Abstract
Abstract. The retention of phosphorus in surface waters through co-precipitation of phosphate with Fe-oxyhydroxides during exfiltration of anaerobic Fe(II) rich groundwater is not well understood. We developed an experimental field set-up to study Fe(II) oxidation and P immobilization along the flow-path from groundwater into surface water in an agricultural experimental catchment of a small lowland river. We physically separated tube drain effluent from groundwater discharge before it entered a ditch in an agricultural field. Through continuous discharge measurements and weekly water quality sampling of groundwater, tube drain water, exfiltrated groundwater, and surface water, we investigated Fe(II) oxidation kinetics and P immobilization processes. The oxidation rate inferred from our field measurements closely agreed with the general rate law for abiotic oxidation of Fe(II) by O2. Seasonal changes in climatic conditions affected the Fe(II) oxidation process. Lower pH and lower temperatures in winter (compared to summer) resulted in low Fe oxidation rates. After exfiltration to the surface water, it took a couple of days to more than a week before complete oxidation of Fe(II) is reached. In summer time, Fe oxidation rates were much higher. The Fe concentrations in the exfiltrated groundwater were low, indicating that dissolved Fe(II) is completely oxidized prior to inflow into a ditch. While the Fe oxidation rates reduce drastically from summer to winter, P concentrations remained high in the groundwater and an order of magnitude lower in the surface water throughout the year. This study shows very fast immobilization of dissolved P during the initial stage of the Fe(II) oxidation process which results in P-depleted water before Fe(II) is completely depleted. This cannot be explained by surface complexation of phosphate to freshly formed Fe-oxyhydroxides but indicates the formation of Fe(III)-phosphate precipitates. The formation of Fe(III)-phosphates at redox gradients seems an important geochemical mechanism in the transformation of dissolved phosphate to structural phosphate and, therefore, a major control on the P retention in natural waters that drain anaerobic aquifers.
Highlights
Eutrophication of freshwater ecosystems following high nutrient loads is a widely recognized water quality problem in agricultural catchments
The measured Fe concentration at sampling locations other than the groundwater may partly be attributed to dissolved Fe(III) colloids or complexed Fe(II) that penetrates through the 0.45 μm filters and dissolve in the acidic media, this is discussed
We show that dynamics in redox processes as well may impact the Fe(II) oxidation process and phosphorus immobilization during flow from groundwater into surface water
Summary
Eutrophication of freshwater ecosystems following high nutrient loads is a widely recognized water quality problem in agricultural catchments. Several studies in fresh water systems suggested that substantial dissolved-phosphate loads in surface waters may originate from exfiltration of shallow or deep groundwater (Holman et al, 2008; Dahlke et al, 2012; Scanlon et al, 2005) This is especially likely to occur in delta areas (Griffioen, 2006; Hayashi and Yanagi, 2009), where the soil water and shallow groundwater is typically pH-neutral to slightly acid, anoxic, and iron-rich. The chemical composition of surface waters in delta areas is normally pH-neutral to slightly alkaline and oxic with low dissolved iron and phosphate concentrations This difference in chemical composition between groundwater and surface water creates strong redox and pH gradients at the groundwater–surface water interface (Frei et al, 2012; Carlyle and Hill, 2001). The oxidation of iron(II) followed by iron(III) hydrolysis and precipitation of iron oxyhydroxides is the dominant chemical reaction (Griffioen, 2006; Gunnars et al, 2002; Kaegi et al, 2010; von Gunten and Schneider, 1991; Baken et al, 2013), that determines the fate of many biochemically important solutes that co-precipitate with the iron oxyhydroxides such as PO4 (Châtellier et al, 2004; Deppe and Benndorf, 2002; Fox, 1989; Lienemann et al, 1999; Mayer and Jarrell, 2000; Voegelin et al, 2013) and AsO4 (Meng et al, 2002; Roberts et al, 2004; Hug and Leupin, 2003)
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