Evaluation of methods for monitoring natural attenuation

The use of monitored natural attenuation as a remediation strategy can reduce costs over active remediation. Current monitoring approaches need to be critically evaluated though, in the context of application to typical field settings, such as heterogeneous alluvial aquifer systems. Spatially varying aquifer properties create non uniform flow fields and greatly influence transport processes, producing complex plume behavior which can confound monitoring results. Highly-resolved simulations of flow and conservative transport in an alluvial aquifer system at the Lawrence Livermore National Laboratory (LLNL) site facilitate a critical review of two monitoring approaches including estimation of mass balance from sampling along the plume centerline and estimation of mass flux from sampling along cross sections. The simulation procedure involves generation of quasi-unconditional transition-probability fields of hydrofacies distributions at the LLNL site, simulation of steady state flow followed by simulation of conservative transport using a highly accurate random walk particle method. The results elucidate limitations and potential pitfalls of the monitoring methods in typically heterogeneous systems. For example, simulations show that because of the system complexity, apparent concentration trends in space and time can easily be falsely attributed to biodegradation when none is occurring. Relying on measured concentrations alone as a means to judge effectiveness of in situ degradation are suspect however, and additional methods should be used to support definitive conclusions regarding remediation objectives