Todd C. Rasmussen

Permeability of Apache Leap Tuff: Borehole and core measurements using water and air

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.

Runoff Curve Numbers for 10 Small Forested Watersheds in the Mountains of the Eastern United States

AbstractEngineers and hydrologists use the curve number method to estimate runoff from rainfall for different land use and soil conditions; however, large uncertainties occur for estimates from forested watersheds. This investigation evaluates the accuracy and consistency of the method using rainfall-runoff series from 10 small forested-mountainous watersheds in the eastern United States, eight annual maximum series from New Hampshire, West Virginia, and North Carolina, and two partial duration series from Georgia. These series are the basis to compare tabulated curve numbers with values estimated using five methods. For nine of 10 watersheds, tabulated curve numbers do not accurately estimate runoff. One source of the large uncertainty is a consistent decrease in storm-event curve numbers with increasing rainfall. A calibrated constant curve number is suitable for only two of 10 watersheds; the others require a variable watershed curve number associated with different magnitude rainfalls or probabilities...

LABORATORY ANALYSIS OF FLUID FLOW AND SOLUTE TRANSPORT THROUGH A FRACTURE EMBEDDED IN POROUS TUFF

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.

Steady fluid flow and travel times in partially saturated fractures using a discrete air-water interface

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.

Unsaturated fractured rock characterization methods and data sets at the Apache Leap Tuff Site

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.

Laboratory characterization of fluid flow parameters in a porous rock containing a discrete fracture

A porous block of volcanic tuff with an embedded fracture was used to perform laboratory-scale water and gas flow experiments for the purpose of obtaining unsaturated fractured rock characterization parameters. Rock matrix properties included porosity, hydraulic diffusivity and the gas-phase diffusion coefficient. Fracture properties included apertures, transmissivity, hydraulic conductivity, and air-entry pressure. Gas tracer experiments using argon and helium are presented for estimating fracture hydraulic conductivity, fracture apertures and rock matrix porosities.

A comparison of linear regression with Clark's Method for estimating barometric efficiency of confined aquifers

An estimate of barometric efficiency, often necessary for aquifer pump test interpretation, is difficult to obtain when there is a trend in water levels over time. Ordinary least squares (OLS) regression methods can be used to determine both the barometric efficiency and a trend function when the form of the trend is known. Clarks (1967) procedure, which does not require specification of the trend functional form, is shown to provide an unbiased and consistent estimate of barometric efficiency. OLS is only advantageous for a short data record providing the form of the trend is known. Clarks method may still be used with short data records with a linear trend when augmented by a recursive correction procedure. Both methods work best with constant barometric efficiency and rapid aquifer-borehole water level equilibration.

A biogeochemical model for metabolism and nutrient cycling in a Southeastern Piedmont impoundment

Abstract While non-point nutrient loads are important determinants of biological productivity in Southeastern Piedmont impoundments, productivity can be attenuated by concomitant sediment loads that reduce the biological availability of these nutrients. A biogeochemical model is proposed that explicitly accounts for the effects of sediment–nutrient interactions on multiple components of phytoplankton metabolism dynamics, including algal photosynthesis and respiration, pH, carbonate speciation, dissolved oxygen, and biochemical oxygen demand. Sediment–nutrient interactions relate nutrient uptake and release to pH, sediment oxygen demand, sediment organic matter, and iron. pH is a state variable in our model, affects sediment–nutrient adsorption, and constrains model parameters. The model replicates water quality observations in a small Southeastern Piedmont impoundment and suggests that pH-dependent sediment–nutrient adsorption dominates both orthophosphate and ammonium dynamics, with phosphate adsorption being controlled by ligand exchange to iron oxides, and ammonium adsorption being controlled by the cation exchange capacity. Sediment organic matter accumulation and decay also affects nutrient availability, and may explain the long-term increase of hypolimnetic dissolved oxygen deficit in Lake Lanier, a large Southeastern Piedmont impoundment.

Apache Leap Tuff INTRAVAL experiments - results and lessons learned

Data from laboratory and field experiments in unsaturated fractured rock are summarized and interpreted for the purpose of evaluating conceptual and numerical models of fluid, heat and solute transport. The experiments were conducted at four scales, in small cores (2.5-cm long by 6-cm across), a large core (12-cm long by 10-cm across), a small block containing a single fracture (20 x 21 x 93 cm), and at field scales in boreholes (30-m long by 10-cm across) at three scales (1/2-, 1- and 3-meters). The smallest scale in the laboratory provided isothermal hydraulic and thermal properties of unfractured rock. Nonisothermal heat, fluid and solute transport experiments were conducted using the large core. Isothermal gas and liquid flow experiments were conducted in the fractured block. Field-scale experiments using air were used to obtain in situ permeability estimates as a function of the measurement scale. Interpretation of experimental results provides guidance for resolving uncertainties related to radionuclide migration from high level waste repositories in unsaturated fractured rock.

Validation studies for assessing unsaturated flow and transport through fractured rock

*The objectives of this contract are to examine hypotheses and conceptual models concerning unsaturated flow and transport through heterogeneous fractured rock and to design and execute confirmatory field and laboratory experiments to test these hypotheses and conceptual models. Important new information is presented such as the application and evaluation of procedures for estimating hydraulic, pneumatic, and solute transport coefficients for a range of thermal regimes. A field heater experiment was designed that focused on identifying the suitability of existing monitoring equipment to obtain required data. A reliable method was developed for conducting and interpreting tests for air permeability using a straddle-packer arrangement. Detailed studies of fracture flow from Queen Creek into the Magina Copper Company ore haulage tunnel have been initiated. These studies will provide data on travel time for transport of water and solute in unsaturated tuff. The collection of rainfall runoff, and infiltration data at two small watersheds at the Apache Leap Tuff Site enabled us to evaluate the quantity and rate of water infiltrating into the subsurface via either fractures or matrix. Characterization methods for hydraulic parameters relevant to Weigh-level waste transport, including fracture apertures, transmissivity, matrix porosity, and fracture wetting front propagation velocities, were developed.