Natural Radiocarbon in Dissolved Organic Carbon in Groundwater

Abstract

Groundwater dating is important to many studies of hydrologic systems, and radiocarbon dating is one of the most common radiometric techniques employed. Chapters 17, 18 and 19 treat theory and application of groundwater dating with dissolved inorganic carbon (DIC). A major source of uncertainty in radiocarbon dating of groundwater is the reconstruction of the geochemical processes in the aquifer involving carbon. In this chapter, we review published studies of the isotope geochemistry of dissolved organic carbon (DOC) in groundwater. We discuss studies of gases in the unsaturated zone, insofar as they impact the organic geochemistry of groundwater. Despite the relatively immature state of knowledge in these areas, importance of such studies to the overall understanding of carbon geochemistry in groundwater is clear. For example, some DOC components may oxidize to DIC within the aquifer, thus affecting the 14C of the DIC. In many groundwater systems, the 14C content of organic fractions will probably be an important link to understanding both the origin of DOC and DIC and the age relations among the soluble carbon species present. To date, the success of the application of radiocarbon dating in hydrogeologic studies has depended almost entirely on 14C measurements of DIC in groundwater. As other chapters in this volume demonstrate, this application has evolved to highly sophisticated and computer-dependent modeling techniques, requiring as much information as possible about sources and sinks of dissolved carbon. However, as DOC is part of the groundwater carbon inventory, groundwater models for 14C dating are incomplete without knowledge of the role of DOC. Additionally, the 14C in certain fractions of DOC may yield independent estimates of age. In contrast to groundwater DIC 14C studies, the hydrologic interpretation of 14C measurements of DOC is still in its infancy. Reasons for this comparatively primitive stage of development are the difficulty of extracting and isolating DOC, the low carbon yield from extraction; hence, the requirement that 14C be measured by accelerator mass spectrometry (AMS), and the complexity of the geochemistry of DOC in groundwater. In view of these handicaps, what compensating advantages make 14C information on DOC worth pursuing?