Selbstreinigung urbaner Flusslandschaften bei Basel : Inwertsetzung revitalisierter und naturnaher Auen- und Feuchtgebiete in den Flussebenen von Wiese, Birs und Rhein

Capacity of self-purification processes of urban river areas near Basel (CH). Improvement of rehabilitated and natural-like wetlands in the ancient river basins of the river Wiese, Birs and Rhine. Introduction and objectives: Urban areas are characterized by an enormous human use density. Passing rivers and their landscapes were subjected and adapted to the human activities. Straightened and obstructed running waters have today become self-evident nationwide. Since the end of the 20th century an increased number of projects have been trying to convert these into natural forms with permitted controlled dynamics (e.g. the rehabilitation of the river Wiese or BirsVital in the area of Basel). Following a new social consent, priority is given to recreation use and sustainability. Such rehabilitation projects in urban areas often encounter different conflicting aims, e.g. between nature conservation and recreation use, or between striving after improved horizontal and vertical permeability (connectivity) and the requirements of groundwater protection. The precepts of a sustainable use advise using the natural self-purification potential within a modern river landscape management by preferring free running waters. In many cases, this won’t be possible in urban areas due to the requirement of space as well as the doubts raised by ground-water protection or involved groups of users. Unfortunately there exist too few studies on rehabilitations in urban areas aiming at a chemical-physical and social optimization. The two MGU projects F2.00 and F1.03 in the period 2000-2002 in the Langen Erlen and 2003-2005 in the Brueglinger Ebene and the Petite Camargue Alsacienne try to remedy these deficiencies by examining under which premises and boundary conditions a revival of the natural wetlands and river landscapes would be possible and how the various requirements for use could be considered. This work focused on the purification potential of flooded, natural wetlands and shallow lakes and shows the efficiency of such systems. The results are discussed against the backdrop of the adoption of such landscape features for the improvement of rehabilitated river landscapes. Methods: Different measuring techniques (according to standardized methods) in different temporal resolutions are used to quantify the purification performance along the running distance of natural horizontal filters and for the characterisation of the water quality in the investigated wetlands. Therefore, in the areas of investigation, monthly samples were generally taken. The determination of the water quality took place on the one hand directly in the field (oxygen saturation, oxygen contents, water temperature, pH value, specific electrical conductivity and turbidity) and on the other hand dissolved organic carbon (DOC) and UV-Absorption at 254 nm (UVA254) as well as cations (K+, Ca2+, Mg2+) and anions (NH4 +, NO3 -, PO4 3-, SO4 2-, Cl-) were measured in the water laboratory of the Geographical Institute. The colony-forming units of E.coli and total coliform germs were determined by means of diaphragm filtration. In the "Hintere Stellimatten" the sampling turned out somewhat different but always with the intention to document the changes within the running water. To investigate the change of the water quality with increasing soil depth in the groundwater recharge area "Hintere Stellimatten", water samples were taken by means of suction candles out of different soil depths. Later – in the constructed wetland of the Brueglinger Ebene – six laterally slit tubing probes divided into three levels were inserted into the substrate down to the tarred bedrock. They served to collect water samples to capture the current chemical-physical conditions and the vertical exchange and conversion processes along the main running distance. The same analytics (with exception of the microbiology and DOC-analysis) as described above are taken with these samples. In order to identify the influence of the surface water on the groundwater quality, water samples were taken (and analyzed in the water laboratory of the Industrial Works of Basel, IWB) monthly or fortnightly in the groundwater well # 8 (distance 400 m) and # 9 (distance 100 m) near the recharge area "Hintere Stellimatten". Almost the same parameters as above were chosen. Results: During the project F2.00 in the "Hintere Stellimatten" in the Langen Erlen area, a procedure to charge the natural-like ground water recharge and protection area (belonging to the IWB) with water from the river Wiese (without using energy) was tested. The purification performance concerning nitrate and ammonium along the running distance of the natural horizontal filter is being proofed. The reduction rates underlie fluctuations and for ortho-phosphates it is even more difficult to see clear tendencies. The number of aerobe mesophilic germs augmented in the recharge area, and E.coli germs consequently decreased by 25-30 %. Turbidity decreased strikingly (especially after the discharge of a high water wave with many suspended matter). Nearly one million m3 water of the river Wiese was discharged in the area, which lead to a surface water/groundwater ratio up to 70 %. In spite of this no problematic changes of the ground water quality in the near groundwater well # 8 and # 9 occurred during the whole period of discharging. The discharge of a high water wave with many suspended matter did not change this either. The artificial water system of the Brueglinger Ebene is divided into an ancient constructed wetland (PG) followed by a shallow lake (Quellsee, QS). The good purification performance of the whole system is achieved especially thanks to the lake, while the contribution of the PG is relatively low: The first (but low) reduction of the nitrogen compounds occurs in the PG, the effects on ortho-phosphates are faint. However, the turbidity is clearly reduced in the PG: even in “clear” water, turbidity falls by almost 50 %, in cloudy water up to 70-80 %. More intensive processes go off in the Quellsee. As a rule, neither nitrate nor ammonium is in it during the summer. Ortho-phosphate is reduced all year round (on average 75.4 % from 0.10 to 0.03 mg/L with regard to the water from the river Birs). The mean reduction of nitrate at the end of the passage through the lake is 77.6 % (15.8 to 3.6 mg/L). The mean removal rate of E.coli germs in the PG is 15.7 % (from 1855 to 1564 CFU/100mL), in the Quellsee, however, 92.7 % (to 135 CFU/100mL) with regard to the discharged water from the river Birs. The Petite Camargue Alsacienne area belongs to the ancient river floodplain of the river Rhine and still shows structures of natural water. It was included as a supplement investigation area in 2005. To preserve this amphibious landscape dynamics are simulated with specific interventions and a hydrological concept. In these water and flooding areas metabolic cycling occurs as in natural wetlands. The following shows the (mean) changes of the most important parameters: in the shallow lake Étang U the concentration of nitrate is reduced from 6.7 mg/L to 4.9 mg/L (-29.5 %), ortho-phosphate from 0.06 mg/L to 0.03 mg/L (-43.9 %) and the loading of E.coli-bacteria sinks by 91.9 % (from 675 to 55 CFU/100mL). Turbidity decreases by 72.6 % from 8.2 to 2.3 FNU. In the larger wetland Grand Marais, the purification performance is even greater: with regard to the discharged water from the river Rhine, the concentration of nitrate is reduced from 7.0 mg/L to 3.2 mg/L (-54.8 %) and ortho-phosphate from 0.06 mg/L to 0.01 mg/L (-78.6 %). The relatively long running distance leads to a decreased loading of E.colibacteria of 87.7 % (from 475 to 58 CFU/100mL). Turbidity decreases by 31.0 % from 8.2 to 5.7 FNU. The smaller wetland Mare des Tritons shows an almost comparable potential of self-purification: with regard to the discharged water from the river Rhine, the concentration of nitrate is reduced from 6.7 mg/L to 3.6 mg/L (-46.7 %) and orthophosphate from 0.06 mg/L to 0.03 mg/L (-53.0 %). The most efficient reduction of E.coli-bacteria occurs here, namely by 94.6 % (from 675 to 36 CFU/100mL), whereas suspended matter is reduced by 83.7 % (turbidity decreases from 8.2 to 1.3 FNU). Conclusions: As the results of the investigations show, a clear improvement of water quality can be achieved even in urban river areas by an optimization of self-purification processes. Especially the loadings of bacteria, nutrients and solid matter can be reduced with a few measures. This offers a cheap and natural option for artificial groundwater recharging. Beside this clear improvement of water quality, such wetlands are important for biodiversity (e.g. variety of niches and ecotopes) and groundwater-protection. At the same time, these areas serve as a (substitute) habitat for wetlands species and as high-quality recreation area for urban residents. Rehabilitations of urban river landscapes are to be endorsed, especially if they facilitate a great connectivity between water and environment with many elements of natural wetlands (e.g. shallow lakes and flooding areas). There are many interesting rehabilitation possibilities within the river basins of the river Wiese, Birs and Rhine leading to natural, attractive river landscapes.