The neglected role of groundwater in the contemporary carbon cycle

The global carbon cycle plays a key role for the earth’ climate. For reliable climate change predictions it is therefore important to accurately model carbon stocks and fluxes across the earth system. Traditionally, researchers have mainly focused on carbon in the oceans, the atmosphere, vegetation and soils. In the past decade, carbon cycle modelers have become increasingly aware of the large carbon emissions from inland waters such as lakes and rivers. A large proportion of the carbon emitted from inland waters originates from groundwater (Drake et al. 2018). Despite being tightly connected to vegetation, soils and inland waters, groundwater is often neglected in landscape and global carbon cycling studies.  Here, I shed light on the role of groundwater in the contemporary carbon cycle, asking how dynamic inorganic carbon concentrations are over decadal time scales and what could be driving these dynamics. 

A look into the relatively sparse literature on carbon in groundwater reveals that groundwater is one of the largest continental carbon pools. Groundwater contains about 1440 Pg carbon, which is more than what sits in the atmosphere and vegetation, and not much less than what is contained in soils (Monger et al. 2015). Carbon is dissolved in groundwater in the form of various organic compounds such as humic substances or inorganic compounds such as carbon dioxide or bicarbonate. Inorganic forms often dominate the carbon pool in groundwater. Inorganic carbon is cycled by weathering, carbonate equilibrium reactions and biogeochemical processes such as respiration (Fig. 1).

Groundwater is typically regarded as a global carbon sink over geologic time scales, but shorter-term dynamics are largely unexplored. Human activities have doubled the global carbon export from land to surface waters in the past 250 years (Regnier et al. 2013). This alarming finding was mainly based on modelling and budget calculations and I started to wonder why there is so little direct observations in groundwater? How have groundwater inorganic carbon concentrations changed in the past decades? Can we really neglect groundwater in the contemporary carbon cycle given that it should account for a great deal of the increasing carbon export from land to water?

An unexpected gold mine for groundwater carbon research

Inorganic carbon concentrations have routinely been measured in the atmosphere and oceans over the past couple of decades. Inorganic carbon is also increasingly monitored in inland waters, but groundwater monitoring is lagging these developments far behind. In other words, any changes occurring in the subsurface remain largely unobserved.

A fundamental challenge in groundwater carbon research is the lack of direct inorganic carbon measurements. Researchers therefore often calculate inorganic carbon concentrations based on pH, alkalinity and water temperature. These variables are routinely measured by many environmental monitoring programs and have yielded valuable insights in long-term changes in inorganic carbon dynamics of inland waters (e.g. Nydahl et al. 2017). Yet, only very few researchers have attempted to look into inorganic carbon dynamics in groundwater (Macpherson 2009).

I happened to dig into groundwater inorganic carbon dynamics by pure coincidence. In 2021, a colleague of mine made me aware of a funding opportunity by the Geological Survey of Sweden (SGU) with a special call on groundwater studies. Until this point, I had mainly focused on carbon cycling in inland waters, but the important role of groundwater as a source of carbon to inland waters made me curios to read the funding call more carefully. It said that SGU would appreciate analyses of the groundwater monitoring data that SGU has been collecting as part of their environmental monitoring program. I was not aware of this data base and it took me only a brief check to realize that SGU was sitting on a gold mine of data.

SGU has been sampling groundwater for water chemical analysis across Sweden since 1962. The database includes thousands of samples from hundreds of sampling sites located all across the country (Fig. 2). In a pilot analysis for my grant proposal, I realized that calculated inorganic carbon concentrations seemed to have increased in many sampling sites. More detailed analyses of 55 sites with consistent sampling confirmed my initial discovery: inorganic carbon concentrations have increased on average by 28% across Sweden between 1980 and 2020. Carbon dioxide (CO₂) has increased even more, by 49% (Fig. 3). What could have caused this substantial increase?

Human impact on groundwater carbon cycling

Inorganic carbon in groundwater comes mainly from two sources. One is weathering from bedrock and soils, the other is the respiration of roots or organisms that live in the soil and break down dead plants or animals. The increase in inorganic carbon can be explained by several mechanisms. Air pollution and sulphur deposition have decreased in recent decades, and this decrease has enhanced the production of bicarbonate during weathering processes. This is because weathering has become less driven by sulphuric acids and more driven by carbonic acid. Recovery from acid deposition could have also increased respiration processes in soils and hence the production of CO₂.

The shift in weathering pathways, however, cannot be the whole explanation because inorganic carbon increased consistently throughout Sweden, even at higher latitudes where historic sulphur deposition has been relatively low. The broad increase across Sweden could alternatively by explained by climate change. On average, Swedish groundwater is 1.5 degrees warmer today than it was 40 years ago. It may be that warmer groundwater has boosted soil respiration. Another factor that could explain the increase in inorganic carbon is that forests have grown better during the past 40 years. The better forests grow, the more plants die and decompose, and the more CO₂ is released into the soil through the roots of trees. Increased CO₂ in the atmosphere could have further stimulated soil respiration.

Avenues for reconciling the role of groundwater in the contemporary carbon cycle

The findings have important consequences for the role of groundwater in the contemporary carbon cycle but also give rise to many new and exciting questions. Increased inorganic carbon in groundwater may indicate that the groundwater's carbon uptake from the atmosphere has increased, but it may also indicate that more carbon has entered surface waters where part of it may be emitted back to the atmosphere (Fig. 1). What do these changes imply for the net-carbon flux between the land surface and the atmosphere? At what time scale does the in- and out-flux respond to environmental changes? Will the inorganic carbon in groundwater fully recover from historic acid deposition, or will it reach a new unprecedented state driven by climate or land use change?

My findings also stress the importance of groundwater as a sentinel and integrator of environmental changes. Carbon observations can track past and ongoing changes in surface carbon uptake and subsurface transformations, but potentially also function as an early warning system of future changes in carbon emissions from receiving surface waters. Groundwater chemistry is monitored in many countries worldwide, but often overlooked in the context of carbon research. In my study, I calculated inorganic carbon concentrations from widely monitored alkalinity, pH and temperature, which worked reasonably well for Swedish groundwater but may be problematic under certain conditions. Direct carbon measurements should further boost our understanding of groundwater carbon dynamics in future studies. With larger efforts in groundwater carbon monitoring and analysis, exciting questions can be tackled such as: How representative are the findings in Sweden for other places in the world? To what extent would the inclusion of groundwater in earth system models improve climate change predictions? How should watersheds be managed to enhance carbon sequestration in groundwater and mitigate climate change?

References

Drake, T. W., Raymond, P. A. & Spencer, R. G. M. Terrestrial carbon inputs to inland waters: a current synthesis of estimates and uncertainty. Limnol. Oceanogr. Lett. 3, 132 –142 (2018).

Macpherson, G. L. et al. Increasing shallow groundwater CO2 and limestone weathering, Konza Prairie, USA. Geochim. Cosmochim. Acta 72, 5581 –5599 (2008).

Monger, C. H. et al. Sequestration of inorganic carbon in soil and groundwater. Geology 43, 375 –378 (2015).

Nydahl, A. C., Wallin, M. B. & Weyhenmeyer, G. A. No long-term trends in pCO 2 despite increasing organic carbon concentrations in boreal lakes, streams, and rivers. Global Biogeochem. Cycles 31, 985 –995 (2017).

Regnier, P. et al. Anthropogenic perturbation of the carbon fluxes from land to ocean. Nat. Geosci. 6, 597 –607 (2013).