where C O2, C NH4,C NO3 and C DOC is the concentration of dissolved oxygen (O2), ammonium (NH4+), nitrate (NO3-) and dissolved organic carbon (DOC), respectively; R O2,R NH4, R NO3 andR DOC is the reaction rate of O2, NH4+, NO3- and DOC, respectively;V AR, V NT,V DN is the maximum specific uptake rates of the substrate of AR, NT and DN, respectively; X AR,X NT and X DN is the biomass of the functional microbial group of promoting AR, NT and DN, respectively; K O2, K NH4,K NO3 and K DOC is the half-saturation constants of O2, NH4+, NO3- and DOC, respectively;K I is the inhibition constant; yO2 is the O2 partition coefficient;U AR, U NT andU DN (U = VX ) is the lumped specific maximum microbial reaction rate of AR, NT and DN, respectively.
2.2.3 Numerical Model and its verification
The numerical modelling code FEFLOW 7.0 (Diersch, 2014) was used to simulate the variably saturated flow and solute transport and reaction by using the PARDISO solver (Schenk and Gärtner, 2004). The model domain was discretized through triangular grid generator, with finer mesh (dx =0.1m) around the surface water-groundwater interface (0 <x <20m) and water-level sensors, medium mesh (dx =0.5m) within 20 <x <100m region and larger mesh (dx =2m) within 100 <x <200m region. The total number of model grid cells was 46679 and the total number of nodes was 23767. In order to keep the computational time within reasonable limits, the automatic time-step control was adopted by setting the initial time step of 0.001d and maximum time step of 0.5d. The model was calibrated by mainly adjusting K through comparing the calculated and measured values of groundwater level and temperature, with the other flow and thermal parameters being the empirical values of sandy loam described by Carsel and Parrish (1988). The calibration and verification periods were respectively Oct 26, 2014-Dec 26, 2014 and Oct 1, 2015-Oct 21, 2015. The solute transport and reaction model was not calibrated and the biochemical parameters were assigned by empirical values too.
2.2.4 Quantification of biogeochemical reaction
Under the water level fluctuation, the consumption of solute j(i.e., O2, NH4+, NO3 – and DOC) at any time can be obtained by the following (M rem-j ):