Geochemical evidence of preferential flow of water through fractures in unsaturated tuff, Apache Leap, Arizona

摘要

The occurrence and significance of aqueous flow through fractures in unsaturated tuff was investigated at the Apache Leap Research Site near Superior, Arizona. Water samples for geochemical and isotopic analysis were collected from water seeping from fractures in a mine haulage tunnel, from the saturated zone in a vertical borehole (USW UZP-4), and from both the unsaturated and saturated zones in an angled borehole (DSB). The geochemistry and C-14 activity of water samples from the DSB suggest that most of the recharge to the saturated zone has occurred through fractures, especially beneath the ephemeral streams. Evidence of substantial recent recharge through fractures was found in saturated-zone samples from the mine haulage tunnel using H-3, delta(34)S and SO42-/Cl- analyses. Evidence of partial imbibition of fracture flow into the rock matrix was found at multiple depths throughout the 147 m unsaturated zone at the DSB using geophysical measurements from the borehole, water-content analyses from core samples, and C-14 and H-3 analyses from pore water extracted front preserved core samples. Post-bomb C-14 activity was measured in pore water near fractures just above the saturated zone. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 22]