Abstract
Abstract. This paper investigates the patterns and controls of aquifer–river exchange in a fast-flowing lowland river by the conjunctive use of streambed temperature anomalies identified with Fibre-optic Distributed Temperature Sensing (FO-DTS) and observations of vertical hydraulic gradients (VHG). FO-DTS temperature traces along this lowland river reach reveal discrete patterns with "cold spots" indicating groundwater up-welling. In contrast to previous studies using FO-DTS for investigation of groundwater–surface water exchange, the fibre-optic cable in this study was buried in the streambed sediments, ensuring clear signals despite fast flow and high discharges. During the observed summer baseflow period, streambed temperatures in groundwater up-welling locations were found to be up to 1.5 °C lower than ambient streambed temperatures. Due to the high river flows, the cold spots were sharp and distinctly localized without measurable impact on down-stream surface water temperature. VHG patterns along the stream reach were highly variable in space, revealing strong differences even at small scales. VHG patterns alone are indicators of both, structural heterogeneity of the stream bed as well as of the spatial heterogeneity of the groundwater–surface water exchange fluxes and are thus not conclusive in their interpretation. However, in combination with the high spatial resolution FO-DTS data we were able to separate these two influences and clearly identify locations of enhanced exchange, while also obtaining information on the complex small-scale streambed transmissivity patterns responsible for the very discrete exchange patterns. The validation of the combined VHG and FO-DTS approach provides an effective strategy for analysing drivers and controls of groundwater–surface water exchange, with implications for the quantification of biogeochemical cycling and contaminant transport at aquifer–river interfaces.
Highlights
The results of this study demonstrate the potential of FODTS observations along a fibre-optic cable buried in the streambed for tracing complex patterns of exchange fluxes across the aquifer–river interface of larger lowland rivers
The results of this study provide strong evidence for the advantage of Fibre-optic Distributed Temperature Sensing (FO-DTS) monitoring in systems where traditional roaming temperature surveys of larger areas or a limited number of temperature profiles in the streambed sediments have a high probability of not capturing the very distinct and localized hotspots of groundwater inflow
Streambed vertical hydraulic gradients (VHG) patterns identified in this study were not suitable for directly determining groundwater– surface water exchange fluxes, when combined with FODTS observations of streambed temperature anomalies, they proved a useful indicator for the discrimination of driving forces and inhibitors of exchange over the aquifer–river interface
Summary
1.1 Motivation: the importance of groundwater– surface water exchange at aquifer–river interfacesHydrological sciences have experienced a significant paradigm shift in recent years, advancing the rather static perception of rivers and aquifers as discrete entities towards a more complex and dynamic understanding of groundwater and surface water as integral components of a streamcatchment continuum (Bencala, 1993; Brunke and Gonser, 1997; Boulton et al, 1998; Boulton, 2007; Sophocleous, 2002; Krause et al, 2009a, 2011a; Woessner, 2000). S. Krause et al.: Investigating patterns and controls of groundwater up-welling al., 1999; Storey et al, 2004; Duff and Triska, 1990; Hinkle et al, 2001; Jones et al, 1995; Findlay et al, 1993, 2003; Fisher et al, 1998; Hill and Cardaci, 2004; Zarnetzke et al, 2011a) as well as (ii) hyporheic flow paths and residence times (Zarnetzke et al, 2011b; Fisher et al, 1998; Bencala et al, 1993; Duff and Triska, 2000; Jones et al, 1995). A detailed understanding of groundwater–surface water exchange flow patterns is essential for the quantitative assessment of biogeochemical cycling at aquifer–river interfaces (White, 1993; Krause et al, 2011a)
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