Impact of Oxidative Environments on the Measured Concentrations of Orthophosphate in Groundwater

Orthophosphate (PO43-) is a vital indicator for groundwater quality and eutrophication control. In the conventional “field sampling-laboratory measurement” workflow, the exposure of water samples to air often leads to systematically underestimated detection results; however, the quantitative attenuation patterns and in-situ coupling mechanisms remain poorly understood. This study investigated the coupled attenuation of PO43- and ferrous iron (Fe2+) in groundwater near an urban lake, Wuhan, using in-situ exposure and surface water mixing experiments. The results demonstrate that within 30 min of exposure to air, the removal rate of PO43- from the aqueous phase reaches 99.7%, with a 50.5% decrease occurring within the first 0–4 min. Furthermore, Fe2+ undergoes rapid oxidation after an 8-minute induction period, and the newly formed iron (hydr)oxides serve as the primary drivers for the rapid attenuation of PO43-. Following the mixing of groundwater with oxidized surface water, the PO43-concentration within 7 minutes was found to be only 5.78% of the theoretical dilution value, confirming the sequestration effect of the “iron curtain” on phosphorus. This study recommends strict oxygen-free preservation measures and acidification of water samples for groundwater PO43- detection to prevent loss of PO43- due to coprecipitation/adsorption.