Spatial and seasonal variations in CH4 in groundwater used for agriculture in central Japan

It is important to estimate the amount of CH4 released following the use of groundwater and to understand the dynamics of groundwater CH4 from the viewpoints of biogeochemical C cycle and global warming. We determined CH4 concentration in 216 groundwaters used for agriculture in Aichi Prefecture, central Japan, and analyzed the spatial and seasonal variations. The percentage of CH4-detected samples and mean CH4 concentration were high in deltaic regions, 88% and 239 μmol L−1(n = 24) or 69% and 166 μmol L−1 (n = 49), low in alluvial fan (0%; n = 12) and mountainous area (35% and 11.7 μmol L−1(n = 22)). Methane was more frequently detected in groundwater with >50 m depth (84%; n = 37). Mean CH4 concentration for CH4-detected samples from >50 m depth was also higher than those from <5 m depth, 248 versus 60.2 μmol L−1. Among groundwater samples with <5 m depth, the probability of CH4 detection was higher under paddy fields. Seasonal variations in the concentration and stable C isotopic ratio of CH4 (δCCH413; −65 to −89‰) were generally small in monitored groundwaters with 4–180 m depth, suggesting that CH4 produced mainly by CO2 reduction was preserved through a year. There were also shallow groundwaters (<5 m depth) showing adverse seasonal changes in CH4 concentration and δCCH413 (up to 20‰). Such shallow groundwaters showed significant CH4 production in a 90-day incubation with maximum rates of 3.03–6.43 nmol day−1. Acetoclastic methanogens were generally absent, while an addition of glucose and especially CO2/H2 enhanced methanogenesis. CH4 oxidizing potential was also observed in an aerobic incubation. These results suggested that the production/oxidation of CH4in situ or at the source was responsible for the seasonal variations in CH4 concentration.