Scientific Investigations Report 2005-5255
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5255
Each of the ground-water discharge study sites examined in this study showed evidence that ground-water discharge is variable in both space and time. Spatial variability of ground-water discharge along the South Fork of the Nooksack River appears to be closely related to the geologic materials adjacent to the streambed. Coarse-grained alluvial fan and landslide deposits are likely in connection with the streambed and are therefore capable of contributing localized flow. In settings like Fishtrap Creek where geologic materials are more homogeneous, spatial variations are more subtle. Slightly larger vertical gradients were present in the upper parts of the drainage basin. Changes in topography or incision of the Fishtrap Creek streambed may cause localized downward flow from the creek that either remains in the hyporheic zone or recharges the aquifer. Variations in vertical hydraulic gradients at ground-water discharge study sites on Fishtrap and Fourmile Creeks suggest differences from one field to the next or from one side of the stream to the other. Such differences may be due to local site conditions such as topography, tile drains, or even wildlife, such as beaver dams.
Temporal variability was more pronounced at each site. Vertical hydraulic gradients generally were upward in the winter and diminishing through the spring and summer. In the late summer, especially during drought conditions, sites, such as Fishtrap and Bertrand Creeks, may have downward vertical hydraulic gradients for a period of time until the next rainy season. Flood events were found to temporarily reverse the flow of water across the streambed as surface-water levels exceed local ground-water levels. During these short-duration events, surface water recharges the aquifer or is held in bank storage until the flood crest passes.
The spatial and temporal variability of surface water/ground-water interactions strongly influence the movement of contaminants into and out of the surface-water system.
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