Methane Emissions from Abandoned Oil and Gas Wells in California.

California hosts ∼124,000 abandoned and plugged (AP) oil and gas wells, ∼38,000 idle wells, and ∼63,000 active wells, whose methane (CH₄) emissions remain largely unquantified at levels below ∼2 kg CH₄ h^-1. We sampled 121 wells using two methods: a rapid mobile plume integration method (detection ∼0.5 g CH₄ h^-1) and a more sensitive static flux chamber (detection ∼1 × 10^-6 g CH₄ h^-1). We measured small but detectable methane emissions from 34 of 97 AP wells (mean emission: 0.286 g CH₄ h^-1). In contrast, we found emissions from 11 of 17 idle wells-which are not currently producing (mean: 35.4 g CH₄ h^-1)-4 of 6 active wells (mean: 189.7 g CH₄ h^-1), and one unplugged well-an open casing with no infrastructure present (10.9 g CH₄ h^-1). Our results support previous findings that emissions from plugged wells are low but are more substantial from idle wells. In addition, our smaller sample of active wells suggests that their reported emissions are consistent with previous studies and deserve further attention. Due to limited access, we could not measure wells in most major active oil and gas fields in California; therefore, we recommend additional data collection from all types of wells but especially active and idle wells.