CO2 distribution in groundwater and the impact of groundwater extraction on the global C cycle

Growing population and wealth, agricultural expansion, changing land use, and mechanized irrigation increase the demand on water resources. Groundwater accounts for about 20% of global water use, with agriculture the largest consumer. Groundwater CO2 partial pressures are typically ∼ 10–100 times higher than atmospheric, so CO2 degassing is an unavoidable consequence of groundwater extraction. For regional studies, very good estimates of groundwater CO2 in clastic aquifers can be calculated for temperate-climate regions from: log 10 ( p CO 2 ) = log 10 ( HCO 3 − ) − pH + 7.749 where the constant is the sum of the negative logarithm (base 10) of the Henryʹs Law constant for CO2 and first association constant for H2CO3 at 13 °C, and titration alkalinity can be substituted for activity of HCO3−. water CO2 is not uniformly distributed in groundwater, being most variable near the water table, lowest at intermediate depth, and highest in deep aquifers where non-potable water is typical. Geochemical speciation modeling is required to assess CO2 released when groundwater equilibrates with the atmosphere. A suite of potable water types were modeled for this purpose, with the result that current groundwater production is estimated to release ∼ 0.01 to 0.03 Pg C year− 1, similar to CO2 from volcanic emissions and ∼ 200 times less than other human-sourced CO2 emissions. Determination of dissolved CO2 depends on accuracy of pH measurements: CO2 released due to groundwater use may be underestimated if reported pH measurements do not represent in situ pH.