A Novel Approach to Estimate Iron Distribution within Different Pore Domains of Structured Media

Abstract

The iron content of soils and aquifer solids usually is quantified using different extraction solutions performed with homogenized samples in a well-mixed batch experiment. For structured media where preferential flow prevails over the matrix flow, however, the iron content determined from homogenized samples may not well represent the iron available for biogeochemical reactions. In this study we performed ammonium oxalate extraction on a core of intact saprolite where physical structure was preserved. An unsaturated flow setup was modified with the intent to allow the extraction under two pore tensions, 15 cm and 0 cm of water, although a malfunctioning vacuum regulator made this more difficult than anticipated. Approximately 85% of the oxalate-extractable Fe was contained within the finer pore domain (matrix potential larger than 15 cm). Less than 15.5% of the extracted Fe mass (an upper bound) was present in domains of pore tension less than 15 cm. To the extent that Fe(III) oxides play an important role in contaminant biogeochemistry and solute transport, their distribution in structured subsurface media is critical to our understanding of these processes.