Optimizing the Operation of the Protective Hydraulic Barrier - Lobau, Vienna, Austria

Summary

Lobau is a riparian area on the outskirts of Vienna, on the northern bank of Danube. It is a ground water protection area for the viennese water supply facilities with the capacity of about 80,000 m3 per day. In the area of interest there are also the oil storage facilities that pose a permanent danger to the water supply. Therefore, a row of wells to provide hydraulic barrier for the ground water supply protection has been bored downstream the oil storage facility. The aim of the study was to optimize the operation of the hydraulic depression wells for the whole range of ground water regime (extreme low - extreme high) so that the leaking contaminants are captured at the minimal drawdown.

The study area has about 72 km2 with the length of 12.7 km NW-SO and width 5.7 km in the SW-NO direction. The surficial as well as hydrological boundaries of the actual target area represent the Danube and its old branches. The time period evaluated were the years 1987 to 1989.

The database of ground water relevant data has been designed and the data from various sources have been standardized and transferred into the database. Statistical and geostatistical methods have been applied in the data evaluation and processing. The software packages HADES (in-house developed package for database design and operation - data in the dBase format), GeoEAS, SURFER, SPSS/PC+ and the MODFLOW model have been applied during the project.

The information of 229 bore-logs has been available for the study area. It has been evaluated to provide the conceptual geological model of the studied quaternary alluvial aquifer. The model consists of three layers that correspond also to the three layers that were used in the MODFLOW simulation. The layers are:

· covering loams, humus (0.5 to 1.0m) and upper fine sands (2-4m);

· saturated sandy gravels close to Danube 15-20 m, in the distatnt zones 10-15 m thick;

· lower loamy fine sands (Donau vicinity up to 20 m, distant zones 5m thick).

Optimizing the Operation of the Protective Hydraulic Barrier - Lobau, Vienna, Austria

Detailed information has been avaliable on the surface water geometry (x,y,z), the permeability data from pump tests were available at about 15 locations, more than 300 d10 values for permeability calculation after Hazen were available with exact location (x,y, depth interval).

The modeling has been performed in two scales. The raw three-layer model is 11900 m long (x-direction) and 4700 m wide (y-direction), and has 5856 nodes with D x varying between 50 and 300 m, and D y ranging form 100 to 200 m. The detailed model focusses on the area of the oil storage facilities and the well row for the hydraulic depression and. The modeling study resulted in the operation plan of the hydraulic barrier at the entire range of ground water flow regimes (extreme high - extreme low).

Die Simulationsstudie Lobau wurde variantenweise durchgeführt. Grundsätzlich wurden zwei Modelle erstellt - ein Grobmodell und ein Detailmodell (siehe Abb. 2). Diese wurden an den zwei Wasserständen geeicht - hoher Grundwasserstand am 30.7.1987 und niederer Grundwasserstand am 17.11. 1991. Mit den geeichten Modellen wurden insgesamt 13 Pumpvarianten simuliert.

Abb.3: Grundwassermodell Lobau - Vergleich der gemessenen und gerechneten Grundwasserstände (Modelleichung) für den 30.7.1987 unbeeinflußt von dem Betrieb der Sperrbrunnenreihe

Die hydraulischen Modelle berücksichtigen das geologische Modell und ermöglichen die dreidimensionale volumetrische Berechnung der Grundwasserströmung im alluvialen Grundwasserspeicher in der Lobau. Die Modelle ermöglichen eine Simulation der Kommunikation zwischen Grundwasser und den Altarmen der Donau.

 Mit Hilfe der Grundwassersimulationen und der statistischen Analyse der hydrologischen Daten wurde eine Betriebsvorschrift für die Sperrbrunnenreihe Lobau erstellt. Es wurden die benötigten Fördermengen in den 12 Sperrbrunnen für den durch 8 Betriebszustände charakterisierten gesamten Schwankungsbereich des Wasserspiegels mit hilfe der numerischen und statistischen Methoden erfaßt. Die erstellte Betriebsvorschrift wird derzeit getestet und aktualisiert.