3.1 Groundwater flow analysis
The infiltration ponds are the primary source of water in the area between the ponds and pumping wells. The pumping wells adjacent to the ponds create a barrier to the flow of water around the ponds. Groundwater level contours have been developed at a depth of -5 mTAW (level 2 wells), focusing only on the area between the ponds and the pumping wells. The flow in this region is outwards from the ponds towards the pumping wells. Fig. 7 shows the variation of groundwater elevations between summer and winter months. The contours have higher values in winter 2015 and 2016 (fig. 7.a and 7.c). Groundwater heads are lower during summer 2015 and 2016 (fig. 7.b and 7.d) indicating a higher vertical gradient between the pond and the aquifer, thereby increasing the flow rate. The direction of flow does not change much over time. However, the horizontal gradient is observed to increase in summer in the northern parts of the infiltration ponds. This hints towards a higher flow rate in summer.
Fig. 7 Contour of hydraulic head near the infiltration ponds at -5 mTAW (Level 2 monitoring wells) during: i) Winter 2015 (01/09/2015), ii) Summer 2015 (07/13/2015), iii) Winter 2016 (01/11/2016) and, iv) Summer 2016 (07/19/2016)
Groundwater level contours at a depth of -5 mTAW (Level 2 monitoring wells) are shown in fig. 8 and the same at -20 mTAW (Level 1 monitoring wells) is shown in fig. 9 for a larger area near the ponds. It can be observed from the contours that the hydraulic heads around the pumping wells are higher during the winters (fig. 8.a, 8.c, 9.a and 9.c) and lower during the summers (fig. 8.b, 8.d, 9.b and 9.d). Similar effect can be observed in groundwater elevations at Level 1 monitoring wells located at -20 mTAW elevation. The groundwater flow is radially inwards towards the location of the pumping wells. The average horizontal gradient in winter is 7.21E-03 whereas that in summer is 8.90E-03 for the level 2 wells and in level 1 wells, the average horizontal gradient in winter is 1.19E-03 whereas that in summer is 1.40E-03 (fig. 10). Both the well levels indicate higher flows from the dunes or areas outside the boundary of pumping wells towards the pumping wells in summer than those in winter.
Fig. 8 Contour of hydraulic head at -5 mTAW (Level 2 monitoring wells) around the pumping wells during: i) Winter 2015 (01/09/2015), ii) Summer 2015 (07/13/2015), iii) Winter 2016 (01/11/2016) and, iv) Summer 2016 (07/19/2016)
Fig. 9 Contour of hydraulic head at -20 mTAW (Level 1 monitoring wells) around the pumping wells during: i) Winter 2015 (01/09/2015), ii) Summer 2015 (07/13/2015), iii) Winter 2016 (01/11/2016) and, iv) Summer 2016 (07/19/2016)
Fig. 10 Horizontal gradients between dunes and pumping wells
Between 3.55 mTAW and 0.55 mTAW elevation, the hydraulic gradient is higher in summer than that in winter for the WP 21, 23 and 24 monitoring wells (fig 11). However, at WP 22, the winter of 2016 showed a higher gradient than the summer gradients. This may be attributed to a less permeable soil lens present in the area. WP 21 shows very high hydraulic gradients in both summer and winter, suggesting the presence of a less permeable lens in the area between 3.55 and 0.55 mTAW in the northern side of the west pond. The occurrence of a shallow low-permeable layer under the western pond is also mentioned by Vandenbohede et al. (2008a) between 3.55 and 0.55 mTAW, even though the lateral extent of the layer in unknown.
Fig. 11: Vertical hydraulic gradient in Well series 21, 22, 23 and 24 for 2015-2016 period between Level 4 (3.55 mTAW) and Level 3 (0.55 mTAW) wells
Fig. 12 Vertical hydraulic gradient in Well series 21, 22, 23 and 24 for 2015-2016 period between Level 3 (0.55 mTAW) and Level 2 (-5 mTAW) wells
Between 0.55 mTAW and -5 mTAW elevation, there is not much variation in gradients (fig 12). The lower part of the aquifer has lower hydraulic conductivity (Vandenbohede et al., 2008b). From the regional groundwater model and the existing local groundwater model (Vandenbohede & Houtte, 2012), it is inferred that the hydraulic conductivity is approximately 20 m day-1 at the top of the aquifer and 1 m day-1 at the bottom. In WP 23, the summer gradients are higher than the winter gradients. This occurrence is due to the high anisotropy in the region.
WP 6.2 is assumed to reflect the conditions exactly underneath the ponds as it is located centrally between two ponds. Vertical hydraulic gradient has been calculated between WP 6.2 and the pond bed to observe the variation of vertical gradient between summer and winter. Fig. 13 shows that the gradients are higher for summer of 2015 and 2016 in comparison to the winter of 2015 and 2016. This also suggests that the rate of infiltration through pond bed is higher in summer as compared to that in winter.
Fig. 13 Vertical hydraulic gradient between WP 6.2 and pond bed
Assuming hydraulic conductivity does not change over time, the rate of vertical flow is directly proportional to the change in vertical hydraulic gradient (eq. 2).
\(q\propto i\)     (2)
During the summer season, a lowering in hydraulic head is observed followed by an increase in vertical gradient. As a result, the vertical flow velocity is expected to be higher in summer. During the winter, the reverse phenomenon is observed. As the hydraulic head rises, the vertical gradient lowers and flow velocity reduces. It is seen from the vertical hydraulic gradients at WP 6.2 that the hydraulic gradient reduces considerable in winter as compared to that in summer.
The average reduction in regional vertical hydraulic gradient in winter as compared to summer is 32 % from 3.55 to 0.55 mTAW depth and 4 % from 0.55 to -5 mTAW. However, Vandenbohede & Houtte (2012) reports that the reduction of infiltration capacity in winter ranges from 33 - 50 %. Thus, the variation in vertical hydraulic gradient alone does not contribute to the overall fluctuation of infiltration rates. Hence, the assumption that hydraulic conductivity is constant over time does not stand valid and it is essential to take into account the variability of hydraulic conductivity as well.