IN SITU MICROCOSM STUDIES OF INTRINSIC AND ENHANCED DEGRADATION OF ORGANIC CONTAMINANTS IN A FRACTURED ROCK AQUIFER AND IN WETLAND POREWATER

In situ microcosms that combine the use of stable isotope probing and passive diffusion water samplers provided a method for determining intrinsic degradation rates and evaluating the effect of enhanced bioremediation technologies, such as biostimulation or bioaugmentation. In situ microcosms were conducted in two hydrogeologic environments that are difficult to simulate in laboratory experiments— a deep, fractured rock aquifer contaminated with perchlorate and the explosives compound RDX, and shallow wetland sediments containing chlorobenzenes and benzene. Microcosms contained Bio-Traps that were pre-loaded with ¹³C labeled RDX or chlorobenzene compounds. In the fractured rock aquifer, the anaerobic consortium WBC-2 and the electron donor lactate were injected into 3 wells that had varying initial contaminant concentrations. Analysis of a series of in situ microcosm devices placed in the wells and removed over time showed zero-order perchlorate degradation rates with half-lives of 21 to 41 days and first-order RDX degradation rates with half-lives of 3.6 to 3.9 days, verifying the feasibility of bioaugmentation for remediation. For the wetland study, in situ microcosm devices pre-loaded with ¹³C labeled compounds were pushed directly into the sediment and allowed to incubate for 8 weeks to evaluate natural attenuation (unamended devices), biostimulation (amended with lactate as an electron donor), and bioaugmentation (amended with lactate and the WBC-2 culture). Removal rates of ¹³C-monochlorobenzene were approximately the same under natural oxic conditions and under methanogenic bioaugmented conditions with half-lives between 34 and 37 days. A difference in degradation mechanism, however, was revealed by a difference in partitioning of ¹³C into biomass and carbon dioxide under natural and bioaugmented conditions. Intrinsic degradation of ¹³C-1,4-dichlorobenzene and ¹³C-benzene also was observed under varying redox conditions by incorporation of ¹³C into carbon dioxide. These microcosm studies allowed quantification of degradation at complex sites where field evaluation was previously hindered by low RDX concentrations (fractured rock site) and by contaminant sources containing both parent compounds and possible metabolites (wetland site).