Implementation of natural attenuation at a JP-4 jet fuel release after active remediation.

After eighteen months of active remediation at a JP-4 jet-fuel spill, a residual of unremediated hydrocarbon remained. Further site characterization was conducted to evaluate the contribution of natural attenuation to control exposure to hazards associated with the residual contamination in the subsurface. Activities included the detailed characterization of ground-water flow through the spill; the distribution of fuel contaminants in groundwater; and the analysis of soluble electron acceptors moving into the spill from upgradient. These activities allowed a rigorous evaluation of the transport of contaminants from the spill to the receptor of groundwater, the Pasquotank River. The transport of dissolved contaminants of concern, that is benzene, toluene, ethyl benzene, xylene isomers (BTEX) and methyl-tertiary-butyl ether (MTBE), into the river from the source area was controlled by equilibrium dissolution from the fuel spill to the adjacent groundwater, diffusion in groundwater from the spill to permeable layers in the aquifer, and advective transport in the permeable layers. The estimated yearly loading of BTEX compounds and MTBE into the receptor was trivial even without considering biological degradation. The biodegradation of hydrocarbon dissolved in groundwater through aerobic respiration, denitrification, sulfate reduction, and iron reduction was estimated from changes in ground-water chemistry along the flow path. The concentrations of target components in permanent monitoring wells continue to decline over time. Long term monitoring will ensure that the plume is under control, and no further active remediation is required.