Methane emissions from groundwater pumping in the USA

Atmospheric methane accumulation contributes to climate change, hence quantifying methane emissions is essential to assess and model the impacts. Here we estimate methane emissions from groundwater pumping in the Los Angeles Basin (LAB), north-eastern Pennsylvania, and the Principal aquifers of the USA using the average concentrations of methane in groundwater and annual groundwater pumping volumes. High average methane concentrations, 44.1 mg/L, and extensive groundwater pumping, ~3.1 x 1011 L/a in the LAB, result in the annual emission of ~2.9 x 10-3 Tg of microbial methane. Ethane emissions in the LAB were 3.5 x 10-6 Tg/a. Lower methane emissions estimated for NE Pennsylvania, ~3.0 x 10-8 Tg/a, reflect lower methane concentrations and groundwater pumping, 0.7 mg/L and 4.67 x 107 L/a, respectively. Methane concentrations and groundwater withdrawals, 1.06 x 1014 L, across the USA enabled the estimation of the total emissions of methane from Principal aquifers (92% of total pumping) of 0.044 Tg/a in the year 2000, which represents a small percentage (~0.2%) of the total annual US methane emissions, but a previously unquantified flux in the global methane budget. Globally, groundwater-pumping methane emissions were estimated to be 0.53 Tg/a, 0.2% of global methane emissions, by adopting a global estimate for groundwater extraction, and an average methane concentration in older groundwater of 0.44 mg/L.