27Dissolved inorganic and organic nitrogen levels are elevated in aquatic systems due to 28 anthropogenic activities. Dissolved organic nitrogen (DON) arises from various sources, and its 29 impact could be more clearly constrained if specific sources were identified and if the molecular-30 level composition of DON were better understood. In this work, the pharmaceutical 31 carbamazepine was used to identify septic-impacted groundwater in a coastal watershed. Using 32 ultrahigh resolution mass spectrometry data, the nitrogen-containing features of the dissolved 33 organic matter in septic-impacted and non-impacted samples were compared. The septic-34 impacted groundwater samples have a larger abundance of nitrogen-containing formulas. 35Impacted samples have additional DON features in the regions ascribed as 'protein-like ' and 36 'lipid-like' in van Krevelen space and have more intense nitrogen-containing features in a 37 specific region of a carbon versus mass plot. These features are potential indicators of dissolved 38 organic nitrogen arising from septic effluents, and this work suggests that ultrahigh resolution 39 mass spectrometry is a valuable tool to identify and characterize sources of DON. 40 41Introduction 42 Groundwater is an important source of freshwater to coastal systems.1, 2 Thus, 43groundwater is a source of nitrogen to coastal waters, especially if it has been impacted by 44 human activities. Dissolved nitrogen is comprised of both inorganic (nitrate, nitrite, and 45 ammonia) and organic forms. The quantitative balance between inorganic and organic nitrogen 46 will depend on various factors, including sources and biological activity, but evidence suggests 47 that dissolved organic nitrogen (DON) can be quantitatively as important as inorganic forms of 48 nitrogen in groundwater.3 For example, Kroeger et al. 3 found that DON comprised 10-93% of 49 total dissolved nitrogen in a set of coastal watersheds. Both the source and environmental 50 conditions influence the bioavailability and processing of DON, with bioavailability ranging 51 from 1-60%.4, 5 Sources of DON include precipitation, agriculture, natural runoff/infiltration, 52 artificial recharge of wastewater, and septic systems. In populated coastal areas, septic systems 53 may be particularly important sources of both inorganic and organic nitrogen.3, 6-8 At present, 54however, it is difficult to link the bulk DON found in impacted aquatic systems to specific 55 sources or processes due to a lack of information about the molecular-level composition of DON. 56 3 Information regarding the molecular-level composition of DON is important to 57 improving understanding the reactivity of the pool of nitrogen in aquatic systems. studies have found such compounds in impacted groundwater, which also serves as the drinking 82 water source for the population.27, 28 Some of these compounds, including carbamazepine (an 83 anti-convulsant) and artificial sweeteners, appear to be persistent and sorb to soils or sediments 84 only to a limi...