Groundwater-fed surface flow path hydrodynamics and nitrate removal in three riparian zones in southern Ontario, Canada

Although the ability of stream riparian buffers to reduce nitrate in groundwater has been promoted, the effectiveness of nitrate removal in riparian zones with upwelling springs and overland flows is not well understood The relationship between groundwater-fed surface flow path hydrodynamics and nitrate removal was investigated in three riparian zones in southern Ontario, Canada. Spring-fed surface flow in white cedar forests at the upland perimeter of the riparian zones occurred as rivulets linked to shallow horizontal pipe systems in peat deposits. These rivulet-pipe systems transported water at rates that were up to 13x faster than in downslope portions of the riparian zones where diffuse flow paths occurred in marshes and areas of mixed cedar-grass vegetation Bromide tracers indicated the exchange of water along surface flow paths between areas of faster flow and storage zones in soil pore-water or in slow moving and stagnant pools of surface water. High nitrate concentrations in upwelling groundwater showed little decline for distances of up to 100 m along rivulet-pipe networks suggesting that these flow paths were ineffective in nitrate removal. Nitrate concentrations declined by 50-95% during the summer in areas of diffuse surface flow in the three riparian zones. Analysis of the distribution of delta(15)N-NO(3) values suggests that denitrification is an important mechanism of nitrate removal. Nitrate concentrations also declined by 25-80% along diffuse surface flow paths in the spring season when the riparian water table was at or above the ground surface. Cold water temperatures (1-6 degrees C) limited biological removal and most of this nitrate decline resulted from dilution by exfiltration of groundwater that had a low nitrate concentration as a result of denitrification during subsurface transport Measurements in one of the riparian zones showed that, despite this nitrate dilution, the increase in runoff volume resulted in an 8x larger nitrate-N flux entering the stream from riparian surface flow paths in the spring vs. summer. This study indicates that riparian zones with groundwater-fed surface flow paths vary in their ability to deplete nitrate A conceptual model linking different surface flow patterns to solute exchanges with riparian soils and water residence time is developed to understand and predict nitrate removal effectiveness. (C) 2010 Elsevier B.V. All rights reserved.