Environmental Factors Affecting Oxidation of Manganese in Pinal Creek, Arizona

The objectives of the laboratory work reported here were to quantify the net rates of removal of manganese [Mn(II)] by streambed sediments collected from a metals contaminated, perennial stream system (Pinal Creek near Globe AZ) and to determine the key variable(s) responsible for the limited removal of Mn(II) observed at this field site. Pinal Creek is characterized by significant spatial gradients in pH, alkalinity, and Mn(II) along its length and by spatial gradients in pH, dissolved oxygen, and Mn(II) across hyporheic zones of varying thickness. These gradients are established by mixing of surface water and entering shallow ground water in the sediments. The mixing of waters in the hyporheic zones define the local chemical environments in which Mn(II) is removed by incorporation into pre-existing and newly formed mineral surfaces through abiotic and biotic processes. As a consequence of the site characteristics, particularly the spatial gradients in the hyporheic zone, the primary chemical parameters included in the test matrix were pH, initial Mn(II) concentration, and dissolved oxygen. Streambed sediments collected from the site were used in laboratory batch investigations of the rate of Mn(II) removal. Results of these studies indicate that removal of Mn(II) within the hyporheic zone primarily occurs via biotic oxidation processes, is approximately first-order with respect to the Mn(II) concentration and inversely proportional to [H], and independent of dissolved oxygen concentration except at very low levels.