Anthropogenic Water Withdrawals Impact Dissolved Inorganic Carbon Fluxes on Local and Continental Scales

Quantifying inorganic carbon fluxes to and from fresh waters is essential as part of understanding ecosystem functioning, potable water quality, and the amount of carbon exported to both the atmosphere and the oceans. Despite this, evidence of how anthropogenic withdrawals of fresh water perturb both land-fresh water-ocean and fresh water-atmosphere carbon fluxes is limited. Using the United States (US) as an exemplar, here we quantify for the first time the impact that both fresh surface water and fresh groundwater withdrawals by major water use sectors can have on land-fresh water-ocean and fresh water-atmosphere inorganic carbon fluxes. Fresh surface water withdrawals across the US during 2015 resulted in an estimated median gross dissolved inorganic carbon (DIC) retention flux of 8.5 (interquartile range: 6.5-11.3) Tg C yr^-1, equivalent to 29% of the total export of DIC to the oceans from US rivers. The median gross retention flux due to fresh groundwater withdrawals was estimated to be 6.5 (interquartile range: 4.9-8.7) Tg C yr^-1, over eight times the magnitude of the DIC flux to the oceans by subterranean groundwater discharging from the US. The median emission of CO₂ from fresh waters to the atmosphere due to degassing of CO₂ supersaturated groundwater following withdrawal was estimated to be 2.2 Tg CO₂ yr^-1 (interquartile range 1.2-4.3), 30% larger than previous estimates. Irrigation and public supply water use sectors contributed 70% and 19% of this total CO₂ emission, respectively. County-level CO₂ emissions from degassing groundwater following withdrawal exceeded the total county-level CO₂ emissions from major emitting facilities across 1,391 counties, many of which were within Western and Midwestern states. This highlighted importance of freshwater withdrawals for DIC fluxes and CO₂ emissions has implications for the accurate development of carbon budgets both across the United States, and for other regions around the world that are associated with significant freshwater withdrawals.