The QuESt project: Assessing the spatiotemporal evolution of iron concentrations in groundwater by nuclear magnetic resonance (NMR): from lab experiments, to reactive transport modeling to field observations

In recent decades, mining-related activities in the Lusatian lignite mining district have led to an extensive pyrite weathering, therewith contributing to the elevation of iron and sulfate concentrations in the groundwater and surface water.

Due to the complicated pathways of pyrite oxidation and the complex spatial distribution of the pyrite-bearing layers, it is difficult to develop a comprehensive restoration plan. Therefore, developing a quick and non-intrusive geophysical measuring technique for estimating pyrite oxidation in various depths and areas is highly desirable. Previous laboratory studies have shown the effect of iron bearing minerals on the nuclear magnetic resonance (NMR) response signal. However, further research is required to link these findings to the subsurface pyrite oxidation state or the accompanied sulfate concentrations in the groundwater.

To this end, column experiments containing different pyrite mass-percentages are performed under various redox conditions. The pyrite oxidation in the columns is measured via the mass balance between the inlet, the initial content, and the outlet. Throughout the experiment, the columns are constantly monitored via laboratory NMR measurements. For modeling purposes, we developed a PHREEQC-based reactive transport model to simulate pyrite oxidation inside the columns. A comparison of the modelling results with the column experiments and their link to the NMR measurements, should be the basis for the future surface-NMR applications in the field. The findings of the QuESt project ultimately enable us to estimate the groundwater contamination due to pyrite oxidation with a NMR-based technique that is less time-consuming and labor-intensive.