THE DISTRIBUTION AND REDOX CHEMISTRY OF IRON IN THE PETTAQUAMSCUTT ESTUARY

A series of high resolution (10 cm) vertical profiles of iron were det ermined across the oxic/anoxic boundary in the Lower Pond of the Petta quamscutt Estuary. Selective chemical treatments and multiple analytic al methods were used to determine the oxidation state and lability of iron across the oxic/anoxic boundary. The vertical distributions of di ssolved and total iron were determined by atomic absorption spectrosco py, and dissolved Fe(II) and reducible iron were determined using a mo dified Ferrozine spectrophotometric method. Well-developed maxima of t otal dissolved iron approximate to 7.5 mu M occurred within the oxic/a noxic transition zone. Analysis of Fe(II) by the FZ method indicates t hat more than 95% of the dissolved iron determined by atomic absorptio n spectroscopy within the maximum is in the form of Fe(II). The concen tration of dissolved Fe(II) ranged from <4 nM in oxygenated surface wa ters to between 7 and 8 mu M at the total dissolved iron maximum. Both dissolved and total iron samples were treated with ascorbic acid to q uantify the fraction of iron that was reducible in this system. Dissol ved iron is quantitatively reduced to Fe(II) by 3.5 m depth, and parti culate iron was almost completely dissolved by 6 m. Thermodynamic spec iation calculations indicate that the dominant species of Fe(TI) in th e anoxic waters is the Fe(HS)(+) complex. In addition, the concentrati on of Fe(II) in the anoxic zone appears to be controlled by precipitat ion of a sulfide phase, the ion activity product for waters below 7 m is in good agreement with the solubility product of mackinawite. The v ertical distribution of oxidation states of the metals indicates non-e quilibrium conditions due to microbiological and chemical processes oc curring in the redox transition zone. A one-dimensional vertical, eddy diffusion model is presented that incorporates redox reactions of iro n, sulfide and oxygen. The modeling suggests the maximum in Fe(II) can be achieved through inorganic oxidation and reduction reactions, howe ver the depth at which the maximum occurs is sensitive to sulfide oxid ation, which appears to be dominated by biological oxidation. The magn itude of the Fe(II) maximum depends on the flux of iron into the basin , and reductive dissolution of particulate iron. (C) 1997 Academic Pre ss Limited.