3.3 Groundwater flow model of the MAR System
WP 6, WP 22 and WP 42 are the monitoring wells located within the pumping well barrier and have been used to observe the impact of variation infiltration rates for summer and winter. Figures 18.a and 18.b show the comparison between simulated heads with summer conditions, heads with winter conditions and observed heads. A satisfactory fit is obtained between the observed and simulated heads in summer months for the summer model as well as in the winter months for the winter model in all the wells (table 3). The maximum Darcy velocity through the recharge ponds in winter is 0.22 m day-1 whereas in summer the maximum Darcy velocity is 0.31 m day-1.
Table 3 Statistical indices showing goodness of fit of the two groundwater flow models at different monitoring wells
Fig. 18 Observed vs simulated heads from MODFLOW model at Torreele MAR facility. a) Results from summer model during summer months (JJA); b) Results from winter model for winter months (DJF)
The models have been developed with constant pumping and infiltration rates to match the respective seasons. The target of these two models is to depict the impact of leakance on the recharge rate and the flow velocity. Leakance is proportional to the hydraulic conductivity of the pond bed. The summer model uses a leakance value which is 30% higher than the leakance value of winter corresponding to the 30% rise in hydraulic conductivity of the pond bed during the summer months. The results obtained from the two models at the respective time zones show a good match. The combined RMSE is 0.74 for WP 22.2, 0.66 m for WP 42.2 and 0.39 for WP 6.2. The variation in accuracy is caused due to the spatial variation of hydraulic parameters in soil. WP 42.2 and WP 22.2 show a higher RMSE as the model assumes a single value for the hydraulic parameters for a single layer. However, WP 6.2 shows a fairly good match as it is located very close to the ponds and it reflects the vertical flux well.
The residence time of water is responsible for the high RMSE in the models. The response of the heads in WP 6.2 to the change in temperature conditions is faster than the other two. The heads in WP 6.2 in December fairly relates to the simulated heads in the winter model. However, in WP 22.2 and WP 42.2 the heads in December are very low and they correspond to the heads in summer scenario. This may be attributed to the distance travelled by the water before it reaches the pumping wells. The variation in residence time occurs due to the presence of a low permeable soil layer on the north side of the west pond. The overall deviation in observed and simulated heads from the models are due to the fact that hydraulic conductivity is constantly varying with time as temperature changes. However, in the models, the conductivity values have been kept constant all throughout each season.
It is observed from the hypothetical scenarios that the mean Darcy velocity through the pond bed during summer is 0.31 m day-1 and that during the winter is 0.22 m day-1 which is at par with the observed recharge rates as calculated. This velocity can also be accounted as the seepage rate or recharge rate. The reduction in winter recharge rate is hence reduced by about 27 %. This suggests that the variation in observed heads is partly influenced by leakance, which is proportional to the hydraulic conductivity of the pond bed.