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.