Revisiting groundwater carbon fluxes to the ocean with implications for the carbon cycle

Compared with riverine systems, the influence of groundwater on the global carbon cycle has remained underexplored. Here, we provide a new estimate of the bicarbonate fluxes from fresh groundwater to the ocean by coupling a statistical and hydrological analysis of groundwater and river samples across the contiguous United States with a study of global groundwater characteristics. We find that the mean HCO₃^– concentration ([HCO₃^–]) in groundwaters exceeds that in surface rivers by a factor of 2–3 throughout the contiguous United States. Based on estimates of fresh groundwater discharge to the ocean and scaling up our estimated mean [HCO₃^–] in groundwaters from the United States and around the world, we arrived at a mean global HCO₃^– flux from groundwaters ranging from 7.4 × 10¹² (25^th percentile)–1.8 × 10¹³ mol/yr (75^th percentile) to 2.8 × 10¹³–8.3 × 10¹³ mol/yr, which is 22%–237% of the global HCO₃^– flux from river systems, respectively. We also estimated that the global carbon flux derived from subsurface silicate weathering could be comparable to 32%–351% that from surficial silicate weathering, depending on groundwater discharge rates. Despite large uncertainties due to data limitation, this study highlights that groundwater weathering could be an important carbon sink in both the short- and long-term carbon cycle. Therefore, additional work on groundwaters is needed to develop a well-constrained view of the global carbon cycle.