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Rifle–A Community Site of Discovery and Accomplishment

Source:  Ken Williams, Susan Hubbard and Dan Hawkes

When it comes to what’s underground (or as earth scientists would describe it, the subsurface), there’s a lot we don’t know. But there are places on earth (and indeed, within the U.S.) that have provided a foundation for expanding our limited knowledge.

Near the town of Rifle, Colorado, lies the primary field site for Phase I of the Subsurface Systems Scientific Focus Area 2.0 (the SFA 2.0, sponsored by DOE’s Office of Biological and Environmental Research—BER. The site’s history as a milling facility for ores rich in uranium and other redox sensitive metals (such as vanadium, selenium, and arsenic) has resulted in low but persistent levels of contamination within subsurface sediments and groundwater that are now under the purview of DOE Legacy Management. BER has conducted a number of major investigations at the Rifle site, including the Rifle Integrated Field Research Challenge (IFRC)—initiated in 2007 to facilitate integrated, field-based subsurface biogeochemical research relevant to uranium mobility—and the SFA 1.0 project, initiated in 2008 to improve the predictive understanding of subsurface flow and transport relevant to metal and radionuclide contaminants.


The Rifle IFRC has placed a premium on developing the site as a community biogeochemical field study site (the “Rifle Community Site”). Its ease of access, location within the Intermountain West, developed infrastructure and safety plans, shallow aquifer and well-constrained hydrology, and juxtaposition to the Colorado River have attracted a global network of scientists to work at the Site or with Site samples. Since 2007, the Rifle Community Site has hosted 18 DOE-BER university-led projects, along with contributions and involvement of four other National Laboratory SFA programs and their international collaborators.

The Rifle Community Site has also served as a “technology incubator” for DOE’s Small Business Innovative Research (SBIR) program, deploying novel approaches for quantifying subsurface processes and data-sharing to validate their performance. Four Phase I and II SBIR projects have been based at Rifle or have utilized data generated from the site since 2007. Research at Rifle has also placed a premium on educating the next generation of ecosystem scientists, with 23 Ph.D. and M.S. degrees awarded (to date) utilizing data derived from the Rifle Community Site; of these, 18 represented the work of female and/or underrepresented minority students. Monthly “friends of Rifle” teleconferences, led by Phil Long (LBNL), bring together university and private sector PIs, DOE program managers, and graduate and postdoctoral students to discuss scientific findings associated with the array of scientific studies under way at the Rifle Site.

Recently, DOE-BER recognized the need for its Subsurface Biogeochemistry Research (SBR) program to expand its focus on biogeochemical cycles beyond contaminant mobility. In response, the SFA 2.0 was established, which focuses on developing approaches and simulation capabilities for quantifying how land use and climate change affect subsurface carbon inputs, flowpaths, subsurface metabolic potential, and ultimately the biogeochemical functioning of a watershed. Not only does the Rifle floodplain provide a wealth of data to launch this new study, it provides an excellent testbed to interrogate how global change affects biogeochemical system functioning. As a semi-arid region within the Upper Colorado River Basin, the Rifle area is threatened by various manifestations of climate change (including drought, diminished snowpack and earlier snowmelt, wildfires, and pest outbreaks), all of which have largely unexplored impacts on ecosystem services provided by the Colorado River corridor and its floodplain.  

As part of the SFA 2.0 at Rifle, insights gained from the legacy of field experimentation there pertaining to organic carbon amendment for contaminant remediation will inform our understanding of natural biogeochemical pathways that mediate elemental cycling tied to modification of natural carbon stocks. Within the aquifer, such pathways are largely seasonal, correlating with excursions in groundwater elevation of 1-2 m associated with elevated discharge in the Colorado River during spring/summer snowmelt. Imbibition of oxygen bubbles within the capillary fringe associated with such excursions, and infiltration of oxygenated snowmelt at the site, are inferred to be the primary contributors to seasonally oxic groundwater. This brief (~3-6 week) period of elevated dissolved oxygen occurs within an otherwise suboxic-to-anoxic environment (i.e., low or no measurable dissolved oxygen) and enables metabolic pathways—such as enhanced rates of organic carbon degradation, conversion of ammonium to nitrate, and oxidation of reduced iron and sulfur minerals—that are otherwise inhibited due to limiting oxygen concentrations.

Climate-induced changes in hydrology that increase the spatial extent and duration of elevated dissolved oxygen in the Rifle aquifer (and others like it) thus have the ability to dramatically impact rates of carbon mineralization, greenhouse gas flux (e.g., nitrous oxide accompanying nitrification and denitrification), and export of metabolic end products to inland waterways. These insights will be essential in constructing and developing an SFA 2.0 genome-enabled biogeochemical watershed simulation capability (GEWaSC)—a predictive framework for understanding how genomic information stored in the subsurface microbiome affects biogeochemical watershed functioning, how watershed-scale processes affect microbial functioning, and the co-evolution of these interactions. Recent SFA 2.0 research at the site is already leading to the development of unprecedented insights into the “Tree of Life” through genome-enabled assessment of subsurface metabolic functionality, including the discovery of 10 candidate phyla about which virtually nothing was heretofore known.

SFA 2.0 research and Rifle Community Site management and infrastructure will continue to foster broad, international  scientific involvement by both national laboratory and university investigators at the Site. It is expected that the Site will play an important role as a staging area, where SFA scientists and “friends of Rifle” can develop critical understanding and capabilities, with a new focus on predicting metabolic and geochemical responses to climate-induced environmental perturbations.