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.