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)