ISOTOPIC COMPOSITION (C-13, C-14, H-2) AND GEOCHEMISTRY OF AQUATIC HUMIC SUBSTANCES FROM GROUNDWATER

Aquatic humic substances from eight confined and unconfined aquifer/aquitard groundwater systems were analyzed for their stable carbon, radiocarbon, deuterium, and elemental contents. Fractionation of groundwater dissolved organic carbon (DOC) showed that humic substances are an important part of the organic solute load in all groundwaters. Groundwater humates are distinct from other terrestrial humates, and are characterized by low oxygen (36%) and high carbon (53%) contents. Elemental data from water table wells suggests this characteristic oxygen depletion is mainly a result of biochemical processes that occur in the vadose zone. The stable carbon isotopic composition of groundwater humates range between -31 and -24%. (PDB) with ann average of -26%., and reflect their terrestrial origins. The deuterium values of groundwater humate vary widely, and are mainly a reflection of 2H/H variability in the continental water cycle. Radiocarbon analyses suggest a predominant soil zone origin for most groundwater humates, although some groundwater systems are influenced to varying degrees by buried peat or coal. Soluble humate 14C mean residence times of up to several hundred years in the vadose zone before recharging to groundwater are due to the mixing of young and old organic carbon sources, rather than advective residence times. © 1990.