River-aquifer exchange fluxes under monsoonal climate conditions
Introduction
The dynamic exchange of water, energy and solutes across the river-aquifer interface affects the ecology of river systems (Brunke and Gonser, 1997), pathways of nutrient cycling (Krause et al., 2009) as well as the transformation and attenuation of nutrients and contaminants (Smith et al., 2009; Zarnetske et al., 2011a, Zarnetske et al., 2011b). Across scientific disciplines interest in the dynamics of river-aquifer exchange and the transition zone between ground- and surface water where differences in chemical, biological and physical properties of the two adjoining compartments result in steep biogeochemical gradients has steadily grown in recent years (Fleckenstein et al., 2010, Krause et al., 2013). River-aquifer interactions were found to have positive as well as negative effects on groundwater and stream water quality (Grasby and Betcher, 2002, Schmidt et al., 2011). High concentrations of contaminants in groundwater can significantly impact surface water quality and vice versa (Kalbus et al., 2007). Groundwater ecosystems often depend on infiltrating surface water that is rich in organic matter as an energy source (Madsen et al., 1991) for biogeochemical reactions.
An important prerequisite to understand the transport of nutrients and contaminants across the river-aquifer interface and the resulting biogeochemical processes in the transition zone is to accurately characterize and asses the exchange fluxes at the river-aquifer interface (Conant, 2004, Greenberg et al., 2002). A broad range of methods exists to quantify groundwater–surface water exchange fluxes (Kalbus et al., 2006). However, the extreme variability of hydrologic conditions in monsoonal systems can make the use of many of them quite challenging. For example direct measurements of exchange fluxes by conventional seepage meters. (Landon et al., 2001, Rosenberry, 2008) are highly impractical under monsoonal conditions. During extreme precipitation events, river discharge can rapidly rise by up to 2 orders of magnitude relative to the discharge under dry conditions making in-stream installations difficult to employ. Additionally extreme flows may result in sediment scour and associated stream bed elevation changes, which further complicates the direct quantification of river aquifer-exchange fluxes in the field.
A commonly accepted conventional method for investigating river aquifer-exchange fluxes is based on monitoring of hydraulic gradients along piezometer transects (Eddy-Miller et al., 2009). The advantage of this method is that in-stream installations are not imperative. However, the head gradients between a piezometer in the riverbed, river bank, riparian zone and the stream alone is often only a weak indicator for the general direction of exchange (Kaeser et al., 2009) and spatial patterns of exchange fluxes are typically much more variable (Schornberg et al., 2010, Lewandowski et al., 2011, Angermann et al., 2012). Scanlon et al. (2002) therefore suggested the application of multiple methods for an accurate assessment of river-aquifer exchange fluxes. An increasing number of studies have combined hydraulic head measurements with the use of heat as a natural tracer and inverse numerical modeling (Eddy-Miller et al., 2009, Anibas et al., 2009, Schmidt et al., 2007, Constantz, 2008, Essaid et al., 2008).
Using heat as a tracer is based on natural temperature differences between ground- and surface water, which result in temperature distributions in the transition zone that are indicative of conductive and advective heat transport processes between the two compartments. Most surface waters show diurnal temperature fluctuations, whereas groundwater temperatures are relatively constant over time. Heat is transported by advection (with the moving water) and conduction (heat exchange due to temperature gradients) through the riverbed sediments (Constantz, 2008). In river reaches where surface water is infiltrating into the aquifer (losing conditions) the diurnal temperature signal from the surface water propagates downward by advective and conductive, heat transport (Graf, 2005). In contrast, in gaining reaches the temperature signal, which is conductively transported downwards, is attenuated by upward advection of groundwater with steady temperature, which dampens the diurnal temperature variation originating from the surface water (Eddy-Miller et al., 2009). Combining both, temperature and head data in the calibration of numerical models of groundwater–surface water interactions can provide more reliable estimates of exchange fluxes as opposed to using head data alone (Anderson, 2005).
In this study, we use head and temperature data from a river-aquifer system in South Korea that is driven by the East-Asian Monsoon to constrain a numerical model of the river-aquifer exchange dynamics. The main objective was to investigate how monsoonal precipitation events and the resulting variability in river discharge affect the dynamics of river-aquifer exchange and the corresponding flux rates. Hydraulic gradients between the river and the aquifer were monitored in a piezometer transect across a typical river reach in the catchment. Temperatures at different depth in the aquifer below the stream were measured in the central piezometer in the Thalweg of the stream. The 2D-model, based on the code HydroGeoSphere, was calibrated to the measured temperature and total head data. To elucidate potential effects of the observed and simulated river-aquifer exchange dynamics on biogeochemical transformations at the groundwater–surface water interface, river and groundwater samples were collected and analyzed for dissolved organic carbon (DOC), nitrate (NO3) and dissolved oxygen saturation (DOsat).
Section snippets
Study area and site
The study area is the Haean-myun Catchment (longitude 128°5′ to 128°11′E and latitude 38°13′ to 38°20′N) located in Yanggu County, Gangwon Province, South Korea. With an agricultural area of 42% of the entire basin area (62.7 km2), the Haean Catchment is one of the major agricultural areas in the region. It contributes significant amounts of agricultural nutrients (nitrogen, phosphorus) to the downstream receiving waters (Kim et al., 2006), which eventually feed into Lake Soyang an important
Precipitation and river discharge
In Fig. 2, panels A and B, the measured rainfall data as well as the corresponding discharge over the measuring period of 250 days are presented. Extreme precipitation events are typically of short duration and high intensity. For example, the first monsoonal event in 2010 on the 5th of July lasted approximately 70 min delivering a precipitation amount of 24.8 mm. At the beginning of August, extreme precipitation events occurred at comparably high frequency until Mid-September. The highest monthly
Simulation of exchange fluxes
The numerical model could successfully be calibrated by inversely estimating the hydraulic conductivities using the parameter estimation code PEST. The statistical measures (Table 3) indicate that the model performs well in predicting both, the hydraulic heads and temperatures. The simulated dynamics in heads and temperatures compare well with the observed dynamics with low RMSE values (<0.04 m for head; <1.75 °C for temperature) and high correlation coefficients (mean = 0.98, min = 0.96 for head;
Summary and conclusions
The main focus of this study was on investigating how monsoonal precipitation events affect the dynamics of river-aquifer exchange and the corresponding flux rates. The specific dynamics of river-aquifer exchange under monsoonal climate conditions were continuously monitored and simulated over several months. We additionally focused on examining potential implications of the investigated river-aquifer exchange fluxes for local water chemistry.
With respect to the water and solute dynamics under
Acknowledgements
This study was carried out within the framework of the International Research Training Group TERRECO (GRK 1565/1), funded by the Deutsche Forschungsgemeinschaft (DFG) at the University of Bayreuth (Germany) and the Korean Research Foundation (KRF) at Kangwon National University, Chuncheon (South Korea). The authors want to thank Sebastian Arnhold, Axel Müller, Bumsuk Seo, Eunyoung Jung, Bora Lee and Heera Lee for their support and translations during field campaigns. We are also grateful to
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