Wetlands, both natural and designed, can be a cost-effective tool for the remediation of groundwater contaminated with volatile organic compounds (VOCs) that discharge into these wetlands. The widespread use of wetlands for remediation has been hindered by uncertainties surrounding the rates and time scales of remediation mechanisms. Volatilization is perhaps the least understood of these removal mechanisms. Initial laboratory and modeling results suggest that volatilization represents a major removal pathway, with the strength and mechanism of volatilization highly dependent on the physiochemical properties of the target compound. Our understanding of the physiochemical drivers of volatilization pathways and rates remain incomplete, however, precluding any predictive modeling techniques to accurately estimate VOC fluxes from wetlands. Questions of fundamental importance to the transport of volatile compounds in saturated shallow, vegetated soils must be addressed in order to close this knowledge gap: What is the balance between gas-phase and transpiration-driven transport of VOCs through wetland plants? To what extent will plant-mediated volatilization vary as a function of vegetation type? How can plant uptake of volatile organic compounds be described appropriately with predictive models, and how can these models be validated in the field? To what extent do volatile compounds partition into subsurface bubbles, how does bubble ebullition contribute to atmospheric fluxes of VOCs, and what is the interplay between bubble dynamics and plant enhanced gas transfer? The PI will perform a comprehensive study into the fundamental dynamics of volatilization through emergent wetland plants. Measurements that will span multiple scales from single stems in controlled experiments to a field pilot study will be carried out. The experimental program will include: (1) systematic laboratory hydroponic studies using multiple volatile tracers and single stems to probe the fundamental relationships between the physiochemical properties of a compound and the rates and mechanisms for its volatilization; (2) greenhouse mesocosm studies to validate predictive models for phytovolatilization in saturated soil conditions and examine interspecies variability in contaminant removal rates; and (3) a pilot field test in a freshwater wetland to quantify spatial and temporal heterogeneities in rates of VOC removal from wetland systems. An important objective of this research is the development of a single-well ?push-pull? technique that will be developed in greenhouse mesocosms and implemented in the field, providing a novel and valuable tool for the assessment of volatilization rates in diverse wetland ecosystems where other mechanisms to determine gas exchange between the rhizosphere of wetland systems and the atmosphere are not feasible. In terms of the broader impacts, the proposed research will address fundamental questions concerning transport of chemicals in saturated soils and in vegetation that have implications across the earth sciences. Proposed technological innovations in measuring groundwater-atmosphere gas exchange will have multidisciplinary applications including the design and management of wetlands and assessment of attenuation in natural wetlands. This work will fund one Ph.D. student and contribute to the education of other graduate students in the PI?s laboratory and laboratories of outside collaborators, including Rutgers University and the Meadowlands Research Institute. Undergraduate students from different schools will be involved via an ongoing REU program. This REU program has been successful in recruiting minority and female students. Undergraduates will be involved via Senior Thesis Research. Outreach to the community is planned via participation in QUEST, a science institute for upper elementary school teachers taught by Princeton University faculty and staff, as well as by participation with the educational programs of the Stony Brook-Millstone Watershed Association, which also engages in active dialogue with municipal officials, citizens, and businesses concerning decisions that affect the local environment. Results of this research will be incorporated in undergraduate (Environmental Engineering Laboratory) and graduate (Water Quality Modeling) courses, and will be disseminated to the scientific community via presentations at National Conferences and peer-reviewed journal publications.
