Spatial and seasonal variations in CH 4 in groundwater used for agriculture in central Japan

Abstract

It is important to estimate the amount of CH 4 released following the use of groundwater and to understand the dynamics of groundwater CH 4 from the viewpoints of biogeochemical C cycle and global warming. We determined CH 4 concentration in 216 groundwaters used for agriculture in Aichi Prefecture, central Japan, and analyzed the spatial and seasonal variations. The percentage of CH 4-detected samples and mean CH 4 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 CH 4 concentration for CH 4-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 CH 4 detection was higher under paddy fields. Seasonal variations in the concentration and stable C isotopic ratio of CH 4 ( δ C C H 4 13 ; −65 to −89‰) were generally small in monitored groundwaters with 4–180 m depth, suggesting that CH 4 produced mainly by CO 2 reduction was preserved through a year. There were also shallow groundwaters (<5 m depth) showing adverse seasonal changes in CH 4 concentration and δ C C H 4 13 (up to 20‰). Such shallow groundwaters showed significant CH 4 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 CO 2/H 2 enhanced methanogenesis. CH 4 oxidizing potential was also observed in an aerobic incubation. These results suggested that the production/oxidation of CH 4 in situ or at the source was responsible for the seasonal variations in CH 4 concentration.