RIDING THE WAVES IN OHIO: AN EVALUATION OF A NITRATE PLUME IN GROUNDWATER

Interactions of the Ohio River flood pool, groundwater in a sand and gravel aquifer, and precipitation have been evaluated over a four-year period to determine the fate of a nitrate plume originating from a manufacturing site in Jefferson County, Ohio. The migration/attenuation behavior of the plume, as identified in prior sampling, did not follow expected trends. The plume was expected to flow to the southeast, tangential to the southward flow of the Ohio River. Instead, the observed plume was elliptical, with its long axis oriented northeast, rather than southeast, along a river reach that flowed south. The center of the plume (where the highest concentrations of nitrate occur) repeatedly shifted northeast to southwest along the plume's axis. In order to evaluate a potential "Natural Attenuation" remediation for the plume, the groundwater system was analyzed in detail. The investigation was designed to evaluate the interaction of river stage, as controlled by locks and dams and flooding events, precipitation infiltration, and sedimentary features in the sand and gravel aquifer.

Results indicate that the plume flow path is controlled by the locations of coarse sand and gravel deposits, likely deposited in paleo-river channels; these channels direct the groundwater flow along the northeast/southwest plume axis. However, the migration of maximum concentrations within the plume (toward and away from the river) is affected by "waves" created by dynamic interaction with precipitation infiltration and the accompanying rises in the river flood pool. Groundwater levels in the highly transmissive aquifer rise quickly (within hours) in response to infiltration, causing groundwater flow toward the river. As surface runoff raises the Ohio River flood pool, the groundwater gradients reverse and flow is away from the river. During non-precipitation periods, flood pool fluctuations result from routine lock releases, creating smaller "waves" of groundwater flow toward and away from the river. This understanding of groundwater flow dynamics is being used to describe the attenuation/migration of the elevated nitrate concentrations, in order to evaluate natural attenuation of the plume.