Sustained treatment: Implications for treatment timescales associated with source‐depletion technologies

Abstract

Sustained treatment is an emerging concept used to describe enhancements in attenuation capacity after the conclusion of the active treatment period for a given source-depletion technology. The term includes mechanisms that lead to contaminant transformation or destruction over extended periods of time, such as endogenous biomass decay, slow diffusion of remedial amendments from low-permeability zones, and the formation of reactive mineral species. This "value-added" treatment continues after the end of capital expenditures at a site, and it provides additional insight in determining if monitored natural attenuation is a viable long-term option for a site. This article identifies several sustained treatment mechanisms, examines technology-specific factors that contribute to sustained treatment, and explores the potential timescales of sustained treatment relative to active treatment. As demonstrated in post-treatment site data obtained during a comprehensive source-depletion technology performance survey, enhanced bioremediation is the most promising in promoting sustained treatment, and this beneficial effect can extend for several years due to factors such as slow biomass decay. There is little evidence that other commonly used technologies (thermal treatment, in situ chemical oxidation, surfactant-enhanced remediation, or cosolvent flushing) result in any significant sustained treatment. An exception would be a cosolvent flushing project where large quantities of biodegradable cosolvent are left in the subsurface at the end of the project, which could result in sustained long-term dechlorination activity. In the case of in situ chemical oxidation, factors that contribute to a higher incidence of concentration rebound mask any potential sustained treatment effects. O

Groundwater Restoration: Large‐Scale Benefits of Small‐Scale Processes

“…The technologies used to achieve the performance presented in Figures 1 and 2 entailed a combination of enhanced reductive dechlorination for source treatment coupled with dilute plume directed groundwater recirculation, both of which are ideal "sustained treatment" (Adamson et al 2011) technologies. Within the source areas, organic carbon delivered to the subsurface supported the development of reducing conditions, provided a source of molecular hydrogen for biological degradation, generated biomass for endogenous carbon recycling, and deposited reduced iron minerals for abiotic treatment; all of which contributed to sustained solvent reduction.…”

Long‐Term Evaluation of Mulch Biowall Performance to Treat Chlorinated Solvents

“…Sustained treatment encompasses the ongoing, long‐term performance of in situ remedial technologies years after initial installation. Adamson et al () introduced the concept of sustained treatment to describe enhancements to the attenuation capacity after the conclusion of the active treatment period. Mechanisms that contribute to contaminant removal over long timeframes may include endogenous biomass decay (Suthersan et al ), slow diffusion of remedial amendments from low‐permeability zones, and the formation of reactive mineral species (e.g., He et al ).…”

“…Mechanisms that contribute to contaminant removal over long timeframes may include endogenous biomass decay (Suthersan et al ), slow diffusion of remedial amendments from low‐permeability zones, and the formation of reactive mineral species (e.g., He et al ). Enhanced bioremediation was identified as a technology that lends itself to sustained treatment (Adamson et al ).…”

Evaluation of Long‐Term Performance and Sustained Treatment at Enhanced Anaerobic Bioremediation Sites

“…This process produces electrons that contribute to the reducing capacity of the aquifer system and can provide reducing equivalents for reductive dechlorination (Yang and McCarty ; Sleep et al ; Adamson and Newell ; Suthersan et al ). Along with decay products, biomass can contribute to the electron pool by excreting degradable molecular products such as extracellular polymeric substances and soluble microbial products (Rittman and McCarty ; Suthersan et al ). Formation of reactive mineral species : Injected amendments create reducing conditions within the treatment zone, which can transform non‐reactive iron mineral species into active (reduced) forms that can mediate abiotic degradation (Kennedy et al ; Brown et al ; He et al ). Amendment diffusion in or out of low‐permeability matrices : Injected amendments can diffuse into low‐permeability zones, potentially promoting contaminant degradation within these zones and reducing the potential for contaminant back diffusion (Adamson et al ). Long‐term persistence of amendments : The design lifetime of many sparingly soluble substrates (e.g., emulsified oils) is 1 to 4 years, but there is an indication that some amendments may be more persistent and thus be able to support biodegradation for even longer periods (Long and Borden ).…”