The source and behavior of iodine dissolving in groundwater in the Mobara gas field, Japan

The groundwater of the Mobara gas-field in Boso peninsula, eastern Japan, contains considerable amount, around 100 mg/L, of iodine besides dissolved methane. Twelve groundwater samples from gas-producing wells to the aquifer within the Kazusa Group in Mobara and Naruto districts have been analyzed for iodine, pH, Eh, HCO3-, NH4+, SiO2(aq), δD, δ18O, δ13CCH4 and seventeen elements. There is a good correlation between the concentrations of the dissolved elements to those in marine algae. The principal source of the iodine is believed to be algae buried in the sediments and the iodine has been discharged to the porefluid during decomposition of the tissue. The molar ratio of I-/Cl- in the groundwater increases from 1.0 x 103 to 2.2 x 103 with changing the depth from 2000 to 1000 meters below the surface and turns to decrease from 2.2 x 103 to 1.8 x 103 with going up from 1000 to 500 meters. This tendency may be caused by the variation of buried organisms due to changing the sedimentary environment. The observed variation in the ratio indicates that the iodine discharged in the pore fluid did not migrate significantly after the deposition of the formation. This paper can be regarded as an English version of Japanese article written by Kamei, which was published on the 'Shigen Chishitsu (Resource Geology)' 51(2)145-151(2000). (author)