Low-level <i>N</i> -nitrosamine occurrence in disinfected drinking water and relationships with standard water quality indicators

<i>N</i>-Nitrosodimethylamine (NDMA) is a probable human carcinogen that can be formed in drinking water treatment systems as a byproduct of chloramination and chlorination. Occurrence of NDMA and other <i>N</i>-nitrosamines in the United States has been previously assessed using a variety of techniques, but few studies have been able to distinguish between concentrations above and below suggested screening levels (<i>e.g.</i>, 0.7 ng L^-1 for NDMA). This study evaluated the presence of NDMA and seven other <i>N</i>-nitrosamines in two drinking water distribution systems in the northeastern United States (<i>n</i> = 42 locations) and assessed factors influencing its occurrence. NDMA was present in 98% of water samples across both systems (MDL 0.15 ng L^-1) with higher concentrations in the system utilizing chloramination (0.39-1.32 ng L^-1) than the system utilizing chlorination (0.20-0.54 ng L^-1). Samples were collected before and after flushing taps, and higher concentrations of NDMA were observed in samples collected prior to flushing, suggesting increased formation due to temporary stagnation. <i>N</i>-Nitrosomorpholine was the only other <i>N</i>-nitrosamine detected in samples taken after tap flushing (5% detection rate; MDL 0.21 ng L^-1), though four additional nitrosamines were detected before flushing in at least one sample. Water quality parameters (<i>i.e.</i>, chlorine residual, dissolved organic carbon, total dissolved nitrogen, specific UV absorbance, pH, temperature, specific conductance) and other disinfection byproducts (trihalomethanes) were measured to assess correlations with NDMA occurrence, and NDMA concentrations were negatively correlated with residual chlorine in both distribution systems. These observations illustrate the potential prevalence of low-level nitrosamine occurrence in disinfected drinking water and provide a framework for system-specific understanding of NDMA occurrence, which can aid in prioritizing locations where further investigation may be needed to mitigate potential exposure risks.