Comparing Groundwater Sampling Devices for Denitrification Assessment Using the N₂/Ar Method

The N₂/Ar method is valuable for studying legacy nitrate and denitrification progress in groundwater systems. It uses dissolved N₂ and Ar concentrations to calculate the amount of N₂ originating from denitrification (excess-N₂). Successfully applying the method requires accurate values of N₂ and Ar concentrations. Therefore, avoiding degassing and atmospheric contamination during groundwater sampling is crucial for reliable results. In this study, we focus on the effect of sampling devices on N₂, Ar, and the resulting excess-N₂ concentrations. To evaluate this effect over a wide concentration range, we sampled 14 observation wells. One sample was collected using a submersible pump and another using a bladder pump. Furthermore, we collected multiple samples with both pumps at a fifteenth site to assess reproducibility. Additionally, we used a point-source bailer for sampling at this site. The major ion concentrations show that the sampling device does not significantly influence the sample chemistry. In contrast, the measured N₂, Ar, and calculated excess-N₂ concentrations significantly differ between the sampling devices. Overall, the samples collected with the submersible pump show the highest N₂ and Ar concentrations, resulting in the highest excess-N₂ concentrations. N₂ and Ar concentrations of the bladder pump samples are lower, resulting in lower excess-N₂ concentrations. The bailer samples show lower N₂ but similar Ar concentrations to the submersible pump samples, leading to the lowest excess-N₂ concentrations. We conclude that a submersible pump is practical and suitable for collecting groundwater samples to assess denitrification by the N₂/Ar method.