Yale Research Team Uses Unique Natural Laboratory, a Watershed, to Study Enhanced Rock Weathering

William Miller-Brown (left) and Quinn Zacharias (right) soil sampling in the upper hayfields of the watershed.

Forty miles south of the Canadian border, hidden in the rolling countryside of northeastern Vermont, lies one of the most well-studied river basins in the United States: the Sleepers River Research Watershed (SRRW). This 111-square-kilometer site has served as a natural laboratory for environmental scientists for over six decades and has hosted seminal studies on snowmelt modeling and streamflow generation as well as long-term work on biogeochemical cycling, isotope hydrology and the impacts of climate change on watershed processes.

Studies were initiated at the site in 1959 by the Agricultural Research Service (ARS) of the US Department of Agriculture, and cooperative research efforts between ARS and the National Weather Service Office of Hydrology began in 1966. In 1979, the US Army Cold Regions Research and Engineering Laboratory (CRREL) began studies at the site, and they were later joined by the US Geological Survey (USGS). In 1990, the USGS took the lead role in managing research operations at the site, where they maintained long-term monitoring datasets that supported numerous studies on water quality, snowmelt processes, and groundwater-surface water interactions. SRRW is now jointly operated by the USGS and CRREL in collaboration with several other Federal Agencies and Universities.

What sets the SRRW apart as a natural laboratory is its characteristic combination of topography, soil types, and mixed land use that includes farmland, northern hardwood forest, and riparian areas. This blend makes it representative of glacially formed upland landscapes, allowing researchers to extrapolate their findings to a wide range of comparable environments. This means that scientific insights from SRRW can illuminate key environmental processes and cycles throughout the northeastern US, as well as other geographically comparable regions, such as southern Quebec and parts of northern and central Europe. 

The unparalleled thoroughness of the long-term hydrologic and biogeochemical datasets from SRRW, coupled with the site’s uniquely representative geographic setting, make it an ideal place for research on how natural biogeochemical processes can be harnessed to remove carbon dioxide from the atmosphere. As discussed below, this type of urgently needed research is now ongoing at SRRW and promises to provide groundbreaking insights into the watershed dynamics of nature-based carbon dioxide removal. 

Enhanced Rock Weathering Field Experiments

Recently, Professor James Saiers at Yale University’s School of the Environment, initiated a project at SRRW focusing on how the weathering of basaltic rock dust can consume CO₂ and how this process influences the biogeochemistry of the watershed. Prof. Saiers and a team of students and staff researchers led by PhD candidate Quinn Zacharias began hydrological, geochemical, and biological measurements in 2022. In June 2023, they applied 400 tonnes of crushed basalt on SRRW pasture and hayfield fields at a rate of 20 tonnes per hectare. This project was part of an ongoing larger initiative investigating the process of enhanced rock weathering (ERW) that is led by scientists from the Yale Center for Natural Carbon Capture (YCNCC), including Professors Noah Planavsky and Peter Raymond. Other partners include the USGS, Vermont State University System, and the University of Maine. For the past few years, the YCNCC has been exploring multiple aspects of ERW through laboratory work, field studies, and modeling to understand the carbon removal potential, co-benefits, and risks of this strategy.

But what is ERW, and how does it work? It turns out that the natural chemical weathering of rocks can convert carbon dioxide into ecologically benign forms of carbon called carbonates. This process starts when CO₂ from the atmosphere dissolves in rainwater, making it slightly acidic. When this weakly acidic water reaches the soil, it combines with soil biological agents such as organic acids and microorganisms to weather rock particles, thus releasing their mineral constituents such as silica, aluminum, calcium, magnesium, iron, sodium, potassium and a host of trace elements such as zinc, molybdenum, and manganese. This chemical weathering process converts the dissolved CO₂ into carbonate molecules (mainly bicarbonate), which can remain stored in geologic or downstream oceanic environments for thousands to millions of years. The weathering rock also acts as a slow-release inorganic fertilizer that enriches soils in major, minor, and trace nutrient elements. To increase the rate of weathering, researchers grind the rock material into a fine powder to enhance or accelerate the mineral dissolution reactions in the soil, hence the name “enhanced rock weathering”. It is now recognized as one of the most promising and scalable carbon dioxide removal methods under investigation.

Early findings

Initial results from the Yale SRRW project were presented at the American Geophysical Union Fall Meeting in December 2024. The American Geophysical Union (AGU) is the world’s largest international Earth science meeting, bringing together tens of thousands of scientists, educators, and students. At the meeting, Dr. Fengchao Sun, an Associate Research Scientist at Yale’s Center for Natural Carbon Capture, delivered a presentation on how the basalt application at the SRRW impacted stream water chemistry. Sun found that a few months after adding the basalt to pasture and hayfield soils, the alkalinity of local stream waters (i.e., their capacity to resist acidification) increased significantly. This increase in alkalinity is likely caused by a flux of bicarbonate produced during basalt weathering. This is supported by noted shifts in elemental ratios that are also indicative of basalt weathering. Sun also noted increased stream water concentrations of trace elements such as arsenic and manganese. The source of these elements remains uncertain and is still under investigation. They may have been released from the basalt itself or may have spread from the surrounding soils due to chemical changes caused by the presence of basalt. This observation underscores the need to monitor water quality impacts carefully following large-scale ERW applications.  

Yale PhD student Wyatt Tatge and Postdoctoral Associate Samuel Shaheen both presented on the development of dynamic models capable of simulating the response of the watershed to the ERW basalt application. The models integrate hydrological and geochemical processes to simulate watershed dynamics, including mineral weathering rates, the residence time of weathering products, and net CO₂ removal. Rob Rioux, another Yale PhD student, presented an initial evaluation of the basalt weathering effects on stream chemistry, with particular emphasis on how weathering products such as calcium and magnesium are transported into streams and the relationship between stream discharge and weathering product concentrations. 

Another key study was presented by Zacharias, who reported on how the basalt addition impacted soil fertility. For this study, Zacharias and colleagues established 64 soil sampling locations across eight transects through the watershed to monitor changes in soil chemistry and vegetation following the ERW application. The transects were aligned parallel to the local hillslope gradient, crossing both pasture and hayfield areas. Four of these transects were in areas treated with basalt, while the remaining four were left untreated to serve as controls. Additionally, a riparian zone at the foot of the transects was monitored to provide insights into how ERW might influence soil fertility from the application site down to the stream banks. In addition to investigating CO₂ removal, this ongoing study aims to provide evidence for the potential agronomic co-benefits of basaltic rock dust applications.

More recent results

In January 2025, Zacharias presented results from his ERW soil-fertility study at the University of Florida’s Department of Geological Sciences seminar series. In this talk, Zacharias gave several key updates on the Yale Sleepers River Watershed ERW study. One of the main observations was that the most consistent effect of the 2023 basalt application was an increase in soil pH. The increase ranged from around 0.1 to 0.3 pH units, with average starting values ranging from around 6.0 to 6.1 in pasture fields and 6.2 to 6.5 in the hayfields. This relatively small but statistically significant increase in soil pH suggests that basalt weathering positively affects soil fertility by centering the soil pH value in the optimal zone for nutrient availability and beneficial microbial communities. 

Other findings noted by Zacharias include the observation that basalt application led to a statistically significant increase in soil calcium, a key plant macronutrient. The results for magnesium were more complex, suggesting a dynamic nutrient transport process that needs further investigation. It was also observed that seasonal and land use effects must be considered when assessing the soil fertility benefits of rock dust applications. For example, Zacharias noted a natural decrease in the baseline soil pH from fall to spring, likely due to soil leaching by snowmelt. He also noted that the pastures showed more heterogeneity in nutrient cycling compared to hayfields, possibly due to the presence of cows and uneven distribution of manure. Topography was also shown to play an important role in nutrient distribution as calcium appears to be leached from mid-slope positions along a transect and pools at the base of the transect slope. The soils’ starting conditions (i.e., pre-application) were also shown to influence the impact of the basalt amendment. For example, it was observed that soils with lower initial pH showed a larger increase in pH after basalt application relative to soils with higher baseline pH.

Zacharias’ ultimate conclusions of his work thus far are that: 

• The basalt amendment produced detectable effects on soil pH, calcium, and magnesium at agronomic application rates (20 tonnes per hectare) in temperate soils, although these effects were relatively small.
• To accurately detect these effects, it is crucial to account for initial conditions, seasonal variations, land use, and slope position.
• A moderate increase in soil pH was the most consistent effect across the agricultural watershed.
• The findings suggest that this amendment may be more effective as a liming agent in temperate soils—whereas, in tropical soils, it might serve both liming and fertilizing functions.

Zacharias and his colleagues have also identified several areas for future investigation, such as: 

• Expanding the analysis to focus on metal cycling and plant metal uptake within the watershed, considering the potential release of metals from the basalt and the counteracting effect of increased pH.
• Examining changes in soil biogeochemistry within the riparian zone, which may be acting as a buffer for agricultural runoff.
• Analyzing multiple years of data to understand the longer-term effects of the basalt application.
• Investigating the potential formation of secondary minerals and their impact on long-term carbon sequestration.

Conclusion

The collective findings of the Yale studies at Sleepers River Research Watershed provide crucial data on the effectiveness and potential impacts of enhanced rock weathering as a carbon dioxide removal strategy. The research indicates that basalt application can indeed accelerate carbon capture through increased weathering rates, as evidenced by changes in stream and soil chemistry. However, the studies also highlight the complexity of watershed responses to such interventions, including potential changes in trace element concentrations and soil fertility. The research also highlights the importance of considering multiple factors such as slope position, land use, and seasonal variations when evaluating the effectiveness of such treatments. As the world seeks sustainable solutions for carbon sequestration and soil improvement, studies like this contribute crucial data to inform future large-scale applications.

It is ultimately fitting that this study is taking place at the Sleepers River Research Watershed, which provides the long-term datasets, historical context, and representative biogeochemical setting needed to produce true scientific breakthroughs in scalable ERW methodologies. The site allows research teams such as Professor Saiers’ Yale group to quantify subtle biogeochemical signals and watershed dynamics, leading to fundamental environmental insights. The Yale study, therefore, highlights the continued importance of SRRW as a world-class natural laboratory. As we face an uncertain climate future, the lessons from Sleepers River offer both caution and hope. The story of SRRW is a testament to the power of patient, methodical science in unlocking nature’s secrets and charting a course through turbulent times. 

James Jerden is an environmental scientist and science writer focused on researching and promoting sustainable solutions to urgent environmental problems. He holds a Ph.D. in geochemistry from Virginia Tech and a Master’s degree in geology from Boston College. Over the past 20 years, James has worked as a research geochemist and science educator. He joined Remineralize the Earth because of their effective advocacy, research, and partnership projects that support sustainable solutions to urgent environmental issues such as soil degradation (food security), water pollution from chemical fertilizers (water security), deforestation, and climate change. As a science writer for RTE, his goal is to bring the science and promise of soil remineralization to a broad, non-technical audience. When not writing, he can be found at his drum set.