Spatially and temporally scaled inverse hydraulic modelling, multi tracer transport modelling and interaction with geochemical processes at a highly transient bank filtration site

Several kinds of managed aquifer recharge techniques provide very good purification of surface water since more than 100 years. In order to maintain a reliable supply of clean water, they are becoming increasingly popular all over the world. These methods require low technical effort. At Aquifer Storage and Recovery and ponded infiltration the recharged amounts are technically controlled. The infiltration water has to be pumped and often pretreated. At bank filtration this is dispensable, the approach, of using existing surface water bodies is even more consequent. Exemplarily, at a test site at Lake Tegel, Berlin, Germany, the hydraulic processes are modelled. By means of 3D long term regional and transient hydraulic modelling it was detected that the existing approaches for determining the leakance induce large errors in the water balance and describe the infiltration zone insufficiently. The leakance could be identified to be triggered by the groundwater table, causing air exchange and intrusion of atmospheric oxygen, which reduces clogging by altered redox conditions by at least one order of magnitude. This causes that changes of the groundwater table are mitigated much more than previously assumed. Taking these findings into account, a transient water balance is determined and bank filtration ratios are quantified. A new inverse modelling concept has been developed and applied to a 3D short term local and transient hydraulic model. It comprises spatially distributed pilot points and overparameterisation constrained by regularisation and calibration to head differences. Significance of the results is demonstrated by cross validation. With this approach the spatial distribution of an aquitard have been identified with high precision. The highly transient and heterogeneous flow conditions are specified and a new viewpoint on the geologic formation of Lake Tegel is obtained. The good fit of modelled and observed breakthrough curves of 18O, chloride and temperature by just using transferred parameters obtained with the previous hydraulic methods, show the very good model performance and predictive capabilities. The intrusion of atmospheric oxygen into the unsaturated zone is identified to be the principal redox determining factor during infiltration. Previously inconsistent and also local geochemical conditions are identified to be determined by interaction of infiltration processes with the spatial extent of the aquitard. A theory for chemical clogging of abstraction wells is developed, identifying the strong vertical redox zoning as principal factor of influence.

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