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

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 (NO_{3⁻ 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 NO_3⁻atm concentrations being obtained for those recharged in rocky, arid or elevated areas with little vegetation and lower NO_3⁻atm concentrations being obtained for those recharged in forested areas with high levels of vegetation. Additionally, many of the NO_3⁻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 NO_3⁻atm.