Lab Studies and Site Characterization for Permeable Reactive Barrier Walls
Numerous and complex geochemical interactions occur in the subsurface that can influence remediation scenarios based on in situ techniques such as reactive iron barrier walls. Differing site geochemistries and iron compositions (which vary widely) can exert significant impacts on contaminant remediation. It is important to understand how these technologies will perform in the subsurface and obtain information that will be useful in properly designing the technology implementation. It is also important to determine the contaminant disappearance half-life (t1/2) in the presence of the actual site geochemistry and the type of reactant that will be used in the reactive zone. Due to these considerations, one aspect of site characterization for reactive barriers is carried out in the lab. Laboratory batch and column tests are used to study the reaction rates and mechanisms, to estimate remedial effectiveness, and to develop design parameters.
Batch tests are typically carried out to select the reactant and to learn whether there is a need for amendments. These are followed by column tests to find the contaminant t1/2. Such tests are best done with the actual contaminated site water. The t1/2 can then be used to calculate the thickness of the reactive zone necessary to achieve the remedial goals in the field emplacement.
Batch tests are usually faster, simpler to set up than columns, and allow rapid comparison of varied parameters on the experimental results. It is simple, for example, to determine which of two types of iron is most effective for remediating a contaminant using batch tests. Three or more replicates for each of the two iron types can be set up in capped tubes or bottles using a constant aqueous volume, contaminant concentration, and iron mass (or surface area). After shaking for some interval, the concentration of the contaminant can be analyzed in each of the replicates and a determination made whether a statistical difference exists in the effectiveness of the two types of iron. Increasing the experimental complexity somewhat by adding multiple sampling intervals allows the determination of the rate of contaminant removal using the equations of classical kinetics. Batch systems are somewhat limited in this regard, however, since shaking of the tubes or bottles negates many of the mass transport and diffusive effects that would limit reactions in unshaken systems. Results from these shaken systems cannot readily be extrapolated to flowing systems, such as exist in an aquifer.
Flowing column tests are usually much more difficult to properly carry out than batch tests. They require specialized equipment, such as the columns themselves and pumps, significant quantities of influent solutions, fairly constant maintenance, and are usually operated longer than batch tests. Due to these constraints they are not as convenient as batch tests for comparing parameters and are best implemented after parameter optimization has been completed using batch tests. Since column tests are flowing systems, they are a step closer to reality than batch experiments, especially when site water is used and the flow rates are scaled to model the field site of interest. Although many columns can only be sampled at the effluent end (typically using a fraction collector) better contaminant fate information can be obtained using columns that have multiple sampling ports along their length. Using these ports, kinetic information on the degradation/remediation processes can be evaluated from concentration versus travel distance (i.e. proportional to time at a given flow rate) down the column. In addition, the changes in concentrations of degradation daughter products, for example DCE and vinyl chloride from the dechlorination of TCE, can be measured. Estimates of the reactive wall width necessary to accomplish the remedial objectives can be obtained in this manner.