Author's personal copy Controlling soluble iron and manganese in a water-supply reservoir using hypolimnetic oxygenation Paul A. Gantzer 1 , Lee D. Bryant 2 , John C. Little* Department of Civil and Environmental Engineering, 418 Durham Hall (0246), Virginia Tech, Blacksburg, VA 24061, United States article info Article history: Received 24 May 2008 Received in revised form 22 October 2008 Accepted 12 December 2008 Published online 25 December 2008 Keywords: Anoxia Bubble plume Hypolimnion Hypoxia Manganese Oxygenation abstract Soluble metals such as iron (Fe) and manganese (Mn) often reach problematic levels in water-supply reservoirs during summer stratification following the onset of hypolimnetic hypoxia. The behavior of soluble and particulate Fe and Mn was studied following the installation of a hypolimnetic oxygenation system in Carvins Cove Reservoir, a water- supply impoundment managed by the Western Virginia Water Authority. During oxygenation, manganese concentrations were very low in the bulk hypolimnion (<0.05 mg l 1 ), but high concentrations (>2.0 mg l 1 ) were still observed in the benthic region close to the sediment, despite near-sediment dissolved oxygen concentrations in excess of 5.0 mg l 1 . Oxygenation appears to affect the location of the oxic/anoxic boundary sufficiently to restrict substantial transport of soluble Mn to the bulk water of the hypo- limnion. However, the position of the oxic/anoxic boundary was not uniformly affected along the reservoir bottom, allowing horizontal transport of soluble Mn from higher elevations in contact with hypoxic sediments. During one summer, when the oxygen system was turned off for a month, the soluble Mn in the bulk hypolimnion increased substantially. Oxygen concentrations were quickly restored after the system was turned back on, but elevated levels of soluble Mn persisted until the sedimentation rate of detritus through the hypolimnion increased. When operated without interruption, the oxygenation system was able to reduce the bulk average hypolimnion soluble Mn concentration by up to 97%, indicating that source water control of soluble Mn and Fe can be accomplished with hypolimnetic oxygenation in water-supply reservoirs. ª 2008 Elsevier Ltd. All rights reserved. 1. Introduction Manganese (Mn) is a nuisance contaminant for the water treatment industry (Stauffer, 1986), which tends to remain in the reduced form, particularly under anoxic or even hypoxic conditions (dissolved oxygen (DO) <16% of saturation; Baden et al., 1995). It is especially prevalent at the oxic/anoxic boundary or redoxcline (Kristiansen et al., 2002; Granina et al., 2004; Koretsky et al., 2006). Manganese has complicated redox kinetics and is very difficult to chemically oxidize in pH envi- ronments typical of natural waters (pH 6–8) (Baden et al., 1995; Kristiansen et al., 2002; Roitz et al., 2002), often persisting in soluble forms despite unfavorable thermodynamics (Balzer, 1982; Dortch and Hamlin-Tillman, 1995). Mn-oxidizing micro- bial communities are often necessary for oxidation (Crittenden et al., 2005; Gabelich et al., 2006). Iron, in contrast, is easily oxidized chemically in the presence of oxygen at the oxic/anoxic boundary (Schaller et al., 1997), which makes it easier to control. * Corresponding author. Tel.: þ1 540 231 8737; fax: þ1 540 231 7916. E-mail addresses: Paul.Gantzer@gmail.com (P.A. Gantzer), lebryan1@vt.edu (L.D. Bryant), jcl@vt.edu (J.C. Little). 1 Tel.: þ1 206 999 1878. 2 Tel.: þ1 540 231 8737; fax: þ1 540 231 7916. Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres 0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.12.019 water research 43 (2009) 1285–1294