Every year some 42,200 tons of hidden CO2 emissions are generated through ground water consumption in Mexico. This is not only due to the operation of pumps powered by fuel or electricity but also to direct release of gas from water following its extraction. The estimate was made in a research project conducted by teams from the University of Petroleum and Energy Studies, India, Technological Institute of Monterrey, Mexico, and the Helmholtz Centre for Environmental Research, Germany.
Mexico counts 653 officially recognized aquifers, and more than 100 of them are overexploited. In certain torrid regions of the country up to 80% of irrigation water comes from underground sources. Water being extracted in large volumes; its stock is gradually depleted so it has to be pumped up from greater depths. Therefore, energy consumption per cubic meter of water increases with time, and so do the associated CO2 emissions.
There is however another, less evident, source of emissions associated with ground water consumption. That water contains dissolved inorganic carbon, mostly in the bicarbonate form. When water is brought to the surface it is subject to different pressure conditions and a shift in chemical equilibrium. As a result, dissolved carbon is partially released into the atmosphere as CO2.
Consumption of ground water thus creates two concurrent sources of emissions: direct, i.e. emerging from water as such, and indirect – related to the production of energy used for pumping.
The international research team decided to estimate the relative contributions of both emission sources. To this end they used data for 2012, 2017 and 2021. Changes in underground water stock were detected through GRACE satellites, and data on bicarbonates concentrations, acidity and temperature was obtained from thousands of water well analyses. Based on that data the team calculated the CO2 volume that can be released from water upon extraction. To estimate the energy component, they considered pumping volumes, water extraction depth, pump efficiency and emissions resulting from the production of energy used by the pumps.
Their estimates demonstrated that the energy component accounts for the major part of the hidden carbon footprint associated with ground water. For the purpose of region-to-region comparison the researches assumed the water extraction depth of 100 m and the pump efficiency rate of 35%. They also took into consideration the gradual decline in carbon intensity of Mexico’s power sector: in 2012 emissions averaged at about 0/50 kg of CO2 per kWh of electricity produced; the figure came down to 0.46 kg in 2017 and to 0.438 kg in 2021. However, this effect is partially cancelled out in districts where ground water stock is severely depleted as water has to be pumped from deeper horizons with more electricity consumed.
This is especially relevant to the country’s northern and central regions which experienced the most drastic reduction of ground water stock between 2012 and 2021. In Chihuahua, direct CO2 emissions from extracted water exceeded 900 tons by 2021, while emissions associated with energy spent on its pumping amounted to 3,200 tons. In Sonora, Zacatecas, San Luis Potosi and Guanajuato the energy-related CO2 emissions component stayed in the range of 1600-3200 tons. The estimate was much lower in the south and along the Gulf of Mexico coast where ground water sources were less depleted and less energy was required for pumping.
Notably, high concentrations of bicarbonates in ground water do not necessarily point to higher emissions. For instance, in Veracruz, Tabasco, Campeche and other more humid regions their concentration may be even greater than in the north, but ground water resources there are more readily replenished and not as quickly depleted. The greatest carbon footprint will thus be found where all the three factors coincide: high extraction volumes, depleted ground water reserves, and significant amount of energy spent on pumping.
The research team estimated total annual CO2 emissions from the use of ground water in Mexico at 42,200 tons on average, corresponding to about 0.009% of the country’s fossil fuel emissions. Given calculation uncertainties, the indicator may range between 35,900 and 48,500 tons per year. Although this may not appear significant on the country-wide scale, the research points to an additional linkage between water resources depletion and energy: as the water pumping depth increases, so does the amount of energy required to extract water, and so does the associated carbon footprint.
This category of emissions can be reduced primarily by cutting down water extraction volumes and pumping energy consumption. The research team suggests increasing irrigation efficiency, using more economical pumps and powering them from low carbon sources. Solar energy could be an option, however the researches warn that it should be adopted in parallel with water intake control, as cheaper energy can reduce the cost of pumping, eventually leading to even greater depletion of ground water reserves.
