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Matrix diffusion is an important mechanism for solute transport in fractured rock. We recently conducted a literature survey on the effective matrix diffusion coefficient, Dem, a key parameter for describing matrix diffusion processes at the field scale. Forty field tracer tests at 15 fractured geologic sites were surveyed and selected for study, based on data availability and quality. Field-scale Dem values were calculated, either directly using data reported in the literature or by reanalyzing the corresponding field tracer tests. Surveyed data indicate that the effective-matrix-diffusion-coefficient factor FD (defined as the ratio of Dem to the lab-scale matrix diffusion coefficient [Dem] of the same tracer) is generally larger than one, indicating that the effective matrix diffusion coefficient in the field is comparatively larger than the matrix diffusion coefficient at the rock-core scale. This larger value could be attributed to the many mass-transfer processes at different scales in naturally heterogeneous, fractured rock systems. Furthermore, we observed a moderate trend toward systematic increase in the emDFmDDF value with observation scale, indicating that the effective matrix diffusion coefficient is likely to be statistically scale dependent. The FD value ranges from 1 to 10,000 for observation scales from 5 to 2,000 m. At a given scale, the FD value varies by two orders of magnitude, reflecting the influence of differing degrees of fractured rock heterogeneity at different sites. In addition, the surveyed data indicate that field-scale longitudinal dispersivity generally increases with observation scale, which is consistent with previous studies. The scale-dependent field-scale matrix diffusion coefficient (and dispersivity) may have significant implications for assessing long-term, large-scale radionuclide and contaminant transport events in fractured rock, both for nuclear waste disposal and contaminant remediation.

Simulation of subsurface heterogeneity is important for modeling subsurface flow and transport processes. Previous studies have indicated that subsurface property variations can often be characterized by fractional Brownian motion (fBm) or (truncated) fractional Levy motion (fLm). Because Levy-stable distributions have many novel and often unfamiliar properties, studies on generating fLm distributions are rare in the literature. In this study, we generalize a relatively simple and computationally efficient successive random additions (SRA) algorithm, originally developed for generating Gaussian fractals, to simulate fLm distributions. We also propose an additional important step in response to continued observations that the traditional SRA algorithm often generates fractal distributions having poor scaling and correlation properties. Finally, the generalized and modified SRA algorithm is validated through numerical tests.

Purpose: The prevalence of peri-implantitis has increased significantly, forcing clinicians to search for ways to prevent it. Laser-microtextured surfaces promote soft tissue attachment and provide a tight seal around implants. Hence, the aim of this study was to examine the clinical, radiographic, and histologic features of ligature-induced peri-implantitis, as well as the effect of surgical treatment of these induced peri-implantitis lesions on laser-microtextured implants in a controlled animal model. Materials and Methods: Six mini-pigs (three males/three females) received 6 implants each (3 resorbable blast textured [RBT] implants and 3 laser-microtextured [LM] implants) in mandibular premolar sites, for a total of 36 implants. Two groups were identified based on the time point of sample analysis. After osseointegration was achieved, metal wire ligatures were placed and left for 12 weeks. Group 1 samples were then obtained, and group 2 samples received rescue therapy following a guided bone regeneration (GBR) protocol. Sample collection in group 2 was completed 12 weeks after the samples were submerged and treated. All samples were analyzed histologically and measurements were taken. Results: Four implants (three RBT, one LM) were lost at early time points because of implant instability. Interimplant distances and soft tissue thicknesses varied subtly between groups. More notable was the mean (± standard error of the mean) crestal bone loss (group 1: 1.860 ± 1.618 mm [LM] and 2.440 ± 2.691 mm [RBT]; group 2: 2.04 ± 1.613 mm [LM] and 3.00 ± 2.196 mm [RBT]) (P < .05), as demonstrated by a paired t test. Histologic pocket depth was also greater at RBT sites than at LM sites (4.448 ± 2.839 mm and 4.121 ± 2.251 mm, respectively, in group 1; and 3.537 ± 2.719 mm and 2.339 ± 1.852 mm, respectively [P < .005] in group 2). Conclusion: LM implants had less crestal bone loss and shallower histologic pocket depth compared with their RBT counterparts. Also, LM implants had higher bone fill when a rescue therapy (GBR) was performed. [ABSTRACT FROM AUTHOR]

To improve understanding of property measurements in heterogeneous media, an energy-based weighting function concept is developed. In (assumed) homogeneous media, the instrument spatial weighting function (ISWF) depends only on the energy dissipation distribution set up by the measurement procedure and it reduces to simply inverse sample volume (uniform weighting) for 1-D parallel flow case (ideal permeameter). For 1-D transient flow in homogeneous media, such as with slug tests, the ISWF varies with position and time, with 95% of the total weighting contained within 115 well radii, even late in the test. In the heterogeneous case, the determination of the ISWF is connected to the problem of determining an equivalent hydraulic conductivity (K), where the criterion for equivalence is based on equal energy dissipation rate rather than equal volume discharge. The discharge-based equivalent K ([K.sub.E]) and the energy-based equivalent K in heterogeneous media ([K.sub.eh]) are not equal in general, with [K.sub.eh] typically above the nodal arithmetic mean K. The possibly more fundamental problem is that as one makes K measurements in heterogeneous media at different locations or on different cores of heterogeneous materials, the ISWF will be heterogeneity dependent, implying that the averaging process resulting in the equivalent K value also varies with position. If the testing procedure is transient, then the averaging process varies with time. This suggests a fundamental ambiguity in the interpretation of hydraulic conductivity measurements in heterogeneous media that may impact how we approach modeling and prediction in a practical sense (Molz 2003). Further research is suggested., Introduction As our ability to simulate flow and transport processes in porous media has increased, the need for improved property measurements has increased also. Measurements almost always must be made [...]

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 [m.sup.3]/month from the bottom half of the aquifer to the upper half. Tracer transport si mulations 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., Background Conventional monitoring wells are often used to obtain information about ground water chemistry and plume geometry. The gathering of information is accomplished by collecting ground water samples for determination [...]

Introduction Inadequate performance of many pump-and-treat systems has been attributed to improper design of the system (USEPA, 1990; Haley et al., 1991). In far too many cases, the underestimation of [...]

Introduction In an earlier paper (Guven et al., 1992), a three-dimensional (3-D), advective based, numerical model was developed for simulating two-well, injection/withdrawal, tracer tests in heterogeneous granular aquifers. The model [...]

Following the grounding of the Exxon Valdez in March 1989, many beaches in the Alaskan waters were fouled with oil. One of these was Kittiwake Beach of Knight Island in Prince William Sound. The U.S. Environmental Protection Agency initiated a research project to evaluate the application of nutrients to the surface of the beach to stimulate biodegradation of the surficial and infiltrated oil by indigenous microbial populations. Nutrients were dissolved in seawater and applied by sprinklers to study plots along the beach. As part of this project. a preliminary, simplified model of the beach hydraulics is developed to describe the advective fate of the applied nutrients. The retention time of nutrients in the beach is estimated to be on the order of 12 h (one tidal cycle) for the application method used. Simulations indicate that application of nutrients through a trench or subsurface drain placed above the high-tide level, rather than directly on the beach by sprinklers, would result in significantly longer retention times. Design guidance for future remediation is provided to ensure nutrient coverage of the entire oiled profile of a permeable beach.

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 [is less than] 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 [is less than] 0.04), compression (p [is less than] 0.003), and posterior bending (p [is less than] 0.03), and more creep under a compressive step load (p [is less than] 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. [DOI: 10.1115/1.1372321]