The Science

Berkeley Lab geochemists and hydrologists who study a mountainous watershed near Rifle, Colorado, discovered that spring snowmelt is essential to the transport of freshly dissolved organic matter (DOM) from the top soil to the part of the Earth’s subsurface that lies above the groundwater table. Because DOM undergoes biological humification over the year, these processes involving this deep vadose zone suggest an annual cycle of DOM degradation and transport at this semiarid floodplain site.

The Impact

Characterizing the dynamics of dissolved organic matter in semiarid regions of Earth’s subsurface is challenging. The researchers obtained insights into transport and humification processes of DOM using several spectroscopic techniques on depth- and temporally distributed pore-waters. This methodology can be applied to other subsurface environments for understanding DOM responses and feedbacks to Earth system processes.

Summary

Scientists studying DOM in surface waters considered it to be the mobile fraction of natural organic matter that falls into or is washed into water bodies. Although it has been extensively studied over many decades, relatively little is known about the dynamics of DOM in the subsurface of semi-arid environments. To understand transport and humification processes of DOM within a semi-arid floodplain at Rifle, Colorado, the researchers applied fluorescence excitation-emission matrix (EEM) spectroscopy, humification index (HIX), and specific ultraviolet (UV) absorbance (SUVA) for characterizing depth and seasonal variations of DOM composition. They found that late spring snowmelt leached relatively fresh DOM from plant residue and soil organic matter down into the deeper vadose zone (VZ). More humified DOM is preferentially adsorbed by upper VZ sediments, while non- or less-humified DOM was transported into the deeper VZ. Interestingly, DOM at all depths undergoes rapid biological humification processes as evidenced by the products of microbial byproduct-like matter in late spring and early summer, particularly in the deeper VZ, resulting in more humified DOM at the end of year. The finding indicates that DOM transport is dominated by spring snowmelt, and DOM humification is controlled by microbial degradation. It is expected that these relatively simple spectroscopic measurements (e.g., EEM spectroscopy, HIX, and SUVA) applied to depth- and temporally distributed pore-water samples can provide useful insights into transport and humification of DOM in other subsurface environments as well.

Principal Investigator

Susan Hubbard
Lawrence Berkeley National Laboratory
sshubbard@lbl.gov

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

This work was supported by the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.

References