Spatiotemporal Variability in N-Nitrosodimethylamine Precursor Levels in a Watershed Impacted by Agricultural Activities and Municipal Wastewater Discharges and Effects of Lime Softening

The Crow River, a tributary of the Mississippi River in Minnesota, U.S.A., that is impacted by agricultural activities and municipal wastewater discharges, was sampled approximately monthly at 12 locations over 18 months to investigate temporal and spatial variations in N-nitrosodimethylamine (NDMA) precursor levels. NDMA precursors were quantified primarily by measuring NDMA formed under the low chloramine dose uniform formation conditions protocol (NDMA_UFC) and occasionally using the high dose formation potential protocol (NDMA_FP). Raw water NDMA_UFC concentrations (2.2 to 128 ng/L) exhibited substantial temporal variation but relatively little spatial variation. An increase in NDMA_UFC was observed for 126 of 169 water samples after lime-softening treatment. A kinetic model indicates that under chloramine-limited UFC test conditions, the increase in NDMA_UFC can be attributed to a decrease in competition between precursors and natural organic matter (NOM) for chloramines and reduced interactions of precursors with NOM. NDMA_UFC concentrations correlated positively with dissolved nitrogen concentration (ρ = 0.44, p < 0.01) when excluding the spring snowmelt period and negatively correlated with dissolved organic carbon concentration (ρ = −0.47, p < 0.01). Overall, NDMA precursor levels were highly dynamic and strongly affected by lime-softening treatment.