Abiotic Degradation of TCE by Naturally Occurring Magnetite can be Important in Large Plumes

Significant quantities of magnetite occur in many aquifers. In these aquifers, abiotic degradation of TCE by magnetite can be an important mechanism for natural attenuation. There is a direct correlation between the quantity of magnetite in aquifer sediments and their magnetic susceptibility. The magnetic susceptibility of aquifer sediment can be characterized with good sensitivity and at low cost using a downhole sonde in conventional monitoring wells at the site.

An empirical relationship has been established between the mass magnetic susceptibility of aquifer sediment and the rate constant for degradation of TCE and cis-DCE in the sediment. In a variety of aerobic unconsolidated sand aquifers, the magnetic susceptibility ranged from 2E-07 to 2E-06 m³ kg^-1. Biological reductive dechlorination of TCE or cis-DCE would not be expected in these aquifers. However, the field-scale bulk rate constants for TCE or cis-DCE degradation varied from 0.1 per year to 0.7 per year with a median near 0.3 per year.

In large plumes, the rate constant for abiotic degradation of TCE by magnetite may be large enough that abiotic degradation is adequate to support monitored natural attenuation (MNA) as a remedy or part of a remedy at the site. If the concentration of TCE is 100 μg/L or less, and the degradation rate constant is 0.3 per year, the concentration can be brought to the MCL in 10 years of travel time along the flow path in the groundwater.