New Method for Measuring Dissolved Ne, Ar, and N2 in Water Using a Plasma Emission Detector: Application for Quantifying Denitrification in Groundwater

Abstract

Groundwater denitrification studies require the careful quantification of excess N₂ to determine that nitrate reduction has taken place. The measurement of corresponding noble gases Ne and Ar quantifies excess air N₂ and in situ degassing. We compare, for the first time, measurement results from a high-precision quadrupole mass spectrometer (QMS) with a low-cost PlasmaDetek (plasma emission detector) GC add-on capable of analyzing all three gases (N₂, Ne, Ar) from a single sample to obtain reliable data for denitrification calculations. Both methods can be used to accurately measure N₂, Ne, and Ar concentrations that are reproducible and overlap within 2σ analytical uncertainty. Moreover, we discuss different groundwater headspace gas collection methods and show that Giggenbach bottles, while limited when storing samples for long-time periods (months or more), are reliable over shorter storage periods (up to 2 or 3 weeks), compared to cold-pressed copper tube collection methods, which are deemed stable over long timeframes (years). This novel plasma detector method will enable laboratories that do not have state-of-the-art noble gas facilities to undertake reliable measurement of dissolved groundwater gases (N₂, Ne, and Ar) for groundwater denitrification studies, understand the implications of natural subsurface nitrate attenuation, and improve catchment nitrate budgets.