Understanding Seasonal Variation of Bioavailability of Residual NAPL in the Vadose ZoneEPA Grant Number: R827133
Title: Understanding Seasonal Variation of Bioavailability of Residual NAPL in the Vadose Zone
Investigators: Holden, Patricia , Keller, Arturo A.
Institution: University of California - Santa Barbara
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 1998 through September 30, 2001
Project Amount: $425,000
RFA: EPA/DOE/NSF/ONR Joint Program on Bioremediation (1998) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Natural biological attenuation in the vadose zone may occur for a wide range of pollutants. However, the physical, chemical and biological factors that control natural biological attenuation are not well understood for most pollutants. Reliance on a weakly understood remediation strategy such as natural attenuation leaves at risk underlying groundwater, hydrologically-connected surface water and thus human and ecosystem health. Pollutant bioavailability, as the composite of mass transfer and biodegradation processes, determines natural biological attenuation. Our hypothesis is that factors controlling pollutant bioavailability over time, and thus natural attenuation, must be understood if natural biological attenuation is to become predictable and reliable. Further, we propose that an accurate understanding of long-term bioavailability can only be gained in the context of the normal moisture fluctuations that occur seasonally in the vadose zone, and by focusing research at the pore scale and on the bulk and mobile form of pollutant, residual non-aqueous phase liquid (NAPL). We will use studies at three scales to study the influence of wetting and drying cycles on residual NAPL mass transfer and biodegradation. Specifically, our objectives are to determine:
- how cycles in water potential affect residual NAPL spreading under abiotic conditions
- how the presence of bacterial exopolymeric substances (EPS) affect NAPL mass transfer and biodegradation as a function of water potential
- the patterns of microbial activity and growth that occur with and without residual NAPL as a carbon source and as a result of wetting and drying cycles
- the mathematical representation of the processes that describe residual NAPL bioavailability in unsaturated porous media
- the functional relationship between the index of bioavailability and environmental conditions such as soil moisture.
To integrate between the mechanistic information that can be derived from controlled laboratory experiments and the empirical information obtained from an analysis of field site data, we will work at both the pore and field scales. Field test data from well-monitored sites of intrinsic or managed bioremediation will be used to evaluate the effect that drying and wetting cycles have on biodegradation rates. Studies to determine the mechanisms involved will be conducted at the pore scale using common organic pollutants present as residual NAPL and acting as carbon sources to microbes. These studies will be conducted using micromodels with a very accurate representation of real pore space. To bridge between the pore scale and the field scale, core scale studies will be performed, using sands and mixtures of 14C-labeled NAPLs. The cores will be non-destructively characterized using X-ray imaging before the biotic experiments.
After characterization, the cores will be inoculated with microbes and degradation will be monitored via 14C-CO2 evolution. Final NAPL saturation and microbial growth will be imaged using X-ray analysis. The results from these three scales will be analyzed quantitatively to provide a mechanistic interpretation of the functional relationship between the index of bioavailability and environmental conditions such as soil moisture
Expected results of this work will be an understanding of the physicochemical and biological mechanisms that determine bioavailability under realistic climatic conditions. The results will be useful to predict the effectiveness of natural biological attenuation and active bioremediation as a function of pollutant properties and moisture regimes. Broadly, we propose that protection of groundwater can be furthered by elucidating seasonal influences on intrinsic biodegradation through studies at the microscale, where pollutants and microbes physically intersect. With an understanding of seasonal influences on intrinsic bioremediation it will be possible to predict the efficacy of natural attenuation and the necessity of engineered or managed remediation for specific vadose zone pollutants.