Monitoring and predicting subsurface biogeochemical hot spots
Abstract
The subsurface zone where river water and groundwater mix (the hyporheic zone) controls hydrologic and biogeochemical fluxes across spatial and temporal scales and has important impacts on water quality. As water and nutrients are exchanged through microbially-active sediments, discrete biogeochemical hot spots (i.e., zones of enhanced reaction rates) often develop throughout the subsurface where conditions favor the reactions, which in turn alter the geochemistry. A number of physical, chemical, and biological processes affect the formation of hot spots, but a fundamental understanding of their development and persistence are hindered by the inability to monitor changes across spatial and temporal scales. We recently installed an innovative, 3D network of redox electrodes at the Theis Environmental Monitoring and Modeling Site (TEMMS), managed by the PI, to investigate the floodplain sediment geochemical response to hydrologic perturbations (e.g., water table fluctuations resulting in a dynamic saturated and unsaturated zones (i.e., a dynamic multiphase system)). We have successfully captured the dynamics of redox conditions during both base flow and storm events, and have demonstrated that redox potential fluctuates most at the transition between low- and high-conductivity sediments. This is directly relevant to the goals of the Environmental Chemistry program within Army Research Office (ARO), which seeks to provide an understanding of where geochemical processes occur in multiphase environments. The objective of this proposed effort is to further understand reaction dynamics and the underlying processes that drive redox potential fluctuations. We propose to generate fundamental insights into the relationships between subsurface redox states, reaction rates, and dissolved organic matter (DOM) chemistry. We propose to quantitatively characterize the relationship between redox measurements and the reaction rate and type of subsurface microbial activity. Our overall objective is to generate fundamental insights into the connections between redox-based inferences of hot spots, DOM chemistry, and microbial respiration. We will pair in-situ redox sensing with spatially-distributed quantification of environmental mRNA from endemic microbial communities, and mesocosm scale measurements of microbial reaction rates (aerobic and anaerobic), to determine their cross-correlation between redox potential and respiration. We investigate both aerobic and anaerobic (e.g., nitrate, iron oxyhydroxides) respirations in addition to quantifying kinetic parameters that will be critical to modeling development of biogeochemical hotspots. In addition, we will integrate redox-informed, ultrahigh-resolution molecular DOM characterization to provide a more robust description of the underlying organics as drivers of microbial activity that result in distinct subsurface redox zonation. In this way, we will demonstrate the potential for combining redox monitoring, groundwater chemistry, and DOM characterization to provide sufficient information to track and ultimately predict the occurrence and magnitude of biogeochemical hotspots in multiphase subsurface systems. The project will benefit human resources and education in science, engineering and mathematics (STEM fields) through training of a PhD student and several undergraduate students. The students will gain expertise in hydrogeology, sedimentology, geochemistry, microbiology, as well as numerical modeling. The project will create the opportunity for mentoring by university faculty and national-laboratory scientists.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 27, 2023
- Source ID
- W911NF2310199
Entities
People
- Mohamad Reza Soltanian
Organizations
- Army Contracting Command
- United States Army
- University of Cincinnati