Origin of Methane and Sources of High Concentrations in Los Angeles Groundwater

Abstract

In 2014, samples from 37 monitoring wells at 17 locations, within or near oil fields, and one site >5 km from oil fields, in the Los Angeles Basin, California, were analyzed for dissolved hydrocarbon gas isotopes and abundances. The wells sample a variety of depths of an aquifer system composed of unconsolidated and semiconsolidated sediments under various conditions of confinement. Concentrations of methane in groundwater samples ranged from 0.002 to 150 mg/L—some of the highest concentrations reported in a densely populated urban area. The δ13C and δ2H of the methane ranged from −80.8 to −45.5 per mil (‰) and −249.8 to −134.9‰, respectively, and, along with oxidation‐reduction processes, helped to identify the origin of methane as microbial methanogenesis and CO2 reduction as its main formation pathway. The distribution of methane concentrations and isotopes is consistent with the high concentrations of methane in Los Angeles Basin groundwater originating from relatively shallow microbial production in anoxic or suboxic conditions. Source of the methane is the aquifer sediments rather than the upward migration or leakage of thermogenic methane associated with oil fields in the basin.

Methane emissions from groundwater pumping in the USA

“…13,37 Within a given lithology group, methane concentrations are typically higher in younger sediments than in older sediments, 13 and higher in older groundwater (pre 1950s) than recently recharged groundwater. 7,13 Previous studies noted that sediment age can be important with respect to redox processes due to the preferential consumption of the most reactive components of sedimentary organic carbon by microbes over time. [38][39][40] In support of this hypothesis, McMahon et al 13 found that for aquifers composed of similar lithologies, concentrations of methane, dissolved organic carbon, and ammonium were higher in the aquifers with younger sediments than in aquifers with older sediments.…”

“…While thermogenic methane was detected locally in the shallow subsurface in conjunction with some of the historic explosions that occurred in the LAB, 46 the methane distribution and isotopic systematics, and groundwater age results indicate that most excess methane in LA groundwater originates from microbial production in anoxic or suboxic conditions in the shallow aquifer systems, rather than the migration of deeper thermogenic methane in the hydrocarbon-bearing formations associated with oil fields in the basin. 7 Groundwater samples were collected in accordance with the protocols established by the USGS National Water-Quality Assessment (NAWQA) 34 and the USGS National Field Manual 35 after purging the equivalent of three casing-volumes from each well with a submersible pump. Samples were analyzed for a suite of inorganic, organic, and isotopic constituents; for a complete description of the sampling, analytical methods, quality assurance, and data.…”

“…Microbial methane formation follows two pathways: via acetate fermentation or CO 2 reduction, typically at shallower depths, by the actions of methanogenic organisms, members of the domain Archaea, on organic matter in anoxic environments, such as wetlands, landfills, and some aquifer sediments and coal deposits. Both thermogenic and microbial methane accumulate in groundwater as a result of generation, migration, and accumulation, [5][6][7][8][9][10] with measured concentrations as high as 150 mg/L in the LAB. 7 Groundwater pumping and the subsequent release of dissolved methane is potentially a significant source of methane emissions that has not been quantified in the global methane budget, 11 although methane emissions from groundwater in the UK have been estimated at 2.2 × 10 −6 Tg/a, about 0.05% of total UK methane emissions.…”

“…Both thermogenic and microbial methane accumulate in groundwater as a result of generation, migration, and accumulation, [5][6][7][8][9][10] with measured concentrations as high as 150 mg/L in the LAB. 7 Groundwater pumping and the subsequent release of dissolved methane is potentially a significant source of methane emissions that has not been quantified in the global methane budget, 11 although methane emissions from groundwater in the UK have been estimated at 2.2 × 10 −6 Tg/a, about 0.05% of total UK methane emissions. 12 Methane and other dissolved gases are typically removed from drinking-water distribution networks by several methods, included vented well caps, cascade, cone, and spray aerators, air stripping, or discharge into open or vented tanks.…”

“…While studying the sources and formation pathways for methane in LAB groundwater, 7 the implications for the high methane concentrations and large volumes of annual groundwater pumping led to the estimation of methane emissions from the LAB, and a comparison with the Marcellus shale region of NE Pennsylvania. Further compilation of recent groundwater methane data, 13 and annual groundwater pumping, 14 enabled estimation of methane emissions from groundwater pumping in the Principal aquifers of the USA (Fig.…”