Collaborative Research: Revealing the Role of Less-Mobile Porosity in Hyporheic Denitrification and Greenhouse Gas Production?

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Streams and rivers have a remarkable cleansing function for natural and human generated contaminants, as microbes living in the streambed can transform these contaminants into less harmful compounds. Excess nitrogen in our terrestrial and aquatic ecosystems is now considered one of the greatest global-scale threats to humanity by degrading water quality and producing a powerful greenhouse gas. This research couples the cleansing function of rivers to this global excess nitrogen issue. Streambed bacteria can break down the reactive nitrogen compounds, primarily releasing non-reactive nitrogen gas that returns harmlessly to the atmosphere. However, a fraction is released as the strong greenhouse gas nitrous-oxide (N2O). Compelling data indicates pockets of longer-term water storage in streambeds, or microzones, create the low-oxygen conditions needed to both break-down dissolved nitrogen and form N2O. New remote sensing techniques of streambed microzones will allow us to better resolve the how nitrogen is attenuated and transformed through river transport, improving evaluations of watershed nutrient mitigation and helping better predict future climate change. Further, this research will dovetail with STEM education via community level partnerships with established outreach institutions. Outreach partners (Impression 5 Science Center and MSUSiFest) specialize in developing, executing and evaluating STEM exhibits and activities for children ages 4-12 and community "life-learners", both of which are key STEM demographics. Project PIs will connect with UConn undergraduate design teams and outreach partners to develop novel groundwater and streambed flow model exhibits and inquiry-based demonstrations designed to harness society's increasing fascination with real-time sensing and interaction. Outreach partners will use these products to illustrate principles of groundwater flow, contaminant transport, and greenhouse gas production, reaching 150,000+ students and community members each year.This project will link and quantify transient storage via dual-domain mass transport principles with the biogeochemical functions of stream sediments to reveal new insights on hyporheic denitrification and stream N2O production. This work is timely because recent global assessments reveal that rivers are major N2O producers, but the mechanism and spatial distribution of production remain unknown. Contrary to existing biogeochemical models for stream sediments, it is hypothesized that nitrate reduction to N2O occurs predominantly within streambed sediments that are oxic in a bulk sense but have local, anoxic less-mobile pore spaces. Largely overlooked in past work, these anoxic microsites must be mechanistically understood in order to upscale freshwater nitrogen dynamics from point, to reach, to basin scales. New observation methods and process-based models are needed to account for the role of anoxic microsites in fluid exchange and nitrogen biogeochemistry. Recently, project team members developed electrical geophysical methods for inference of less-mobile parameters, as the electric field can directly sense spatially variable solute dynamics in less-mobile porosity. Other team members have focused on developing labeled 15N tracer methods to reveal residence time controls on denitrification. These techniques will be combined to unlock the presence and function of anoxic microsites. The workplan comprises controlled laboratory experiments, numerical modeling, and field experiments at an established research site in the Ipswich Watershed, MA, USA. Our work will directly connect new process-based understanding to existing river network nitrate models, extending and capitalizing on previous NSF LINXII research. Specifically, the intrinsic properties of less-mobile pore space will be characterized, the existence of anoxic microsites and denitrification occurring in anaerobic microsites will be quantified, and multi-scale patterns of river nitrogen biogeochemistry will be enhanced. Overall, this work will transform the current understanding of hyporheic microsite processes, providing new mechanistic models of the role of hyporheic zones on watershed solute transport, nitrogen cycling and greenhouse gas production. The proposed research will address big questions about some very small places in our watersheds by quantifying hydrodynamic exchange with previously uncharacterized less-mobile hyporheic pore space.