EFFECTS OF ENHANCED VEGETATION AND WETLANDS ON THE INTERCEPTION OF CONTAMINANTS AT THE GROUNDWATER-SURFACE WATER INTERFACE

The effects of dry-land and wetland vegetation can play an important role in controlling the exchange of contaminants across the groundwater-surface water interface. This paper describes an ongoing field study being conducted by the U.S. Department of Agriculture at a test site in Nebraska, in which selective vegetation plantings are being used to mitigate the impacts of contaminated groundwater discharge to a naturally gaining stream.

At the test site, preliminary studies demonstrated that the naturally occurring vegetation along a gaining stream was capable of intercepting several contaminants identified in the groundwater feeding the stream. The contaminants of concern in the groundwater included carbon tetrachloride, as a result of historic grain storage and fumigation activities upgradient of the headwaters, and nitrate contributed from surrounding agricultural lands. To capitalize on these observations, the natural vegetation was augmented by the planting of selected dry-land species, and the creation of a in-stream wetland, in an 8-acre area surrounding the headwaters area.

To document the effects of the vegetation enhancements on contaminant migration via the groundwater-surface water interface, a comprehensive strategy has been implemented to monitor the site since 2006. The key elements of this multi-faceted program have included: hydrologic monitoring, involving continuous tracking of water levels and stream flow rates; the measurement of sap flow rates to investigate the movement of fluids within the vegetation; and direct water and vegetation sampling for contaminant and geochemical analyses. The investigation results from the past seven years quantitatively demonstrate that significant and beneficial changes have occurred in the hydrologic, geochemical, and biological characteristics of the groundwater and surface water environments, and in the transport, transformation, and reduction of contaminants within these systems, in response to the vegetation enhancements.

The experience gained from this test has provided both fundamental and practical information pertinent to the application of enhanced vegetation (and wetlands), for manipulation of the hydrologic processes, and potential contaminant mitigation, in other similar near-surface environments.