Nonuniform Groundwater Discharge across a Streambed: Heat as a Tracer

Abstract

Time series analysis of con nuous streambed temperature during a period of 47 d revealed that discharge to a stream is nonuniform, with strongly increasing ver cal fl uxes throughout the top 20 cm of the streambed-aquifer interface. An analy cal solu on to the transient heat transport equa on was used to analyze several pairs of observed amplitude damping with depth. A nonuniform pa ern in discharge across the stream width was also observed, which could have been caused by lateral or horizontal fl ow. Head measurements in a meadow area and below the stream showed strong convergence of fl ow near the streambed. Seepage meter measurements in the middle of the stream o en resulted in highly variable fl ux esmates, which could have been caused by hyporheic fl ow due to the presence of a gravel layer. Discharge and recharge to the stream at the bank near the meadow was rela vely steady throughout the period. On the other hand, discharge to the stream at the opposite bank near a steep hillslope decreased signifi cantly toward the end of the period (early June), which was a ributed to a drop in the water table on this side of the stream. The results from the me series analysis were compared with seepage meter measurements and the results from a steady-state analy cal solu on to the heat transport equa on. The diff erent methods agreed on the pa ern of discharge across the stream width, and the mean values during the studied period generally agreed well but with diff erent ranges.Abbrevia ons: SWI, sediment-water interface. The European Union (EU) Water Framework Direc ve requires EU membercountries to administer their water resources in an integrated fashion, where the ecological statuses of diff erent water bodies are treated together. In this respect, groundwater-surface water interactions play a central role (Dahl et al., 2007). In the Skjern River Catchment (SRC) in Denmark, groundwater-stream and groundwater-lake interactions have been investigated at various scales. For example, Kidmose et al. (2011) modeled lake-scale seepage patterns at a fl ow-through lake in the upper parts of the SRC to see how seepage is aff ected by lake bathymetry and the depositional character of the lake bed sediments. On-land and off shore geophysical explorations have helped to improve the inclusion of the lake in the regional groundwater fl ow model, improving the model's ability to simulate the observed seepage patterns.Heat has been used in many studies as a natural tracer to assess groundwater-surface water exchange at various scales. Th e seasonal changes in temperature contrasts between the surface water and deeper groundwater make it possible to quantify the direction and magnitude of the exchange (Constantz, 2008). At the kilometer scale, distributed temperature sensing has been used to study groundwater-lake and groundwater-stream interactions (Selker et al., 2006a,b). At the meter scale, point or profi le measurements of temperature have been popular for some time (Constantz, 2008). A number of investigations have relied on ...

Detecting groundwater discharge dynamics from point-to-catchment scale in a lowland stream: combining hydraulic and tracer methods

“…Based on the VTP measurements, vertical groundwater fluxes were estimated by fitting the steady-state analytical solution of the one-dimensional conduction-convection equation (Bredehoeft and Papadopulos, 1965) to the measured temperature data as described by Schmidt et al (2007) and Jensen and Engesgaard (2011):…”

“…As previous studies (Jensen and Engesgaard, 2011;Karan et al, 2013) in the same area only detected moderate seasonal changes in streambed temperatures, the steady-state conditions were assumed to be valid for the study period in June. For each VTP, T s was given as the temperature measured by the uppermost sensor, and the constant groundwater temperature of 8 • C (T g ) was assumed at a depth of 5 m (L).…”

“…Groundwater fluxes at specific point locations have been measured in rivers by use of seepage metres, revealing large heterogeneity within metres (Landon et al, 2001;Langhoff et al, 2006;Rosenberry, 2008). This spatial heterogeneity has been confirmed by use of temperature as a natural tracer (Conant, 2004), where punctual vertical groundwater fluxes have been estimated from vertical sediment temperature profiles (VTPs) using the steady-state analytical solution to the 1-D conduction-convection equation (Schmidt et al, 2007;Jensen and Engesgaard, 2011) and streambed temperature time series (Hatch et al, 2006;Keery et al, 2007).…”

Continental hydrosystem modelling: the concept of nested stream–aquifer interfaces

“…Coupled with in situ measurements, two methods, based on heat transport governing equations, are used to quantify stream-aquifer exchanges (Anderson, 2005): 1. Analytical models (Stallman, 1965;Anderson, 2005) are widely used to invert temperature measurements solving the 1-D heat transport equation analytically under simplifying assumptions (sinusoidal or steady boundary conditions and homogeneity of hydraulic and thermal properties) (Anibas et al, 2009(Anibas et al, , 2012Becker et al, 2004;Hatch et al, 2006;Jensen and Engesgaard, 2011;Keery et al, 2007;Lautz et al, 2010;Luce et al, 2013;Rau et al, 2010;Schmidt et al, 2007;Swanson and Cardenas, 2011). …”

Observing temporal patterns of vertical flux through streambed sediments using time-series analysis of temperature records

“…In these studies, streambed hydraulic conductivity has been derived by normalizing the heat transport modeling flux results to observed hydraulic gradients and, as such, the heat transport modeling results were not field-validated. Two field studies presented in the literature compare SW-GW exchange rates derived from the one-dimensional heat transport model with direct measurements of flux across the streambed interface, showing mean values over time were in general agreement Jensen and Engesgaard, 2011). To date, only one example of a controlled laboratory verification of the heat transport model exists in the literature (Munz et al, 2011).…”