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.

Kinetics of N<sub>2</sub>O production and reduction in a nitrate-contaminated aquifer inferred from laboratory incubation experiments

“…The unconfined aquifer consists of pleistocene, highly permeable carbonate-free sands and gravels with a thickness of 20-40 m underlain by impermeable cretaceous clays. More information about the soils, the hydrology and the land use of the research site is given by Frind et al (1990), Deurer et al (2008) and von der . All recent studies were conducted within a groundwater flowpath strip equipped with multilevel sampling wells .…”

“…With respect to denitrification efficiency, Weymann et al (2008) revealed the considerable difference between heterotrophic and autotrophic denitrification by determination of "excess nitrogen" in groundwater samples. Hence, high NO − 3 concentrations are limited to the top few metres of the aquifer, but the rest of the aquifer is almost NO − 3 -free (Frind et al, 1990;. The recent research activities in the FFA focused on the dynamics of N 2 O in the system groundwater -unsaturated zone.…”

“…Research activities were conducted within the unconsolidated sandy Fuhrberger Feld aquifer (FFA) in northern Germany. The FFA has been a subject of extensive research activities since the 1980s (reviewed in Korom, 1992), since the catchment is in an area of conflict between drinking water supply on the one hand and agricultural activities causing considerable inputs of pollutants via seepage, especially of nitrate, on the other hand (Kölle et al, 1985;Frind et al, 1990). In the FFA, substantial microbially mediated processes and reactions like denitrification and desulfurication (heterotrophic reduction of sulfate, Korom, 1991Korom, , 1992Böttcher et al, 1991) occur, strongly influencing groundwater geochemistry.…”

Recovery of groundwater N2O at the soil surface and its contribution to total N2O emissions

“…The aquifer is unconfined and consists of 20-40 m of pleistocene, highly permeable carbonate-free sands and gravels underlain by impermeable cretaceous clays. More information about the research site is given by Frind et al (1990) and . In the FFA, substantial microbially mediated processes and reactions like denitrification and desulfurication occur, strongly influencing groundwater chemistry.…”

Kinetic modeling of microbially-driven redox chemistry of radionuclides in subsurface environments: coupling transport, microbial metabolism and geochemistry

“…Microbially-mediated redox reactions have Baedecker and Back, Jakobsen and Postm~b een documented in various subsurface environments (e.g., 1979% 1979bFrind et al, 1990;Chapelle, 1993;Fish, 1993;Kent et al, 1994;Vroblesky and Chapelle, 1994;B fike and Denver, 1995;Chapelle et al, 1995;Cozza.relli et al, 1995;Essaid et al, 1995;Heron and Christensen, 1995). Microbial degradation of organic matters, derived either naturally or from human activity, is a driving force in many subsurface geochemical systems (e.g-, Chapelle, 1993).…”