Abstract:
Where cities are located above productive aquifers and are far from surface water supplies, groundwater is usually the primary freshwater source. However, groundwater pollution in industrialized sites is a general problem in a variety of European cities. Most of them are located at river basins and use groundwater for water supply from local aquifer systems. The investigation of groundwater contamination, given the relative inaccessibility of the subsurface, is typically restricted by the number of monitoring wells and therefore detailed characterization, using conventional approaches, is economically not feasible at many sites.
A new integral approach for the investigation of groundwater contamination has been recently developed at the Center of Applied Geosciences, University of Tübingen. Using this integral approach, the concentration in a pumping well is measured as a function of time. This procedure increases the sampling volume and reduces the effect of small scale variability that may bias point measurements. This method is therefore capable of reaching the aquifer volume located between monitoring wells, avoiding the risk of missing narrow contaminant plumes. Average concentrations and total mass flow rates are obtained through an inversion procedure, providing reliable estimates of water quality and source strength, respectively.
This thesis is entirely focused on the development and application of analytical and numerical tools for the inversion of data obtained through integral pumping tests. A derivation of the fundamental equation, partially based on previous results, is provided here and generalized to account for advective transport and linear retardation within 3D heterogeneous aquifers. Both existing analytical and numerical approaches have been further developed, tested and applied to a wide range of field scale conditions. Within the analytical approach, the existing solutions have been generalized to fully account for the groundwater velocity, i. e. without assuming perfect radial flow during pumping. Additionally, the integral approach is further analysed through the classical theory of integral equations by means of Abel´s integral transform providing a new closed-form solution. The novel results are then compared, yielding a general methodology for dimensioning the optimal pumping duration. Within the numerical framework, less restrictive conditions are considered (e. g. heterogeneous aquifers, multiple-well pumping tests). A new numerical algorithm (an updated version of the original code C1, extensively rewritten) is developed, implemented (in the code CSTREAM), tested and applied to real data. A number of field-scale applications have been evaluated, in cooperation with our partners in the projects SAFIRA and INCORE yielding consistent results. Within the project SAFIRA, the integral approach is applied for the first time to a multi-layer aquifer system. In a more theoretical framework, CSTREAM is also used to quantify the effects of heterogeneity and variability of boundary conditions.
The results of this Thesis show that this approach has a great potential within both applied framework for evaluation and investigation of real contaminated aquifers and theoretical or basic research to further develop conceptual models for the evaluation of concentration-time data obtained in pumping wells.