Riparian zone flowpath dynamics during snowmelt in a small headwater catchment

The hydrology of the near-stream riparian zone in upland humid catchments i s poorly understood. We examined the spatial and temporal aspects of ripari an flowpaths during snowmelt in a headwater catchment within the Sleepers R iver catchment in northern Vermont. A transect of 15 piezometers was sample d for Ca, Si, DOC, other major cations, and delta(18)O. Daily piezometric h ead values reflected variations in the stream hydrograph induced by melt an d rainfall. The riparian zone exhibited strong upward discharge gradients. An impeding layer was identified between the till and surficial organic soi l. Water solute concentrations increased toward the stream throughout the m elt. Ca concentrations increased with depth and DOC concentrations decrease d with depth. The concentrations of Ca in all piezometers were lower during active snowmelt than during post-melt low flow. Ca data suggest snowmelt i nfiltration to depth; however, only upslope piezometers exhibited snowmelt infiltration and consequent low delta(18)O values, while delta(18)O values varied less than 0.5 parts per thousand in the deep riparian piezometers th roughout the study period. Ca and delta(18)O values in upslope piezometers during low streamflow were comparable to Ca and delta(18)O in riparian piez ometers during high streamflow. The upland water Ca and delta(18)O may expl ain the deep riparian Ca dilution and consistent delta(18)O composition. Th e temporal pattern in Ca and delta(18)O indicate that upland water moves to the stream via a lateral displacement mechanism that is enhanced by the pr esence of distinct soil/textural layers. Snowmelt thus initiates the flux o f pre-melt, low Ca upland water to depth in the riparian zone, but itself d oes not appear at depth in the riparian zone during spring melt. This is de spite the coincident response of upland groundwater and stream discharge. ( C) 1999 Published by Elsevier Science B.V. All rights reserved.