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.