Upward movement of plutonium to surface sediments during an 11-year field study.

An 11-year lysimeter study was established to monitor the movement of Pu through vadose zone sediments. Sediment Pu concentrations as a function of depth indicated that some Pu moved upward from the buried source material. Subsequent numerical modeling suggested that the upward movement was largely the result of invading grasses taking up the Pu and translocating it upward. The objective of this study was to determine if the Pu of surface sediments originated from atmosphere fallout or from the buried lysimeter source material (weapons-grade Pu), providing additional evidence that plants were involved in the upward migration of Pu. The (240)Pu/(239)Pu and (242)Pu/(239)Pu atomic fraction ratios of the lysimeter surface sediments, as determined by Thermal Ionization Mass Spectroscopy (TIMS), were 0.063 and 0.00045, respectively; consistent with the signatures of the weapons-grade Pu. Our numerical simulations indicate that because plants create a large water flux, small concentrations over multiple years may result in a measurable accumulation of Pu on the ground surface. These results may have implications on the conceptual model for calculating risk associated with long-term stewardship and monitored natural attenuation management of Pu contaminated subsurface and surface sediments.

An interpretation of potential scale dependence of the effective matrix diffusion coefficient.

Matrix diffusion is an important process for solute transport in fractured rock, and the matrix diffusion coefficient is a key parameter for describing this process. Previous studies have indicated that the effective matrix diffusion coefficient values, obtained from a large number of field tracer tests, are enhanced in comparison with local values and may increase with test scale. In this study, we have performed numerical experiments to investigate potential mechanisms behind possible scale-dependent behavior. The focus of the experiments is on solute transport in flow paths having geometries consistent with percolation theories and characterized by multiple local flow loops formed mainly by small-scale fractures. The water velocity distribution through a flow path was determined using discrete fracture network flow simulations, and solute transport was calculated using a previously derived impulse-response function and a particle-tracking scheme. Values for effective (or up-scaled) transport parameters were obtained by matching breakthrough curves from numerical experiments with an analytical solution for solute transport along a single fracture. Results indicate that a combination of local flow loops and the associated matrix diffusion process, together with scaling properties in flow path geometry, seems to be an important mechanism causing the observed scale dependence of the effective matrix diffusion coefficient (at a range of scales).

Implications of observed and simulated ambient flow in monitoring wells.

A recent paper by Hutchins and Acree (2000) has called attention to ground water sampling bias due to ambient (natural gradient-induced) flows in monitoring wells. Data collected with borehole flowmeters have shown that such ambient flows are ubiquitous in both confined and unconfined aquifers. Developed herein is a detailed three-dimensional model of flow and transport in the vicinity of a fully penetrating monitoring well. The model was used to simulate a measured ambient flow distribution around a test well in a heterogeneous aquifer at the Savannah River Site (SRS) near Aiken, South Carolina. Simulated ambient flows agreed well with measurements. Natural flow was upward, so water entered the well mainly through high K layers in the lower portion of the aquifer and exited through similar layers in the upper portion. The maximum upward discharge in the well was about 0.28 L/min, which implied an induced exchange of 12 m3/month from the bottom half of the aquifer to the upper half. Tracer transport simulations then illustrated how a contaminant located initially in a lower portion of the aquifer was continuously transported into the upper portion and diluted throughout the entire well by in-flowing water. Even after full purging or micropurging, samples from such a well will yield misleading and ambiguous data concerning solute concentrations, location of a contaminant source, and plume geometry. For all of these reasons, use of long-screened monitoring wells should be phased out, unless an appropriate multilevel sampling device prevents vertical flow.

Elastic and viscoelastic properties of the human pubic symphysis joint: effects of lateral impact joint loading.

The human pelvis is susceptible to severe injury in vehicle side impacts owing to its close proximity to the intruding door and unnatural loading through the greater trochanter. Whereas fractures of the pelvic bones are diagnosed with routine radiographs (x-rays) and computerized tomography (CT scans), non-displaced damage to the soft tissues of pubic symphysis joints may go undetected. If present, trauma-induced joint laxity may cause pelvic instability, which has been associated with pelvic pain in non-traumatic cases. In this study, mechanical properties of cadaveric pubic symphysis joints from twelve normal and eight laterally impacted pelves were compared. Axial stiffness and creep responses of these isolated symphyses were measured in tension and compression (perpendicular to the joint). Bending stiffness was determined in four primary directions followed by a tension-to-failure test. Loading rate and direction correlated significantly (p<0.05) with stiffness and tensile strength of the unimpacted joints, more so than donor age or gender. The impacted joints had significantly lower stiffness in tension (p <0.04), compression (p<0.003), and posterior bending (p<0.03), and more creep under a compressive step load (p<0.008) than the unimpacted specimens. Tensile strength was reduced following impact, however, not significantly. We concluded that the symphysis joints from the impacted pelves had greater laxity, which may correlate with post-traumatic pelvic pain in some motor vehicle crash occupants.

Effects of kyphosis and lordosis on the remaining lumbar vertebral levels within a thoracolumbar fusion: an experimental study of the multisegmental human spine.

This study was done to determine the motion of the whole lumbar spine after internal fixation and the effect of kyphosis and lordosis on the remaining vertebral levels. Baseline motion analysis of sagittal, frontal, and transverse planes was done to determine the intact range of motion. Three fusion configurations were tested: neutral position (0 degrees), 4.6 degrees +/- 2.0 kyphosis, and -6.2 degrees +/- 3.6 lordosis. The sagittal and frontal plane relative rotation of the instrumented segments (T12/L2) decreased an average of 74% and 60%, respectively, as compared with intact testing. Sagittal plane motion at the remaining segments increased for all fusion configurations when compared with intact motion and reached statistical significance at the L4/L5 level. No significant differences were found between fusion configurations (ie, fused neutral, kyphosis, and lordosis).

Investigation of video techniques for dynamic measurements of relative vertebral-body motion in vitro.

The precision and accuracy of techniques for analysis of lumbar spinal-segment motions were evaluated using a specially designed orthogonal axis fixture and video image processing. Triplicate validation tests were performed by monitoring precisely controlled motions of the triaxial measurement device. The errors of the overall measurements were found to be less than 0.5 mm for axial and 1 degree for rotational displacements. The system accuracy was within 0.230 mm based on the calculation of the points on a calibration frame. The geometry of the calibration frame was designed with the capability to analyze the 3-D motion properties of a swine lumbar spine. Statistical analyses of the validation test data showed that the precision and accuracy of this motion-analysis system offer new opportunities for the measurement of total-length spinal-motion profiles. The system was then applied utilizing fresh thawed swine spines to test the limits of relative-motion measurements for the centers of adjacent vertebrae in a coupled unit. Lateral-medial and anterior-posterior video images of spines and the triaxial measurement devices were made simultaneously while applying cyclic axial and torsional loads. The distribution of data from six spines illustrated biological variability of relative displacements for adjacent coupled vertebrae.

A circuit analog model for studying quantitative water relations of plant tissues.

Using arrays of resistors and capacitors, a lumped circuit analog of plant tissue is developed. The circuit elements of the analog are identified in terms of physiological variables (hydraulic conductivities, water capacities, and cell dimensions) which can be measured in the laboratory. With the aid of a circuit simulation subroutine, the model was solved to predict water potential distributions as a function of position and time in plant tissues of three, six, and nine cells. Results presented for the six-cell case indicate that local equilibrium may or may not occur depending on the actual values of tissue hydraulic conductivities, water capacities, and the rate of change of water potential at the tissue boundaries. However, present measurements and estimates of tissue parameters suggest that local equilibrium is more the rule than the exception. Membrane resistance is an especially important parameter because it serves to isolate the vacuoles from the cell walls in addition to increasing the natural vacuole response time to changes in water potential.The proposed model should be useful in studying water transport processes in roots, stems, and leaves. Nonhomogeneity can be taken into account easily. Nonlinearity (changes in circuit parameter values with potential) which is known to occur in plant tissues could be incorporated also if the required information were available.

Growth-induced Water Potentials in Plant Cells and Tissues.

A physical analysis of water movement through elongating soybean (Glycine max L. Merr.) hypocotyls was made to determine why significant water potentials persist in growing tissues even though the external water potentials were zero and transpiration is virtually zero. The analysis was based on a water transport theory modified for growth and assumed that water for growing cells would move through and along the cells in proportion to the conductivity of the various pathways.Water potentials calculated for individual cells were nearly in local equilibrium with the water potentials of the immediate cell surroundings during growth. However, water potentials calculated for growing tissue were 1.2 to 3.3 bars below the water potential of the vascular supply in those cells farthest from the xylem. Only cells closest to the xylem had water potentials close to that of the vascular supply. Gradients in water potential were steepest close to the xylem because all of the growth-sustaining water had to move through this part of the tissue. Average water potentials calculated for the entire growing region were -0.9 to -2.2 bars depending on the tissue diffusivity.For comparison with the calculations, average water potentials were measured in elongating soybean hypocotyls using isopiestic thermocouple psychrometers for intact and excised tissue. In plants having virtually no transpiration and growing in Vermiculite with a water potential of -0.1 bar, rapidly growing hypocotyl tissue had water potentials of -1.7 to -2.1 bars when intact and -2.5 bars when excised. In mature, nongrowing hypocotyl tissue, average water potentials were -0.4 bar regardless of whether the tissue was intact or excised.The close correspondence between predicted and measured water potentials in growing tissue indicates that significant gradients in water potential are required to move growth-associated water through and around cells over macroscopic distances. The presence of such gradients during growth indicates that cells must have different cell wall and/or osmotic properties at different positions in the tissue in order for organized growth to occur. The mathematical development used in this study represents the philosophy that would have to be followed for the application of contemporary growth theory when significant tissue water potential gradients are present.

Location of the low temperature water flow barrier in stems.

Experiments are described indicating the magnitude and location of the low temperature barrier to lateral water flow in stems of cotton (Gossypium hirsutum L. ;Auburn 7-683'). Rehydration of wilted stem tissues was performed at 6 C and 32 C. Compared with the 32 C control, a 13-fold increase in the rehydration halftime was recorded at 6 C when water entered the secondary phloem tissues across the vascular cambium from the secondary xylem. However, only a 3-fold increase in the rehydration halftime occurred when water entered phloem tissues through the cortex, and most of this increase was due to the higher viscosity of water at the lower temperature. These results show that the cambial region of an intact cotton stem markedly resists the radial flow of water at lower temperatures. This resistance was not demonstated by other stem tissues.

Temperature effects on radial propagation of water potential in cotton stem bark.

Low temperature affects the lateral movement of water across the xylem-phloem boundary in intact cotton stems. There is a reduction in the effective diffusion coefficient relating free energy flux to water potential gradients with an associated increase in resistance to water flow. Detached phloem and excised leaves do not show this effect of low temperature. Experiments on stem section halves indicate that the effect is probably associated with the cambial region.

Rehydration versus Growth-induced Water Uptake in Plant Tissues.

Experiments show that the rate of water uptake by living tissues external to mature xylem of cotton stems (Gossypium hirsutum L. Auburn 7-683) is very similar to the corresponding curves for leaf tissue. In both cases one obtains a two-phase curve with phase I corresponding to passive rehydration and phase II pertaining to active growth.A theory of water movement in plant tissue first proposed by Philip allows one to make a more rigorous distinction than made previously between phase I and phase II. This theory is applied explicitly to water uptake by leaf disks and results in a simple expression for the time required for phase I completion. Because the time required varies as the square of the disk radius, it is essential to use a standad disk size in water uptake studies of a particular tissue.Additional analysis indicates that clear temporal distinction cannot be made between phase I and phase II. Different portions of the leaf disk rehydrate at significantly different rates, resulting in a grey zone with phase I and phase II occurring simultaneously in different parts of the disk.