fall below

; and (3) The electron acceptor iron(III) is available.

Table 1: Critical concentrations which are controlling the sequential initialization of the redox sequence. Values were derived from field observations. Table must be read row-wise (e.g.

denitrification is initiated if 1) oxygen contents drop below Ccrit for oxygen and 2) if nitrate is present).

= 5.0 x 10^-6 mol/L; = 4.0 x 10^-7 mol/L; = 5.0 x 10^-6 mol/L.

The critical concentrations were formulated based on evaluation of depth profiles for redox-sensitive solutes which were taken at the Schlöpnerbrunnen II site in the Lehstenbach catchment (Knorr and Blodau, 2009; Knorr et al., 2009). Intervals for the activation of redox processes are overlapping, meaning that multiple processes can occur simultaneously which can be approved under laboratory as well as under field conditions (Knorr and Blodau, 2009; Knorr et al., 2009).

Availability of oxygen can be seen as a key component, controlling the process composition within wetland ecosystems. Processes like aerobic respiration or nitrification only occur if oxygen is

oxygen nitrate iron(III) Sulfate

aerobic respiration >0 - - -

denitrification >0 - -

iron(III) reduction >0 -

sulfate reduction >0

MATERIALS AND METHODS available. Other processes, like denitrification iron(III)- or sulfate-reduction are only initiated under anoxic conditions where oxygen concentrations are very low. Along a subsurface flow path, availability of oxygen varies as the hydrological boundary conditions change. Within the unsaturated zone, depleted oxygen is being replaced by diffusion of atmospheric oxygen and availability of oxygen for microbial catalyzed reactions is high. In the saturated zone dissolved oxygen concentrations are low because the resupply by diffusion is being inhibited by pore water, which acts as an effective diffusion barrier. Therefore, in the biogeochemical model oxygen availability was used as a key variable that either triggers or suppresses redox-sensitive processes. Along a sub-surface flow path, availability of oxygen was coupled to the transient pressure heads which were available as part of the virtual wetland modeling. For each PHREEQC sub-section simulation of a sub-surface flow path, the corresponding pressure head was estimated for the start location of the sub-section. Pressure heads were related to a certain oxygen concentration according to Figure 8. If the pressure head of the sub-section is located within zone 1 (unsaturated zone with negative pressure heads), the oxygen availability is at a maximum due to the uninhibited diffusion of atmospheric oxygen. Within zone 2 (saturated zone with positive pressure heads), oxygen contents are decreasing with increasing pressure heads representing increasing inhibition of oxygen diffusion with depth. Oxygen concentrations in sub-section simulations that are located either within zone 1 or 2 were set to a constant value reflecting that rapid resupply of oxygen prevents its depletion by oxygen consuming processes. For sub-sections that are located within zone 3 (deeper saturated zone with pressure heads above 0.25 m) oxygen is not assigned as a constant boundary condition. Instead, oxygen is set as an initial condition where the residual oxygen contents of the preceding sub-section are used as initialization. Within zone 3, where atmospheric diffusion is disrupted, oxygen can be totally depleted due to oxygen consuming processes. The relationship shown in Figure 8 was derived from observed oxygen-depth profiles taken at the Schlöppnerbrunnen II site in the Lehstenbach catchment (Knorr et al., 2009). Aside from an adequate electron acceptor (e.g. oxygen, nitrate, iron(III) or sulfate), microbial catalyzed reduction processes require a carbon source that is available to microorganisms. For the carbon rich systems studied here unlimited availability of carbon was assumed.

MATERIALS AND METHODS

Figure 8: Typical oxygen depth profile observed for a wetland site of the Lehstenbach catchment.

Profile was used to assign oxygen boundary conditions to the different PHREEQC sub-section simulations based on transient model output of the virtual wetland model.

RESULTS AND DISCUSSION