Dissolved organic matter in the unsaturated zone: the view from the cave

Soil organic matter content is typically a few percent of the total soil composition. Diffuse recharge can mobilise some of this soil-derived organic matter. While soil pore water dissolved organic matter (DOM) concentrations are up to 100 ppm, the resulting groundwater dissolved organic matter concentration is typically less than 2ppm. Dissolved organic matter transported from the soil can be both biodegraded and sorbed to minerals, and the relative importance of these two processes in the unsaturated zone is poorly understood. Caves in karstified limestone uniquely provide direct access to water percolating from the soil to the groundwater. Cave percolation waters can be analysed for their DOM concentration and character. This provides insights into the extent and type of biological and chemical processing of DOM during transport from the soil to the groundwater. We determine the concentration and characteristics of DOM in cave percolation waters using liquid chromatography (LC-OCD) and optical spectrophotometry (fluorescence and absorbance). We sample DOM from multiple caves in SE Australia (Cathedral Cave, Wellington; South Glory and Harrie Wood Caves, Yarrangobilly), permitting comparison of unsaturated zone DOM properties at different depths (up to 30m below land surface) and different climate zones (montane and temperate). We use caves with long-term hydrological monitoring programs so that DOM in waters of contrasting residence times can be compared. Additionally, we compare these cave percolation water DOM characteristics to those from local and regional groundwater, sampled from nearby wells. Our results will help improve our understanding of how DOM is processed from soil to groundwater, and is also relevant to speleothem scientists interested in using organic matter preserved in speleothems as a paleoclimate or paleoenvironmental proxy. Plain Language Summary When plants die, they break down to organic matter, which forms part of the soil. When this organic matter is washed out of the soil and into the subsurface, we know very little about what happens next. Partly it is because we can't see and measure what is happening. There is a solution. We can use caves as observatories. We can collect the organic matter in the water which enters the caves, and analyse it back in the laboratory. There, we can determine not only the cocntration of organic matter, but also its chemical composition. Why is this important? There's lot of organic matter in soil. But only one or two organic molecues per million water molecules are present in groundwater. Where does it all go? One idea is that it is used as food by subterranean microbes. Another is that is sorbed to minerals. By measuring the chemical composition of organic matter in cave drip waters, we can work out which is more important, and help understand why there is so little organic matter in groundwater.

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