RADIOCARBON ANALYSIS OF MICROBIAL RNA TO DETERMINE CARBON SOURCES IN ARSENIC CONTAMINATED PLEISTOCENE AQUIFERS IN BANGLADESH

Widespread arsenic (As) consumption from contaminated aquifers has been endemic in Bangladesh. Geogenic As is released into the groundwater through microbial reduction of iron oxides coupled to the oxidation of organic carbon. People have historically drilled inexpensive, shallow wells (<33 meters deep) that pump from the Holocene aquifer. This aquifer often contains high As levels and is associated with young advected organic carbon. However, due to this contamination, people have recently been drilling deeper wells to tap into the low As Pleistocene aquifer. In the area sampled for this study, (Site M), the Holocene and Pleistocene aquifers are separated by a clay layer (27 – 40 meters), which is thought to protect the Pleistocene aquifer. However, some wells tapping the intermediate Pleistocene aquifer (~50-100 meters) at Site M have high As levels as well. The goal of this work is to determine the source of carbon fueling microbial metabolism in a high As Pleistocene well at Site M. Extracting and radiocarbon dating microbial RNA can indicate whether young carbon (0-1k years) is coming around the clay from the shallow Holocene aquifer or whether old carbon (>40k years) is leeching from or through the clay layer itself. 300 μg of RNA have been extracted from a filter that pumped 33,860 liters of groundwater at 51 meters in duplicate. These radiocarbon dates will help identify the source of organic carbon fueling arsenic release in the Pleistocene aquifer and may provide insight towards the long-term viability of Pleistocene aquifer wells as a source of water in Bangladesh.