Assessing Colloidal Forms of Phosphorus and Iron in the Tualatin River Basin

Abstract

AbstractAlthough colloids can affect the transport and chemistry of trace constituents in some aquatic systems, there are few estimates of the mass of given constituents in colloidal forms. The existence, variability, and origin of colloidal Fe oxide, and its effect on P chemistry, was evaluated in the Tualatin River Basin of northwestern Oregon. Watershed streams were regularly sampled during 1992 and 1993. The concentrations of P and Fe in the colloidal size class (0.05–1.0 µm) were determined by filtration. Elemental composition of the suspended material was assessed using scanning electron microscopy/energy‐dispersive spectroscopy (SEM/EDS). Colloidal P and Fe ranged from 0 to 48% and 2 to 77% of the total P and total Fe, respectively. Since filtration underestimates colloidal concentrations, these are conservative estimates. Concentrations of P and Fe in colloidal form were correlated (r2 = 0.83; P < 0.001) but total P, total Fe, and total suspended sediments were not correlated. SEM/EDS analysis showed that, in addition to P and Fe, the colloids contained Si, Al, and Ca. The colloids were enriched with P and Fe relative to Al and Si. Evidence suggests that the colloidal P and Fe particles form as groundwater‐ or sediment‐released Fe(II) is oxidized to Fe(III), which associates with P, either as coatings on the surface of colloidal clays and organics or as homogeneous particles.