Tracing atmospheric nitrate in groundwater using triple oxygen isotopes: evaluation based on bottled drinking water

Abstract

Abstract. The stable isotopic compositions of nitrate dissolved in 49 brands of bottled drinking water collected worldwide were measured, to trace the fate of atmospheric nitrate (NO3− atm) that had been deposited into subaerial ecosystems, using the 17O anomalies (Δ17O) of nitrate as tracers. The use of bottled water enables collection of groundwater recharged at natural, background watersheds. The nitrate in groundwater had small Δ17O values ranging from −0.2‰ to +4.5‰ n = 49). The average Δ17O value and average mixing ratio of atmospheric nitrate to total nitrate in the groundwater samples were estimated to be 0.8‰ and 3.1%, respectively. These findings indicated that the majority of atmospheric nitrate had undergone biological processing before being exported from the surface ecosystem to the groundwater. Moreover, the concentrations of atmospheric nitrate were estimated to range from less than 0.1 μmol L−1 to 8.5 μmol L−1 with higher NO3−atm concentrations being obtained for those recharged in rocky, arid or elevated areas with little vegetation and lower NO3−atm concentrations being obtained for those recharged in forested areas with high levels of vegetation. Additionally, many of the NO3−atm-depleted samples were characterized by elevated δ15N values of more than +10‰. Uptake by plants and/or microbes in forested soils subsequent to deposition and the progress of denitrification within groundwater likely plays a significant role in the removal of NO3−atm.

Quantifying the effects of clear-cutting and strip-cutting on nitrate dynamics in a forested watershed using triple oxygen isotopes as tracers

“…However, the importance of atmospheric nitrate as the source of increased nitrate in the stream water has not been evaluated to date. Quantitative evaluation of the source of increased nitrate in stream water subsequent to artificial clear-cutting and strip-cutting will improve our understanding of N cycling in forested soils prior to artificial alternations, as well as the mechanisms that regulate the direct discharge of NO − 3 atm deposited onto surface ecosystems (Durka et al, 1994;Ohte et al, 2004;Costa et al, 2011;Nakagawa et al, 2013). Thus, in this study, we conducted further isotope analysis of archived stream water samples to clarify the source of increased nitrate.…”

“…The natural stable isotopic composition of nitrate has been widely applied in the determination of the sources of nitrate in natural freshwater systems (Wada et al, 1975;Durka et al, 1994;Williard et al, 2001;Burns and Kendall, 2002;Campbell et al, 2002;Michalski et al, 2004;Ohte et al, 2004;Hales et al, 2007;Barnes et al, 2008;Burns et al, 2009;Tsunogai et al, 2010;Tobari et al, 2010;Ohte et al, 2010;Barnes and Raymond, 2010;Tsunogai et al, 2011;Nestler et al, 2011;Curtis et al, 2011;Costa et al, 2011;Pellerin et al, 2012;Dejwakh et al, 2012;Yue et al, 2013;Ohte, 2013;Lohse et al, 2013;Thibodeau et al, 2013;Nakagawa et al, 2013 (Michalski et al, 2003;Morin et al, 2008;Alexander et al, 2009). Using the 17 O signature defined by the following equation (Miller, 2002;Kaiser et al, 2007) …”

“…In our previous study, we determined the 17 O values of nitrate in aerobic groundwater worldwide to trace the fate of NO − 3 atm that had been deposited onto and passed through natural background watersheds (Nakagawa et al, 2013). The results of that study revealed that nitrate in groundwater had small 17 O values ranging from −0.2 ‰ to +4.5 ‰ ; therefore, we estimated the average mixing ratio of NO − 3 atm to total nitrate in the groundwater samples to be 3.1%.…”

“…The stable isotopic compositions were determined by converting the nitrate in each sample to N 2 O using the chemical method originally developed to determine the 15 N / 14 N and 18 O / 16 O ratios of seawater and freshwater nitrate (McIlvin and Altabet, 2005), with slight modifications Tsunogai et al, 2010;Konno et al, 2010;Tsunogai et al, 2011;Yamazaki et al, 2011;Nakagawa et al, 2013). Then, the stable isotopic compositions of N 2 O were determined using a continuous-flow isotope ratio massspectrometry (CF-IRMS) system Hirota et al, 2010).…”

<sup>17</sup>O excess traces atmospheric nitrate in paleo-groundwater of the Saharan desert

“…3 in modern groundwater, lakes, and wet depositions (Darrouzet-Nardi et al, 2012;Dejwakh et al, 2012;Li et al, 2010;Michalski et al, 2005;Michalski et al, 2004b;Tsunogai et al, 2011;Tsunogai et al, 2010;Nakagawa et al, 2013).…”