Overview

At a time when demand for water is dramatically increasing because of population growth, industrialization, and expansion of irrigated agriculture, freshwater resources supplied by headwater streams and their surrounding watersheds are being threatened by severe pollution from anthropogenic releases of nutrients and trace metals such as mercury (Hg). More than 9,000 waterbodies in the continental United States are impaired by Hg. Mercury is the second leading cause of impaired waters—including locations in the Tennessee River Basin—and is responsible for fish consumption advisories in all 50 states (U.S. EPA 2011, 2013).

The economic and societal importance of headwater streams and their surrounding watersheds is exemplified by the Tennessee River Basin (see Fig. 1). Located in the southeastern United States, this river basin consists of a series of nested watersheds that supports ~4.5 million people by supplying water for power generation, industry, recreation, agriculture, and human consumption (Bohac and Bowen 2012) and represents the most intensively used freshwater Water Resource Region (WRR) in the contiguous United States, with estimated withdrawals of >280,000 gallons a day per square mile. Preserving these freshwater resources for future use requires developing a deeper understanding of the structure and function of watersheds and the processes that govern pollutant transformations in aquatic ecosystems.

Research findings during Phase I of the Critical Interfaces Scientific Focus Area (SFA) project have led to the realization that transient storage zones (TSZs), and more specifically, metabolically active TSZs (MATSZs), are investigation. While TSZs are surface and subsurface locations (e.g., hyporheic zone) that delay the downstream flow of water in comparison to the main channel, MATSZs are microbially active TSZs, such as the interstitial spaces of hyporheic zone streambed sediments and the pore space present in the microbiome of instream biofilm. MATSZs exhibit very different biogeochemical environments (e.g., redox conditions) compared with flowing stream or streambed, making them important “hot spots” that account for a substantial proportion of the diverse and intensified biogeochemical activity in watersheds.

The SFA project is progressively advancing our understanding of the factors that influence watershed structure and function using Hg and the East Fork Poplar Creek (EFPC) watershed as representative use cases. The EFPC watershed offers a unique niche to the Environmental System Science (ESS) program by being nested in the most intensively used freshwater WRR and serving as a representative low-order freshwater stream system, which is a segment of stream orders with relevance to the largest proportion of the total stream length in United States. Led by Oak Ridge National Laboratory (ORNL), this project is supported by the ESS program of the Office of Biological and Environmental Research (BER) within the Department of Energy’s (DOE) Office of Science. In FY21, the Critical Interfaces SFA team (1) added new capabilities to the Advanced Terrestrial Simulator (ATS) modeling software by extending the Advective Dispersion Equation with Lagrangian Subgrid (ADELS) model from conservative tracers to multicomponent reactive transport, (2) refined our Transient Availability Model (TAM) to include Monod-type kinetics for methylation and demethylation, (3) examined how nutrient amendments (nitrate and/or phosphate) altered periphyton community structure and function, (4) continued to explore the transcriptional regulation of hgcA under different conditions, and (5) used density functional theory (DFT) calculations to determine the role of metal sulfide surfaces in dimethylmercury (DMeHg) formation. We advanced our understanding of ligand exchange reactions controlling Hg (II) uptake and methylmercury (MeHg) production by determining the solution-phase configurations and metal interaction of methanobactin from Methylocystis sp. strain SB2 with Hg and other group 12 metals. Our results demonstrate that a linear model is consistent for Hg(II) coordination versus a tetrahedral model for zinc (II) and cadmium (II). Collectively, the aforementioned activities are providing a deeper understanding of Hg transformations in EFPC and allowing us to gain the process knowledge needed to improve predictions of Hg transformations at the scale of individual stream reaches and small watershed catchments.

Integrated, Multiscale Research Approach

Critical Interfaces SFA research encompasses three themes—ecosystem processes, microbial community processes, and biogeochemical processes—and a research activity involving field-scale model integration.

Ecosystem Processes. Through a combination of field- and laboratory-scale studies, research investigates Hg biogeochemical transformations in hyporheic zone sediments and the influence of nutrient additions on net MeHg production and microbial community composition in field-derived periphyton biofilms.

Microbial Community Processes. Research seeks to (1) understand the contributions of known Hg-methylating organisms to observe Hg methylation rates and extents in biofilm lifestyles, using synthetic and natural microbial communities; (2) determine the breadth and depth of Hg-methylating species; and (3) determine the biochemical roles of the proteins (HgcA and HgcB) that facilitate MeHg production.

Biogeochemical Processes. Research elucidates key biogeochemical mechanisms controlling Hg bioavailability and microbial transformation of inorganic Hg to MeHg in simplified, but field-relevant, laboratory experiments. Activities include (1) investigating complex biogeochemical processes and their interactions controlling Hg species transformation and availability for cellular uptake and methylation and (2) using molecular-scale computational approaches to elucidate key biogeochemical mechanisms governing Hg speciation and microbial transformations.

Field-Scale Model Integration. Improves stream reach-to-watershed reactive transport modeling of contaminant and nutrient export. Activities include estimating the volume of TSZs and MATSZs in EFPC and the mass transfer between TSZs and the creek channel, using nonreactive and reactive tracers to parameterize the field-scale model.

References

• Bohac, C. E., and A. K. Bowen. 2012. Water use in the Tennessee Valley for 2010 and projected use in 2035. Tennessee Valley Authority. Chattanooga, Tenn.
  
• U.S. EPA. 2013. 2011 National Listing of Fish Advisories (Technical Fact Sheet). U.S. Environmental Protection Agency.
• U.S. EPA. 2011. 2010 Biennial National Listing of Fish Advisories (Technical Fact Sheet). U.S. Environmental Protection Agency.