ARE PLEISTOCENE AQUIFERS IN BANGLADESH SEALED OR BEING RECHARGED? A COMBINED ANALYSIS OF RADIOCARBON SIGNATURES OF DIC, DOC, AND RNA WITH GEOCHEMICAL DATA

In Bangladesh, Pleistocene-aged aquifers are typically low in aqueous Arsenic (As). 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 determine if the Pleistocene aquifers in Araihazar, Bangladesh are being recharged from the Holocene aquifers above or are sealed and have minimal vertical interactions. Geochemical characterization of the cations and anions in groundwater were used to assess the geochemical mixing. Radiometric dating of the DIC, DOC, and RNA were used to constrain the sources of reactive organic carbon in the low arsenic Pleistocene aquifers. 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‰. However, the radiocarbon signatures of the RNA and DOC tell a different story. The DOC (n=8) which has an average Δ¹⁴C of -386.4±234.5‰ which is equivalent to an average age of 3,912 years is much younger than the DIC. 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 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.