Origin of methane in high-arsenic groundwater of Taiwan – Evidence from stable isotope analyses and radiocarbon dating

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH4). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH4 and its hydrological implications, stable carbon isotope ratios (δ13C) and radiocarbon (14C) ages of exsolved CH4, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ13CCH4δ13CCH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ13CCH4=38.2‰)(δ13CCH4=38.2‰) from the Ilan plain, all CH4 samples analyzed were produced via microbially mediated CO2 reduction. Many δ13CDIC values are considerably greater than −10‰ and even up to ∼10‰ due to Rayleigh enrichment during CO2 reduction. (2) Almost all the 14C ages of CH4 samples from the shallow aquifer (I) (<60 m depth) are greater than the 14C ages of coexisting DIC and sediments, suggesting the presence of CH4 from underlying aquifers. (3) The 14C ages of coexisting CH4, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH4 and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH4 and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH4 sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO2 and CH4 from underlying strata.