Assessing hyporheic zone dynamics in two alluvial flood plains of the Southern Alps using water temperature and tracers

International audienceWater temperature can be used as a tracer for the interaction between river water and groundwater, interpreting time shifts in temperature signals as retarded travel times. The water temperature fluctuates on different time scales, the most pronounced of which are the seasonal and diurnal ones. While seasonal fluctuations can be found in any type of shallow groundwater, high-frequency components are more typical for freshly infiltrated river water, or hyporheic groundwater, and are thus better suited for evaluating the travel time of the youngest groundwater component in alluvial aquifer systems. We present temperature time series collected at two sites in the alpine floodplain aquifers of the river Brenno in Southern Switzerland. At the first site, we determine apparent travel times of temperature for both the seasonal and high-frequency components of the temperature signals in several wells. The seasonal signal appears to travel more slowly, indicating a mixture of older and younger groundwater components, which is confirmed by sulphate measurements. The travel times of the high-frequency component qualitatively agree with the groundwater age derived from radon concentrations, which exclusively reflects young water components. Directly after minor floods, the amplitude of temperature fluctuations in an observation well nearby the river is the highest. Within a week, the riverbed is being clogged, leading to stronger attenuation of the temperature fluctuations in the observation well. At the second site, very fast infiltration to depths of 1.9m under the riverbed could be inferred from the time shift of the diurnal temperature signal

Assessing residence times of hyporheic ground water in two alluvial flood plains of the Southern Alps using water temperature and tracers

International audienceWater temperature can be used as a tracer for the interaction between river water and groundwater, interpreting time shifts in temperature signals as retarded travel times. The water temperature fluctuates on different time scales, the most pronounced of which are the seasonal and diurnal ones. While seasonal fluctuations can be found in any type of shallow groundwater, high-frequency components are more typical for freshly infiltrated river water, or hyporheic groundwater, and are thus better suited for evaluating the travel time of the youngest groundwater component in alluvial aquifer systems. We present temperature time series collected at two sites in the alpine floodplain aquifers of the Brenno river in Southern Switzerland. At the first site, we determine apparent travel times of temperature for both the seasonal and high-frequency components of the temperature signals in several wells. The seasonal signal appears to travel more slowly, indicating a mixture of older and younger groundwater components, which is confirmed by sulphate measurements. The travel times of the high-frequency component qualitatively agree with the groundwater age derived from radon concentrations, which exclusively reflects young water components. Directly after minor floods, the amplitude of temperature fluctuations in an observation well nearby the river is the highest. Within a week, the riverbed is being clogged, leading to stronger attenuation of the temperature fluctuations in the observation well. At the second site, very fast infiltration to depths of 1.9 m under the riverbed could be inferred from the time shift of the diurnal temperature signal

Untersuchung der Flusswasserinfiltration in voralpinen Schottern mittels Zeitreihenanalyse

Kurzfassung: Grundwasserfassungen in der Nähe von Flüssen können durch die Infiltration von Flusswasser beeinflusst werden. Aus Sicht des Trinkwasserschutzes interessiert vor allem, welcher Anteil des geförderten Wassers aus dem Fluss stammt und wie lange das Flussinfiltrat im Grundwasserleiter verbleibt, bevor es gefördert wird. Hierzu können Markierversuche durchgeführt werden, die jedoch bei größeren Flüssen mit einem erheblichen Stoffeintrag verbunden sind. Als Alternative zu Markierversuchen stellen wir Methoden vor, um aus Zeitreihen der elektrischen Leitfähigkeit und der Temperatur quantitative Aussagen zu Mischungsverhältnissen und Aufenthaltszeiten abzuleiten. Wir empfehlen ein mehrstufiges Vorgehen bestehend aus: (1) einer qualitativen Analyse, (2) der spektralen Ermittlung des saisonalen Temperatur- und Leitfähigkeitsverlaufs, (3) einer Kreuzkorrelationsanalyse und (4) der nicht-parametrischen Dekonvolution der Zeitreihen. Wir wenden diese Methoden an drei Standorten im Grundwasserstrom des Thurtales im schweizerischen Mittelland an. An Standorten ohne gute Flussanbindung oder mit exfiltrierenden Verhältnissen können die aufwändigen Zeitreihenanalysen nicht angewendet werden, die Messreihen zeigen jedoch die entsprechenden Verhältnisse an. An Standorten mit dauerhafter Flussinfiltration kann aus den Zeitreihen die Durchbruchskurve eines Markierversuches rekonstruiert werden, ohne einen künstlichen Markierstoff in den Fluss geben zu müsse

Contributions of catchment and in-stream processes to suspended sediment transport in a dominantly groundwater-fed catchment

Suspended sediments impact stream water quality by increasing the turbidity and acting as a vector for strongly sorbing pollutants. Understanding their sources is of great importance to developing appropriate river management strategies. In this study, we present an integrated sediment transport model composed of a catchment-scale hydrological model to predict river discharge, a river-hydraulics model to obtain shear stresses in the channel, a sediment-generating model, and a river sediment-transport model. We use this framework to investigate the sediment contributions from catchment and in-stream processes in the Ammer catchment close to Tübingen in southwestern Germany. The model is calibrated to stream flow and suspended-sediment concentrations. We use the monthly mean suspended-sediment load to analyze seasonal variations of different processes. The contributions of catchment and in-stream processes to the total loads are demonstrated by model simulations under different flow conditions. The evaluation of shear stresses by the river-hydraulics model allows the identification of hotspots and hot moments of bed erosion for the main stem of the Ammer River. The results suggest that the contributions of suspended-sediment loads from urban areas and in-stream processes are higher in the summer months, while deposition has small variations with a slight increase in summer months. The sediment input from agricultural land and urban areas as well as bed and bank erosion increase with an increase in flow rates. Bed and bank erosion are negligible when flow is smaller than the corresponding thresholds of 1.5 and 2.5 times the mean discharge, respectively. The bed-erosion rate is higher during the summer months and varies along the main stem. Over the simulated time period, net sediment trapping is observed in the Ammer River. The present work is the basis to study particle-facilitated transport of pollutants in the system, helping to understand the fate and transport of sediments and sediment-bound pollutants.</p

Sorption and transformation of the reactive tracers resazurin and resorufin in natural river sediments

Resazurin (Raz) and its reaction product resorufin (Rru) have increasingly been used as reactive tracers to quantify metabolic activity and hyporheic exchange in streams. Previous work has indicated that these compounds undergo sorption in stream sediments. We present laboratory experiments on Raz and Rru transport, sorption, and transformation, consisting of 4 column and 72 batch tests using 2 sediments with different physicochemical properties under neutral (pH=7) and alkaline (pH=9) conditions. The study aimed at identifying the key processes of reactive transport of Raz and Rru in streambed sediments and the experimental setup best suited for their determination. Data from column experiments were simulated by a travel-time-based model accounting for physical transport, equilibrium and kinetic sorption, and three first-order reactions. We derived the travel-time distributions directly from the breakthrough curve (BTC) of the conservative tracer, fluorescein, rather than from fitting an advective-dispersive transport model, and inferred from those distributions the transfer functions of Raz and Rru, which provided conclusive approximations of the measured BTCs. The most likely reactive transport parameters and their uncertainty were determined by a Markov chain–Monte Carlo approach. Sorption isotherms of both compounds were obtained from batch experiments. We found that kinetic sorption dominates sorption of both Raz and Rru, with characteristic timescales of sorption in the order of 12 to 298 min. Linear sorption models for both Raz and Rru appeared adequate for concentrations that are typically applied in field tracer tests. The proposed two-site sorption model helps to interpret transient tracer tests using the Raz–Rru system.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Copernicus Publications on behalf of the European Geosciences Union. The published article can be found at: http://www.hydrology-and-earth-system-sciences.net/home.html

Numerische Methoden zur Simulation des reaktiven Mehrkomponententransports im Grundwasser

Available from TIB Hannover: RA 1064(95) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Investigating riparian groundwater flow close to a losing river using diurnal temperature oscillations at high vertical resolution

River-water infiltration is of high relevance for hyporheic and riparian groundwater ecology as well as for drinking water supply by river-bank filtration. Heat has become a popular natural tracer to estimate exchange rates between rivers and groundwater. However, quantifying flow patterns and velocities is impeded by spatial and temporal variations of exchange fluxes, insufficient sensors spacing during field investigations, or simplifying assumptions for analysis or modeling such as uniform flow. The objective of this study is to investigate lateral shallow groundwater flow upon river-water infiltration at the shoreline of the riverbed and in the adjacent riparian zone of the River Thur in northeast Switzerland. Here we have applied distributed temperature sensing (DTS) along optical fibers wrapped around tubes to measure high-resolution vertical temperature profiles of the unsaturated zone and shallow riparian groundwater. Diurnal temperature oscillations were tracked in the subsurface and analyzed by means of dynamic harmonic regression to extract amplitudes and phase angles. Subsequent calculations of amplitude attenuation and time shift relative to the river signal show in detail vertical and temporal variations of heat transport in shallow riparian groundwater. In addition, we apply a numerical two-dimensional heat transport model for the unsaturated zone and shallow groundwater to obtain a better understanding of the observed heat transport processes in shallow riparian groundwater and to estimate the groundwater flow velocity. Our results show that the observed riparian groundwater temperature distribution cannot be described by uniform flow, but rather by horizontal groundwater flow velocities varying over depth. In addition, heat transfer of diurnal temperature oscillations from the losing river through shallow groundwater is influenced by thermal exchange with the unsaturated zone. Neglecting the influence of the unsaturated zone would cause biased interpretation and underestimation of groundwater flow velocities. The combination of high resolution field data and modeling shows the complex hydraulic and thermal processes occurring in shallow riparian groundwater close to losing river sections as well as potential errors sources for interpreting diurnal temperature oscillations in such environments

Numerical simulation of streamline-oriented transport: flow calculation, grid generation and transport calculation

A method is presented to construct two-dimensional flownets for the numerical simulation of transport in porous media. The flow problem is solved using a mixed-hybrid Finite Element method. For the construction of lines orthogonal to the streamlines pseudopotentials are used. Pseudopotentials are evaluated using a standard Finite Element method. The grid generator determines stagnation points and uses local values of the streamfunctions for the construction of streamlines. Both interior wells and anisotropic conditions can be handled. The grid is used for the simulation of multicomponent reactive transport, using a Finite Volume approach with slope limiter stabilization of advection-dominated transport. The reactive sub-problem is solved by the solver for differential-algebraic systems DASSL. Coupling of reactive processes and advective-dispersive transport is done by the operator-split method. Injection of a biodegradable substrate into an injection well is taken as test example for comparison of grids. The streamline-oriented grid leads to a significant improvement in the approximation of transverse mixing compared to a rectangular grid, leading to different results in the prediction of reactive transport. (orig.)Available from TIB Hannover: RN 7077(236) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman