Influence of an upstream dam on riparian zone hydrology and shallow groundwater nitrate dynamics

Abstract

<p>This study used detailed spatio-temporal hydrologic measurements, groundwater chemistry data (dissolved nitrate, chloride, oxygen and sulphate concentrations) and denitrification potential measurements to assess the impact of a dam on shallow (<150 cm) groundwater movement and nitrate patterns in a downstream riparian zone. The results show that the upstream dam management practices supported stream inputs to the riparian zone by maintaining stream stage above the water table levels in the adjacent riparian zone. These stream inputs limited the extent of the summer water table draw-down in the riparian zone. Furthermore, the timing of upstream dam management practices influenced the timing of large shifts in riparian hydrology, including in-bank flood events and a mid-summer water table gradient reversal. This riparian zone experienced three distinct hydrologic regimes in the spring, summer and fall when riparian zone water table gradients were dominated by the hillslope, stream and upstream marsh respectively. These regimes altered nitrate patterns near the field by changing the balance of high nitrate inputs from the field and intermediate nitrate inputs from depth. Stream stage patterns were only one factor influencing hydrology and nitrate patterns at the site. During all three hydrologic regimes, the mid riparian zone was dominated by low nitrate inputs from depth and acted as a hydraulic barrier to field-to-stream groundwater transport. Nitrate removal by denitrification was observed at this site but was limited by low nitrate fluxes. Therefore, this riparian zone's role as a physical barrier to field-to-stream nitrate transport may be more important than its role as nitrate sink. These results can contribute to the enhancement of current conceptual models linking landscape features to riparian zone hydrology and nitrate removal function.</p>