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 similar to 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)(pCO(2)) = log(10)(HCO3-) - pH + 7.749|where the constant is the sum of the negative logarithm (base 10) of the Henry's Law constant for CO2 and first association constant for H2CO3 at 13 degrees C, and titration alkalinity can be substituted for activity of HCO3-, Groundwater 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 similar to 0.01 to 0.03 Pg C year(-1), similar to CO2 from volcanic emissions and similar to 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. (C) 2009 Elsevier B.V. All rights reserved.