In-situ monitoring of microbial activity and biodegradation during solute transport in porous media

Abstract

Over the last decade, luminescence measurements have been used primarily to detect and quantify specific organic pollutants and heavy metals, and in a few cases for monitoring microbial activity. In this study, a fiber-optic luminescence detection system was developed to examine the relationship between microbial activity and the resultant impact on biodegradation and transport of substrate in porous media. This system allows rapid, real-time, and non-destructive measurements of in-situ luminescence from a specific lux reporter microbial population in porous media. An understanding of the formation and dynamics of bioactive zones is very important for in-situ bioremediation applications because it is in these zones that the remediation process is optimal. This study also examined the location and size of a biologically active zone in response to changes in local substrate and electron acceptor availability. Results show that when DO was not a limiting factor, the bioactive zone encompassed the entire system. However, as the availability of DO became limiting for the higher-00 experiments, the size of the bioactive zone shrank and was ultimately limited to the proximity of the substrate source. Furthermore, a decrease in the size of the bioactive zone enhanced the rate of substrate biodegradation per unit area. This study also investigated the impact of several coupled factors including substrate concentration, pore-water velocity, and initial cell density on solute biodegradation and transport behavior for a system influenced by three stressors, microbial lag, microbial growth, and cell transport. Results showed that temporal changes in biodegradation potential, and therefore attendant substrate transport behavior, were influenced by microbial lag, growth, dissolved oxygen limitations, and cell elution. As a result, substrate transport behavior was non-steady except for relatively short residencetime conditions wherein substrate degradation exhibited quasi first-order behavior. Cell transport and elution was important, especially under significant growth conditions. Under such conditions, the majority of the cells in the system (60 to 90%) was distributed in the solution phase where most of the biodegradation took place. This study illustrates the complex behavior that can be associated with microbially mediated processes, and which should be included in solute transport models to accurately predict the fate of contaminants in the subsurface environment.