Before in situ remediation can be implemented at a hazardous waste site, bench-scale or field-scale feasibility studies are required. These are typically conducted in static batch-bottle microcosms. An alternative approach, continuous-flow column studies, is rarely used by the remediation industry. Although scientifically constituting the “gold standard” approach to studying transport and reaction phenomena in saturated media, column studies are avoided because of their considerable costs, complexity and difficulty in performing multiple replicates, and the operator time required. Although batch-bottle tests may be adequate for qualitative screening of remedial design options, they are generally considered to have poor quantitative predictive power. In contrast, column studies are expected to produce both reliable qualitative and quantitative data, as they create a more realistic reflection of subsurface realities and the associated difficulty of delivering the remedial agent to where the contaminants of concern reside.
On the small scale, the in situ microcosm array (ISMA) technology answers this challenge by creating a platform for standardized flow-through sediment column experiments, thus making the more sophisticated continuous-flow evaluation method more accessible to the Department of Defense and environmental restoration industry.
The ISMA is the hybrid of a laboratory treatability study and a field pilot trial. The device contains all of the components necessary for it to autonomously conduct a flow-through sediment column treatability study in the subsurface. All components—columns, pumps, electronics, etc.—have been miniaturized and assembled to fit within a 4-inch groundwater well. During operation, the ISMA is suspended in a well for approximately 4-8 weeks, during which time it operates autonomously collecting groundwater directly from the subsurface formation and feeding it into the array of microcosms. The ISMA can accommodate up to 10 sediment column microcosms, allowing for the side-by-side testing of 10 remediation strategies under truly identical conditions, or the testing of fewer strategies in replicate experiments to assess reproducibility. Throughout the deployment period, all the groundwater entering the ISMA is collected in column-specific, individual effluent capture vessels, and analyzed in the laboratory after retrieval of the ISMA from the well.
The main advantages the ISMA offers are: (1) reduced cost when compared to alternatives; (2) the opportunity to generate data on field performance of remediation technologies with no risk of negative impacts on the aquifer; (3) the ability to screen multiple, mutually exclusive, treatment options in parallel; and (4) to do so in situ using fresh groundwater, drawn in real-time from the subsurface formation, thereby reflecting the ambient hydrogeochemistry and microbiology of the target environment. Limitations include that the current embodiment does not enable intermittent or continuous monitoring of conditions prevailing in the device during field incubation. Further, the construction of sediment microcosms may result in experimental bias and potential inactivation of sensitive anaerobic microorganisms. As any other small-scale feasibility assessment tool, the ISMA technology is incapable of assessing site heterogeneities that are known to influence the outcome of remediation efforts.
Two demonstration deployments of the ISMA were conducted, one evaluating three different in situ remediation strategies for treatment of perchlorate, and the other evaluating three different strategies for treatment of two co-contaminants, hexavalent chromium [Cr(VI)] and trichloroethene (TCE). Where applicable, ISMA-generated results were compared to and found consistent with complementary data sets produced from batch-bottle treatability studies, laboratory column studies, and field pilot trials.
Results indicate that the ISMA is a cost-effective and suitable alternative to contemporary treatability or feasibility study methods. Qualitatively, results from ISMA and batch-bottle studies led to similar conclusions: both indicated that bioaugmentation was effective at treating the perchlorate (Site 1) and Cr(VI) and TCE (Site 2). This conclusion is consistent with the results from all relevant site-specific data sets, including (1) data gathered in the laboratory at Arizona State University from both complementary batch-bottle studies and flow-through column studies; (2) results generated from a batch-bottle study conducted by an outside consulting firm; and (3) results generated from a field pilot trial. A quantitative comparison of first-order degradation rate constants found that batch bottles overestimated field rates by over an order of magnitude (>10), while the degradation rates observed in the ISMA differed from those observed in the field only by a factor of two. This result indicates that the ISMA more accurately reproduces field phenomena and may potentially be used to quantitatively and accurately assess the field performance of in situ remediation technologies.
The ISMA costs are similar to a traditional bottle treatability study conducted in static (batch) mode, but notably lower than both a laboratory column test and field pilot trial. Furthermore, the standardized, modular components of the ISMA can be used as a platform for conducting column studies in the lab as well. This usage mode can serve to reduce costs of a laboratory column study, thereby making the more sophisticated flow-through evaluation method more accessible to environmental restoration professionals.
The ISMA is a new platform for conducting column studies in the laboratory and in the field. The standardized column format allows for the performance of experiments in multiple replicates, which is of great importance because of the large variability associated with microcosm experiments. The technology’s high degree of automation reduces the requirement for constant monitoring by an operator. Its application in the subsurface helps to create quasi-field conditions in the device and eliminates to a large degree the need for maintaining expensive laboratory space; in situ operation may serve to reduce laboratory artifacts introduced by removal of groundwater from the subsurface. In situ operation also yields degradation rates that are more consistent with observed field rates, which will benefit decision-making in the remedial design phase of site cleanup. The cost evaluation showed that an ISMA deployment is only marginally more expensive than a contemporary batch-bottle experiment but drastically less expensive than the alternatives, a contemporary laboratory column study and a field pilot trial.