Developing a microbial community structure index (MCSI) as an approach to evaluate and optimize bioremediation performance.

Much attention is placed on organohalide-respiring bacteria (OHRB), such as Dehalococcoides, during the design and performance monitoring of chlorinated solvent bioremediation systems. However, many OHRB cannot function effectively without the support of a diverse group of other microbial community members (MCMs), who play key roles fermenting organic matter into more readily useable electron donors, producing corrinoids such as vitamin B12, or facilitating other important metabolic processes or biochemical reactions. While it is known that certain MCMs support dechlorination, a metric considering their contribution to bioremediation performance has yet to be proposed. Advances in molecular biology tools offer an opportunity to better understand the presence and activity of specific microbes, and their relation to bioremediation performance. In this paper, we test the hypothesis that a specific microbial consortium identified within 16S ribosomal ribonucleic acid (rRNA) gene next generation sequencing (NGS) data can be predictive of contaminant degradation rates. Field-based data from multiple contaminated sites indicate that increasing relative abundance of specific MCMs correlates with increasing first-order degradation rates. Based on these results, we present a framework for computing a simplified metric using NGS data, the Microbial Community Structure Index, to evaluate the adequacy of the microbial ecosystem during assessment of bioremediation performance.

Utilization of waste materials, non-refined materials, and renewable energy in in situ remediation and their sustainability benefits.

In the ramp-up to integrating sustainability into remediation, a key industry focus area has been to reduce the environmental footprint of treatment processes. The typical approach to integrating sustainability into remediation projects has been a top-down approach, which involves developing technology options and then applying sustainability thinking to the technology, after it has been conceptualized. A bottom-up approach allows for systems thinking to be included in remedy selection and could potentially result in new or different technologies being considered. When using a bottom-up approach, there is room to consider the utilization of waste materials, non-refined materials, and renewable energy in remediation technology-all of which generally have a smaller footprint than processed materials and traditional forms of energy. By integrating more systems thinking into remediation projects, practitioners can think beyond the traditional technologies typically used and how technologies are deployed. To compare top-down and bottom-up thinking, a traditional technology that is considered very sustainable-enhanced in situ bioremediation-is compared to a successful, but infrequently deployed technology-subgrade biogeochemical reactors. Life Cycle Assessment is used for the evaluation and shows the footprint of the subgrade biogeochemical reactor to be lower in all seven impact categories evaluated, sometimes to a significant degree.

Design and performance of subgrade biogeochemical reactors.

Subgrade biogeochemical reactors (SBGRs), also commonly referred to as in situ bioreactors, are a unique technology for treatment of contaminant source areas and groundwater plume hot spots. SBGRs have most commonly been configured for enhanced reductive dechlorination (ERD) applications for chlorinated solvent treatment. However, they have also been designed for other contaminant classes using alternative treatment media. The SBGR technology typically consists of removal of contaminated soil via excavation or large-diameter augers, and backfill of the soil void with gravel and treatment amendments tailored to the target contaminant(s). In most cases SBGRs include installation of infiltration piping and a low-flow pumping system (typically solar-powered) to recirculate contaminated groundwater through the SBGR for treatment. SBGRs have been constructed in multiple configurations, including designs capable of meeting limited access restrictions at heavily industrialized sites, and at sites with restrictions on surface disturbance due to sensitive species or habitat issues. Typical performance results for ERD applications include 85 to 90 percent total molar reduction of chlorinated volatile organic compounds (CVOCs) near the SBGR and rapid clean-up of adjacent dissolved contaminant source areas. Based on a review of the literature and CH2M's field-scale results from over a dozen SBGRs with a least one year of performance data, important site-specific design considerations include: 1) hydraulic residence time should be long enough for sufficient treatment but not too long to create depressed pH and stagnant conditions (e.g., typically between 10 and 60 days), 2) reactor material should balance appropriate organic mulch as optimal bacterial growth media along with other organic additives that provide bioavailable organic carbon, 3) a variety of native bacteria are important to the treatment process, and 4) biologically mediated generation of iron sulfides along with addition of iron pyrite sands as an abiotic polishing step within the reactor has been observed to be an efficient treatment train for chlorinated solvent sites.