New Experimental Tools to Use Noble Gases as Artificial Tracers for Groundwater Flow

Abstract

Labeling groundwater by injecting an artificial tracer is a standard and widely used method to study groundwater flow systems. Noble gases dissolved in groundwater are potentially ideal artificial tracers, as they are not subject to biogeochemical transformations, do not adsorb onto the aquifer matrix, are colorless, and have no negative impact on the quality of groundwater resources. In addition, combining different noble-gas species in multi-tracer tests would allow direct analysis of the spatio-temporal heterogeneity of groundwater flow systems. However, while the handling of noble gases is safe and straightforward for injection into groundwater, conventional methods to analyse dissolved noble gases tend to be impractical for groundwater tracer tests. The sampling and subsequent lab-based analysis of dissolved noble gases are laborious, expensive and time intensive. Therefore, only researchers with access to specialized noble-gas labs have attempted such tracer tests. The recently developed gas-equilibrium membrane-inlet mass spectrometers (GE-MIMS) allow efficient on-site analysis of dissolved gases at high temporal resolution. The GE-MIMS instruments thereby eliminate most of the analytical and logistical constraints of conventional lab-based techniques and therefore provide new opportunities for groundwater tests using artificially injected gases. We used a GE-MIMS to systematically test the applicability of He, Kr, and Xe as artificial groundwater tracers. These gas species were injected into groundwater as Dirac-like pulses at three piezometers located at various locations upstream of a pumping well, where dissolved gas concentrations were continuously monitored with the GE-MIMS instrument. The groundwater travel times observed in these tracer tests ranged from a few hours to several weeks, and were consistent with the groundwater flow field at the experimental test site. Travel times determined from the noble gas tracer tests were also consistent with those obtained traditional dye tracers.

Lake surface cooling drives littoral-pelagic exchange of dissolved gases

“…2 ). Noble gas tracer transport provided a physical quantification of gas dynamics free of biogeochemical reactions ( 39 ). After the injection on 25 November (03:50 UT), the krypton concentration at M0 reached ~1 μM, which is three orders of magnitude larger than the natural background concentration in equilibrium with the atmosphere ( Fig.…”

“…We performed a 2-day campaign from 24 to 26 November 2020 to quantify the lateral gas transport in the two limbs of the convective circulation. Two portable mass spectrometers miniRUEDI (Gasometrix, Switzerland) measured the krypton and methane [among other gases ( 38 , 39 )] concentration on site with an analytical uncertainty of 1 to 3% ( 38 ). Each spectrometer received lake water pumped from specific depths.…”

Quantifying Carbon Cycling across the Groundwater–Stream–Atmosphere Continuum Using High-Resolution Time Series of Multiple Dissolved Gases

“…21−23 Notably, technological advances in portable mass spectrometers 24,25 have facilitated the on-site and quasi-real-time quantification of different dissolved gas species, proving to be significantly beneficial for the high-resolution monitoring of carbon dynamics in aquatic systems. 24,26,27 In this study, we used a recently developed gas-equilibrium membrane-inlet mass spectrometer ("miniRUEDI", see Brennwald et al) 24 to quantify multiple dissolved gas concentrations at a representative groundwater−stream− atmosphere transect. First, we integrated high-resolution time series data of dissolved noble gas concentrations into a numerical model to simultaneously constrain groundwater discharge and stream−atmosphere gas exchange rates.…”

“…For instance, dissolved noble gases like 222 Rn and 4 He were used to quantify groundwater discharge into streams, , and diurnal variations of dissolved N 2 , Ar, Kr, and atmospheric He concentrations assisted in evaluating air–water gas exchange rates. , Accurately quantifying these rates is vital for understanding carbon physical transport and exchange across the GSAC. Moreover, dissolved O 2 , dissolved CO 2 –O 2 pair, and dissolved CH 4 time series provide insights into biogeochemical processes in aquatic systems. − Notably, technological advances in portable mass spectrometers , have facilitated the on-site and quasi-real-time quantification of different dissolved gas species, proving to be significantly beneficial for the high-resolution monitoring of carbon dynamics in aquatic systems. ,, …”