Identification of small-scale low and high permeability layers using single well forced-gradient tracer tests: fluorescent dye imaging and modelling at the laboratory-scale

Tracer movements in a straight uniform flow: New equations for the advective part considering the distortion of flow lines around the well

Methods for interpreting tracer tests often rely on equations assuming a natural regional flow which is straight and uniform. The main purpose of this project was to develop more realistic equations for the advective part of contaminant transport, by including the flow lines distortion occurring in the vicinity of the injection well. The complex potential of the flow during a tracer test was calculated by superimposing the complex potential of a natural, straight and uniform flow distorted by the presence of a passive well, and the complex potential of a radial flow corresponding to an isotropic injection alone. The equations were developed for a horizontal plan and the calculated complex potential yielded a groundwater velocity field, and after that a formula connecting the position of the advancing front of the tracer plume to the injection duration. These new equations were then tested with numerical simulations. A two-dimensional aquifer plan was modeled and set in order to numerically solve particle tracking and travel time computation of the moving front within a reasonable calculation time. This model provided a comparison of times needed to fully recover the tracer plume previously injected, the one calculated with the new equations and the one calculated with former equations neglecting the impact of the well presence on the groundwater flow field. The results showed that the new equations are significantly more precise, in particular when the injection rate is sufficiently low compared to the natural regional flow rate, with a relatively large well diameter and in the vicinity of the injection well. Three different plume shapes could be visualized numerically, and those shapes depend on the value of a parameter △ which compares the velocity component caused by the injection in the well and the component caused by the natural regional flow.

Contaminant occurrence and migration between high- and low-permeability zones in groundwater systems: A review

In recent decades, water quality problems that impact human health, especially groundwater pollution, have been intensely studied, and this has contributed to new ideas and policies around the world such as Low Impact Development (LID) and Superfund legislation. The fundamental to many of these problems is pollutant occurrence and migration in saturated porous media, especially in groundwater. Such environments often contain contrasting zones of high and low permeability with significant differences in hydraulic conductivity (~10^-4 and 10^-8 m/s, respectively). High-permeability zones (HPZs) represent the primary pathways for pollutant transport in groundwater, while low-permeability zones (LPZs) are often diffusion dominated and serve as both sinks and sources (i.e., via back-diffusion) of pollutants over many decades. In this review, concepts and mechanisms of solute source depletion, contaminant accumulation, and back-diffusion in high- and low-permeability systems are presented, and new insights gained from both experimental and numerical studies are analyzed and summarized. We find that effluent monitoring and novel image analysis techniques have been adroitly used to investigate temporal and spatial evolutions of contaminant concentration; simultaneously, mathematical models are constantly upscaled to verify, optimize and extend the experimental data. However, the spatial concentration data during back-diffusion lacks diversity due to the limitations of pollutant species in studies, the microscopic mechanisms controlling pollutant transformation are poorly understood, and the impacts of these reactions on contaminant back-diffusion are rarely considered. Hence, most simulation models have not been adequately validated and are not capable of accurately predicting pollutant fate and cleanup in realistic heterogeneous aquifers. Based on these, some hypotheses and perspectives are mentioned to promote the investigation of contaminant migration in high- and low-permeability systems in groundwater.

Acceleration and centralization of a back-diffusion process: Effects of EDTA-2Na on cadmium migration in high- and low-permeability systems

Pollutant accumulation in the low-permeability zones (LPZs) in groundwater systems is regarded as a secondary source, and its consequent back-diffusion can extend the timeframe of pump-and-treat remediation. However, the bioavailability and mobility of heavy metals and the medium characteristics can be changed during the process. This study investigated the accumulation and back-diffusion law of toxic metals and the effects of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) on them by implementing a series of tank experiments. In these experiments, a cadmium solution was injected first, and deionized water or EDTA-2Na constantly washed the system consisting of different medium layers. The experimental results showed that the cadmium breakthrough curves had some concentration gradient reverse points where the curves fluctuated with elution by deionized water, which did not exist when EDTA-2Na was the eluent. In these scenarios, the mass of accumulated cadmium in the media before elution was large, with a value of 931 mg (153 mg/kg), when the low-permeability medium was clay. However, when EDTA-2Na was injected together with cadmium, the value dropped to 319 mg (52.3 mg/kg), greatly reducing the cadmium accumulation. Additionally, the use of EDTA-2Na as an eluent resulted in the appearance of a secondary peak in the breakthrough curve, showing that EDTA-2Na accelerated and centralized the back-diffusion. Notably, the reduced cadmium accumulation in LPZs with the elution by EDTA-2Na was partly due to a reduced adsorption capacity of the clay minerals. The above results can advance the technology related to pump-and-treat remediation.

Quantification of Uncertainties from Image Processing and Analysis in Laboratory-Scale DNAPL Release Studies Evaluated by Reflective Optical Imaging

Subsurface DNAPL (dense non-aqueous phase liquid) contamination from (un-) intentional spilling typically leads to severe environmental hazards. A large number of studies have demonstrated the relevance of DNAPL source zone geometry for the determination of contaminant plume propagation in groundwater. Optical imaging represents a promising non-invasive method for identifying DNAPL saturation without disturbing multiphase flow dynamics. However, workflow and image analysis methodologies have not been sufficiently developed or described for general application to related experimental efforts. For example, the choice of dye(s) used for phase colorization affects image processing and can bias final estimations of DNAPL saturations. In this study, we perform a series of DNAPL migration and entrapment studies in transparent tanks that are filled with three different types of porous media. Different dyes are used and raw images are acquired. Subsequently, these are used to evaluate a suite of image processing and analysis approaches, which are organized into a workflow. Our approach allows for us to identify key image processing and analysis steps that introduce the most error. Applicable dye configurations led to uncertainties of up to 41% depending on the selection of processing steps. Based on these findings, it was possible to delineate a flexible framework for image processing and analysis that has the potential for transfer and application in other tank experiment setups.