Seasonal Variation of Infiltration Rates Through Pond Bed in a Managed Aquifer Recharge System

Abstract

Managed Aquifer Recharge (MAR) has become a powerful tool for increasing water supplies around the world. The Torreele Waste Water Treatment Plant (TWWTP) in Belgium uses MAR to recharge the aquifer with treated wastewater in order to sustain the potable water supply on the Belgian coast. The treated water from the TWWTP is transported to two infiltration ponds where it is recharged into a 30m deep phreatic aquifer by the natural process of infiltration under gravity. One of the challenges of the MAR facility is the reduced infiltration rates during the winter season when pond water temperatures vary from 4 °C to 10 °C. The infiltration capacity is approximately 50.5 to 100 % higher in summer as compared to that in winter. Several factors including pumping rate around ponds, natural recharge, tidal influences of the North Sea and pond-water temperature were identified as potential causes for the variation of recharge rate. This study involves the identification of the predominant factor influencing the rate of infiltration through the pond bed. Correlation statistics and linear regression analysis have been used to determine the sensitivity of infiltration rate to the aforementioned factors. Results showed that water temperature has the maximum impact on infiltration rate. Cyclic variations in water viscosity, occurring as a result of seasonal temperature changes, influence the saturated hydraulic conductivity of the pond bed. A lower infiltration rate through the pond bed is observed during the winter months due to increased viscosity, which results in a decline in saturated hydraulic conductivity of the sandy soil. Temporal changes in the vertical hydraulic gradient is another factor that could alter infiltration rate as the pumping rate around the pond causes fluctuations in the groundwater level. Two groundwater flow models have been developed in visual MODFLOW to simulate the water movement under the pond bed. The response of groundwater levels to artificial recharge from the pond under hypothetical scenarios for summer and winter months are assessed to obtain the differences in flux and track the effects of variation of hydraulic conductivity during the two seasons. It is observed from the models that higher leakance through infiltration pond bed in summer corresponds to the reduced heads in the monitoring wells and lower leakance corresponds to the higher heads, hence proving the impact of temperature influenced hydraulic conductivity in variation of infiltration rate.