Epsilon Archive for Student Projects

Abstract

The understanding of longitudinal heterogeneity in pre-reservoirs is crucial for the development of a sophisticated monitoring program which shall be able to represent the water bodies in a resource efficient way. The present study was conducted at two prereservoirs in the Harz mountains (Germany), the Rappbode and Hassel pre-reservoirs which are connected to the largest drinking water reservoir in Germany (Rappbode reservoir). Analyzed were the routine monitoring data from the bi-weekly sampling of the inflow and the outflow of the water bodies. Additionally, in summer 2012, two sampling campaigns of the transects were conducted (6 sampling points per pre-reservoir). Focus was laid on the heterogeneity of major ions, nutrients and dissolved metals, algae (chlorophyll concentration) and CO2 and CH4 concentrations. A further field of investigation was the emission of these GHG from the pre-reservoirs, using a floating chamber during the transect sampling campaigns. It was found out that the current monitoring program represents the pre-reservoir considerably well. However, a heterogeneous distribution of the algae was discovered, making it not suitable for the representation of the reservoirs if only a few spots are sampled. Ebullition fluxes were recognized in the transition zone of the pre-reservoirs,
contributing to an emission of the GHG to the atmosphere. The findings of this study confirm the use of the inflow and outflow of the reservoirs as routine spots but to for an extensive and more precise conclusion a more sustainable way in the monitoring should be found.

POPULAR SCIENCE SUMMARY
Water reservoirs make up a considerable amount of fresh water bodies in our landscapes and their importance flood protection, drinking water production, recreation etc.
increases. Reservoirs show characteristics of both streams and lakes; the inflow zone is still very similar to a stream because the water has a higher flow velocity so that particulate matter is transported along with it and a higher nutrient availability is found. In the transition zone the water is already flowing with a decreased velocity and particles will settle. The deep water zone close to the dam, the lacustrine zone, mainly shows characteristics of a lake with very low water movement, long residence times and a low concentration of dissolved nutrients. In some cases pre-reservoirs are installed in front of larger water reservoirs to act as buffers, especially if the downstream reservoir is used for drinking water abstraction and a high water quality is required. For the collection of long term data sets, a monitoring program is set up which provides information on the behavior and processes in the reservoirs.
In this study two pre-reservoirs in the Harz mountains (Germany) have been investigated with two major research questions in mind. First, data from the relatively new
monitoring program of the Hassel- and Rappbode pre-reservoirs were analyzed to see how well the current routine monitoring spots at the inflow and outflow of the reservoirs represent the reservoir even though heterogeneous conditions can be expected as I described above. A second research question dealt with the emission of greenhouse gases from the two reservoirs. As greenhouse gases are known to be a great contributor to climate change, the emission coming from water bodies receives more and more attention in the field of natural sciences.
The results show that for many parameters the inflow and the outflow are representing the reservoir well because the change between these two points is considerably small.
However, the biological parameters (chlorophyll distribution) is very heterogeneous and no conclusions about the chlorophyll distribution can be drawn from the routine monitoring.

The greenhouse gas emissions of the reservoirs showed that the reservoirs are both emitters of methane. It was even possible to document methane bubbles (ebullition).
These bubbles move from the sediment and while reaching the water surface the very high point emission of the gas can be documented. For carbon dioxide a net drawdown was found but as we sampled during the day and photosynthesis required
carbon dioxide a diurnal sampling has to be conducted before it can be identified if the reservoirs are sinks or sources of carbon dioxide.

To have a proficient and sustainable monitoring of the water reservoirs in the Harz mountains, it is important to understand if the current undertakings to monitor the water
bodies are representable. As this study started to analyze the long term data collection which seems to be appropriate at the current sampling points, future data sets can be
build up on that.