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

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.