1. Introduction
Managed Aquifer Recharge (MAR) is a relatively new method to increase potable water supplies in Belgium. MAR provides a sustainable solution to increasing water supply, without compromising the water quality, and often uses treated wastewater, agricultural return flows or excess water from streams (Massmann et al., 2008). The main objective of the St-André MAR facility is to meet the water demands of the surrounding area, replenish the overexploited aquifer and restore the groundwater table in the unconfined dune aquifer (Vandenbohede, Van Houtte, & Lebbe, 2008a). Infiltration rate (IR) is the most important parameter controlling recharge capacity in the St-André Managed Aquifer Recharge facility. A higher infiltration rate through the pond bed signifies better recharge capacity and in turn increase groundwater availability (Sheng & Zhao, 2015). However, data collected from this facility reveals that the infiltration rate is lower in the winter and higher in the summer (Vandenbohede & Houtte, 2012), indicating that the total volumes of water recharged by the MAR system are lower during the winter.
The variation of the vertical flow velocity of groundwater may occur as a result of changes in aquifer properties such as lower hydraulic gradient, reduced hydraulic conductivity of aquifer media and reduced leakance through pond bed occurring as a result of reduced conductivity of the bed during the winter. Suzuki (1960) and Stallman (1965) started the development of computation of infiltration rates from observed temperatures near the land surface with the idea that low temperatures are effective at reducing hydraulic conductivity of the soil media. Constantz, Thomas, & Zellweger (1994) studied the influence of diurnal variations in stream temperature on streamflow loss and groundwater recharge. They observed a trend of diurnal variation of stream temperature and seepage and hypothesized that the two are related and that regardless of timescale, a significant change in stream temperature can cause a measurable response in seepage loss in a reach. Their study yielded to the conclusion that a fluctuation in stream temperature changes the hydraulic conductivity of the underlying sediments in a losing stream. The temperature of streambed layer is controlled by the temperature of the stream and is dependent on factors such as depth, thickness, degree of saturation and rate of flow of water through it. For both reaches, the predicted influence of stream temperature on hydraulic conductivity of bed accounted for all the variation in the seepage loss. Similar behavior was reported again by Constantz and Thomas (1997), Constantz (1998), and Heilweil and Watt (2011).
Lin, Greenwald, & Banin (2003) stated that infiltration rates (IR) in a large-scale effluent recharge facility can be affected by factors such as physical clogging, biological clogging, temperature variations, entrapped air, dispersion and the swelling of clay. With the assumption that all factors controlling IR are constant and only temperature is the variable property, they studied the impact of temperature in the variation of infiltration rate through a natural porous media. Based on the hypothesis that relative infiltration rate is driven by temperature or temperature-controlled properties such as density and viscosity, the study could generate expression of IR in terms of viscosity at a given temperature with reference to 25°C. However, the entire variability of IR could not be accounted for with just viscosity and changes in entrapped air content was stated as an alternate explanation to the variation of IR. Similar study has been published by Braga, Horst, & Traver (2007) on the effect of temperature on the infiltration rate through an infiltration basin BMP (best management practice) to develop a methodology to simulate varying infiltration rates. The study has been done though a model that uses Green-Ampt equation for infiltration, assuming that all precipitation inputs the system and the only outflow from the system is infiltration. Also, it is assumed that there is no infiltration through the sidewalls of the BMP. The modified Green-Ampt equation used in this paper assumes that there is no ponding at the surface such that the infiltration rate is always the infiltration capacity. Braga et al. (2007) shows a strong relationship between hydraulic conductivity and mean bed temperature. They found that during warmer period the infiltration rate increases by as much as 56%.
Racz, Fisher, Schmidt, Lockwood, & Huertos (2012) studied the spatial and temporal infiltration dynamics during managed aquifer recharge (MAR) and noticed a seasonal trend in hydraulic conductivity variation. The study reported that the daily temperature of water reduced in amplitude and shifted in phase as depth from surface increased. However, Racz et al. (2012) did not consider any possible relation between temperature and hydraulic conductivity variation and was more interested in the various possibilities that could generate spatial and temporal variations in the MAR system. This study suggested that variable source area concept can be extended to infiltration as a framework for describing spatial and temporal variability.
(Loizeau et al., 2017) have studied the combined involvement of water temperature and air entrapment on infiltration rate variations at a scale of infiltration basins. They employed both experimental design and modelling work to address the issue. The key assumption made for this study was that when one component or an ensemble of components are analyzed for their impact on hydraulic conductivity, all other factors are treated as constants. Their primary hypothesis was that the basin response to infiltration is dependent on temperature contrast between surface and groundwater and on air entrapped in the porous media. The study involved three infiltration experiments for injection of warm and cold surface water at an initial dry basin and injection of temperate surface water in an initially wet basin. Unsaturated and saturated flow models were developed using the 2D Richards equation. The experiments and model simulations verified that temperature-contrast and air entrapment could significantly impact infiltration rates and the effects are similar in magnitude. Loizeau et al. (2017) showed a very critical understanding of variation of infiltration rates from a management point of view by stating that all temporary reduction in rates should not be attributed to clogging. There are multiple ways of misinterpretation of infiltration rates and the measurements must be made for a long enough time to show some trends in variation.
Vandenbohede and Houtte (2012) studied the influence of temperature variation of infiltration water in the managed aquifer recharge facility in Belgium and stated that temperature variations have a number of consequences on the operation and management of the MAR system. They made a few hypotheses justifying the variation of infiltration rates such as temperature variations causes change in hydraulic conductivity since pore water density and viscosity are temperature dependent; temperature can be used to study residence time of infiltrated water, which was causing fluctuation in infiltration rates; and that recharge water had in most cases a different composition than the pore water triggering a number of reactions which are influenced by temperature. The paper employed a hypothetical 2D groundwater flow and heat transport model to test 5 alternating scenarios which would represent different temperature conditions and inflow/extraction rate in the system. MOCDENS3D in combination with SEAWAT was used to simulate the conditions of St-André MAR system. Results of the flow and transport model were compared to time series of hydraulic head in observation wells, of chloride concentration in the extraction wells and of temperature observations in observation wells. It was found that placing the extraction wells deeper or lowering the extraction to infiltration ratio for a given system, increased the temperature influence outside the well battery but remained limited to the immediate vicinity of the MAR system. Heat conduction smoothened the temperature variations in the aquifer. Close to the pond and in the shallow part of the aquifer, short-term temperature variations of the infiltration water persisted. Temperature variations influence hydraulic conductivity, and this resulted in a larger infiltration capacity in summer than in winter by 1.7 times.
The motive of this paper was to provide a process-based understanding of the variation of infiltration rates and indicate the factors that influence this variation by using statistical and numerical models for different seasonal scenarios. The objectives were to (a) identify possible factors that influence the variation in infiltration capacity in this site, (b) develop a relationship between the predominant factor and the infiltration rate, and (c) develop MODFLOW models to simulate water movements below the ponds and assess hypothetical scenarios for summer and winter to verify flux and flow velocity during the two seasons. Finally, a quantitative evaluation was provided to account for the variations in the recharge rates.