COMBINED RADIOCARBON AND SEQUENCING ANALYSES TO UNDERSTAND CARBON SOURCES IN GROUNDWATER SYSTEMS (Invited Presentation)

Microbial activity has a large impact on both anthropogenic and geogenic contaminants in groundwater systems. In these systems it is critical to understand the carbon sources utilized by the microbes in order to constrain the biogeochemical process. For example, in high arsenic reducing aquifers, iron reduction coupled to the oxidation of organic matter is the critical step. However it has taken combined radiocarbon and sequencing analysis of the DNA and RNA to better understand the carbon sources. In the shallow high arsenic aquifers of Bangladesh, dissolved organic carbon (DOC) is slowly advected into the aquifers over 100’s of years and utilized by microbial communities. In Bangladesh, deeper, Pleistocene-aged aquifers are typically low in Arsenic and utilized as a safe community-wide drinking water source. Radiocarbon analysis of DIC from the Araihazar region of Bangladesh indicates the water is ~10,000 years old and recharged during the Last Glacial Maximum. However, radiocarbon analysis of DOC and RNA indicate that younger DOC is entering the aquifer and being utilized by the microbial community. This result is further corroborated by metagenomic and metatranscriptomic results. This indicates that radiocarbon and sequencing analysis of RNA can be a sensitive indicator of aquifer flow paths and recharge. Utilization of the same methods in an alluvial aquifer in Colorado highlighted the importance of autotrophy where reducing and oxic groundwaters mixed. Furthermore, the results illustrated the importance of dissolved over sedimentary organic matter for active microbial communities. In all cases, carefully selecting aquifers and wells for more in depth molecular and radiocarbon analyses of both RNA and DNA can lead to new and unexpected details about groundwater flow systems.