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TERENO Newsletter 2/2024
TERENO Newsletter 2/2024
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Hyporheic zone dynamic

The test site is located at the Selke river. The Selke is a tributary to the Bode and drains a catchment of 458 km². The test site is an approximately 1 km long reach of the Selke, 3 km downstream of the Meisdorf gauge station. The reach characterized by a near natural morphology with meanders, pool-riffle sequences and point and mid-channel bars.

 

Research activities at the Selke test site focus on the pattern and dynamics of water flow and solute transport within in the morphological features (e.g. gravel bars). It is hypothesized that the residence time of stream water in the streambed significantly influences solute turnover and stream metabolism. Reactions in the streambed are essentially control by the redox conditions. For instance: denitrification can only occur at oxygen concentration lower than 1 mg/l. One other key parameter is streambed temperature. Temperature controls microbially mediated reactions and is a crucial parameter for reactive transport modeling.

 

The key objective at the Selke test site is to understand which hydraulic and morphological factors control the reactivity of the hyporheic zone.

 

At the Selke test site stream and streambed temperature is continuously monitored with multilevel temperature probes. The temperature data is used for reactive transport models and also as natural tracer to estimate direction and magnitude of water flow. Water level in the stream and pressure in the stream and the shallow aquifer is monitored with pressure transducers to derive the dynamics of hydraulic gradients. Electrical conductivity is continuously monitored in the stream and the streambed and used as natural tracer to derive travel times in the streambed. Oxygen is measured in the streambed applying newly developed vertical optical oxygen profilers. The gauging station Meisdorf is equipped with a multi parameter probe and automated water samplers for water quality monitoring.

 

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Understanding of DOC, water and solute fluxes within a the catchment of a drinking water reservoir

The Rappbode Reservoir provides drinking water to more than 1 Mio people in central eastern Germany. Having a maximum depth of almost 90 m and a maximum volume of about 100 Mio m3, this reservoir is the largest drinking water reservoir in Germany. The three major tributaries (Bode, Hassel & Rappbode) into the reservoir, each of them being separately damed by smaller pre-dams, are contrasted by different land use characteristics with large differences in nutrient load and DOC export.

 

Given the relatively short residence times within the reservoirs the dynamics in the water body are tightly coupled to the biogeochemical processes in their catchment. Accordingly, the aim of this observation is to study the interactions between catchment and surface waters. We are interested in quantifying the exports of carbon and nutrients from the catchment and understanding the driving variables of this flux of matter from the terrestrial environment into surface waters. Moreover, we study how these exported nutrients or organic carbon compounds are processed within the lacustrine ecosystem.

 

A currently highly important topic in our research is the dynamics of dissolved organic carbon (DOC) in drinking water reservoirs. A consistent trend of increasing concentrations of DOC has been observed over the past 20 years in the Rappbode Reservoir leading to a degrading water quality and problems in the drinking water preparation in the nearby water works.

 

At nine locations in and around the reservoir system we are continuously observing dynamics in physical, chemical and biological parameters. This includes stations at all tributaries, at the connecting infrastructures in between the pre-dams and the main reservoirs, as well as within the main reservoir and its outlet. Besides automated sampling stations we also conduct a detailed limnological monitoring programme in the water bodies on a fortnightly sampling regime. Close cooperations have been initialised with the reservoir authority (Talsperrenbetrieb Sachsen-Anhalt) and the drinking water company (Fernwasserversorgung Elbaue Ostharz) leading to an active exchange of data and knowledge.

 

Active research projects are focused on the dynamics of DOC, nutrient removal in the pre-dams, and numerical modelling of hydrodynamics and plankton dynamics.

 

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Quantification of water and solute fluxes within and out of an agriculturally intensively used catchment

In the Sauerbach headwater catchment (aerial view - see picture) decades of intensive agricultural usage have let to a significant deterioration of groundwater and surface water quality. The Sauerbach headwater catchment is located with the Bode catchment (52°04’, 11°16’), which is part of the “Magdeburger Börde” region a fertile and intensively used agricultural lowland area. The Sauerbach drains a surface area of 1.4 km2. The stream length from the spring to the gauging station at the catchment outlet is 1.1 km.

 

Research activities at the Sauerbach site address the spatial and temporal dynamics of water and solute fluxes in the unsaturated and saturated zones, between ground- and surface water and in the stream itself with a specific focus on agricultural chemicals. The general goal is the assessment of water flow and solute transport in a lowland headwater catchment. This brings together hydrologists, hydrogeologists and soil scientist combining monitoring, field experiments and numerical modelling of coupled soil water-groundwater-surface water systems.

 

Data from the site suggest that water quality and quantity are strongly driven by threshold behavior with different sources that dynamically contribute surface and subsurface flows to the stream channel. The specific geology at the site and the location of the GW table in different parts of a complex multi-aquifer system control the dynamics of subsurface water and nutrient inputs to the Sauerbach.

 

In this context, the travel time distribution (TTD) of water flowing to the stream is seen as a scalable key descriptor of water and solute fluxes in the catchment. Water flow dynamics (e.g. shifting flow paths) and related variability in water storage lead to a strongly transient behavior of the TTD. This contradicts the common assumption of time-invariant TTD as a static catchment descriptor. The work at the Sauerbach catchment will emphasize the characterization of transient TTD and their relationship to catchment water and solute fluxes.

 

Water quality deterioration within the aquifers is a result of recent and historical fertilizer and pesticide inputs. The concentration of nutrients and pesticides is assumed to be generally related to groundwater age. A longer residence time should result in a more pronounced attenuation of dissolved substances. On the other hand, the solute input functions are not constant with time. For example, the use of fertilizers reached a peak during the 1970s to the 1980s. Older groundwater can be affected by these peaks and may reflect quite a different spectrum of agrochemicals. The work at the Sauerbach will cast new light on the water quality-age relationship and quantify the solute contributions from different subsurface regions that are connected to the receiving stream by dynamically shifting flow paths.

 

Finally, the river bed is seen as an important reactor, mediating the flux of agricultural chemicals. An additional objective of the work at the Sauerbach is the quantification of solute turnover in the river bed and to evaluate the relative importance of geologically controlled GW-SW interactions versus hyporheic exchange.

 

The Sauerbach site is highly instrumented including:

  • Gauging Station with multiparameter probe and automated sampler
  • Climate station and 3 additional precipitation logger
  • 10 observation wells with automated water level measurements
  • 2 instream loggers for water level and electrical conductivity measurements
  • 5 probes for multilevel temperature measurements in the streambed
  • Lysimeter station

 

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