Geochemistry of iron in the Salton Sea, California

Abstract

The Salton Sea is a large, saline, closed-basin lake in southern California. The Sea receives agricultural runoff and, to a lesser extent, municipal wastewater that is high in nutrients, salt, and suspended solids. High sulfate concentrations (4x higher than that of the ocean), coupled with warm temperatures and low-redox potentials present during much of the year, result in extensive sulfate reduction and hydrogen sulfide production. Hydrogen sulfide formation may have a dramatic effect on the iron (Fe) geochemistry in the Sea. We hypothesized that the Fe(II)-sulfide minerals should dominate the iron mineralogy of the sediments, and plans to increase hypolimnetic aeration would increase the amount of Fe(III)-oxides, which are strong adsorbers of phosphate. Sequential chemical extractions were used to differentiate iron mineralogy in the lake sediments and suspended solids from the tributary rivers. Iron in the river-borne suspended solids was mainly associated with structural iron within silicate clays (70%) and ferric oxides (30%). The iron in the bottom sediments of the lake was associated with silicate minerals (71% of the total iron in the sediments), framboidal pyrite (10%), greigite (11%), and amorphous FeS (5%). The ferric oxide fraction was <4% of the total iron in these anaerobic sediments. The morphological characteristics of the framboidal pyrite as determined using SEM suggest that it formed within the water column and experiences some changes in local redox conditions, probably associated with alternating summer anoxia and the well-mixed and generally well-aerated conditions found during the winter. The prevalence of Fe(II)-sulfide minerals in the sediments and the lack of Fe(III)-oxide minerals suggest that the classic model of P-retention by Fe(III)-oxides would not be operating in this lake, at least during anoxic summer conditions. Aeration of the hypolimnion could affect the internal loading of P by changing the relative amounts of Fe(II)-sulfides and Fe(III)-oxides at the sediment/water interface. © 2008 Springer Science+Business Media B.V.