Seasonal Influences on the Molecular and Metal Binding Properties of Groundwater & Surface Water Dissolved Organic Matter (DOM) in an Alpine Watershed Located in Central Colorado

Abstract

The nature and behavior of surface water and groundwater chemistry in alpine watersheds is influenced by snowmelt-driven hydrology. This study investigated the effects of seasonal variability in groundwater and surface water dissolved organic matter (DOM) on trace metal binding behavior. Samples were collected from surface water and groundwater in an alpine watershed located in Central Colorado during the spring-snowmelt, summer, and fall seasons for general water chemistry and DOM characterization. We used the isolated fulvic acid (FA) component of DOM for spectroscopic characterization and Cu-DOM binding experiments to relate seasonal variability associated with groundwater DOM to its presence in surface water. Optical spectroscopy results of FA isolated from surface water had specific ultraviolet absorbance (SUVA254) values that ranged from 4.00 to 4.71 L mg-1 m-1 during spring snowmelt and from 1.76 to 2.41 L mg-1 m-1 during baseflow, and the fluorescence index (FI) ranged from 1.35 to 1.38 during spring snowmelt and from 1.48 to 1.60 during baseflow. Groundwater SUVA254 values ranged from 0.27 to 0.62 L mg-1 m-1 during baseflow and from 0.27 to 0.96 L mg-1 m-1 during spring snowmelt, while FI ranged from 1.77 to 2.73 during spring snowmelt and from 1.82 to 2.27 during baseflow in 2020. The range of spectroscopic indices suggests that DOM sources and associated molecular compositions in streams change from aromatic and allochthonous during spring/summer to lower aromaticity and autochthonous in fall/winter. DOM-Cu binding measurements using a cupric ion-selective electrode (ISE) showed that low-SUVA254 values (0.62 to 2.05 L mg-1 m-1) correspond to greater free copper ions in solution (17% Cu2+) in comparison to higher SUVA254 (2.18 to 4.71 L mg-1 m-1) with a smaller proportion of free copper ions (ex. 3%) in surface water samples. The relationship between SUVA254, Cu2+, and seasonality indicates that DOM in streams during baseflow conditions exhibits similar groundwater DOM characteristics. Our results indicate that seasonal variability exists in streams and is not as pronounced in groundwater. An improved understanding of variable DOM composition in streams related to snowmelt and baseflow conditions are important for the continued improvement of DOM parameterization in geochemical and toxicological models.