CO 2 distribution in groundwater and the impact of groundwater extraction on the global C cycle

Abstract

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 CO 2 partial pressures are typically ∼ 10–100 times higher than atmospheric, so CO 2 degassing is an unavoidable consequence of groundwater extraction. For regional studies, very good estimates of groundwater CO 2 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 Henry's Law constant for CO 2 and first association constant for H 2CO 3 at 13 °C, and titration alkalinity can be substituted for activity of HCO 3 −. Groundwater CO 2 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 CO 2 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 CO 2 from volcanic emissions and ∼ 200 times less than other human-sourced CO 2 emissions. Determination of dissolved CO 2 depends on accuracy of pH measurements: CO 2 released due to groundwater use may be underestimated if reported pH measurements do not represent in situ pH.