BayCEER Workshop 2022

Phosphorus mobilization and interactions with iron in response to redox dynamics in a drained wetland area

P 4.9 in Poster Session

Riparian wetlands are highly reactive biogeochemical interfaces and can play a crucial role in the retention and mobilization of nutrients and contaminants. Due to fluctuating hydrogeochemical conditions, this critical zone can strongly influence the fate of phosphorus, which is controlled by the redox status of Fe (hydr)oxides. Previously adsorbed or coprecipitated P may release from reductive dissolution of Fe-oxides under anoxic conditions. In a study area located in the basin of granitic, apatite-rich mountains in north-eastern Bavaria, Germany, the drainage system releases a significant amount of dissolved Fe and P and may cause eutrophication in a recreational lake.

This work aimed to address the enrichment of groundwater-borne P and its response to changing redox conditions. We established a monitoring site with four groundwater wells to identify the processes which govern Fe and P dynamics in a reducing geochemical environment. Multilevel sampling by dialysis bags has been conducted in GWs to investigate water chemistry at the redox interface. In addition, soil sequential extractions have been applied to address the interaction between GW P and Fe with surrounding sediment.

The measured concentration in the drainage water was up to 13 µmol/L (0.4 mg/L) for SRP and 170 µmol/L (9.4 mg/L) for Fe2+. Among four groundwater wells, the average SRP showed significant difference and ranged between 1.2 and 14.3 µmol/L, while the redox potential varied from oxidizing to weakly reducing. Our preliminary results indicated that reductive dissolution of P-rich aquifer sediments and organic-P mineralization in the entire region lead to geogenic P and Fe accumulation. However, the local redox dynamics result from hydrological fluctuations that control the measured groundwater P and Fe concentrations. We found that both P-retention capacity and soil P-content were the highest at the groundwater surface. Therefore, the soil capillary fringe may be considered as a critical zone for P mobilization.

Keywords: phosphorus mobilization redox dynamics