Anthropogenic Water Withdrawals Modify Freshwater Inorganic Carbon Fluxes across the United States

Abstract

Quantifying inorganic carbon fluxes to and from freshwater environments is essential for the accurate determination of the total amount of carbon exported to both the atmosphere and oceans. However, understanding of how anthropogenic freshwater withdrawals perturb land-freshwater-ocean and freshwater-atmosphere inorganic carbon fluxes is limited. Using the United States (US) as an exemplar, we estimate that fresh surface water withdrawals across the country during the year 2015 resulted in a median gross dissolved inorganic carbon (DIC) retention flux of 8.2 (uncertainty range: 6.7–9.9) Tg C yr^–1, equivalent to 28.3% of the total export of DIC to the oceans from US rivers. The median gross retention flux due to fresh groundwater withdrawals was 6.9 (uncertainty range: 5.3–8.8) Tg C yr^–1, over eight times the magnitude of the DIC flux to the oceans by US subterranean groundwater discharge. The degassing of CO₂ supersaturated groundwater following withdrawal emitted 3.6 (uncertainty range: 2.2–5.5) Tg of CO₂ yr^–1, 112% larger than previous estimates. On a county level, these CO₂ emissions exceeded CO₂ emissions from major emitting facilities across 45% of US counties. Reported results and a data analysis framework have important implications for the accurate development of carbon budgets across the US and around the world.