Accurate prediction of future climate hinges on understanding and regulating greenhouse gas (GHG) fluxes, particularly methane (CH₄), a potent GHG responsible for a significant portion of radiative forcing. Arctic lakes are a substantial and climate-sensitive source of atmospheric CH₄, with emissions comparable to those from high-latitude wetlands. Climate warming is projected to increase CH₄ emissions from Arctic lakes by a factor of 2-3 by the end of the 21st century, potentially creating a strong positive climate feedback. However, the sensitivity of these emissions to climate change remains highly uncertain due to a limited understanding of the mechanisms controlling CH₄ cycling in Arctic lakes. Traditionally, CH₄ emissions from lakes are considered to be governed by the balance between in-lake production and oxidation. However, high CH₄ concentrations in lake waters can also arise from terrestrial CH₄ influx via groundwater discharge. This pathway is particularly relevant in the Arctic, where abundant wetlands (CH₄ production hotspots) and constrained water flow paths within the shallow active layer lead to substantial inorganic and organic carbon delivery to surface waters. While the impact of terrestrial carbon inputs via groundwater on lake carbon cycling is acknowledged, the significance of groundwater inflows for CH₄ emissions has been understudied. Previous research on individual lakes in Alaska demonstrated the importance of groundwater as a CH₄ source, suggesting that groundwater inputs could entirely sustain summer CH₄ evasion rates. However, mire CH₄ production and export are heavily dependent on factors like temperature, water table depth, active layer thickness, and topography. Therefore, observations from single lakes or seasons may not fully capture the large-scale or year-round importance of groundwater CH₄ inputs. This study addresses this gap by investigating the role of groundwater discharge on lake CH₄ emissions across multiple lakes and seasons in the Arctic region of Sweden.

Several studies have highlighted the importance of various factors in the methane cycle of arctic lakes. Bastviken et al. (2004) established a global estimate of methane emissions from lakes, emphasizing their dependence on lake characteristics. DelSontro et al. (2018) explained the spatial distribution of surface water methane through the interaction of physical transport and biological processes. Research on methane oxidation in Alaskan lakes by Martinez-Cruz et al. (2015) and Thottathil et al. (2018) has advanced our understanding of this key process. The role of groundwater in delivering dissolved organic matter to Arctic coastal waters has been documented by Connolly et al. (2020). Prior research on individual Alaskan lakes by Dabrowski et al. (2020) and Paytan et al. (2015) showed the significant contribution of groundwater discharge to lake methane. However, the need for studies encompassing multiple lakes and seasonal variability has been emphasized to better assess this process. The influence of hydrological and environmental factors on mire methane production and export has also been extensively studied (O’Connor et al., 2019; Saarnio et al., 1997; Lupascu et al., 2012). Studies on carbon cycling in Arctic lakes have demonstrated the significant role of external inputs through groundwater (Kling et al., 1992; Striegl & Michmerhuizen, 1998). The dataset used in this study included methane fluxes from the Boreal-Arctic Wetland and Lake Methane Dataset (BAWLD-CH4) (Kuhn et al., 2021), and data on sediment production and oxidation were collected from several Arctic lake studies (Hershey et al., 2015; Bretz & Whalen, 2014; Lofton et al., 2014; Gentzel et al., 2012; Cunada et al., 2021).

This study combined measurements of methane (CH₄) and radon (²²²Rn) in ten lakes and adjacent groundwater in the Arctic region of Sweden to quantify groundwater CH₄ inputs during the ice-free season. Radon-222 (²²²Rn), a natural tracer of groundwater, was employed to estimate groundwater inflow rates. The study area, located in the Torneträsk catchment in Arctic Sweden, lies within the discontinuous permafrost zone. Ten small lakes were selected across a precipitation gradient to investigate seasonal patterns of groundwater CH₄ inputs. Groundwater inflow rates were quantified using a ²²²Rn mass-balance approach, which considers sources (groundwater inflow, diffusion from sediments, inlet streams, in situ ²²⁶Ra decay) and sinks (atmospheric evasion, outlet streams, radioactive decay) of ²²²Rn. The study assumed steady-state conditions over a short period (1-3 days) based on the ²²²Rn residence time in the water column. A Monte Carlo analysis was conducted to account for uncertainties associated with the ²²²Rn mass balance. This analysis involved generating 1000 values of ²²²Rn flux for each lake and then using these values, along with randomly selected values of ²²²Rn and CH₄ concentrations in groundwater, to estimate 1000 values of groundwater flows and CH₄ fluxes. The final groundwater inflow and CH₄ input values reported are the median and 25th and 75th percentiles of the simulations. Surface water samples (lake water, inlet, and outlet streams) and groundwater samples were collected during summer and autumn. ²²²Rn activity was determined using a Durridge RAD7 monitor, and dissolved CH₄ concentrations were measured using a gas chromatograph. In situ measurements of physicochemical parameters (temperature, dissolved oxygen, specific conductivity, pH) were also taken. Groundwater samples were collected from mire areas at the lake shoreline using a direct-push piezometer. Discharge from inlet and outlet streams was measured using an electromagnetic flow meter or salt slug injections. Wind-speed, rainfall, air temperature, and air pressure data were obtained from weather stations near each lake. Lake sediment cores were collected, and sediment incubation experiments were used to constrain ²²²Rn inputs and to obtain an independent estimate of the ²²²Rn concentration in groundwater. Lake bathymetry was determined using echo sounding, and catchment characteristics were derived from a digital elevation model and vegetation maps. Atmospheric fluxes of ²²²Rn and CH₄ were calculated using a wind-based gas transfer velocity model. Statistical analyses (ANOVA, PLS regression, multiple linear regression) were conducted to identify relationships between groundwater inflows and other variables.

The study found a consistent enrichment of CH₄ and ²²²Rn in groundwater compared to surface waters and inlet streams. Groundwater CH₄ concentrations were significantly higher than in lake waters or streams. The high CH₄ concentrations in groundwater suggest that even relatively low groundwater inflows can substantially impact lake CH₄ budgets. There was no significant difference in groundwater CH₄ concentrations between seasons. ²²²Rn served as an effective tracer for quantifying groundwater inflows, revealing that groundwater discharge was a major water source for most lakes. Groundwater inflows varied among lakes and seasons, being higher in summer than in autumn. Groundwater CH₄ inputs to lakes significantly exceeded CH₄ inputs through inlet streams, exceeding them by up to an order of magnitude in summer. Potential total CH₄ emissions from the lakes, considering both diffusion and ebullition fluxes, were of a similar order of magnitude as groundwater CH₄ inputs, indicating that groundwater discharge could sustain total lake CH₄ emissions at a regional scale. There was a strong positive correlation between groundwater discharge rates and total CH₄ emissions during summer, but not in autumn. Spatial variation in groundwater inflows was linked to lake depth and wetland cover in the catchment, although the relationship between mire cover and groundwater inflow was negative. Partial least squares regression indicated that lake depth, wet area coverage in the catchment, and catchment slope were good predictors of groundwater inflow. Seasonal variation in groundwater CH₄ inputs was influenced by hydrological (groundwater recharge) and biological (CH₄ production) factors, with higher inflows and inputs in summer due to snowmelt and increased CH₄ production.

This study's findings highlight the significant and often overlooked role of groundwater discharge as a driver of CH₄ emissions from Arctic lakes at a regional scale. The magnitudes of groundwater CH₄ inputs are comparable to other CH₄ sources and sinks in Arctic lakes, emphasizing the importance of this pathway in lake CH₄ cycling. The spatial variability in groundwater inflows, primarily influenced by lake depth and catchment characteristics, demonstrates the importance of considering landscape patterns of CH₄ production. The seasonal variation in groundwater CH₄ inputs, modulated by hydrological and biological factors, underscores the need for comprehensive temporal characterization of groundwater discharge. The strong positive correlation between groundwater inflow rates and CH₄ evasion in summer suggests that higher discharge in early summer contributes significantly to higher summer emissions. However, the relative contribution of biological factors warrants further research. These results challenge previous assessments that rely solely on in-lake processes for estimating CH₄ emissions, as groundwater CH₄ inputs can be a substantial external source.

This study demonstrates the significant role of groundwater discharge as a driver of methane emissions from Arctic lakes, a factor often overlooked in previous studies. Spatial patterns of groundwater inflow are mainly determined by physical-hydrological characteristics of the catchment-lake system. Seasonal variations are influenced by both hydrological and biological factors. The study highlights the need to incorporate groundwater CH₄ inputs into regional assessments and Earth system models to improve climate predictions, especially considering that climate change is likely to exacerbate groundwater CH₄ inputs to lakes through factors such as permafrost thaw and increased precipitation. Further investigation into the relative importance of biological processes and a refined understanding of hydrological pathways are crucial for advancing our ability to predict future CH₄ emissions from Arctic lakes.

While this study provides valuable insights into the role of groundwater discharge in methane emissions from Arctic lakes, several limitations should be considered. The study focused on a specific region in Arctic Sweden, and the findings may not be directly generalizable to all Arctic regions, particularly those with different permafrost conditions, hydrological regimes or landscape features. The mass balance approach used to estimate groundwater inflow rates relies on certain assumptions, including steady-state conditions, which may not always hold true, particularly over longer time scales. Furthermore, although ebullition was accounted for in the total CH₄ emission estimates, it was not directly measured in the study. Finally, this study did not explicitly assess the isotopic signature of methane in groundwater and in the lake waters. Such measurements may provide additional insights on the source of methane in the lakes.