Beyond flux: persistent geophysical-geochemical signatures of methane leakage from a legacy well

Legacy wells represent a long-term risk to the integrity of subsurface energy and carbon storage systems, yet leakage detection remains challenging due to the episodic nature of surface gas fluxes and strong modulation by meteorological and near-surface conditions. Here we present an integrated field investigation of methane leakage from a legacy petroleum well, demonstrating that leakage generates a coherent and persistent geophysico-chemical footprint in the shallow subsurface that extends beyond the zone of detectable surface emissions. We combine multi-year surface gas flux measurements with soil gas and soil geochemical indicators and near-surface geophysical imaging (electrical resistivity tomography and electromagnetic induction). Methane fluxes exhibit steep spatial decay and strong temporal variability, reflecting short-term leakage activity. In contrast, geophysical properties define a broader footprint associated with sustained changes in subsurface state, including pore fluid conductivity and moisture structure. Soil geochemical indicators show the most persistent response, recording cumulative alteration driven by repeated gas migration and oxidation, carbonate buffering, and ion exchange processes. These observations reveal three coupled expressions of a single leakage footprint, operating over contrasting timescales: surface flux as a short-term, dynamic signal; geophysical anomalies as an intermediate-memory integrator of subsurface state; and soil geochemistry as a long-memory archive of cumulative leakage impacts. Importantly, the geophysico-chemical footprint remains detectable under conditions where surface flux measurements alone provide weak or ambiguous evidence of leakage. The results demonstrate the value of integrating geophysical and geochemical observations into monitoring strategies for legacy wells and subsurface storage projects, improving confidence in leakage detection, delineation, and long-term storage integrity assessment.