Biogeochemical phosphorus cycling in groundwater ecosystems – Insights from South and Southeast Asian floodplain and delta aquifers

• Published in: The Science of the total environment 2018-12, Vol.644, p.1357-1370
• Main Authors: Neidhardt, Harald, Schoeckle, Daniel, Schleinitz, Anna, Eiche, Elisabeth, Berner, Zsolt, Tram, Pham T.K., Lan, Vi M., Viet, Pham H., Biswas, Ashis, Majumder, Santanu, Chatterjee, Debashis, Oelmann, Yvonne, Berg, Michael
• Format: Article
• Language: English
• Subjects: Aquatic phosphorus cycling; Asian floodplain and delta regions; Ecological and Environmental Phenomena; Ecosystem; Environmental Monitoring; Groundwater; Groundwater - chemistry; India; Phosphate; Phosphorus - analysis; Reducing aquifers; Stable oxygen isotopic signature in phosphate; Vietnam; Water Pollutants, Chemical - analysis
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2018.07.056

The biogeochemical cycling of phosphorus (P) in South and Southeast Asian floodplain and delta aquifers has received insufficient attention in research studies, even though dissolved orthophosphate (PO43−) in this region is closely linked with the widespread contamination of groundwater with toxic arsenic (As). The overarching aim of this study was to characterize the enrichment of P in anoxic groundwater and to provide insight into the biogeochemical mechanisms underlying its mobilization, subsurface transport, and microbial cycling. Detailed groundwater analyses and in situ experiments were conducted that focused on three representative field sites located in the Red River Delta (RRD) of Vietnam and the Bengal Delta Plain (BDP) in West Bengal, India. The results showed that the total concentrations of dissolved P (TDP) ranged from 0.03 to 1.50 mg L−1 in groundwater, with PO43− being the dominant P species. The highest concentrations occurred in anoxic sandy Holocene aquifers where PO43− was released into groundwater through the microbial degradation of organic carbon and the concomitant reductive dissolution of Fe(III)-(hydr)oxides. The mobilization of PO43− may still constitute an active process within shallow Holocene sediments. Furthermore, a sudden supply of organic carbon may rapidly decrease the redox potential, which causes an increase in TDP concentrations in groundwater, as demonstrated by a field experiment. Considering the subsurface transport of PO43−, Pleistocene aquifer sediments represented effective sinks; however, the enduring contact between oxic Pleistocene sediments and anoxic groundwater also changed the sediments PO43−-sorption capacity over time. A stable isotope analysis of PO43−-bound oxygen indicated the influences of intracellular microbial cycling as well as a specific PO43− source with a distinct isotopically heavy signal. Consequently, porous aquifers in Asian floodplain and delta regions proved to be ideal natural laboratories to study the biogeochemical cycling of P and its behavior in groundwater environments. [Display omitted] •Overall high PO43− conc. in groundwater of the Red River Delta and the Bengal Delta•PO43− represented the dominant P species in anoxic groundwater reaching 1.5 mg L−1.•PO43− release via reductive dissolution of Fe-oxides coupled with OM mineralization•PO43− is subject to microbial cycling, subsurface transport, and immobilization.•Stable O isotope analysis in PO43− as tool to unravel P cycling and