The Differential Geochemical Behavior of Arsenic and Phosphorus in the Water Column and Sediments of the Saguenay Fjord Estuary, Canada

Abstract

The distribution and partitioning of dissolved and particulate arsenic and phosphorus in the water column and sediments of the Saguenay Fjord in Quebec, Canada, are compared. In addition, selective and/or sequential extractions were carried out on the suspended particulate matter (SPM) and solid sediments to contrast their geochemical behaviors in this natural aquatic system. Results of our analyses show that both arsenic and soluble reactive phosphate are actively scav- enged from the water column by settling particles. Upon their accumulation at the sediment-water interface some As and P may be released to porewaters following the degradation of organic matter to which they are associated. The porewater concentrations are, however, limited by their strong affinity for authigenic, amorphous iron oxyhydroxides which accumulate in the oxic sediments near the sediment-water interface. The geochemical behavior of arsenic and phosphorus diverge most strikingly upon the development of anoxic conditions in the sediments. Following their burial in the anoxic zone, amorphous iron oxyhydroxides are reduced and dissolved, releasing phosphate and arsenic to the porewaters. We observed, however, that porewater arsenic concentrations increase at shallower depths than phosphate in the sediments. The reduction of arsenate, As(V), to arsenite, As(III), and its desorption prior to the reductive dissolution of the carrier phase(s) may explain this observation. Driven by the strong concentration gradient established in the suboxic zone, phosphate diffuses up towards the oxic layer where it is readsorbed by authigenic iron oxyhydroxides. In the organic-rich and rapidly accumulating sediments at the head of the Fjord, porewater sulfate depletion and the resulting absence of a sulfide sink for Fe(II), may lead to the formation of vivianite in the fermentation zone, a potential sink for phosphate. Arsenite released to the porewaters in the suboxic and anoxic zones of the sediments diffuses either down, where it is adsorbed to or incorporated with authigenic iron sulfides, or up towards the oxic boundary. Arsenite appears to migrate well into the oxic zone where it may be oxidized by authigenic manganese oxides before being adsorbed by iron oxyhydroxides present at the same depth. Whereas, in the absence of authigenic carbonate fluorapatite precipitation, the ability of oxic sediments to retain mineralized phosphate is a function of their amorphous iron oxyhydroxide content, arsenic retention may depend on the availability of manganese oxides, the thickness of the oxic layer and, its co-precipitation with iron sulfides at depth.