EAR-PF: Quantifying methane reactivity and turnover in the subterranean estuary: combined in-situ and ex-situ isotope tracer approaches

At the interface of land and sea, subterranean estuaries (STEs) serve as subsurface transition zones. STEs can support a variety of biogeochemical reactions that influence concentrations and transport of nutrients and carbon. High concentrations of methane (CH4), an important greenhouse gas and major player in the global carbon cycle, has been observed in the coastal subsurface.

However, the way that this CH4 pool is transformed and transported within STEs remains unconstrained. This study will provide a novel understanding of how CH4 is cycled in the subsurface, specifically its turnover time and the transformations controlling concentration and fate, just prior to release to surface waters. These research products represent fundamental constraints, which are critical to determining fluxes of CH4 from the subsurface and the role of STEs in the global carbon cycle.

Dr. Stephanie Wilson will build upon previous work conducted by multiple research groups along the east coast, to further scientific understanding of CH4 cycling and exchanges between the subsurface and the overlying water or atmosphere. The results of this study will be directly applicable to ongoing research and modeling efforts, providing fundamental information about CH4 to inform calculations of fluxes and emissions from the subsurface, with implications beyond the fields of groundwater hydrology and geochemistry.

Moreover, the project is designed to optimize the reach of its findings and develop human capital. Results will be disseminated at conferences, departmental seminars, and published in scientific journals. Dr.

Wilson will incorporate undergraduate students in both field and lab research efforts with the goal of creating an inclusive environment that promotes personal growth and scientific learning. Research findings will be made available to the public via several avenues designed to engage K-12 students, educators, the local community, and the larger public via a variety of engagement activities.

Subterranean estuaries (STEs) are important transition zones within the subsurface that host a variety of biogeochemical reactions. They are dynamic systems influenced by both watershed and tidal drivers. Within the STE, biogeochemical reactions control the speciation and concentration of subsurface nutrients and carbon, therefore, they determine the fate of these analytes when they are released to the overlying water and/or the atmosphere.

There is a growing body of literature reporting high concentrations of methane (CH4) in the subsurface along coastal margins; however, the mechanisms of transformation and transport of this pool remain unknown. There is, therefore, a limited understanding of how this subsurface CH4 pool exchanges with the overlying water and/or atmosphere. The STE may act as a passive interface whereby CH4 generated within the aquifer moves through the STE conservatively, or CH4 inventories may be modified via consumption or production within the STE on timescales faster than net transport.

The lack of context and defined constraints controlling the STE CH4 pool represents a knowledge gap essential to determining fluxes of CH4 from the subsurface and the role STEs play in the global carbon cycle. The project includes examination of the transformations and transport of CH4 in STEs using a three-pronged approach that combines in situ and ex situ stable isotope labeled tracer experiments with the characterization of subsurface geochemical gradients.

The work addresses rates and mechanisms. Tracer experiments will be conducted in several STEs along the east coast of the US parsed into the two dominant coastal STE types, wetland and sandy, and spanning a spectrum of hydrologic forcings. Results from this study will provide a novel understanding of how CH4 is cycled in the subsurface, specifically its turnover time and the transformations controlling concentration and fate, just prior to release to surface waters.

Specifically, this work will provide information regarding the CH4 pool rate of turnover, transformation, and transport. These products represent fundamental constraints, which are critical to determining fluxes of CH4 from the subsurface and the role of STEs in the global carbon cycle. This novel information will be directly applicable to ongoing research determining the role of transition zones in greenhouse gas dynamics and global climate change.

Research products will provide fundamental information, which will reach beyond the fields of groundwater hydrology and geochemistry. Results will provide fundamental constraints on modeling atmospheric fluxes of CH4 meditated by the STE and the project is designed to optimize the reach of its findings and develop human capital.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.