Nuclear magnetic resonance at the laboratory and field scale as a tool for detecting redox fronts in aquifers

Surface nuclear magnetic resonance (NMR) provides information not only on the lithology and water transport characteristics of aquifer systems but also on the state of iron mineralization within the pore space. This feature makes surface NMR a potential tool for the observation of changing redox conditions in the aquifer, which control the type and oxidation state of iron minerals and, relatedly, the buffering or release of pollutants that may pose a threat to drinking water resources. Our study aims on testing this potential and focuses on pyrite contained in the matrix of a sandy aquifer that serves as a natural denitrification buffer in a drinking water catchment area with intensive agricultural use. We observe a significant change in the surface-NMR relaxation times related to the pyrite content in the subsurface, which is most obvious at the transition from the pyrite-free oxic zone and the pyrite-bearing reducing zone. Complementary laboratory experiments using core material from the study site, including laboratory NMR and geochemical analyses, verify that the influence of pyrite content predominates the NMR relaxation behavior rather than pore size, porosity, or carbon content. The mean relaxation times measured in the sediment without pyrite exceed those with pyrite by approximately 20%–100%. We conclude that surface NMR can serve as a capable tool to locate and monitor the pyrite-based denitrification buffer. However, for general applicability within the given framework, more research is needed to address the natural ambiguity among iron content, its redox state and speciation, and pore size.