Rohit Salve

Various field studies have concluded that shallow groundwater in weathered bedrock underlying hillslopes can contribute to both base and stormflow and thus dominate runoff. The processes associated with recharge from the ground surface, through this unsaturated zone, have received little study, yet they influence runoff dynamics, the chemical evolution of water, and moisture availability. Here we use five measurement systems to document soil and rock moisture dynamics within a 4000 m²zero‐order basin in which all runoff occurs through weathered argillite. At this site, the weathered bedrock zone (in which the groundwater fluctuates by 8 m seasonally) varies in depth from ∼4 m at the base of the hillslope to nearly 19 m near the hill top. An aggregate‐rich, porous, 0.5 m thick soil overlies the weathered bedrock. We find that during the first rains of the wet season, water rapidly travels meters into the weathered bedrock zone. Consistently, however, groundwater at some places responds quickly to the first major storm, well before the wetting front has been detected much beyond about 1 m. Furthermore, throughout the wet season, the lower portion of the unsaturated weathered bedrock shows little or no moisture change. These observations suggest a fracture‐dominated flow path, leading to a highly variably groundwater response across the hillslope for a given storm. Seasonal changes in rock moisture content are greatest in the first 5 to 10 m depth and may exceed the magnitude of moisture changes in the soil, suggesting that it could constitute a significant unmapped moisture reservoir.

As the scope of hydrologic investigations extend deeper into the subsurface profile, and increasingly include fractured rock, there is a growing need for techniques that can accurately monitor saturation changes at a high spatial and temporal resolution in this environment. We have developed a technique, the Electrical Resistance Sensor Array System (ERSAS), to track moisture dynamics in vadose zone regions that include both fractured rock and soil. The performance of ERSAS was compared with the time domain reflectometry (TDR) technique under controlled and field conditions. We found that ERSAS was effective in determining patterns of saturation changes along vertical soil/rock profiles. Because of the small size of individual sensors, it was able to resolve travel times associated with a wetting front and peak saturation better than TDR. In addition, ERSAS is significantly cheaper than the TDR system, and the sensor arrays are relatively easier to install in the subsurface profile. Published in 2011 by John Wiley & Sons, Ltd.

The primary objective of this study was to assess the impact of two grassland soils and precipitation regimes on soil‐moisture dynamics. We set up an experiment in a greenhouse and monitored the soil moisture dynamics in mesocosms planted with Avena barbata , an annual species found in California grasslands. By repeating the precipitation input at regular intervals, we were able to observe plant manipulation of soil moisture during well‐defined periods during the growing season. We found that the amount of water partitioned to evapotranspiration, seepage, and soil storage varied among different growth stages. Furthermore, both soil type and precipitation regime had a significant impact on redistributing soil moisture. Whereas in the low‐precipitation treatments most water was released to the atmosphere as evapotranspiration, major losses from the high‐precipitation treatment occurred as gravity drainage. Observations from this study emphasize the importance of understanding intraseasonal relationships between vegetation, soil, and water.

One potential consequence of global climate change and rapid changes in land use is an increased risk of flooding. Proper understanding of floodwater infiltration thus becomes a crucial component of our preparedness to meet the environmental challenges of projected climate change. In this paper, we present the results of a long‐term infiltration experiment performed on fractured ash flow tuff. Water was released from a 3 × 4 m² infiltration plot (divided into 12 square subplots) with a head of ∼0.04 m, over a period of ∼800 days. This experiment revealed peculiar infiltration patterns not amenable to current infiltration models, which were originally developed for infiltration into soils over a short duration. In particular, we observed that in part of the infiltration plot, the infiltration rate abruptly increased a few weeks into the infiltration tests. We suggest that these anomalies result from increases in fracture permeability during infiltration, which may be caused by swelling of clay fillings and/or erosion of infill debris. Interaction of the infiltration water with subsurface natural cavities (lithophysal cavities) could also contribute to such anomalies. This paper provides a conceptual model that partly describes the observed infiltration patterns in fractured rock and highlights some of the pitfalls associated with direct extension of soil infiltration models to fractured rock over a long period.

To better understand preferential flow in fractured rock, we carried out an in situ field experiment in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of ~22 m³ of ponded water (at a pressure head of ~0.04 m) over a period of 7 months, directly onto a 12 m² infiltration plot located on a fractured welded tuff surface. As water was released, changes in moisture content were monitored along horizontal boreholes located in the formation ~19‐22 m below. Distinct flow zones, varying in flow velocity, wetted cross‐sectional area, and extent of lateral movement, intercepted the monitoring boreholes. There was also evidence of water being diverted above the ceiling of a cavity in the immediate vicinity of the monitoring boreholes. Observations from this field experiment suggest that isolated conduits, each encompassing a large number of fractures, develop within the fractured rock formation to form preferential flow paths that persist if there is a continuous supply of water. In addition, in fractured welded tuffs the propensity for fracture‐matrix interactions is significantly greater than that suggested by existing conceptual models, in which flow occurs along a section of fracture surfaces. An overriding conclusion is that field investigations at spatial scales of tens of meters provide data critical to the fundamental understanding of flow in fractured rock.

A key issue regarding the performance of the potential high‐level radioactive waste repository at Yucca Mountain, Nevada, is the likelihood of precipitation percolating a vertical distance of ∼300 m through fractured unsaturated rock into drifts containing waste packages. Water enhances waste package corrosion and is required for transport of released radionuclides. To evaluate the propensity for seepage into tunnels at Yucca Mountain, a 5‐m‐diameter, 2.7‐km‐long tunnel, commonly referred to as the Cross Drift (CD), was excavated in 1998, branching off from the main Exploratory Studies Facility tunnel. Sections of this tunnel have been isolated from ventilation for extended periods over the last 4 years. We present continuous measurements of relative humidity and temperature and periodic observations of liquid water in the CD over two periods. During this observation duration the terminal section of the drift was partitioned into four sections by bulkheads, and ventilation to these sections was minimized to a few days. We compare these observations to results from analytical and numerical models to investigate processes associated with the movement of water vapor between the tunnel bore and the surrounding fractured rock formation. Observations from this effort indicate that fractures can be primary paths for unsaturated zone vapor flow in the immediate vicinity of emplacement drifts. Observations, measurements, and analysis indicate the need for a model that includes fracture‐dominated vapor transport as a significant contributor to total water flow into the drifts.

This paper presents the design of the passive‐discrete water sampler (PDWS) which has been developed to facilitate investigations of flow partitioning in fractured rocks. The PDWS continuously isolates seeping water into discrete samples while monitoring the seepage rate. The PDWS was used in a flow and transport experiment that investigated fracture‐matrix interactions. During the experiment, a mix of conservative tracers with significantly different diffusion coefficients (lithium bromide [LiBr] and pentafluorobenzoic acid [PFBA]) was introduced along a fault located in fractured tuffs, and water seeping through the lower end of the fault was collected by the PDWS and analyzed for tracer concentrations. Preliminary results from this investigation show that samples of effluent captured by the PDWS effectively retained temporal changes in the chemical signature, while providing seepage rates.