Sorption of Groundwater Dissolved Organic Carbon onto Minerals

Our understanding of groundwater organic matter (OM) as a carbon source or sink in the environmental carbon cycle is limited. The dynamics of groundwater OM is mainly governed by biological processing and its sorption to minerals. In saturated groundwaters, dissolved OM (DOM) represents one part of the groundwater organic carbon pool. Without consideration of the DOM sorption, it is not possible to quantify governing groundwater OM processes. This research explores the rate and extent of DOM sorption on different minerals. Groundwater DOM samples, and International Humic Substances Society (IHSS) standard solutions, were analysed. Each was mixed with a range of masses of iron coated quartz, clean quartz, and calcium carbonate, and shaken for 2 hours to reach equilibrium before being filtered through 0.2 μm for total dissolved organic carbon (DOC) and composition analysis by size-exclusion chromatography-organic carbon detection (LC-OCD). Sorption isotherms were constructed and groundwater DOM sorption were compared to the sorption of IHSS standards. Initial results suggest that for the IHSS standards, the operationally-defined humic substances fraction had the strongest sorption compared to the other LC-OCD fractions and total DOC. Some samples exhibited a small increase in the low molecular weight neutral (LMW-N) aqueous concentration with increasing humic substances sorption. This gradual increase observed could be the result of humic substances desorbing or their breakdown during the experiment. Further results comparing these IHSS standards with groundwater samples will be presented. In conjunction with complementary studies, these results can help provide more accurate prediction of whether groundwater OM is a carbon source or sink, which will enable the management of the groundwater resources as part of the carbon economy.