THE SHALLOW WEATHERED FRACTURE LAYER IN THE NEAR-STREAM ZONE

The bedrock profile of an east–central Pennsylvania watershed contains a shallow weathered fracture layer. To begin quantifying the effects of this fracture layer on ground–water movement, two valley cross sections were chosen for detailed investigation. One section represents the typical upland watershed situation where the regional ground water is in contact with the stream for only part of the year, while at the other section the regional ground water is in perennial contact with the stream. Rock cores extracted from the sections reveal a pattern of extensive fracturing to approximately 50 ft directly under the stream thinning to a relatively uniform 30–ft deep fracture layer away from the channel. Limited seismic investigations support these observations. Piezometers within each cross section show that hydraulic conductivity differs between the shallow fracture layer and the relatively unfractured zone below and that there is great variability in conductivity within the fracture layer itself. Lateral flow generally occurs within the fracture layer to within approximately 50 ft of the stream; here a converging flow pattern indicates ground–water discharge to the stream. Under wet conditions, the stream in each section serves as a point of ground‐water discharge, while under dry conditions, the more upland section shows no ground–water discharge to the land surface. Rather, subsurface flow is likely occurring beneath and parallel to the channel controlled by a discharge point at some downstream location. Finally, water–quality samples from the piezometers indicate a direct effect of overlying land use on water quality within the shallow fracture layer. Copyright © 1990, Wiley Blackwell. All rights reserved