INFILTRATION IN WELDED FRACTURED TUFFS

Flow through a surface (infiltration) is an important boundary condition that controls subsequent subsurface flow and transport processes. Usually, infiltration is associated with flow through surface soils. As such, current theories of infiltration are based on flow through porous media, which predict a sluggish advance of the wetting front. These theories are not well suited to fractured rocks, which have a marked contrast in hydrologic properties, largely attributed to the presence of fractures within a low- permeability matrix. In this paper, we present results from an infiltration test conducted on the surface of fractured rock.

This investigation was performed in fractured welded tuff. The test bed is located ~190 m below the ground surface of Yucca Mountain, where a cavity has been excavated into the walls of the Cross Drift tunnel. A 3 X 4 m2 infiltration plot, located on the floor of the cavity, was divided into 12 square subplots of similar size (i.e., 1 m2). Within the boundary of the infiltration plot, fractures were concentrated at the two ends of the plot with few visible fractures in the middle.

Ponded water (i.e., with a head of ~4 cm) was released into the infiltration plot over a period of ~800 days in four distinct phases, and infiltration rates were measured in the subplots. During Phase 1, ponded release of water along the entire infiltration plot continued uninterrupted for 6 months. In the second phase, which extended for six months, ponded water was released into two of the subplots (while the remaining 10 remained dry). During Phase 3, ponded release of water was resumed in all 12 subplots for the next 12 months. During the final phase, the infiltration in six of the twelve subplots was perturbed by a brief interruption to the supply of ponded water and by alteration of the infiltration surface. This particular sequence of phases allowed for the observation of spatial and temporal patterns in infiltration rates.

The spatial and temporal pattern of water entry into the fractured tuff was different from that predicted by current infiltration theories for soil. Through rigorous analytical and numerical analyses, we developed alternative conceptual models that address the unique features of fractured rocks observed in our studies.