The spatiotemporal dynamics of denitrification in groundwater are still not well-understood because of a lack of efficient methods to quantify this biogeochemical reaction pathway. Previous research used the ratio of N2 to argon (Ar) to quantify net production of N2 via denitrification by separating the biologically generated N2 component from the atmospheric-generated components. However, this method does not allow the quantification of the atmospheric components accurately because the differences in gas partitioning between N2 and Ar are being neglected. Moreover, conventional (noble) gas analysis in water is both expensive and labor-intensive. We overcome these limitations by using a portable mass spectrometer system, which enables a fast and efficient in situ analysis of dissolved (noble) gases in groundwater. By analyzing a larger set of (noble) gases (N2, He, Ar, and Kr) combined with a physically meaningful excess air model, we quantified N2 originating from denitrification. Consequently, we were able to study the spatiotemporal dynamics of N2 production due to denitrification in riparian groundwater over a six-month period. Our results show that denitrification is highly variable in space and time, emphasizing the need for spatially and temporally resolved data to accurately account for denitrification dynamics in groundwater.