Dissolved black carbon (DBC) is one of the major components of dissolved organic carbon in water bodies and plays a crucial role in the oceanic carbon cycle. Photodegradation is considered one of the main sinks of DBC in natural water bodies. However, to estimate the total amount of DBC removed from the natural environment due to photodegradation, the critical data of its apparent quantum yield (AQY) is needed. To address this data gap, this study conducted sunlight simulation experiments in the laboratory using both fresh river (Ogeechee River) and coastal seawater (from the southeastern Atlantic Bight) region. Based on the linear correlation between DBC and CDOM, the study explored the degradation process of DBC and calculated its AQY. The results showed that within the research range of 280-600 nm, the AQY of B5CA, B6CA and DBC in the river water and offshore coastal seawater samples decreased exponentially with increasing wavelength, and there was no significant difference in the DBC AQY spectra of DBC betweenobtained from the two water samples. The results of this study provide a solid foundation for the subsequent calculation of the degradation of DBC in global or regional water bodies in the natural environment fresh and marine waters.

Wildfires can release pyrogenic dissolved organic matter (pyDOM) to the forest watershed, which may pose challenges for water treatment operations downstream due to the formation of disinfection byproducts (DBPs). In this study, we assessed the physio-chemical properties of pyDOM (e.g., electron-donating and -accepting capacities; EDC and EAC) and their contributions to the formation of DBPs using (1) 10 lab samples produced from various feedstocks and pyrolysis temperatures, and (2) pre- and post-fire field samples with different burning severities. A comprehensive suite of DBPs—four trihalomethanes (THMs), nine haloacetic acids (HAAs), and seven N-nitrosamines—were included. Our results showed that the non-purgeable organic carbon, total nitrogen, specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), EDC, and EAC of pyDOM from lab chars decreased with increasing pyrolysis temperature; however, E₂/E₃ values increased with pyrolysis temperature, suggesting the release of lower molecular weight pyDOM from chars. Both pyDOM from lab chars and field samples exhibited the formations of lower THMs and HAAs, but higher N-nitrosamines with increasing pyrolysis temperature or burning severity, which was attributed to the decrease in SUVA₂₅₄ and changes in the nitrogen precursor structures. Field samples were further reduced electrochemically. Our results demonstrated that reduced pyDOM can promote the formation of both carbonaceous and nitrogenous DBPs. Overall, the higher N-nitrosamine formation potential and the presence of two previously overlooked N-nitrosamines (nitrosodiethylamine, N-nitrosodi-n-propylamine) in the post-fire samples and pyDOM specimens from higher temperature chars raised concerns for water quality given that N-DBPs are highly toxic, especially for forest watersheds during post-fire recovery.

It is well understood that the frequency and intensity of wildfires is increasing globally. When these fires spread into the wildland-urban interface (WUI), changes in fuel composition, loading, and combustion conditions can result in emissions which may ultimately impact public health. However, the overall potential impacts of WUI fires on watersheds are not well understood; our current knowledge of WUI fires is based upon extrapolations and inferences from what is known of wildfires and urban fires as separate entities. Over the past three years, our group has begun to identify and quantify specific pyrogenic organic compounds present in surface waters after fires. We have found that in both stream water and ash leachate, a suite of aromatic polycarboxylic acids can be quantified at levels equivalent to as much as 0.1% of the DOC present in certain samples. We have also confirmed the presence of these compounds in effluents from WUI fire events. Further, data from mass spectrometry analysis indicates that hundreds of additional organic compounds produced during the combustion processes can be mobilized into surface waters after incidences of WUI fires. This presentation will provide an overview of the recent efforts at understanding the potential for contamination of surface waters after WUI fires, including evaluation of select pyrogenic organic compounds and assessment of potential associated toxicological impacts.

Fire is a global natural phenomenon and a driving force in the Earth system. Fire is a persuasive disturbance to the burned ecosystem because fire thoroughly changes its physical, chemical, and biological characteristics and processes that can take decades to recover (e.g., vegetation). The cycling of many macronutrients is among the processes being affected by fire, especially the relatively nonvolatile elements, because internal recycling of plant biomass pools is a main input to soil bioavailable nutrient pool. In this project, we aim to qualify and quantify the fire residues and its legacy impact on phosphorus cycling in the post-fire ecosystem.

Combining field sampling, lab-scaled investigation, and literature reivews, we identified a clear pattern of how fire changes phosphorus cycling in the post-fire ecosystem and recognized internal and external factors controlling P bioavailability to plants.

N-nitrosodimethylamine (NDMA) is a chloramination disinfection byproduct, notorious for its potent carcinogenicity and toxicity. The sources of NDMA precursors in watersheds are diverse. This study investigated the geospatial distributions of NDMA precursors in watersheds in Minnesota with various land use characteristics. A geostatistical analysis was performed to evaluate relationships between NDMA formation potential and anthropogenic activities. The possibility of relating organic matter properties, specifically, information derived from fluorescence excitation/emission matrices (EEMs) to NDMA formation was explored. Water samples were collected along rivers in major watersheds in Minnesota during Summer 2022. NDMA formation was measured via the uniform formation conditions (UFC) test, which represents average disinfection conditions applied in US drinking water treatment plants. Different geospatial patterns of NDMA precursors were observed across watersheds impacted by various land use. No significant change in NDMA observed upstream and downstream of wastewater treatment plant (WWTP) discharges. NDMA was positively correlated with the number of feedlots (ρ = 0.48, p < 0.01), total weight of animals (ρ = 0.54, p < 0.01), and total number of domestic WWTPs (ρ = 0.44, p < 0.01), indicating these are potential sources of precursors. NDMA was positively associated with fluorescent region IV (ρ = 0.7, p < 0.01), and negatively associated with total fluorescence and fluorescent region V (ρ = -0.55, p < 0.01) in EEM spectra, suggesting region IV is indicative of contributors to NDMA formation while organic matter in region V may serve as competitor for available chloramines during UFC testing.