NITRATE DYNAMICS IN RIPARIAN FORESTS - GROUNDWATER STUDIES

This study was conducted to assess the removal of groundwater nitrate (NO3-) in different soil drainage classes within three riparian forests located in Rhode Island. A solution of NO3- and a conservative tracer [either bromide (Br-) or chloride (Cl-)] was applied in the growing and the dormant seasons to trenches upgradient of wetland locations with hydric soils (poorly and very poorly drained soils) and transition zone locations with somewhat poorly and moderately well-drained soils located immediately upslope of the wetlands. To assess removal, the change in groundwater concentrations of NO3- relative to the concentration of the conservative tracer was observed in monitoring wells located in each soil drainage class from June 1989 through April 1990. Removal of groundwater NO3- was consistently high in the wetland locations, generally in excess of 80% in both growing and dormant seasons. In the transition zones, attenuation was less than 36% during the growing season, and ranged from 50 to 78% in the dormant season. Attenuation in the transition zones was positively correlated with water table elevations. Transition zone attenuation was high in the dormant season relative to the growing season likely because high water tables during the dormant season caused the contaminant plume to be exposed to soil with higher organic matter. The results suggest that both wetlands and transition zones between wetlands and uplands can be important sinks for groundwater NO3-.