Soil-derived dissolved organic matter cycling at terrestrial-aquatic interfaces: Implications for wetland-dominated landscapes, stormwater control measures, and drinking water supply

Abstract

Terrestrial-aquatic interfaces (TAIs) are transitional zones between the terrestrial landscape and aquatic ecosystems (e.g., soil-water, floodplain-river, upland-wetland). Water movement across TAIs, known as hydrologic connectivity, mediates the transport and transformation of biogeochemically significant substrates, such as carbon and nutrients. Dissolved organic matter (DOM) is a soluble and reactive form of carbon comprised of organic molecules derived from allochthonous (e.g., soils, plants) and autochthonous (e.g., algae, microbes) sources. Soils are source of DOM found at TAIs. However, hydrologic connectivity of soils located at TAIs can be spatiotemporally variable. This dissertation seeks to quantify how variable hydrologic connectivity influences the transport and transformation of soil-derived DOM across TAIs. Using a combination of laboratory and field based methods, soil-derived DOM was characterized in three different soil ecosystems (1) urban stormwater control measures, (2) wetland-dominated landscapes, and (3) forested drinking supply watersheds. First, anthropogenic stressors in urban landscapes are known to alter DOM cycling but few studies have explored DOM cycling from stormwater control measures (SCM) soils. I leached water-soluble organic matter (WSOM), a proxy for soil-derived DOM, from the soils of SCMs to explore how SCMs design influences DOM cycling in urban settings. There were low quantities of WSOM, regardless of SCM type. The composition of WSOM was similar to other WSOM studies in natural soil ecosystems. The composition of WSOM was more microbial-like than SCM surface water, highlighting how the route of water movement through an SCM (e.g., runoff retained in an SCM as surface water versus runoff filtering through engineered soils) influences the composition of DOM exported to downstream aquatic ecosystems when SCMs are hydrologically connected during storm events. Second, wetlands in low-relief landscapes have dynamic TAIs, but few studies have quantified DOM release as a result of seasonal groundwater saturation and rapid water level changes during precipitation events. I simulated vertical groundwater saturation on intact wetland soil cores over a 40-day laboratory experiment and analyzed the concentration and composition of DOM in soil porewater. Porewater DOM concentrations were sustained over the 40-days, supporting the hypothesis that wetland soils can act as quasi-infinite sources of DOM. As experimental saturation progressed, DOM composition shifted towards more aromatic, plant-like organic matter signatures. I then performed in-situ sampling of porewater and wetland surface water at two wetlands during an early winter rain event. As wetland water levels rose and the soil-water interface expanded outwards, surface water DOM tended to be more dynamic while porewater DOM concentrations were stable. A simple water and DOM mass balance suggests that groundwater inputs sustained wetland surface water DOM during the rain event. Finally, high levels of DOM can lead to the formation of disinfection byproducts (DBP) during drinking water treatment which pose a threat to human health. Further work is needed to quantify and predict the DBP formation potential of soil-derived DOM to inform watershed management and protect drinking water quality. I leached Water-Extractable Organic Matter (WEOM), similar to WSOM, from soils collected in drinking supply watersheds. Chlorinating WEOM samples demonstrated that soil-derived DOM has the potential to exceed DBP regulatory limits. WEOM composition and watershed characteristics were explanatory variables of WEOM DBP formation potential. Together, these findings further our understanding of how variable hydrologic connectivity influences soil-derived DOM cycling at TAIs in both natural and engineered soil systems with implications for carbon cycling and water quality.