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The authors of a new book on groundwater hydraulics and hydrology describe a theoretical foundation for modeling time-periodic groundwater flow.

Rapid fluid pressure changes in unsaturated media due to perturbations at or near the earth-atmosphere interface are commonly attributed to preferential or bypass flow. An alternative mechanism for the rapid propagation of pressure perturbations in unsaturated media is the kinematic response, which results from the nonlinear relationship between unsaturated hydraulic conductivity and water content. The relationship between pressure wave velocities and fluid velocities is described using kinematic wave theory, presented for the Brooks - Corey and van Genuchten -Mualem formulations. The kinematic mechanism predicts fluid pressure pulse velocities to be substantially faster than unsaturated fractured rock tracer velocities for both formulations. A hydraulic form of the advection-diffusion equation based on Richards' Equation is presented that uses the hydraulic diffusivity and kinematic velocity to predict the hydraulic response in unsaturated fractured rock to pressure head perturbations. Pressure pulse velocity monitoring may be an additional tool for estimating unsaturated hydraulic properties in low permeability media.

Groundwater flow and contaminant transport through the unsaturated zone continue to receive attention as former nuclear weapons development sites are being characterized for determining decommissioning options, and geologic sites are being considered for deep disposal of nuclear and hazardous waste. The ability to make decisions with regard to cleanup, site suitability, and anticipated performance of disposal or storage facilities is dependent upon the characterization and modeling of unsaturated flow and transport features, events, and processes. Hazardous materials may be mobilized by infiltrating water and move downward to affect groundwater resources. These contaminants may also move upward or laterally by capillary movement or as vapor to the accessible environment. Substantial experience exists with respect to near-surface partially saturated porous media, traditionally the domain of the soil scientists and geotechnical engineers, but does not exist for fractured rock in either near-surface or deep, complex, unsaturated systems that are now being characterized and modeled. The ability to represent these complex systems composed of both a matrix and a fracture component is being tested through a variety of field experiments and comparisons to natural analogues. The present focus is on evaluating conceptual flow and transport models and quantifying the spatial and temporal parameters for these models using laboratory and field methods, and environmental tracer analyses. Modeling of these dynamic and transient systems has followed two paths, porous equivalency and discrete fracture models. In general, characterization methods and modeling are in the confirmation stage with the greatest lack of knowledge being the interaction between fracture and matrix properties for flow and transport.

Water movement above the water table, yet below the root zone, is predominantly downward because of gravitational forces. This vertical movement is frequently affected by lower permeability geologic units that divert water horizontally. Also, vapor-driven transport of water can be important in very dry climates. Shallow soil-water is affected by plant uptake of water, and also by evaporation from the soil surface. These processes are characteristic of unsaturated media, i.e., zones where the soil materials are not saturated with water. In these cases, the surface tension of water causes adherance to soil surface, and soil-water pressures are generally negative. Each soil material has a distinctive relationship between the negative fluid pressure (or tension) and water content. The unsaturated hydraulic conductivity is lower than the hydraulic conductivity under saturated conditions because not all available pore space contributes to fluid flow. Deep soil-water movement is also affected by macropores (large pores) that can provide preferential flow paths under very wet conditions, but may not significantly contribute to flow otherwise. Keywords: deep percolation; unsaturated zone; vadose zone; unsaturated hydraulic conductivity; characteristic curves; capillary rise

Field and laboratory methods for estimating and interpreting parameters obtained from field borehole and laboratory core experiments are examined using permeability data interpreted from air and water injection tests in variably saturated fractured tuff at the Apache Leap Tuff Site in central Arizona. The tuff at the field site has a matrix porosity of approximately 17.5% and contains numerous near-vertical fractures at an average spacing of 1.3 m. More than 270 m of 6.4-cm-diameter oriented core were collected from boreholes drilled to a maximum depth below the surface of 30 m and at a vertical angle of 45°. Laboratory estimates of absolute permeabilities using air and water as the test fluids were acquired at a range of matric potentials for 105, 5-cm-long core segments extracted at approximately 3-m intervals containing no obvious fractures. Field scale estimates of fractured rock permeabilities using air and water as test fluids were obtained at ambient matric suctions and water saturated conditions, respectively. The field tests were conducted along 3-m intervals within boreholes with the intervals centered on core sampling positions. Borehole and core permeabilities demonstrate substantial spatial variability, with variations exceeding three orders of magnitude. Laboratory core data show a strong relationship between permeabilities using saturated water and oven-dry air injection tests with the latter demonstrating the Klinkenberg effect. The influence of matric suction on permeabilities is used to demonstrate that relative permeabilities do not sum to a constant for a wide range of matric suction. Only weak relationships exist between permeabilities measured in boreholes versus cores for both water and air. Permeabilities measured in boreholes using air are shown to provide good estimates of permeabilities measured using water into initially unsaturated, fractured rock at the Apache Leap Tuff Site.

Laboratory experiments were conducted to determine the flow and transport properties of a fractured porous tuff block measuring 20 × 20 × 50 cm. One porous ceramic plate was placed immediately above a fracture and two other plates were placed on either side of the fracture above the rock matrix. The plates control the pressure head applied to the upper surface of the fractured rock block. Laboratory results are simulated using the boundary integral method for a single saturated fracture with an assumed uniform transmissivity embedded within a porous tuff block. The simulation is used to identify the saturated hydraulic properties of the fracture. Matrix hydraulic conductivity is estimated as 50 × 10−9 m s−1 and the fracture transmissivity is estimated as 5.0 × 10 −9 m2 s−1. Much of the flow which exits the fracture at the lower surface first passes through the rock matrix even though a direct contact with a porous plate is present. Travel times and breakthrough curves are calculated by integrating the inverse velocity along a streamline, and then summing over all streamlines. Observed breakthrough curves were used to estimate fracture dispersivities which ranged from 0.0207 to 8.01 m. Breakthrough curves deviated from simulation results due to significant channeling of fracture flow.

Fracture flow under conditions of partial fluid saturation is studied where a portion of the fracture is filled with water and the remaining portion is filled with air. A discrete air-water interface is used to separate the saturated from the drained regions within the fracture. A laboratory flow experiment is presented which demonstrates the interface concept. The steady state air-water interface position is observed by maintaining a circular water source at constant positive pressure head between two vertical glass plates. An analytic solution to the flow problem is compared to the observed interface position and to simulated results which assume that a constant capillary pressure head can be assigned along the interface. Simulations performed for a hypothetical vertical fracture indicate zones of positive and negative pressure head in the saturated zone as well as an air-filled zone. For the hypothetical fracture examined, fluid travel times are not substantially changed as fracture saturation decreases. This result may not hold generally for all fractures.

Aquifer hydraulic properties are commonly estimated using aquifer tests, which are based on an assumption of a uniform and constant pumping rate. Substantial uncertainties in the flow rate across the borehole-formation interface can be induced by dynamic head losses, caused by rapid changes in borehole water levels early in an aquifer test. A system is presented that substantially reduces these sources of uncertainty by explicitly accounting for dynamic head losses. The system which employs commonly available components (including a datalogger, pressure transducers, a variable-speed pump motor, a flow controller, and flowmeters), is inexpensive, highly mobile, and easily set up. It optimizes the flow rate at the borehole-formation interface, making it suitable for any type of aquifer test, including constant, step, or ramped withdrawal and injection, as well as sinusoidal. The system was demonstrated for both withdrawal and injection tests in three aquifers at the Savannah River Site. No modifications to the control system were required, although a small number of characteristics of the pumping and monitoring system were added to the operating program. The pumping system provided a statistically significant, constant flow rate with time. The range in pumping variability (95% confidence interval) was from +/- 2.58 x 10(-4) L/sec to +/- 9.07 x 10(-4) L/sec, across a wide range in field and aquifer conditions.

A field heater experimental plan is presented for investigating hydrologic transport processes in unsaturated fractured rock related to the disposal of high-level radioactive waste (HLW) in an underground repository. The experimental plan provides a methodology for obtaining data required for evaluating conceptual and computer models related to HLW isolation in an environment where significant heat energy is produced. Coupled-process models are currently limited by the lack of validation data appropriate for field scales that incorporate relevant transport processes. Presented in this document is a discussion of previous nonisothermal experiments. Processes expected to dominate heat-driven liquid, vapor, gas, and solute flow during the experiment are explained, and the conceptual model for nonisothermal flow and transport in unsaturated, fractured rock is described. Of particular concern is the ability to confirm the hypothesized conceptual model specifically, the establishment of higher water saturation zones within the host rock around the heat source, and the establishment of countercurrent flow conditions within the host rock near the heat source. Field experimental plans are presented using the Apache Leap Tuff Site to illustrate the implementation of the proposed methodology. Both small-scale preliminary experiments and a long-term experiment are described.

Performance assessment of high-level nuclear waste containment feasibility requires representative values of parameters as input, including parameter moments, distributional characteristics, and covariance structures between parameters. To meet this need, characterization methods and data sets for interstitial, hydraulic, pneumatic and thermal parameters for a slightly welded fractured tuff at the Apache Leap Tuff Site situated in central Arizona are reported in this document. The data sets include the influence of matric suction on measured parameters. Spatial variability is investigated by sampling along nine boreholes at regular distances. Laboratory parameter estimates for 105 core segments are provided, as well as field estimates centered on the intervals where the core segments were collected. Measurement uncertainty is estimated by repetitively testing control samples. 31 refs., 10 figs., 21 tabs.

The siting and reclamation of hazardous waste disposal facilities may require the examination of solute transport through networks of fractures within lithic units. Water intake at the surface of a fractured rock medium may provide a transport mechanism for the movement of hazardous wastes. A dual-chambered fractured rock infiltrometer (FRI) is used to provide in situ measurements of water and air intake rates for isolated fracture segments. The FRI is used to apply a known head to an exposed fracture segment and to provide a precise means for measuring the intake rate. Water intake rates for fractures in an exposed, densely welded tuff are highest at early time, then fall to a constant rate. Analytical procedures are developed to estimate fracture apertures using the Green and Ampt model and Poiseuille's law. Calculated fracture apertures from water intake data range from 1.0 to 33.7 μm and are approximately lognormally distributed. An analytical expression relating air intake data to fracture aperture is derived using the Schwartz-Christoffel transformation. Estimated apertures using this experimental technique range from 10.0 to 37.2 μm and appear to fit the same lognormal distribution.