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TRANSPORT PROCESSES WITHIN THE HYPORHEIC ZONE
N. Saenger, B. Weiler, M. Lenk
Technical University of Darmstadt, Water
Resources Research and River Ecology, Rundeturmstraße 1, 64283 Darmstadt,
Germany,
tel. 0049/6151-162549, fax: 0049/6151-163223,
e-mail: saenger@hrzpub.tu-darmstadt.de
ABSTRACT
Interactions between surface water and the hyporheic zone are important for the ecosystem of streams. The parameter and processes within this zone develop their own gradients, varying with space and time. One very important process is the transport of fluid through this zone. The mean velocities can be determined with tracer experiments and depend on the morphology of the river bed (topography, sediment structure, poresystem).
Keywords: hyporheic zone, transport processes,
interactions with surface water
INTRODUCTION
In stream ecosystems structure and function, interactions between surface water and groundwater play an important role (see e.g.. Gibert et al. 1994): alluvial sediments with high hydraulic conductivity and short residence times enable subsurface flowpaths to transport water, with physicochemical properties similar to the surface water, into deeper alluvial layers underneath the riverbed and the floodplain. Lying underneath the riverbed, the hyporheic zone connects the two ecosystems stream and groundwater. The hyporheic zone can be distinguished from its surrounding environments because it combines feature of both. Parameter develop their own gradients (Brunke & Gonser 1997).
Within an interdisciplinary research project, the processes taking place are analysed in order to describe and quantify the ecological relevance of the interactions between surface water and the hyporheic zone. In this paper, velocities within the hyporheic zone of the River Lahn, Germany will be discussed.
STUDY SITE
The study site is located at the medium-scaled River Lahn, Germany, which is a right-sided tributary of the middle part of the river Rhine. It is situated near Marburg, about 53 km from the source with a drainage area of 453 km². The mean discharge amounts to MQ=7,3 m³/s, the MNQ is 0,567 m³/s (Deutsches Gewässerkundliches Jahrbuch, Rheingebiet, Teil III, 1993); the medium gradient of the Upper Lahn is 2,36 ?. The study site is about 400 m long and includes two pool-riffle-sequences, with a sewage treatment plant inlet in between.
METHODS
Different methods are used to analyse the exchange, e.g. tracer injection in the stream and in the sediment, measurement of hydraulic heads with piezomanometers and freeze-cores to understand the sediment structure (Saenger & Lenk in press) as well as the survey of the dynamic of temperature within the river bed (Lenk & Saenger in press). Here, tracer experiments, where tracer is injected in the sediment, will be discussed.
To extract porewater from the hyporheic zone at up to eight different depths (5, 15, 25, 45, 55, 65, 75 and 95 cm), special multilevel probes were developed. Fig. 1 shows the stainless steel probe with a length of 50 cm and four extraction levels. Each extraction level is connected by teflon tubes with a sampling collector, so that all samples can be collected simultaneously. The pipe-system is operated at low pressure of max. 0,2 bar. Experiments in the laboratory showed that the pressure is low enough to avoid short circuits of interstitial flow at the sampling probes and that the mean velocities of water flow is not affected by the vacuum.
The tests are used to analyse tracer experiments, physical parameters (Fischer et al. (1998), Borchardt & Fischer in press) and microbiological activity (Bolanos et al.1998). For the tracer experiments 50 ml of porewater was evacuated at each extraction level and analysed with a Turner Fluorometer. Fiftytree multilevel probes were installed in cross sections in the sediment of the two riffle-pool-sequences, three cross sections per riffle and one cross section in the pool.
Fig. 1: Multilevel probe
EXPERIMENTS
To study the sediment structure, probes were taken out as freeze-cores. Sieve analysis were made, permeability and porosity were calculated as shown by Beyer & Schweiger (1969) and Hölting (1992). The permeability ranges from 10-2 to 10-4 m/s and the average effective porosity is about 24%.
The reference pool-riffle-sequence upstream from the sewage treatment plant was selected to analyse the transport processes within the hyporheic zone. On the riffle multilevel probes are installed in cross sections where up- and downwelling is expected. The three cross sections I, II (both expected downwelling zones)and III (expected upwelling zone) have a distance in drift of about 22m. The acceptance of in- and exfiltration could be proved when analysing the measurements of hydraulic heads and tracer experiments, where tracer was injected in the surface water (Saenger & Lenk in press). This relates to the topography and morphology of the river bed (pool-riffle-sequence) as shown e.g. by Thibodeaux & Boyle (1987).
In each cross section, four probes with extraction levels at depth of 5, 15, 25 and 45 cm and one probe with extraction levels of 5, 15, 25, 45, 55, 65, 75 and 95cm were considered. They were installed as shown in fig.2. The distances between the probes were about 1m.
Fluorescein tracer was injected in the hyporheic zone 1m in front of the midstream multilevel probe at a depth of about 20cm (cross section I and II) and 30cm (cross section III). Tracer was injected for about 20min and porewater was extracted with the probes for 7h. The expermiments took place in October 1998 at a discharge of about 10,7m³/s.
Fig. 2: Arrangement of probes in each of the three cross sections
RESULTS
The tests were analysed with a Turner Fluorometer and, where tracer was detected, throughflow curves could be drawn. The time taken for half the tracer to pass the probe as well as the time when the tracer peak occured as determined and a mean velocity calculated (KÄSS 1992). The results showed, that the velocities within the interstitial water varied in space: at cross section I, tracer was found in the two probes in succession in drift and the velocities varied from 0,5 - 1,0 m/h in the depth of 25cm and from 1,4 to 1,8m/h in the depth of 45cm. As the tracer was injected at a depth of about 20cm, the flume`s transport can be characterized as horizontal and downwelling.
In cross section II, tracer was detected in the depth of 10, 15 and 25cm with mean velocities varying between 0,37 to 0,4 m/h. Also, the flume reached the far right probe in a depth of 5cm with a velocity of about 1,1m/h. So, in this cross section, the flume is transported horizontal and is more upwelling than downwelling.
In cross section III, only in the depth of 25cm could tracer be found. As the tracer injection was at the depth of 30 cm, a slight upwelling could be seen; this has to be varified. The velocity was about 0,5m/h.
CONCLUSION AND OUTLOOK
The experiments show, that with this method mean velocities in the hyporheic zone can be estimated. In comparison, the velocities within the hyporheic zone are about 10-4 of the velocity of the surface water.
Further experiments have to be done to verify these results and to examine the influence of the variability of surface and groundwater flow on the flow velocities in the interstitial pores, on the amount of water going through the hyporheic zone as well as on the flow directions.
In additon, the impact of the sediment structure and of colmation processes on the interactions between surface water, hyporeic zone and groundwater has to be estimated.
LITERATURE
Beyer W. & K.H. Schweiger (1969): Zur Bestimmung des entwässerbaren Porenanteils der Grundwasserleiter. Wasserwirtschaft Wassertechnik (WWT), 19.Jg., Heft 2.
Bolanos, M.C., S. Brandt, V. Rosenkranz, D. Werner (1998): Nitrifikations- und Denitrifikationsprozesse im hyporheischen Intertitial eines Fließgewässers am Beispiel der Lahn. Deutsche Gesellschaft für Limnologie (DGL), Tagungsbericht 1997 (Frankfurt/M.), Krefeld.
Borchardt, D. & J. Fischer (in press): Three-dimensional patterns and processes in the RIVER LAHN (Germany): Variability of abiotic and biotic conditions. SIL Proceedings Dublin.
Brunke, M. & T. Gonser (1997): The ecologic significance of exchange processes between rivers and groundwater. Freshwater Biology 37.
Deutsches Gewässerkundliches Jahrbuch, Rheingebiet, Teil III, 1993
Fischer, J., C. Hellwig, D. Borchardt (1998): Räumliche und zeitliche Variabilität im Stoffhaushalt der Lahn. Deutsche Gesellschaft für Limnologie (DGL), Tagungsbericht 1997 (Frankfurt/M.), Krefeld.
Gibert, J., D.L. Danielopol, J.A. Stanford (1994): Groundwater Ecology. Academic press, San Diego, California.
Hölting, B. (1992): Hydrogeologie, 4. Auflage, Stuttgart.
Käss W. (1992): Geohydrologische Markierungstechnik. Berlin, Stuttgart.
Lenk, M. & N. Saenger (in press): Exchange processes in the river bed and their influence on temperature variations. SIL Proceedings Dublin 1998.
Saenger, N. & M. Lenk (in press): Hydraulic head and tracer experiments - two techniques to examine the hydraulic exchange through a riffle. SIL Proceedings Dublin 1998.
Thibodeaux, L.J. & J.D. Boyle (1987): Bedform-generated convective transport in bottom sediment. Nature Vol. 325.