Insights in groundwater organic matter from Liquid Chromatography-Organic Carbon Detection

Understanding the processes that control the concentration and characteristics of organic matter in groundwater has important implications for the terrestrial global carbon budget. Liquid Chromatography – Organic Carbon Detection (LC-OCD) is a size-exclusion based chromatography technique that separates the organic carbon into molecular weight size fractions of biopolymers, humic substances, building blocks (degradation products of humic substances), low molecular weight acids and low molecular weight neutrals. Groundwater and surface water samples were collected from a range of locations in Australia representing different surface soil, land cover, recharge type and hydrological properties. At one site hyporheic zone samples were also collected from beneath a stream.The results showed a general decrease in the aromaticity and molecular weight indices going from surface water, hyporheic downwelling and groundwater samples. The aquifer substrate also affected the organic composition. For example, groundwater samples collected from a zone of fractured rock showed a relative decrease in the proportion of humic substances, suggestive of sorption or degradation of humic substances.This work demonstrates the potential for using LC-OCD in elucidating the processes that control the concentration and characteristics of organic matter in groundwater