Hidden CO₂ from falling aquifers: National-scale pumping emissions and bicarbonate degassing in Mexico

Groundwater depletion increases the carbon cost of irrigation through two linked controls: falling water tables raise pumping lift and electricity-related CO₂ per unit of water, and extraction of HCO₃⁻-rich groundwater can add a geochemical CO₂ flux as dissolved inorganic carbon re-equilibrates during use. These mechanisms compound as depletion deepens, making intensively irrigated drylands especially sensitive. Mexico is a compelling Latin American case because it is among the region’s largest agricultural groundwater users, with primary irrigated production concentrated in arid-to-semiarid basins where GRACE indicates substantial storage declines. We quantified national groundwater-related CO₂ emissions for 2012, 2017, and 2021 by integrating GRACE groundwater storage anomalies (referenced to a 2002 baseline) with representative bicarbonate concentrations and electric pumping energy. GRACE anomalies intensified to < −75 cm by 2021 across arid to semi-arid, groundwater-irrigated basins of northern Mexico and the central plateau (major production regions in Chihuahua, Sonora, Zacatecas, Guanajuato, and San Luis Potosí—where groundwater HCO₃⁻ typically spans ~150–600 mg L⁻¹, consistent with these controls, state hotspots exceeded 3,200 t CO₂e yr⁻¹ for pumping (notably Chihuahua) and 900 t CO₂e yr⁻¹ for bicarbonate-associated emissions (Chihuahua and Sonora). Combined emissions increased from 2012 to 2021, dominated by pumping with a minor bicarbonate-associated component. Hierarchical clustering separated a high-emission irrigated-dryland group from a lower-emission group, indicating that depletion intensity and hydrochemical enrichment jointly structure the national pattern. In the global context, Mexico’s national pumping emissions are far below national inventories such as the USA irrigation estimate (12.6 Mt CO₂e yr⁻¹; 2018) and India’s groundwater-linked estimate (32–132 Mt CO₂ yr⁻¹; 1996–2016), while hotspot area intensities reported for northern/central Mexico are comparable to the USA per-area irrigation-energy benchmark. These results support integrating groundwater-driven emissions into water–energy planning and prioritizing mitigation that couples lower-carbon electricity for pumping with reductions in irrigation demand and shifts away from high water-demand cropping systems in the most depleted basins.