2.5 Visual MODFLOW Model description of MAR System
Visual MODFLOW was used in this study to develop a groundwater flow model representing the area close to the infiltration ponds for the period of January 2015 to March 2016 to verify the variation of infiltration rates. Two models were developed to represent the conditions for summer and winter seasons. Constant pumping and infiltration rates were used in each model to observe the impact of leakance through pond bed. The primary objective is to observe the modelled heads at the location of the monitoring wells and verify how closely they match the observed heads under controlled circumstances.
The model area is 1100 m long, 630 m wide and is centered on the infiltration ponds. The model has been slightly rotated to match the north boundary to the coast of North Sea (fig. 5). It is divided into 110 columns and 63 rows each cell having a 1 m x 1 m area. The aquifer thickness is 31 m and is divided into 32 layers. All layers are of 1 m depth except for between 3 and 4 mTAW where they are of 0.5 m depth. The sources of water in this model are recharge through the pond beds and natural recharge in the area. There are 137 extraction wells surrounding the ponds, which act as the sinks.
Fig. 5 Layout of St-André MAR MODFLOW model
The hydraulic conductivity and specific yield of the layers have been set based on borehole measurements obtained at the location of WP6 and from the parameters used in previously developed models for the area (Vandenbohede & Houtte, 2012). A low permeable zone was introduced in the model just under the bed of both ponds to match the effect of vertical gradient in the area. Vandenbohede et al. (2008b) also mentioned the presence of a low permeable layer under the west pond. Fig. 6 shows a two-dimensional vertical cross-section of the model with the various hydraulic conductivity zones with hydraulic conductivity values as shown in Table 1. Boundary conditions have been set based on groundwater flow models developed previously for St-André MAR facility (Vandenbohede & Houtte, 2012) and from the groundwater flow analysis from this study.
MODFLOW Lake package (LAK3) has been used to model the infiltration ponds in both models. The lake bottom is at 6.24 mTAW. The average precipitation is 345 mm/year and the evaporation are 450 mm/year in both the models. Heads have been simulated at monitoring wells WP 6.2, WP 22.2 and WP 42.2. Both the models have been calibrated to match the heads in the aforementioned monitoring wells. The variations of hydraulic conductivity of the pond bed has been implemented in the models by changing the leakance parameter. Pond bed leakance is expressed as (eq. 2),
\(L=\frac{K_{z}}{b}\)     (2)