Sarah E. Dickson

Differential transport and dispersion of colloids relative to solutes in single fractures

This work employed numerical experiments simulating colloid and solute transport in single parallel-plate fractures, using the random walk particle tracking method, to demonstrate that (1) there exists an aspect ratio of the colloid radius to half the fracture aperture, delta(o), where the average velocities of colloids and solutes are similar. When delta>delta(o), the velocity distribution assumption is satisfied, and the fact that the ratio of the colloid transport velocity to the solute transport velocity, tau(p), decreases as delta increases is well documented in the literature. However, when delta<delta(o), the velocity distribution assumption is violated, and tau(p) increases as delta increases and (2) the Taylor dispersion coefficient and its extension by James and Chrysikopoulos [S.C. James, C. V. Chrysikopoulos, J. Colloid Interface Sci. 263 (2003) 288] will overestimate the colloid dispersion coefficient significantly. Additionally, numerical experiments simulating colloid and solute transport in variable-aperture fractures demonstrated that tau(p) and D(L,)(coll)/D(L,)(solute) decrease with increasing CoV, and the anisotropy ratio only plays a minor role compared to the CoV. These observations have important implications towards the interpretation of colloid transport in both porous and fractured media.

Mass flux from a non-aqueous phase liquid pool considering spontaneous expansion of a discontinuous gas phase

The partitioning of non-aqueous phase liquid (NAPL) compounds to a discontinuous gas phase results in the repeated spontaneous expansion, snap-off, and vertical mobilization of the gas phase. This mechanism has the potential to significantly affect the mass transfer processes that control the dissolution of NAPL pools by increasing the vertical transport of NAPL mass and increasing the total mass transfer rate from the surface of the pool. The extent to which this mechanism affects mass transfer from a NAPL pool depends on the rate of expansion and the mass of NAPL compound in the gas phase. This study used well-controlled bench-scale experiments under no-flow conditions to quantify for the first time the expansion of a discontinuous gas phase in the presence of NAPL. Air bubbles placed in glass vials containing NAPL increased significantly in volume, from a radius of 1.0 mm to 2.0 mm over 215 days in the presence of tetrachloroethene (PCE), and from a radius of 1.2 mm to 2.3 mm over 22 days in the presence of trans-1,2-dichloroethene (tDCE). A one-dimensional mass transfer model, fit to the experimental data, showed that this expansion could result in a mass flux from the NAPL pool that was similar in magnitude to the mass flux expected for the dissolution of a NAPL pool in a two-fluid (NAPL and water) system. Conditions favouring the significant effect of a discontinuous gas phase on mass transfer were identified as groundwater velocities less than approximately 0.01 m/day, and a gas phase that covers greater than approximately 10% of the pool surface area and is located within approximately 0.01 m of the pool surface. Under these conditions the mass transfer via a discontinuous gas phase is expected to affect, for example, efforts to locate NAPL source zones using aqueous concentration data, and predict the lifetime and risk associated with NAPL source zones in a way that is not currently included in the common conceptual models used to assess NAPL-contaminated sites.

Removal of Methyl-tert-butyl Ether from Water by a Pulsed Arc Electrohydraulic Discharge System

The removal of methyl-tert-butyl ether (MTBE) from water by a pulsed arc electrohydraulic discharge (PAED) system was investigated experimentally at the laboratory scale. The effects of arc electrode gap, detention time and initial solution pH on the efficacy of MTBE treatment by PAED were investigated. A 0.3 kJ/pulse spark-gap-type power supply was employed in combination with a 3.0 L flow-through reactor. Experimental results showed that: 1) oxidation reactions are induced by this PAED system; 2) removal efficiency increases with increasing cumulative input energy (kWh/m3 or kJ/L); 3) removal efficiency decreases with increasing arc electrode gap; and 4) initial solution pH does not have a significant effect on the efficacy of MTBE removal by PAED when the water matrix is buffered prior to treatment.

Water Security Assessment Indicators: The Rural Context

An increasing number of factors pose challenges to the development and management of water resources in rural, remote, or otherwise marginalized (RRM) communities. Indicators and indices have been developed for evaluation, prioritization, and decision-making at local and supra-local scales. Indicators and indices are useful assessment tools as they simplify the modeling process and provide results in an accessible format. The purpose of this paper is to consolidate a list of indicators (n = 176) from a review of community- and basin- level indices and a selection of other literature within a water security framework for RRM communities. A detailed discussion of each of the six dimensions within the framework is provided. This paper concludes with some general remarks on the standards used for evaluation, the reliance upon historical and field data, suggestions for improving the descriptive clarity where it is lacking, and the prospect of these indicators getting used by community members.

Characteristics of Pulsed Arc Electrohydraulic Discharge for Eccentric Electrode Cylindrical Reactor using Phosphate-Buffered Saline Water

Pulsed arc electrohydroulic discharge (PAED) has been proposed as a water treatment technology for the removal of chemical and microbial contaminants. In this work, we examined the fundamental characteristics of a PAED system with an eccentric electrode cylindrical reactor. Phosphate-buffered saline (PBS) water was used in lieu of tap water, because the conductivity of PBS is much higher than that of tap water. The results show that the voltage and current waveforms generated in PBS and tap water are very different due to the higher conductivity of PBS. Strong pressure waves and UV emission were also observed in PBS.

Proposed method: incorporation of fractured rock in aquifer vulnerability assessments

Groundwater from fractured rock aquifers serves as a source of drinking water around the world; however, wells in fractured rock can be susceptible to contamination. Aquifer vulnerability assessments are tools used for identifying the risk potential in groundwater sources, as well as efficiently allocating resources for source water protection. Existing methods include DRASTIC, GOD, EPIK, AVI, COP, and ISI. Some approaches do not consider risks posed by fractures, while others were developed for karstified regions; all are ill suited to fractured formations. This work proposes a new vulnerability assessment method incorporating quantitative portions of existing methods together with fractured rock characteristics. The proposed method is applied to a study area in Acton-Georgetown, Ontario; the DRASTIC and AVI methods are also applied for comparison. DRASTIC and AVI methods yield significantly different results from each other and from the proposed method. The proposed method demonstrates the heavy influence the fractured rock has on vulnerability, highlighting the need for its inclusion in vulnerability assessments.

A phenomenological model for particle retention in single, saturated fractures.

Fractured aquifers are some of the most poorly characterized subsurface environments despite posing one of the highest risks to the protection of potable groundwater. This research was designed to improve the understanding of the factors affecting particle transport through fractures by developing a phenomenological model based on laboratory-scale transport data. The model presented in this research employed data from over 70 particle tracer tests conducted in single, saturated, variable-aperture fractures that were obtained from the natural environment and fractured in the laboratory or cast from epoxy in the laboratory. The particles employed were Escherichia coli RS2-GFP and microspheres. The tracer experiments were conducted in natural (dolomitic limestone and granite) as well as epoxy replicas of the natural fractures. The multiple linear regression analysis revealed that the most important factors influencing particle retention in fractures are the ratio of the ionic strength of solution to collector charge, the ratio of particle to collector charge, and the ratio of advective to diffusive forces as described by the Peclet number. The model was able to reasonably (R(2)  = 0.64) predict the fraction of particles retained; however, it is evident that some factors not accounted for in the model also contributed to retention. This research presents a novel approach to understanding particle transport in fractures, and illustrates the relative importance of various factors affecting the transport mechanisms. The utility of this model lies in the increased understanding of particle transport in fractures, which is extremely useful for directing future research.

Colloid retention mechanisms in single, saturated, variable-aperture fractures.

The characterization of fractured aquifers is commonly limited to the methodologies developed for unconsolidated porous media aquifers, which results in many uncertainties. Recent work indicates that fractured rocks remove more particulates than they are conventionally credited for. This research was designed to quantify the number of Escherichia coli RS2-GFP retained in single, saturated, variable-aperture fractures extracted from the natural environment. Conservative solute and E. coli RS2-GFP tracer experiments were used to elucidate the relationships between dominant retention mechanisms, aperture field characteristics, and flow rate. A non-destructive method of determining a surrogate measure of a coefficient of variation (COV(S)) for each fracture was used to better understand the transport behaviour of E. coli RS2-GFP. The results from this research all point to the importance of aperture field characterization in understanding the fate and transport of contaminants in fractured aquifers. The mean aperture was a very important characteristic in determining particulate recovery, so were matrix properties, COV(s), and flow rate. It was also determined that attachment is a much more significant retention mechanism than straining under the conditions employed in this research. Finally, it was demonstrated that the dominant retention mechanism in a fracture varies depending on the specific discharge. An improved understanding of the mechanisms that influence the fate and transport of contaminants through fractures will lead to the development of better tools and methodologies for the characterization of fractured aquifers, as well as the ability to manipulate the relevant mechanisms to increase or decrease retention, depending on the application.

Pathways to a Water Secure Community

Community-based water security, defined herein as “the sustainable access to affordable and reliable quantities of water of suitable quality to enable all persons to lead healthy, dignified, and productive lives, including neighbours and future users”, has not been paid a significant amount of attention to date. However, in light of current global access to drinking water and sanitation facilities, wastewater treatment coverage, the importance of water for food, energy and industry, and the impacts of climate change on the hydrological cycle, water (in)security is extremely important at the local scale and a potential threat to (de)development. While community water security can be difficult to incorporate into the water security continuum due to differences in scale, water secure communities are a building block for sustainable watersheds. After identifying the different aspects of water security at the community level, it is concluded that a comprehensive, systems approach coupled with capacity for sustainable local change is key for sustained and sustaining community water security.

On the Importance of Gravity in DNAPL Invasion of Saturated Horizontal Fractures.

Invasion percolation (IP) models of dense non-aqueous phase liquid (DNAPL) invasion into saturated horizontal fractures typically neglect viscous and gravity forces, as it is assumed that capillarity dominates in many situations. An IP model simulating DNAPL invasion into saturated horizontal fractures was modified to include gravity as a local effect. The model was optimized using a genetic algorithm, and demonstrated that the inclusion of gravity is important for replicating the architecture of the DNAPL invasion pattern. The optimized gravity-included simulation showed the DNAPL invasion pattern to be significantly more representative of the experimentally observed pattern (80% accuracy) than did the optimized gravity-neglected simulation (70% accuracy). Additional simulations of DNAPL invasion in 360 randomly generated fractures were compared with and without gravity forces. These simulations showed that with increasing fracture roughness, the minimum difference between simulations with and without gravity increases to 35% for a standard deviation of the mid-aperture elevation field (SDz ) of 10 mm. Even for low roughness (SDz = 0.1 mm), the difference was as high as 30%. Furthermore, a scaled Bond Number is defined which includes data regarding DNAPL type, media type and statistical characteristics of the fracture. The value of this scaled Bond Number can be used to determine the conditions under which gravity should be considered when simulating DNAPL invasion in a macroscopically horizontal fracture. Finally, a set of equations defining the minimum and maximum absolute percentage difference between gravity-included and gravity-neglected simulations is presented based on the fracture and DNAPL characteristics.