Phosphorus retention at a redox transition zone in a floodplain aquifer of the Red River Delta in Vietnam

Harald Neidhardt1, Sebastian Rudischer1, Yvonne Oelmann1, Magnus Schneider2, Elisabeth Eiche2, Martyna Glodowska3, Emiliano Stopelli4, Alex Lightfoot4, Bhasker Rathi5, Vu Duyen6, Pham Thi Kim Trang6, Pham Hung Viet6, Michael Berg4
1 Arbeitsgruppe Geoökologie, Universität Tübingen
2 Institut für Angewandte Geowissenschaften, KIT
3 Arbeitsgruppe Geomikrobiologie, Universität Tübingen
4 Abteilung Wasserressourcen & Trinkwasser, Eawag
5 Center for Applied Geoscience, Tübingen University
6 CETASD, Vietnam National University

1.9 in Wasserqualität in Grundwasser und Flusseinzugsgebieten

26.03.2020, 17:00-17:15, Weißer Saal

An increasing number of publications has recently focussed on the fate of phosphorus (P) in aquifer systems, but still little is known regarding the controlling processes. The aim of this study was to investigate mechanisms of P retention under field conditions in the floodplain of the Red River in Vietnam. Due to intense pumping activities in the surroundings of Hanoi for the municipal water supply, reducing groundwater with dissolved P concentrations exceeding 2 mg L-1 is laterally migrating from a Holocene into an adjacent Pleistocene porous aquifer. As a consequence, a redox transition zone has formed at the boundary of the Holocene and the Pleistocene aquifer where groundwater P concentrations gradually decrease to below 0.01 mg L-1 (Neidhardt et al. 2018).

Pleistocene aquifer sediments were recovered from this transition zone to a depth of 46 meters during a drilling campaign in 2018. Due to prolonged contact with the reducing groundwater, the colour of the Pleistocene sediments has partially changed from orange to grey, which indicates a pronounced shift in the composition of the Fe-minerals in respective sediments. To assess the influence of these mineralogical changes on the retention of dissolved P from groundwater, sedimentary P pools were characterized with an optimized five-step sequential extraction scheme. Total P (Pt) was additionally assessed after microwave-assisted aqua regia digestion. Based on their colour, aquifer sediment samples were subdivided into three classes: orange (n=19), grey (n=6) and a mixture of both colours at the sub-cm scale (n=13). Mean Pt concentrations in the orange aquifer sediments (260 mg kg-1) were significantly higher as in the mixed (120 mg kg-1) and the grey colored sediments (100 mg kg-1). Strongly surface adsorbed P and P associated with Ca- and FeII-minerals formed the dominant P pools in the grey aquifer sediments. In the orange and mixed colored sediments, P occluded in FeIII-hydroxides represented the dominant P pool.

In summary, the observed compositions of P pools along the vertical profile reflected different P retention mechanisms in the aquifer sediments of the transition zone in dependence of the mineralogical composition. Adsorption but also occlusion of P in secondary FeIII-hydroxides dominated in the orange aquifer sections, whereas incorporation of P into secondary Ca- and FeII-minerals represented the principal P retention mechanism in the greyish sediments. Interestingly, the mixed colored sediments showed a similar P pool composition as the orange sediments despite the overall lower Pt concentrations. The extraction results provided an indirect insight into the geochemical processes controlling P concentrations in groundwater of Asian floodplain aquifers. Such knowledge regarding immobilization pathways can be highly valuable regarding the remediation of anthropogenically P-contaminated aquifer systems that threaten surface waters with eutrophication.

Neidhardt, H., Schoeckle, D., Schleinitz, A., Eiche, E., Berner, Z. A., Trang, P., Lan, V., Viet, P., Biswas, A., Majumder, A., Chatterjee, D., Oelmann, Y., Berg, M.: (2018): Biogeochemical phosphorus cycling in groundwater ecosystems – Insights from South and Southeast Asian floodplain and delta aquifers. - Sci. Total Environ., 644: 1357-1370.

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