H107-01
Combined Radiocarbon Analysis of DIC, DOC, and RNA coupled with Geochemical Data to Understand Pleistocene Aquifer Recharge in Bangladesh

Friday, 11 December 2020: 04:00
Virtual
Brian Mailloux1, Amana Mohiuddin1, Sarah Blau1, Shahreen Hossain1, Camille Gowda1, Tyler Ellis2, Bruce Buchholz3, Kazi Matin Ahmed4, Imtiaz Choudhury4, Benjamin C Bostick5, Rajib Hassan Mozumder2, Afsana Akter1, Elizabeth Trembath-Reichert6 and Alexander van Geen2, (1)Barnard College, Environmental Science, New York, NY, United States, (2)Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, United States, (3)Lawrence Livermore National Laboratory, Livermore, United States, (4)Dhaka University, Geology Department, Dhaka, Bangladesh, (5)Lamont-Doherty Earth Observatory of Columbia University, Center for Climate and Life, Palisades, NY, United States, (6)Arizona State University, Tempe, AZ, United States
Recorded Presentation
Abstract:
In Bangladesh, Pleistocene-aged aquifers are typically low in Arsenic (As) concentrations. Wells that tap the deep Pleistocene aquifers are used to mitigate the damaging public health effects from consuming groundwater from shallower, high As Holocene-aged aquifers. However, a small number of wells tapping the Pleistocene aquifer contain elevated levels of As. The goal of this study is to perform a broad geochemical characterization of the cations and anions in groundwater across the aquifers coupled to the radiocarbon signatures of DIC, DOC, and RNA in order to better constrain the potential for reactive organic carbon to enter the low arsenic Pleistocene aquifers. Groundwater has been collected from Araihazar, Bangladesh located 25km east of the urban capital, Dhaka. Principal component analysis utilizing anions and cations on over 1100 samples indicate the Holocene and Pleistocene aquifers have unique aqueous geochemical signatures. This is consistent with the DIC radiocarbon data; the Pleistocene aquifer (n=31, average depth 704 feet) has an average Δ14C of -704.2±134.7‰ whereas the Holocene aquifer (n=12, average depth 51 feet) has an average Δ14C of -13.13±38.2‰. This is in contrast to the much younger DOC (n=8) which shows an average Δ14C of -386.4±234.5‰ which is equivalent to an average age of 3,912 years. Furthermore, novel analysis of the radiocarbon in microbial RNA from three wells in the Pleistocene have Δ14C of -483, -503, and -545‰. This indicates that the young DOC entering the aquifer is being utilized by the native microbial populations. The geochemistry and the DIC radiocarbon data alone would indicate that the Pleistocene aquifer is sealed from recent recharge. However, the radiocarbon of the DOC and RNA indicates that younger reactive carbon is entering the deeper Pleistocene aquifer and being utilized by a diverse microbial community making the few observations of high arsenic potentially more worrisome.
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