Modeling of multicomponent transport with microbial transformation in groundwater: The Fuhrberg Case

Abstract

This paper serves the twofold purpose of demonstrating a numerical technique that appears to have potential in studies of nonlinear reactive transport problems and of presenting a case study where the implications of reactive transport processes are profound. The case study relates to the Fuhrberger Feld aquifer in northern Germany, which provides the bulk of the water supply for the half‐million city of Hannover. Part of the aquifer receives a strong influx of nitrate from agricultural activities. The nitrate in the aquifer is microbially transformed into sulfate, the transformation depending on the availability of reduced sulfur compounds present in the sediment, and the sulfate is also transformed further. The individual transformations occur within distinct reaction zones in the aquifer. The relevant dynamic processes can be formulated as a nonlinear transport problem involving multiple interacting species in dissolved or solid form. The system is simulated using a finite element‐based technique that generates a symmetric coefficient matrix for the transport equation, while providing second‐order accuracy in time. The form of the matrix allows the use of a highly efficient and robust symmetric conjugate gradient solver. The technique provides an ample spatial resolution capacity at reasonable cost and handles grids with irregular geometry in two or three dimensions. The convenience and versatility of conventional finite elements is retained. A vertical‐section simulation identifies different reaction zones in the aquifer and gives insight into the effect of the controlling parameters. The simulation study is a first step in the development of a prognosis for the fate of the groundwater resource.