ABSTRACT
Colloidal iron (Fe) oxides have been described in several systems including lakes, rivers, and groundwaters. The Tualatin river watershed of northwest Oregon is the site of intensive studies of phosphorous (P) dynamics. Colloids may have been ignored or underestimated in the past due to analytical difficulties in their measurement and identification. Filtration, the most common method for separation of soluble and solid phases, can underestimate the extent of colloidal constituents in such ways. While colloids have been described in natural systems, there are few estimates of the relative mass of Fe or trace elements in colloidal pools. A significant fraction of the P and Fe oxides in suspended sediments of the Tualatin river system are in the colloid size fraction. The lower portion of the mainstem of the river is characterized by low flow and high biological activity. Silicate may be important in stabilizing the colloids and inhibiting the release of solid phase P during recrystallization of the Fe oxides.
