Rohit Salve

Liquid-release tests in unsaturated fractured welded tuffs: I. Field investigations

Wetting-front movement, flow-field evolution, and drainage of fracture flow paths were evaluated within the Topopah Spring welded tuff at Yucca Mountain, Nevada. Equipment and techniques were developed for in situ quantification of formation intake rates, flow velocities, seepage rates, and volumes of fracture flow paths. Localized injections of liquid into a low-permeability zone (LPZ) and a high-permeability zone (HPZ) along a borehole were detected in two boreholes below the point of injection. For the LPZ tests, water did not seep into an excavated slot that defined the lower boundary of the test bed, and the liquid-intake rate under constant-head conditions was observed to steadily decrease by two orders of magnitude. In the HPZ, liquid-intake rates under constant-head conditions were significantly higher and did not exhibit a strong systematic decline. HPZ tests were also conducted under a range of constant-flow conditions. Slot seepage rates showed intermittent responses and the percentage of injected water recovered in the slot increased as each test progressed. A maximum of 80% of the injected water was recovered during high-rate injection tests. The flow path volumes were found to increase during the course of each HPZ test. The data collected during these field tests has provided useful information for developing an understanding of liquid flow in unsaturated fractured welded tuff. It has also provided a basis for quantitative comparisons with numerical simulations of liquid-release experiments, as demonstrated in a companion paper [J. Hydrol. 256 (2002)].

Fault-matrix interactions in nonwelded tuff of the Paintbrush Group at Yucca Mountain

To investigate the potential for fast flow through altered tuff of the nonwelded unit of the Paintbrush Group (PTn) at Yucca Mountain, Nevada, we carried out in situ field experiments using water released directly into the matrix and along a minor subvertical normal fault at Alcove 4 in the Exploratory Studies Facility (ESF). During the experiments, changes in moisture content were monitored within the test bed, and a slot excavated below the test bed was visually inspected for seepage. Our field tests suggest that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would be expected to dominate. Once wetted, the matrix is able to retain the moisture over a period of months. As saturation increases in the matrix, less water imbibes along the fault and more water travels farther along the fault. From this observation, we infer that a sequence of infiltration events separated by periods of up to a few months could convey water over increasing distances along the fault.

Water flow within a fault in altered nonwelded tuff

In situ flow experiments were conducted at Alcove 4 in the Exploratory Studies Facility at Yucca Mountain to investigate flow dynamics in a small normal fault located in zeolitically and argillically altered tuffs of the nonwelded rocks of the Paintbrush group. Water was released at constant head into a packed-off interval straddling the fault within a horizontal borehole. Changes in water-intake rate into the interval were monitored along with saturation in the formation using boreholes installed with electrical resistivity probes. For water released into the fault the water-intake rate gradually fell from ∼200 to 50 mL min−1 over a period of 41 hours of cumulative release time spread over 17 days of testing. Resistance changes recorded by electrical resistivity probes in matrix rock on either side of the fault interval suggest that flow in the fault spread out laterally in the fault plane. The time for the wetting front to travel l m in the fault was initially ∼0.15 days. Subsequent tests in the fault showed the response time to be dependent on wetting history, with a faster response observed in a wetter system. Data analysis using the numerical code TOUGH2/ITOUGH2 reveals that a simple decline in permeability caused by simulated clay swelling can explain the experimental observations.

Field tracer transport tests in unsaturated fractured tuff

This paper presents the results of a field investigation in the unsaturated, fractured welded tuff within the Exploratory Studies Facility (ESF) at Yucca Mountain, NV. This investigation included a series of tests during which tracer-laced water was released into a high-permeability zone within a horizontal injection borehole. The tracer concentration was monitored in the seepage collected in an excavated slot about 1.6 m below the borehole. Results showed significant variability in the hydrologic response of fractures and the matrix. Analyses of the breakthrough curves suggest that flow and transport pathways are dynamic, rather than fixed, and related to liquid-release rates. Under high release rates, fractures acted as the predominant flow pathways, with limited fracture-matrix interaction. Under low release rates, fracture flow was comparatively less dominant, with a noticeable contribution from matrix flow. Observations of tracer concentrations rebounding in seepage water, following an interruption of flow, provided evidence of mass exchange between the fast-flowing fractures and slow- or non-flowing regions. The tests also showed the applicability of fluorinated benzoate tracers in situations where multiple tracers of similar physical properties are warranted.

Observations of preferential flow during a liquid release experiment in fractured welded tuffs

Author(s): Salve, Rohit | Abstract: 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.

On the physics of unstable infiltration, seepage, and gravity drainage in partially saturated tuffs

To improve understanding of the physics of dynamic instabilities in unsaturated flow processes within the Paintbrush nonwelded unit (PTn) and the middle nonlithophysal portion of the Topopah Spring welded tuff unit (TSw) of Yucca Mountain, we analyzed data from a series of infiltration tests carried out at two sites (Alcove 4 and Alcove 6) in the Exploratory Studies Facility (ESF), using analytical and empirical functions. The analysis of infiltration rates measured at both sites showed three temporal scales of infiltration rate: (1) a macro-scale trend of overall decreasing flow, (2) a meso-scale trend of fast and slow motion exhibiting three-stage variations of the flow rate (decreasing, increasing, and [again] decreasing flow rate, as observed in soils in the presence of entrapped air), and (3) micro-scale (high frequency) fluctuations. Infiltration tests in the nonwelded unit at Alcove 4 indicate that this unit may effectively dampen episodic fast infiltration events; however, well-known Kostyakov, Horton, and Philip equations do not satisfactorily describe the observed trends of the infiltration rate. Instead, a Weibull distribution model can most accurately describe experimentally determined time trends of the infiltration rate. Infiltration tests in highly permeable, fractured, welded tuff at Alcove 6 indicate that the infiltration rate exhibits pulsation, which may have been caused by multiple threshold effects and water-air redistribution between fractures and matrix. The empirical relationships between the extrinsic seepage from fractures, matrix imbibition, and gravity drainage versus the infiltration rate, as well as scaling and self-similarity for the leading edge of the water front are the hallmark of the nonlinear dynamic processes in water flow under episodic infiltration through fractured tuff. Based on the analysis of experimental data, we propose a conceptual model of a dynamic fracture flow and fracture-matrix interaction in fractured tuff, incorporating the time-dependent processes of water redistribution in the fracture-matrix system.

A probe for measuring wetting front migration in rocks.

To facilitate investigations of flow dynamics in unsaturated fractured rocks, we have designed a probe to qualitatively monitor saturation changes in borehole environments. This flexible probe is able to fit the cylindrical shape of boreholes and requires no backfill. Field results suggest that the design of this electrical resistivity probe (ERP) is suitable for determining changes in relative wetness along boreholes in unsaturated fractured rock environments. The performance of these probes was compared with that of psychrometers under a variety of tests; these probes were found to be significantly cheaper in addition to providing data that had a greater spatial and temporal resolution.

Infiltration into fractured bedrock

Infiltration into fractured bedrock Rohit Salve and Teamrat A. Ghezzehei Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. Robert Jones Earth Sciences Division, Sandia National Laboratories Rohit Salve and Teamrat A. Ghezzehei, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA (R Salve@lbl.gov, TAGhezzehei@lbl.gov), Robert Jones, Earth Sciences Division, Sandia National Laboratories

Development of a Wet Plume Following Liquid Release along a Fault

To investigate unsaturated flow through a fault located within fractured welded tuff, we performed in situ field experiments in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of approximately 82000 L water for a period of 17 mo directly into a near-vertical fault under both constant positive head (at about 0.04 m) and decreasing fluxes. As water was released into the fault, changes in moisture content were monitored in the formation while a large cavity excavated below the test bed was visually inspected for seepage. We observed that water (introduced along the fault) maintained the fault as the primary vertical flowpath, while the adjacent fractured rock served to move water laterally and vertically. Further, unlike primary flowpaths along the fault, flow was not persistent along the secondary flowpaths. While this field experiment provided preliminary insights about the flow field associated with a fault, it also demonstrated the need to investigate the role of infill material and secondary fractures in diverting flow from gravity-driven fast flow toward flowpaths in which lateral flow may occur.

Flow processes in a rangeland catchment in California.

Emerging hydrology-related issues in California grasslands have directed attention towards the need to understand subsurface water flow within a complex, dynamic system. Tensiometers and neutron probes evaluated the subsurface hydrology of a rangeland catchment. Hydrological processes within the catchment varied both in space and time. Spatial variability was evident along the vertical profile and between the catchment slopes. Temporal variability in processes coincided with the seasons (i.e., wet winter, dry summer, and spring). From a water-balance equation developed for the catchment, we determined that there was significant variability both spatial and temporal in the amount of soil moisture lost to evapotranspiration and deep seepage. During the 16 month monitoring period there was a total of 50 cm of rainfall that fell in the catchment of which 9-55 cm was lost to evaporation and 37-79 cm to deep seepage. A simple deduction of the losses (evaporation and deep seepage) from the input (rainfall) shows that all monitored locations had a substantial decrease in the amount of water that was stored in the soil profile. DOI:10.2458/azu_jrm_v53i5_salve