Methane in groundwater used for Japanese agriculture: Its relationship to other physico‐chemical properties and possible tropospheric source strength

The concentration of CH 4 in 131 groundwater samples used for agriculture in Aichi Prefecture, central Japan, averaged 1.58 mgC l −1 for those water samples above detection (>0.006 mgC l −1 ), with the highest value of 18.4 mgC l −1 . Methane was detected in more than half of the groundwater samples. The amount of CH 4 released to the atmosphere because of agricultural groundwater use was estimated to be 2.00 × 10 7 gC yr −1 for a cultivated area of 8.61 × 10 4 ha, or about 1.4% of the CH 4 production in paddy fields derived from soil organic matter in the same geographic area. Distribution of measurements of redox potential (Eh), chemical oxygen demand (COD), Fe, Mn, NH 4 ‐N, and NO 3 ‐N was clearly different between the CH 4 ‐detected and undetected samples; Eh values and NO 3 ‐N concentrations were lower while the other four factors were higher in the CH 4 ‐detected samples.