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

Abstract

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 (NDMAUFC) and occasionally using the high dose formation potential protocol (NDMAFP). Raw water NDMAUFC concentrations (2.2 to 128 ng/L) exhibited substantial temporal variation but relatively little spatial variation. An increase in NDMAUFC 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 NDMAUFC can be attributed to a decrease in competition between precursors and natural organic matter (NOM) for chloramines and reduced interactions of precursors with NOM. NDMAUFC 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.

Animal Feedlots and Domestic Wastewater Discharges are Likely Sources of N-Nitrosodimethylamine (NDMA) Precursors in Midwestern Watersheds

“…Consistent with our previous findings, an elevated NDMA UFC concentration was observed for the sample from the headwaters of the North Fork of the Crow River (NF1, 49.5 ± 14.9 ng/L), then gradually decreased downstream with increasing river discharge. Relatively low NDMA UFC concentrations were observed in the South Fork of the Crow River (9.9 ± 1.6 ng/L) and the Mississippi River sample (8.1 ± 0.6 ng/L).…”

“…The negative correlation between Region V (humic acid-like) and NDMA UFC indicates that humic acid-like substances are the primary competitors with NDMA precursors for chloramines. This is consistent with increased NDMA UFC after lime softening , because this process removes the high MW fractions of NOM (i.e., humic acids). The weak association between NDMA UFC concentrations and region III indicates that fulvic acid does not significantly affect NDMA formation.…”

“…Further study of discharges from animal operations is needed to confirm their role as potentially important sources of NDMA precursors and to identify the predominant precursor compounds as possible targets for source reduction strategies. Furthermore, this study only represented a snapshot of conditions throughout four watersheds, and NDMA precursor concentrations can be highly dynamic. , …”

“…Raw water samples were filtered through a GF/F glass fiber filter (Whatman, WHA1825047, 0.7 μm, 47 mm) before the chloramination test . To further investigate the effect of lime softening on NDMA formation, lime-softened samples were prepared by using river water samples collected from different watersheds following a protocol mimicking the lime softening process applied at the St. Paul Regional Water Services drinking water treatment plant . Details for the lime softening experiments are provided in the Supporting Information.…”

“…A geostatistical analysis was performed to extract information about anthropogenic activities around each sampling location to assess relationships between land use and the NDMA formed using the UFC test (NDMA UFC ). Because previous work in our laboratory identified an increase in NDMA formation following lime softening, the effect of lime softening on NDMA formation was further explored in this study. In addition, correlations between selected fluorescence peak intensities and NDMA formation using the UFC test were examined.…”