(497e) Evidence of Natural Attenuation of Hexavalant Chromium in Groundwater Plumes at Tinker Air Force Base

Alexander, M. L., Science Applications International Corporation
Bowen, S., Tinker AIr Force Base

An assessment of hexavalent chromium and related groundwater data was undertaken for two adjacent plumes emanating from landfill areas at Tinker AFB, with the objective of determining whether natural attenuation of the hexavalent chromium would be a viable remedial alternative for the site. Existing groundwater data, including total and hexavalent chromium, total organic carbon, ferrous iron, and groundwater quality parameters such as dissolved oxygen and oxidation-reduction potential, were reviewed.

For the northern plume, the data indicate that hexavalent chromium levels are increasing in the central and downgradient area of this plume. The increasing hexavalent chromium trends in down-gradient wells would suggest that the plume may be expanding longitudinally in that area; however, an active containment/extraction well is likely controlling this end of the plume. This preliminary review suggests that MNA may not be applicable to the northern plume because the contamination source does not appear to be exhausted or controlled.

For the southern plume, declining hexavalent chromium levels are exhibited in two monitoring wells in the middle of the plume, while steady concentrations are exhibited in one extraction well, and a possible increasing trend is exhibited in another extraction well. The locations of the wells may explain these differences since the up-gradient and periphery of the plume show declining trends, while the downgradient center of the plume indicates a steady or increasing trend. This interpretation would be consistent with a plume for which the majority of the source area mass has been released into the plume and the higher concentration area has detached from the source and is moving downgradient. Groundwater modeling efforts are being used to evaluate whether this contamination is one continuous plume, or perhaps two separate smaller plumes. MNA may be suitable at the southern plume since up-gradient plume concentrations appear to be declining.

Data gap field efforts are in progress to obtain additional groundwater quality data specific to hexavalent chromium attenuation assessment. Sampling is being performed to obtain data that includes ferrous iron, sulfide, organic carbon, and chromium (total and hexavalent) concentrations in both groundwater and saturated zone soil. This data will provide a more complete geochemical picture of the status and potential for reduction reactions to control the hexavalent chromium at this site.