The growing United States population means increasing waste production and a corresponding increase in the use and release of contaminants of emerging concern (CECs). The increasing volume and changing composition of waste poses new challenges for waste management and the protection of ecosystem, surface water, and groundwater resources. In the U.S., most domestic solid waste is disposed of in landfills, and domestic wastewater is treated by wastewater treatment plants (WWTPs) or onsite wastewater treatment systems (OWTSs). While OWTSs, the majority of which are conventional septic systems, account for the minority of wastewater treatment in the U.S., they present a significant pollution risk because they are not subject to the same treatment level or discharge regulatory standards as municipal WWTPs. Landfill leachate is also an important source of environmental contamination because most existing landfills in the U.S. are closed, unlined landfills that lack engineered systems to enhance refuse degradation or collect leachate. To mitigate the pollution risks of these effluents, it is important to understand initial wastewater composition and how to identify and trace environmental contamination.In this study, I generated background molecular composition data for landfill leachate and domestic wastewater effluents and developed chemical tracers for septic system impacted streams. I used ultrahigh resolution, Fourier-transform ion cyclotron resonance mass spectrometry, to molecularly characterize the dissolved organic matter (DOM) of septic system wastewater and septic system wastewater-impacted surface waters. I also analyzed traditional water quality markers such as CECs, chloride, nitrate isotopic signatures, and nutrients. Additionally, I molecularly characterized landfill leachate DOM and analyzed similar chemical markers to those used in the septic system study to understand composition. The goals in the main septic system study were to better understand the composition and natural processing of septic system wastewater and to develop new chemical wastewater tracers while assessing traditionally used tracers. The landfill leachate study addressed the lack of nontargeted leachate composition data. Determining initial molecular composition is necessary to understand the consequences of discharge to the environment and to design leachate treatments.