Collaborative Research: Development of a Submersible, Autonomous Rn-222 Survey System

Intellectual Merit: Submarine groundwater discharge (SGD) is quickly gaining recognition as an important delivery mechanism of new and recycled nutrients to the coastal ocean. Chemical tracers such as 222Rn and radium isotopes offer excellent utility at detecting groundwater discharge zones and quantifying associated fluxes in nearshore (shallow) waters, but the traditional approaches to sampling and measuring these tracers become progressively less useful as the water column deepens, stratification strengthens, and physical mixing becomes more complex. In deeper waters (1) of the continental shelf where outcropping geological units can focus SGD, and (2) around critical habitats like coral reef ecosystems, one’s ability to measure these tracers is limited to grab sampling-scale resolution. Such resolution is generally insufficient to understand the pathways, driving forces, and rates of these discharges, nor is it conducive to quantifying associated nutrient delivery fluxes. Prior to assessing the global significance of SGD, then, there exists great need for a tool capable of in situ, continuous measurement of geochemical tracers of SGD in deeper waters of the continental shelf.

This proposal outlines a plan to develop a submersible system capable of in situ 222Rn analysis while deployed from a remotely-operated vehicle (ROV) or autonomous underwater vehicle (AUV). Such a system would allow researchers to conduct high-resolution radon surveying through 3-D grids of bottom water and later return to sites of interest to measure a 222Rn time-series in order to quantify SGD fluxes. The system design relies on a new technique to sparge radon, while submerged, from the water for analysis via bubbling a closed air loop through a contained water column. Preliminary evidence shows this to be a viable approach. Remaining design components relate to relatively routine engineering challenges of configuring electronic and structural systems to function properly at pressures up to 2000 m of water depth.
This work will also design and construct a discrete sample collection system that will allow for grab samples to validate the continuous radon measurements and also provide additional geochemical evidence of SGD. Such samples will allow subsequent analysis of pH, alkalinity, nutrients, major and trace metals, dissolved organic carbon, and radium isotopes. The water sampling system is based on a simple modular design that builds upon concepts developed through proven sampling systems and several recent sampling system development projects. Critical design components focus on maintaining sample cleanliness and integrity.

Following design, construction, and bench testing of the radon system and the discrete sample collection system, field tests and calibrations will be conducted from pier deployments, rosette deployments from a ship of opportunity, and full scale testing via an AUV in the coastal waters of Cape Cod, MA, and during a trial survey of Puerto Rican coral reefs.

Broader Impacts: One significant impact of this project will be the development of a new research tool available for scientists from around the globe to implement in field campaigns to better understand SGD cycling in areas of mid- to deep-water of the continental shelf (e.g., coral reefs, karst/carbonate platforms, and relict river channels) and associated nutrient and trace metal fluxes. This system will become part of WHOI’s National Deep Submergence Facility, making it widely available to the scientific community. Eventually, through lessons learned from this project and future work to extend the depth rating of the system, a similar prototype could follow to enhance the study of mid-ocean ridge hydrothermal circulation (especially flank flows), and associated heat and trace metal flows.

This project also offers the opportunity to involve students directly in the science of SGD biogeochemical influences as well as instrument design and development. The WHOI engineering team will sponsor a two-semester, senior Design Clinic team of 3-4 undergraduate female engineering students from Smith College’s Picker Engineering Program to be equally beneficial to both students and mentors for engineering collaboration. The WHOI PIs will also mentor a MIT/WHOI Joint Program Ph.D. student, while an undergraduate student will be mentored at Coastal Carolina University. These students will gain valuable experience in instrument design and construction, prototype testing, laboratory analysis, and field work, while presenting their work in international journals and peer-reviewed publications.