A novel method for analysing phosphorus-bound stable oxygen isotope ratios in groundwater ecosystems

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The role and fate of phosphate (PO4) in groundwater ecosystems has only recently come into scientific focus. In contrast to common assumptions, groundwater can locally maintain considerable concentrations of PO4 because of biogeochemical processes. One approach to study these processes is the assessment of PO4 pools in the solid (i.e. aquifer matrix) and aqueous phase (i.e. groundwater) and to monitor changes therein over time. Lately, the analysis of the stable oxygen isotope composition of PO4 (δ18OPO4) in operationally defined PO4 pools emerged as a novel and promising tool. By combining δ18OPO4 analysis of operationally defined PO4 pools with isotopic labelling experiments, we are able to monitor microbial cycling as well as abiotic processes that control in concert the fate of PO4 in groundwater. Despite the countless possible experimental possibilities arising from this approach, no rigorously tested analytical protocol exists to date for groundwater systems combining sequential extractions with δ18OPO4 analysis. At present, available δ18OPO4 analysis protocols for soil and sediments focus on sample preparation aiming to eliminate organic matter and other impurities from extracted PO4. What is missing is a combined testing of potential signal alterations and selectivity of the respective extraction steps. Such a rigorous method testing is important as aquifer materials and groundwater may exhibit a wide range of physicochemical properties (e.g. contents of organic matter, calcium carbonates, prevailing redox state and pH) that could interfere with the isotopic analysis and signal interpretation. The objective of this project is to develop and validate a methodological approach to reliably determine and interpret δ18OO4 signals in solid (i.e. aquifer material) and aqueous (i.e. groundwater) PO4 pools in groundwater systems. The proposed test scheme will allow to evaluate if and to which extent limitations exist regarding the applicability for samples with a high concentration of organic matter, calcium carbonates, and samples obtained from aquifers with iron-reducing conditions. This tool will be highly useful for isotopic labelling and incubation experiments, but also for the determination of natural isotopic abundances in groundwater and aquifer materials. In summary, the project will provide a novel methodological approach that will be highly useful to trace the biogeochemical cycling and fate of PO4 in groundwater systems.