Magnetic monitoring of environmental contamination and remediation

Abstract:

As our communities continue to intensify their interactions with groundwater resources, there is an increasing need for novel, low-cost methods for monitoring the status of contaminated sites and our efforts to remediate them. Rock magnetic methods are often overlooked as a tool for such environmental applications. However, many organic contaminants are attenuated in the environment through combinations of microbially mediated redox reactions of Fe-bearing phases. Additionally, some engineering remediation strategies use Fe-bearing mineral phases as hosts for adsorbing or re-mineralizing otherwise soluble contaminants. Thus, rock magnetic measurements that constrain the composition, concentration, and grain size distribution of Fe-bearing phases in the environment can help trace the reactions that attenuate contaminants in aquifer settings. Here we report two such case studies that highlight the utility of magnetic methods in environmental monitoring. The first examines the long-term natural attenuation of petroleum-based hydrocarbons in a deposit of glacial outwash sands at the National Crude Oil Spill Fate and Natural Attenuation Research Site in Bemidji, MN. Measurements of magnetite inside the contaminant plume show evidence of accelerated oxidation (maghemitization) and reduction (anoxic dissolution) as compared to magnetite outside the plume. Further, magnetic measurements exclude the occurrence of magnetic Fe-sulfides and siderite. The second case study uses mineral magnetism to monitor iron-enhanced sand filters used to remove excess phosphorous from storm water in urban and suburban settings across the Twin Cities metropolitan area. Magnetic methods trace the evolution of the active component of the filters, shavings of cast iron metal, as they oxidize into magnetite, hematite, goethite, and lepidocrocite. These secondary minerals each have their own pH-dependent phosphorous affinity and their formation is accompanied by volume changes, both of which impact the effectiveness and longevity of the filters. Low temperature magnetic measurements exclude the presence of vivianite, indicating that the phosphorous is being adsorbed rather than remineralized. These examples demonstrate the potential contributions that mineral magnetism can make to environmental monitoring.