Ultraviolet-Visible and Fluorescence Analyses Reveal the Spatial and Seasonal Variability of Dissolved Organic Matter through the Vadose Zone to Groundwater at the Rifle, Colorado River Floodplain Site

Abstract:

Dissolved organic matter (DOM) is a complex and poorly understood mixture of natural organic compounds that play important roles in terrestrial C transport and biogeochemical cycles, and its reactivity makes it sensitive to seasonal variations and longer term climate change. As a component within the LBNL Science Focus Area 2.0, this study is designed to determine the spatial and temporal variability of DOM concentrations and characteristics throughout the vadose zone and groundwater within a semi-arid floodplain at Rifle, Colorado. Three sets of vertically stratified pore water samplers and wells were installed along a groundwater flow transect. These installations allowed acquisition of vertically- and temporally-resolved pore water samples from the vadose zone, capillary fringe, and saturated zone from April 2013 to May 2014. Ultraviolet-visible absorbance (UVA) and fluorescence excitation-emission matrix (EEM) spectroscopy are being applied to trace the changes in DOM characteristics. Initial results indicate that the aromatic C contents (%) of DOM vary with depth and season and exhibit patterns distinct from groundwater. EEM analysis identified fulvic- and humic-like substances as the major fluorescent components of DOM in pore water samples. The concentrations of fulvic- and humic-like matter decreases with depth within the vadose zone, and increases from Spring and Summer to Fall, then decreases in Winter. The trend is consistent with UVA results. Microbial by-product-like components in DOM show higher concentrations in the vadose zone, and decrease from Spring to Winter. Fulvic- and humic-like substances are the only detectable fluorophore components in the groundwater samples. The results from both UVA and EEM suggest that (1) aromatic C or fulvic- and humic-like matter are preferentially adsorbed within shallower sediments during transport; and (2) microbial transformations of DOM composition may occur in the vadose zone, particularly during late Spring and early Summer.