The WaterWorm: a low-cost, low power sensor for the detection of dissolved CH4 in glacial meltwater

An unknown source of methane (CH₄) was recently discovered under the Kangerlussuaq sector of the Greenland Ice Sheet (GrIS). CH₄ is transported dissolved in meltwater from the subglacial environment to the margin of the ice sheet, where it rapidly degasses to the atmosphere. Existing knowledge gaps concern the magnitude of emissions, seasonal patterns and spatial variations along the margin of the GrIS, which require long-term monitoring and large-scale measurement campaigns at multiple meltwater outlets. A limiting factor for such studies in remote areas is that CH₄ analysers (laser spectroscopy) are power-hungry, maintenance-intensive, and expensive. To overcome these obstacles, we are developing a low-cost, low power sensor for measuring dissolved CH₄ in subglacial meltwater systems in the MetICE project: the WaterWorm.

The WaterWorm is based on a metal oxide sensor (MOS) designed for CH₄ detection (Figaro TGS2611-E00), which is highly sensitive to variations in relative humidity (RH) and temperature. In the WaterWorm, the MOS is encased in a hydrophobic but gas-permeable silicone tube, ensuring a stable and fully saturated headspace (100% RH) during submergence. We calibrated the analogue output (in mV) of the submerged WaterWorm against a reference CH₄ analyser (μGGA, GLA-331, LGR Research) connected to a dissolved gas extraction system (DGES, LGR Research) in temperature-controlled laboratory experiments by stepwise enrichment of the water with CH4. These calibration tests showed that the sensor output (set at two readings per minute) is proportional to dissolved CH₄ at constant humidity and temperature.

During fieldwork near Kangerlussuaq, Greenland, in summer 2021, a field baseline calibration was performed in a meltwater stream on the surface of the GrIS at ambient CH₄ concentrations. WaterWorms were deployed for ten weeks in the meltwater of a small outlet of the Isunnguata Sermia glacier with known CH₄ export and stable meltwater temperatures (0.0 - 0.1°C) to test the sensor under field conditions. Throughout this period, the WaterWorms measured elevated dissolved CH₄ concentrations with diurnal variations that corresponded to similar diurnal variation in gaseous CH₄ measurements performed with the reference CH₄ analyser.

The WaterWorm is a promising and cost-efficient option for the seasonal monitoring of dissolved CH₄ in glacial meltwater. With material costs of only 150€, the WaterWorm can be left unattended in the field and positioned directly at the ice edge. This makes the sensor suitable for a large-scale CH₄ monitoring network along the margin of the GrIS. The next steps involve material tests to build WaterWorms for applications in other aquatic environments and at different water depths.