CO2 RELEASE FROM GROUNDWATER AND HUMAN POPULATION GROWTH

The compounding factors of growing population (especially in water-scarce regions), growing wealth of the population, agricultural expansion in response to demand for food, changing land use from native vegetation to cropland and managed rangeland, and increasing use of mechanized irrigation will strain water resources. With increasing pollution of surface waters, decreasing river flow, sedimentation filling water-supply reservoirs, and decrease in number of dams built (in the USA), available surface sources of potable water may be decreasing, and world dependence on groundwater will increase. Groundwater now constitutes about 20% of the global water use, with agriculture using most of that volume. Because, for economic reasons, development of groundwater supply usually begins with the shallowest potable water, new water wells are likely to tap unconfined aquifers. Dissolved CO₂ in unconfined groundwater is typically one to two orders of magnitude higher than dissolved CO₂ in surface water, and so degassing of groundwater CO₂ into the atmosphere is an unavoidable consequence of groundwater production. At current groundwater production rates, CO₂ degassing probably releases on the order of 0.74 to 0.87 Pg of carbon into the air every year. This is at least seven to eight times more than annual CO₂ from volcanic emissions, and about an order of magnitude less than the sum of other anthropomorphic CO₂ emissions.