Colloid migration in symmetrical non-uniform fractures: particle tracking in three dimensions

A three-dimensional model for the migration of colloids in a saturated fracture is presented, which considers the motion of colloids as a result of advection and diffusion, as well as colloid-surface interactions at the fracture walls. This model is successfully incorporated into a three-dimensional particle tracking algorithm that tracks particles within a continuum and allows consideration of the migration of colloids in symmetrical, three-dimensional, non-uniform fractures. The framework is general enough to incorporate non-local interactions that provide colloid motion relative to the fluid. The algorithm is verified against classical Taylor dispersion, and its generalization to a sorbing phase, in a uniform fracture and shows excellent agreement with theory. A simple, non-uniform fracture that has an analytically tractable velocity field is also considered, and both quantitative and qualitative comparisons are made with the uniform fracture case. The modelling of more complex fracture geometries is also discussed and a particular case is implemented within the particle tracking framework.     

Migration of colloids in discretely fractured porous media: effect of colloidal matrix diffusion

Numerical simulations of colloid transport in discretely fractured porous media were performed to investigate the importance of matrix diffusion of colloids as well as the filtration and remobilization of colloidal particles in both the fractures and porous matrix. To achieve this objective a finite element numerical code entitled COLDIFF was developed. The processes that COLDIFF takes into account include advective-dispersive transport of colloids, filtration and remobilization of colloidal particles in both fractures and porous matrix, and diffusive interactions of colloids between the fractures and porous matrix. Three sets of simulations were conducted to examine the importance of parameters and processes controlling colloid migration. First, a sensitivity analysis was performed using a porous block containing a single fracture to determine the relative importance of various phenomenological coefficients on colloid transport. The primary result of the analysis showed that the porosity of the matrix and the process of colloid filtration in fractures play important roles in controlling colloid migration. Second, simulations were performed to replicate and examine the results of a laboratory column study using a fractured shale saprolite. Results of this analysis showed that the filtration of colloidal particles in the porous matrix can greatly affect the tailing of colloid concentrations after the colloid source was removed. Finally, field-scale simulations were performed to examine the effect of matrix porosity, fracture filtration and fracture remobilization on long-term colloid concentration and migration distance. The field scale simulations indicated that matrix diffusion and fracture filtration can significantly reduce colloid migration distance. Results of all three analyses indicated that in environments where porosity is relatively high and colloidal particles are small enough to diffuse out of fractures, the characteristics of the porous matrix that affect colloid transport become more important than those of the fracture network. Because the properties of the fracture network tend to have greater uncertainty due to difficulties in their measurement relative to those of the porous matrix, prediction uncertainties associated with colloid transport in discretely fractured porous media may be reduced.     

A multi-directional tracer test in the fractured Chalk aquifer of E. Yorkshire, UK

A multi-borehole radial tracer test has been conducted in the confined Chalk aquifer of E. Yorkshire, UK. Three different tracer dyes were injected into three injection boreholes and a central borehole, 25 m from the injection boreholes, was pumped at 330 m(3)/d for 8 days. The breakthrough curves show that initial breakthrough and peak times were fairly similar for all dyes but that recoveries varied markedly from 9 to 57%. The breakthrough curves show a steep rise to a peak and long tail, typical of dual porosity aquifers. The breakthrough curves were simulated using a 1D dual porosity model. Model input parameters were constrained to acceptable ranges determined from estimations of matrix porosity and diffusion coefficient, fracture spacing, initial breakthrough times and bulk transmissivity of the aquifer. The model gave equivalent hydraulic apertures for fractures in the range 363-384 microm, dispersivities of 1 to 5 m and matrix block sizes of 6 to 9 cm. Modelling suggests that matrix block size is the primary controlling parameter for solute transport in the aquifer, particularly for recovery. The observed breakthrough curves suggest results from single injection-borehole tracer tests in the Chalk may give initial breakthrough and peak times reasonably representative of the aquifer but that recovery is highly variable and sensitive to injection and abstraction borehole location. Consideration of aquifer heterogeneity suggests that high recoveries may be indicative of a high flow pathway adjacent, but not necessarily connected, to the injection and abstraction boreholes whereas low recoveries may indicate more distributed flow through many fractures of similar aperture.     

Applied tracer tests in fractured rock: Can we predict natural gradient solute transport more accurately than fracture and matrix parameters?

Applied tracer tests provide a means to estimate aquifer parameters in fractured rock. The traditional approach to analysing these tests has been using a single fracture model to find the parameter values that generate the best fit to the measured breakthrough curve. In many cases, the ultimate aim is to predict solute transport under the natural gradient. Usually, no confidence limits are placed on parameter values and the impact of parameter errors on predictions of solute transport is not discussed. The assumption inherent in this approach is that the parameters determined under forced conditions will enable prediction of solute transport under the natural gradient. This paper considers the parameter and prediction uncertainty that might arise from analysis of breakthrough curves obtained from forced gradient applied tracer tests. By adding noise to an exact solution for transport in a single fracture in a porous matrix we create multiple realisations of an initial breakthrough curve. A least squares fitting routine is used to obtain a fit to each realisation, yielding a range of parameter values rather than a single set of absolute values. The suite of parameters is then used to make predictions of solute transport under lower hydraulic gradients and the uncertainty of estimated parameters and subsequent predictions of solute transport is compared. The results of this study show that predictions of breakthrough curve characteristics (first inflection point time, peak arrival time and peak concentration) for groundwater flow speeds with orders of magnitude smaller than that at which a test is conducted can sometimes be determined even more accurately than the fracture and matrix parameters.     

Incidence of injury in children and adolescents with intellectual and developmental disability

Introduction: Children and adolescents living with intellectual and developmental disability (IDD) have a higher risk of experiencing morbidities and premature death when compared to children and adolescents living without IDD. Childhood injuries are a leading cause of morbidity and death, yet there are limited studies that explore the prevalence of childhood injuries for individuals living with IDD. The purpose of this study was to analyze Ontario health administrative data to identify and compare rates of injury resulting in hospitalization in children and adolescents living with and without IDD. Methods: This is a cross-sectional study of all Ontarians aged 0–19 years with and without IDD. The outcome of interest was the rate of injury resulting in hospitalization. Results: This study found that children and adolescents with IDD had 1.79 (CI 1.66, 1.92) times higher rates of both intentional and unintentional injuries that resulted in hospitalization when compared to children and adolescents without IDD. Hospitalizations for self-harm related injuries were 3.16 (CI 3.09, 3.23) times higher in the IDD group. Conclusion: Children and adolescents with IDD have a higher risk of sustaining serious injuries, particularly injuries resulting from self-harm. Practical Applications: This study provides evidence of increased injury related hospitalizations for children and adolescents with IDD when compared to their peers without IDD.     

Multiphase flow and transport through fractured heterogeneous porous media

The migration of Dense, Non-Aqueous Phase Liquid (DNAPL) and dissolved phase contamination through a fractured heterogeneous porous medium has been investigated through the use of a multiphase compositional model. The sensitivity of the timescales of migration and the distribution of contaminant in the subsurface to the mean permeability, the variance of the permeability, and the degree of fracturing of the domain were examined. It was found that increasing the mean permeability of the domain allowed the DNAPL to penetrate deeper into the subsurface, while decreasing the mean permeability caused the DNAPL to pool at shallower depths. The presence of fractures within the system was found to control the infiltration only in the most fractured domain. Moment analysis of the nonwetting phase showed that large-scale movement had ceased after approximately 9 years (maximum duration of the source-on condition was approximately 4.5 years). This tended to be due to a redistribution of the DNAPL towards a residual configuration, as was evidenced by the gradual trending of average nonwetting phase saturations within the domain to a static value. The dissolved phase plume was found to migrate at essentially the same rate as the nonwetting phase, due to the reduced relative permeability of lenses containing DNAPL, and due to diffusive losses of mass to the matrix of fractured clay and silty-clay lenses. Some exceptions to this were found when the DNAPL could not overcome the displacement pressure of a lens, and could not by-pass the lens due to the lack of available driving force after the source had been shut off.     

Prioritizing occupational injury prevention in the construction industry: injury severity or absence?

PROBLEM: National occupational injury prevention goals often prioritize the reduction of serious injuries. This study analyzed whether this prioritization is credible in respect to lost-time injuries and short and long term work absence, and the implications this has for injury severity-based versus injury absence-based prevention approaches. METHOD: The data consisted of national and work-site specific injury and absence data from construction workers in Denmark, including workers from the Copenhagen Metro construction sites, during the period 2000-2001. RESULTS AND IMPACT ON INDUSTRY AND GOVERNMENT: Absence length was significantly dependent on the type of injury. Sprains and strains were most prevalent and accounted for approximately one third of injuries and absence. Fractures accounted for one sixth of injuries and the greatest proportion of long-term absence. The results give credibility to the need for targeting sprains and strains in injury and absence prevention initiatives.     

Prediction of some in situ tracer tests with sorbing tracers using independent data

Some recent converging tracer tests with sorbing tracers at the Asp? Hard Rock Laboratory in Sweden, the TRUE tests, have been predicted using only laboratory data and hydraulic data from borehole measurements. No model parameters were adjusted to obtain a better fit with the experiments. The independent data were fracture frequency and transmissivity data obtained in the field and laboratory data on sorption and matrix diffusion. Transmissivity measurements in five boreholes in the rock volume containing the region surrounding the injection and collection points show that there is a high frequency of water conducting fractures. Of 162 packed off sections with 0.5 m packer distances, 112 were found to have a transmissivity above the detection limit. The specific flow-wetted surface (FWS) of the rock mass could be estimated from these data. The transmissivities were found to be reasonably well described by a lognormal distribution. Laboratory data on diffusion and sorption properties together with the hydraulic data were used to "predict" the residence time distribution (RTD) of the sorbing tracers. The results were compared with the experimental breakthrough curves. In these experiments, the water residence time is very small compared to the residence time of the sorbing tracers due to their diffusion and sorption within the rock matrix. We thus could neglect the influence of the water residence time in our predictions. Therefore, no information on water residence times or on "dispersion" was needed. The dispersion of the sorbing tracers is caused by the different sorbing tracer residence times in different pathways. The sorbing tracer residence time is determined by the ratio of flowrate to the flow-wetted surface in the different pathways and not by the water residence time. Assuming a three-dimensional flow pattern and using the observed fracture frequency and flowrate distribution, breakthrough curves for three strongly sorbing tracers were predicted. Only the laboratory data, the transmissivity measurements and the pumping flowrate were used in the predictions. No information on the water residence time as obtained by the nonsorbing tracers was used. The predictions were surprisingly accurate.     

First-passage-time transfer functions for groundwater tracer tests conducted in radially convergent flow

Forced-gradient groundwater tracer tests may be conducted using a variety of hydraulic schemes, so it is useful to have simple semi-analytic models available that can examine various injection/withdrawal scenarios. Models for radially convergent tracer tests are formulated here as transfer functions, which allow complex tracer test designs to be simulated by a series of simple mathematical expressions. These mathematical expressions are given in Laplace space, so that transfer functions may be placed in series by simple multiplication. Predicted breakthrough is found by numerically inverting the composite transfer function to the time-domain, using traditional computer programs or commercial mathematical software. Transport is assumed to be dictated by a radially convergent or uniform flow field, and is based upon an exact first-passage-time solution of the backward Fokker–Planck equation. These methods are demonstrated by simulating a weak-dipole tracer test conducted in a fractured granite formation, where mixing in the injection borehole is non-ideal.