The efficiency of in situ remediation technologies that involve injecting remediation agents into the subsurface is often limited by the ability to obtain an efficient sweep of these amendments within the target zone of contamination. Poor amendment sweep efficiencies are typically the result of injected amendments seeking preferential flow paths within zones of higher permeability. This leads to bypassing lower permeability strata and, if sufficient contamination exists within these lower permeability strata, rebounding contaminant concentrations within a groundwater aquifer following treatment. Methods designed to mitigate the potential for preferential flow and bypassing effects would increase remediation efficiencies and reduce costs of environmental restoration efforts.
The primary objective of this research was to provide information that will advance the state of the science in enhanced delivery of remedial agents to heterogeneous contaminated zones by improving the subsurface sweep efficiency of injected remediation agents (e.g., chemical oxidants and bioamendments) using viscous solutions of water-soluble polymers.
Methods that were utilized to meet the project objectives included batch studies, column studies, 2-D vertical tank experiments, and 3-D numerical simulations. The 3-D multicomponent, multiphase, compositional simulator, UTCHEM, was used in testing experimental conditions to optimize the experimental design of the 2-D tank experiments. UTCHEM also was used to analyze experimental data and as a tool to identify and further elucidate important mechanisms of amendment emplacement, with and without polymer addition, within heterogeneous aquifer systems. UTCHEM has the unique capability of simulating the subsurface transport of numerous amendment types, including polymer-amended fluids.
Investigations revealed that the xanthan polymer may be compatible with both permanganate and persulfate oxidants, but the xanthan-permanganate pair is most promising for future use. Both polymers are compatible with bioaugmentation remediation, but neither polymer was demonstrated to be an effective electron donor to achieve complete dechlorination. The confirmation of the presence of simple reducing sugar compounds resulting from xanthan biodegradation led researchers to infer that the use of xanthan polymers in the subsurface should not result in long-term deleterious effects on groundwater quality. Polymer injections will result in some clogging near the injection zone, but this is not expected to significantly influence field application. Batch and column tests were helpful in constructing numerical models in up-scaled systems (2-D tanks). The UTCHEM model was able to successfully simulate 2-D experimental data for layered heterogeneous systems. Experimental data from intermediate-scale 2-D systems and from hundreds of numerical simulations suggest that polymer floods are very effective at improving sweep efficiency in layered systems and that performance is improved in systems with more layers. Additional 2-D experiments confirmed that a polymer-oxidant flood enhanced mass removal and reduced post-remediation mass flux; whereas, the effectiveness of treatment depends on the delivery method.
Polymer flooding shows considerable promise for improving delivery of remediation agents in heterogeneous media where contaminants reside in hydraulically inaccessible zones. Knowledge gained from this effort is being utilized during field testing under ESTCP project ER-200912.