Increased sediment oxygen uptake caused by oxygenation-induced hypolimnetic mixing

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 3 6 9 2 e3 7 0 3

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Increased sediment oxygen uptake caused by oxygenation-induced hypolimnetic mixing Lee D. Bryant*, Paul A. Gantzer 1, John C. Little Department of Civil and Environmental Engineering, 418 Durham Hall, Virginia Tech, Blacksburg, VA 24061, USA

article info

abstract

Article history:

Hypolimnetic oxygenation systems (HOx) are increasingly used in lakes and reservoirs to

Received 23 December 2010

elevate dissolved oxygen (O2) while preserving stratification, thereby decreasing concen-

Received in revised form

trations of reduced chemical species in the hypolimnion. By maintaining an oxic zone in

1 April 2011

the upper sediment, HOx suppress fluxes of reduced soluble species from the sediment

Accepted 12 April 2011

into the overlying water. However, diminished HOx performance has been observed due to

Available online 19 April 2011

HOx-induced increases in sediment O2 uptake. Based on a series of in situ O2 microprofile

Keywords:

sediment-water interface as a function of HOx operation. These data were used to deter-

Sediment-water flux

mine how sediment O2 uptake rate (JO2 ) and sediment oxic-zone depth (zmax) were affected

and current velocity measurements, this study evaluates the vertical O2 distribution at the

Sediment oxic zone

by applied oxygen-gas flow rate, changes in near-sediment mixing and O2 concentration,

Microprofile

and proximity to the HOx. The vertical sediment-water O2 distribution was found to be

Hypolimnetic oxygenation

strongly influenced by oxygenation on a reservoir-wide basis. Elevated JO2 and an oxic

Lake and reservoir management

sediment zone were maintained during continuous HOx operation, with zmax increasing linearly with HOx flow rate. In contrast, JO2 decreased to zero and the sediment became

In situ

anoxic as the vertical O2 distribution at the sediment-water interface collapsed during periods when the HOx was turned off and near-sediment mixing and O2 concentrations decreased. JO2 and zmax throughout the reservoir were found to be largely governed by HOxinduced mixing rather than O2 levels in the water column. By quantifying how JO2 and zmax vary in response to HOx operations, this work (1) characterizes how hypolimnetic oxygenation affects sediment O2 dynamics, (2) contributes to the optimization of water quality and management of HOx-equipped lakes and reservoirs, and (3) enhances understanding of the effect of mixing and O2 concentrations in other systems. ª 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Dissolved oxygen (O2) has been identified as one of the most critical environmental factors controlling water quality and associated ecological conditions (Hondzo et al., 2005). Aquatic ecosystems, hydropower plants, and water quality are all negatively affected by depleted O2 concentrations (Beutel and

Horne, 1999). Water-quality standards typically require O2 > 150 mmol L
1 to protect aquatic life (EPA, 2000). Hydropower plants are usually required to meet these minimum O2 levels in the water they discharge downstream (Mobley et al., 2000a). Long-term O2 depletion in fish habitats has been shown to cause significant declines in fish populations as a result of endocrine system disruptions and reproductive

* Corresponding author. Present address: Department of Civil and Environmental Engineering, Box 90287, 121 Hudson Hall, Duke University, Durham, NC 27708, USA. Tel.: þ1 919 660 5034; fax: þ1 919 660 5219. E-mail addresses: [email protected] (L.D. Bryant), [email protected] (P.A. Gantzer), [email protected] (J.C. Little). 1 Present address: Gantzer Water Resources LLC, 14816 119th Place NE, Kirkland, WA 98084, USA. 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.04.018

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 3 6 9 2 e3 7 0 3

impairment, with low O2 possibly having more of an effect than anthropogenic chemicals (Wu et al., 2003). Organic or nutrient loading of thermally stratified lakes and reservoirs may lead to extensive depletion of O2 in the deeper hypolimnetic water. Hypolimnetic O2 depletion can result in the release of soluble chemical species from the sediment, decreasing water quality and increasing drinking-water treatment costs (Gantzer et al., 2009a). Oxygen depletion in lakes and reservoirs is largely controlled by sediment O2 uptake which is regulated by near-sediment hydrodynamics and the intrinsic sediment O2 demand (Veenstra and Nolen, 1991; O’Connor et al., 2009). Understanding the processes controlling the sediment O2 uptake rate (JO2 ) and other sediment-water fluxes is crucial for optimizing water quality and successfully managing lakes and reservoirs (Zhang et al., 1999; Beutel, 2003). Hypolimnetic oxygenation systems (HOx) are used increasingly by drinking water and hydropower utilities to replenish O2 and decrease concentrations of soluble metals, such as iron (Fe) and manganese (Mn), and other chemical species in source water while preserving stratification (McGinnis and Little, 2002; Beutel et al., 2007; Gantzer et al., 2009b). While several types of systems are used for hypolimnetic oxygenation (Singleton and Little, 2006; Moore and Christensen, 2009), this study is based on the performance of a bubble-plume diffuser HOx. Bubble-plume HOx release oxygen gas from diffusers positioned near the reservoir bottom and impart relatively low levels of mixing within the hypolimnion to maintain overall thermal structure. These HOx thereby increase hypolimnetic O2 concentrations while suppressing mixing of the epilimnion and hypolimnion and preventing destratification (Wu¨est et al., 1992; McGinnis et al., 2004). Ideally, an HOx increases O2 availability both in the water column and in the upper sediment to prevent the release of reduced chemicals into the hypolimnion (Zaw and Chiswell, 1999; Beutel 2003). The balance between the amount of O2 supplied to the sediment via JO2 and the amount consumed via various sediment biogeochemical processes (e.g., benthic mineralization of organic matter and oxidation of reduced chemicals) governs sediment oxic-zone depth (zmax) and sediment O2 availability (Glud et al., 2007). Oxygenation may, however, cause excessive O2 uptake in the hypolimnion and sediment due to HOx-induced increases in hypolimnetic O2 concentrations and turbulent mixing (Moore, 2003; Gantzer et al., 2009b). JO2 is a function of the O2 concentration driving force across the diffusive boundary layer (DBL), a mm-scale laminar layer immediately above the sediment-water interface (SWI; Jørgensen and Revsbech, 1985). Turbulence in the overlying water governs DBL thickness (dDBL; Lorke et al., 2003; Bryant et al., 2010a) and hence directly affects JO2 . Despite the potential influence that HOx may have on sediment O2 uptake, reservoir-specific JO2 measurements are rarely available and HOx are often designed based on O2 depletion rates measured prior to installation of the systems (Moore et al., 1996; Mobley et al., 2000b; Beutel et al., 2007). While the relationship between near-sediment current velocity and JO2 has been investigated (Gundersen and Jorgensen, 1990; Mackenthun and Stefan, 1998), little work has been done to quantify how HOx operations affect JO2 and

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the vertical distribution of O2 at the SWI. Most studies performed have been laboratory based (Moore et al., 1996; Beutel, 2003). However, it has been shown that JO2 can be significantly affected by variations in natural turbulence (Lorke et al., 2003; Bryant et al., 2010a) and laboratory studies may not capture actual HOx-driven conditions. The research presented here is therefore based on in situ O2 sediment-water microprofiles and near-sediment current velocity to address five key themes related to the influence of HOx on sediment O2 uptake. This study characterizes how (1) the vertical O2 distribution at the SWI and (2) sediment O2 dynamics (quantified by JO2 ) are affected by HOx operations in a drinking-water-supply reservoir. It also evaluates (3) the sediment oxic zone (quantified by zmax) as a function of HOx oxygen-gas flow rate and (4) spatial variation in the influence of the HOx. Finally, (5) broader impacts on reservoir water quality are assessed. Focus is first placed on a multi-day in situ campaign that shows how JO2 and the vertical O2 distribution at the SWI respond to turning the HOx off for w48 h and then back on. Building on these results, a multi-year data set is then used to quantify the influence of HOx on sediment O2 conditions using a broader range of flow rates. To the authors’ knowledge, this is the first study to assess in situ how HOx-induced variation in nearsediment mixing and O2 concentrations influence JO2 and the sediment oxic zone on a reservoir-wide scale.

2.

Materials and methods

2.1.

Study site

This research focused on Carvins Cove Reservoir (CCR), which is managed by the Western Virginia Water Authority to provide drinking water to the county of Roanoke, Virginia, USA. CCR is a stream-fed lake that has been managed as a drinking-watersupply reservoir since the late 1940s. CCR is eutrophic and has a maximum depth of 23 m, width of w600 m, and length of w8000 m (Fig. 1). In 2005, a bubble-plume line diffuser HOx was installed in the deepest section of the reservoir near the water treatment plant withdrawal (Fig. 1) to replenish O2 depleted during summer stratification and to minimize soluble Fe and Mn in the source water (McGinnis and Little, 2002; Gantzer et al., 2009a). The CCR HOx delivers pure oxygen gas over a wide range of flow rates, providing considerable operational flexibility. Thus, it was possible to vary O2 and artificiallyinduced mixing in the reservoir by changing the applied HOx flow rate. Data were collected from 2005 through 2008 to assess how the HOx affects CCR, with substantial improvement in water quality observed since HOx operations began (Gantzer et al., 2009a,b). Research presented here on HOx-induced variation in the vertical O2 distribution at the SWI extends the work of Gantzer et al. (2009a,b), which focused on watercolumn conditions, by evaluating the influence of HOx on sediment O2 dynamics and the sediment oxic zone.

2.2.

Data collection and analysis

This study is based on in situ data obtained during field campaigns using a microprofiler (MP4; Unisense A/S) to obtain profiles at the SWI and an acoustic Doppler current profiler

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end of the HOx; Fig. 1). During May to August 2007, monthly MP4 microprofile measurements were also obtained downstream, alongside, and 1000 m upstream of the HOx at sites CCR-1, CCR-2, and CCR-6, respectively, while the HOx was operated at four different oxygen-gas flow rates increased incrementally on a monthly basis from 17 to 70 m3 h
1. Each day that O2 microprofile measurements were obtained, conductivity, temperature, and O2 as a function of depth (CTD) profiles of the water column were measured at each sampling location using a Seabird Electronics SBE 19plus profiler. The SBE 19plus has a 4-Hz sampling rate and an O2 sensor response time of 1.4 s.

2.2.1.

Fig. 1 e Map of Carvins Cove Reservoir (CCR) showing linear bubble-plume hypolimnetic oxygenation system (HOx) and sampling sites (near-field locations CCR-1 (0 m; relative to start of HOx lines) and CCR-2 (189 m); midreservoir locations CCR-3 thru CCR-6 (683, 1011, 1373, and 1814 m, respectively); back-reservoir location CCR-7 (3000 m)).

(ADCP; Teledyne RDI, Inc.) for current velocities in the water column. During the primary field campaign in 2008, HOx flow was typically maintained at 51 m3 h
1 (or w1580 kg O2 d
1). However, the HOx was turned off for w48 h during two experimental campaigns (each w1e2 weeks in duration) in June and August 2008 to track the response of the vertical O2 distribution at the SWI and corresponding JO2 . The relatively short period that the HOx was turned off was planned to preserve reservoir water quality by minimizing effects on hypolimnetic O2 levels. Previous work by Gantzer et al. (2009b) showed that turning the HOx off for a longer period (e.g., several weeks) can impair water quality substantially due to the accumulation of reduced compounds as the hypolimnion becomes hypoxic. During the first campaign, the MP4 microprofiler and ADCP were deployed alongside the HOx at site CCR-2 (Fig. 1) from June 19 to 26. The HOx was turned off from June 19 to 21. Data were downloaded and batteries for the MP4 and the ADCP were exchanged daily. A similar campaign was performed in August at the mid-reservoir site CCR-6, w1000 m upstream of the end of the HOx, and data were collected almost continuously from August 18 to 30. The HOx was turned off from August 19 to 21. While analogous results were obtained from both campaigns, reported results are based largely on the August CCR-6 campaign due to insufficient background data for the June CCR-2 campaign. In addition to these multi-day campaigns, MP4 and ADCP data were collected monthly from June to September 2008 at CCR-2, CCR-6, and CCR-7 (located w2000 m upstream of the

O2 microprofiles

The in situ autonomous MP4 microprofiler equipped with microsensors (O2 and temperature) was used to obtain microprofiles at the SWI. The O2 microsensor (OX-100; Unisense A/S) had a 100-mm tip diameter and depth resolution, fast response time (90% in <8 s), and negligible stirring sensitivity. The O2 microsensor was a Clark-type sensor with an internal reference and guard cathode. The temperature microsensor (TP-100; Unisense A/S) was a thermo-coupled sensor with a tip diameter and depth resolution of w200 mm, measurement resolution of 0.1 mV per  C, and a 90% response time of <3 s. Profiles were measured nearly continuously (exceptions include while data were downloaded and a period on August 26e27 when a storm prevented use of the boat to download data) during the multi-day campaigns and obtained in duplicate at each CCR sampling location during monthly measurements. Profiles were obtained as follows: 10-mm resolution from 10 cm to 1 cm above the SWI, 1-mm resolution from 1 cm to 0.5 cm above the SWI, and 0.1-mm resolution from 0.5 cm above the SWI to 0.5 cm below the SWI. The SWI location was determined as described by Bryant et al. (2010a,b). A video camera was used periodically to ensure that the MP4 remained stable and did not sink. Ten measurements were typically taken at each depth. For the multi-day campaigns, however, three measurements were obtained per depth due to data-storage limitations during overnight deployments. Following a pause between measurements for equilibrium to be established at each depth, microsensor data were collected at a rate of 1 Hz. Time required to obtain a full profile was w50e70 min and profiles are referenced by the time when the microsensor encountered the SWI. A two-point, linear calibration of the O2 microsensor was performed using zero readings from anoxic sediment and Winkler titration of water sampled immediately above the sediment using a Kemmerer bottle and/or sediment cores. zmax was designated as the depth where O2 drops to <3 mmol L
1.

2.2.2.

Current velocity

Velocity profiles were collected using a 1200 kHz Workhorse Rio Grande ADCP equipped with four transducers in a janus configuration with a beam angle of 20 . The ADCP, positioned adrift alongside the research vessel and facing downward from the water surface, profiled the water-column depth using a 1-m bin size. Samples were obtained in a multi-ping mode with 50 samples per ensemble at a rate of 2 Hz. Accuracy of velocity measurements was 0.25% of water-plus-boat velocity 0.25 cm s
1. ADCP motion relative to the sediment

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was measured using bottom tracking. The boat remained stationary during measurements and boat velocity was negligible. Near-sediment velocities were of primary interest; however, the ADCP has a blanking distance near the sediment due to interference from the angled acoustic beams reflecting off the sediment surface, thereby contaminating the acoustic signal returned to the ADCP. The near-sediment blank zone is w6% of the total water-column depth, as defined by RDI, or approximately the bottom 1 m in CCR. Hence, near-sediment velocities were evaluated at w2 m above the sediment.

2.2.3.

O2 microprofile analyses

JO2 and dDBL were evaluated based on O2 microprofile data using Fick’s law (Rasmussen and Jørgensen, 1992):


h i vC
vC
Cbulk
CSWI


1 mmolm
2 d JO2 ¼
4Ds

¼
D

¼D
dDBL vz sed vz water water (1) where 4 is sediment porosity (m3 voids m
3 total volume), Ds is the diffusion coefficient for O2 in sediment (m2 s
1), D is the diffusion coefficient for O2 in water (m2 s
1), vC/vz is the linear O2 concentration gradient in the DBL water above or in the sediment immediately below the SWI (mmol m
4), Cbulk is the O2 concentration in the bulk water (mmol L
1), and CSWI is the O2 concentration at the SWI (mmol L
1). JO2 and depth z are defined positive downwards into the sediment. Sediment cores from the primary sampling locations were used to evaluate 4 following Dalsgaard et al. (2000) and 4 values of 0.95e0.97 were obtained for the upper 1 cm of sediment. Values for D were based on D ¼ 1.97  10
9 m2 s
1 at 20  C, correcting for temperature using the Stokes-Einstein relationship (Li and Gregory, 1974; Arega and Lee, 2005). Ds was defined as Ds ¼ 4D to correct for sediment tortuosity as a function of 4 (Berg et al., 1998; Glud, 2008; Bryant et al., 2010b). The temporal change in O2 concentration (vC/vt) was evaluated for the series of in situ O2 profiles by comparing profiles immediately before and after one another and calculating the rate of change in O2 at each depth. vC/vt was found to be on average <5% of JO2 , establishing that measured profiles were at quasi-steady state. Fick’s law may be applied to either water- or sediment-side data to evaluate JO2 and dDBL (Rasmussen and Jørgensen, 1992; Bryant et al., 2010a,b). Diffusive transport in the sediment is given by the second term in Eq. (1) and in the water by the third and fourth terms. JO2 was evaluated as a function of (vC/ vz)sed from sediment O2 porewater data using the second term in Eq. (1). Because JO2 estimates were based on porewater data, water-side data were used for dDBL estimates to allow for an independent comparison with JO2 . Water-side dDBL was estimated as a function of the measured (vC/vz)water in the DBL using the third and fourth terms in Eq. (1).

2.2.4.

Turbulence estimations

Turbulence has been shown by Lorke et al. (2003) to have a more direct influence on dDBL and JO2 estimates than current velocity. Turbulence is characterized by the dissipation rate of turbulent kinetic energy, e (W kg
1), which is frequently estimated by applying the inertial dissipation method (Grant et al., 1962) to near-sediment velocity data, as performed by Bryant et al. (2010a) and Lorke et al. (2003) using acoustic

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Doppler velocimeter (ADV) data measured at 10 cm and 1 m above the sediment, respectively. However, as discussed in Section 2.2.2, obtaining near-sediment velocity measurements was restricted by the limited accuracy of the ADCP approaching the sediment. Estimates of velocity at w2 m above the sediment are thus a relative measure of nearsediment mixing conditions. To quantify turbulence levels in the absence of more precise near-sediment velocity data, a correlation between dDBL, the viscous boundary layer (the cm-scale region immediately above the DBL), and friction velocity (u*) was used to estimate e. According to Wu¨est and Lorke (2003), dDBL and viscous boundary layer thickness (dn) are related by:

dDBL ¼ dv

 1=3 D ½m v

(2)

where v is the kinematic viscosity of water (m2 s
1). In turn, dv is defined as a function of u* (Schlichting, 2000) via: dv ¼

11v ½m u

(3)

Combining Eqs. (2) and (3) allows u* to be related to dDBL:  1=3 D   v m s
1 dDBL

11v u ¼

(4)

The u* values estimate the frictional stress of currents on the sediment and, similar to e, characterize near-sediment turbulence. Estimated u* values obtained via Eq. (4) were used to calculate e using the law-of-the-wall assumption (Lorke et al., 2003): e¼

i u3 h
1 W kg kh

(5)

where k (the von Karman constant) is 0.41 and h is height above the sediment. For these estimates, e was evaluated at an assumed h ¼ 10 cm to obtain near-sediment e predictions and to allow for direct comparison with e based on ADV data measured in a similar system at h ¼ 10 cm (Bryant et al., 2010a). This comparison verified that e estimates for this study were typical for a freshwater lake.

3.

Results and discussion

3.1.

Vertical O2 distribution at SWI

Substantial variation in the vertical O2 distribution on both sides of the SWI was observed in response to halting oxygen flow for w48 h during the 2008 campaigns. Profiles (each of w50 min duration) were collected almost continuously over the course of both multi-day campaigns in 2008. A summary of profile results during the August CCR-6 campaign is presented in Fig. 2. Prior to the point at which the HOx was turned off on August 19 at w15:00, O2 concentrations were relatively high both in the water immediately above the sediment (w125 mmol L
1) and within the sediment porewater (w80 mmol L
1 at the SWI with zmax of 0.8 mm; Fig. 2a). A constant Cbulk and a well-defined DBL are also evident,

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signifying active near-sediment turbulence. However, as indicated by the 17:39 profile, O2 started to become depleted from the sediment and overlying water within a few hours of turning the HOx off (Fig. 2b). Approximately 5 h after halting oxygenation, the sediment and overlying water had become completely anoxic (profile 20:02). A transient oxic period occurred on August 21 (profiles 15:11, 15:58; Fig. 2c) when near-sediment O2 and zmax increased immediately after HOx operation resumed, which may be attributed to a large internal wave induced by turning on the HOx. Excluding this brief period, however, conditions remained almost completely anoxic until August 29 (Fig. 2c,d). A relatively rapid increase in O2 is then observed on both sides of the SWI, with the vertical O2 distribution returning to a structure similar to that at the beginning of the campaign (Fig. 2a). Contour plots using a Kriging interpolation scheme were created based on the full 255-profile series of O2 microprofile data obtained during the August CCR-6 campaign, background microprofiles, and corresponding CTD hypolimnetic O2 data (Fig. 3). While O2 concentrations near the SWI changed substantially in response to turning off the HOx for w48 h (Fig. 3a), O2 remained relatively constant (w200 mmol L
1) at 8 cm above the sediment (Fig. 3b) and the bulk water column was affected negligibly (Fig. 3c). The fact that O2 in the overlying water remained largely unaffected while the sediment became anoxic emphasizes how sediment O2 uptake depends on continual operation of the HOx. Although the HOx was turned off for only w48 h, it took w8 days for the vertical O2 distribution at the SWI to be restored during the August CCR-6 campaign (Figs. 2c,d and 3a). This delayed response may be attributed both to time required for a uniform flow pattern in the hypolimnion to be reestablished and to localized sediment resuspension effects (discussed further in Section 3.4). During a six-year study in an HOx-equipped reservoir similar to CCR, it was observed that it typically took w1 week for ‘steady-state’ O2 conditions to return in the water column after resuming oxygenation, with substantially higher initial O2 depletion rates in the hypolimnion (Gantzer, 2002; Gantzer et al., 2009a). The considerable influence that HOx operations had on the vertical sediment-water O2 distribution mid-reservoir at CCR6 (Figs. 2 and 3) was also evident in the near field at CCR-2 (Fig. 4). A strong correlation was found between HOx operation and parameters quantifying the vertical O2 distribution (O2 at 5 cm above the sediment (C5), CSWI, and zmax) during both the June and August 2008 campaigns. At CCR-6, as CSWI dropped from w80 to 0 mmol L
1 and C5 decreased from w200 to 50 mmol L
1 after turning off the HOx on August 19, zmax also rapidly decreased from 1 to 0 mm as O2 was depleted from the sediment (Fig. 4a). Apart from the brief oxic period on August 21 after the HOx was turned back on, sediment porewater and the SWI remained anoxic and C5 remained low until August 29 Fig. 2 e Summary of in situ dissolved oxygen (O2) profile data obtained at the sediment-water interface (SWI; at depth 0 mm) with the MP4 microprofiler during the August 2008 CCR-6 campaign. The HOx was turned off on August 19 (w15:00) and turned back on w48 h later on August 21 (w12:00). Profile data obtained prior to turning the HOx off are shown in (a). Profiles characterizing the periods when

the HOx was turned off and then back on are shown in (b) and (c, d), respectively. (b) Following the halt of HOx operations, O2 rapidly depleted from the sediment and the overlying water column. (c, d) An oxic vertical O2 distribution was not re-established until August 29, 8 days after the HOx was turned back on.

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Fig. 3 e Contour plots of O2 concentrations at the SWI (a; MP4 microprofile data), in the water overlying the sediment (b; MP4 microprofile data), and in the hypolimnion (c; CTD (conductivity-temperature-O2 as a function of depth) profile data). Data from the full set of 255 MP4 O2 profiles obtained during the August 2008 on/off campaign as well as from monthly profiles measured before and after the campaign are presented in (a, b). Shaded region indicates period when HOx was turned off. While O2 is rapidly depleted from the sediment and water immediately above the SWI after the HOx was turned off (a), O2 concentrations were only minimally affected at w8 cm above the sediment (b) and remained relatively constant in the bulk hypolimnion (c). In (a) and (b), depth represents distance above (L) or below (D) the SWI, which is indicated by the dashed line. Water-column depth in (c) is also characterized by distance above (L) the sediment.

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when oxic sediment conditions were re-established. Similar results were observed at CCR-2 after turning off the HOx on June 19 as CSWI and C5 both dropped to 0 mmol L
1 and zmax decreased from 0.5 to 0 mm (Fig. 4b). The response time for the re-establishment of an oxic sediment-water O2 distribution was also delayed at CCR-2, taking w5 days. A transient oxic period after resuming oxygenation was not observed at CCR-2 perhaps due to enhanced sediment resuspension in this nearfield region after turning on the HOx (discussed in Section 3.4). Due to equipment issues and the fact that O2 depletion occurred more rapidly than expected following the halt of HOx operation, data characterizing the initial O2 depletion phase were not obtained during the preliminary June campaign (Fig. 4b). Data measured before and after each multi-day campaign (e.g., August 12 and September 14; Fig. 4a) compare well with average CSWI and zmax values during normal continuous HOx operation (CSWI ¼ 94  38 mmol L
1 and zmax ¼ 1.4  0.6 mm at CCR-6 (n ¼ 22) and 61  45 mmol L
1 and 0.7  0.4 mm (n ¼ 15) at CCR-2).

3.2.

force at the SWI and suppression of the DBL resulting from increased O2 and turbulence levels, respectively, in the lower hypolimnion (Eq. (1); Bryant et al., 2010a,b). Sediment O2 dynamics (as characterized by JO2 ) and water-side controls on diffusive flux (as characterized by CSWI and dDBL) were affected considerably by HOx operations, as shown by CCR-6 data in Fig. 5a. During ongoing oxygenation, dDBL was suppressed while JO2 and CSWI remained elevated. Average summer JO2 and dDBL at CCR-6 were 12.5  7.6 mmol m
2 d
1 and 1.6  0.9 mm (n ¼ 22), respectively. However, in response to turning off the HOx on August 19, dDBL increased from 0.6 mm to the point of becoming undefined (Fig. 5a; no discernable DBL was measurable at dDBL >5 mm; hence, a nominal maximum dDBL ¼ 5 mm was assumed). Simultaneously, JO2 decreased from 12.5 to 0 mmol m
2 d
1 and CSWI dropped from w80 to 0 mmol L
1 as diffusive transport of O2 was restricted and O2 was depleted from the sediment. Per Eq. (1), as dDBL increases to the point of becoming undefined in the absence of turbulent mixing and the O2 concentration driving force

HOx-induced variation in sediment O2 dynamics

While HOx are designed to remediate problems caused by hypolimnetic O2 depletion, conceptually these systems should also increase sediment O2 uptake via enhanced O2 flux into the sediment due to an elevated O2 concentration driving

Fig. 4 e Variations in O2 concentrations at the SWI (CSWI), at 5 cm above the SWI (C5), and within the sediment (as characterized by sediment oxic-zone depth zmax) in response to turning off the HOx. Similar trends are observed in data from (a) the August 2008 campaign at CCR-6 and also (b) the June 2008 campaign at CCR-2. During the period of zero flow when the HOx was not in operation, O2 concentrations dropped both at 5 cm above and directly at the SWI and the sediment became anoxic as O2 was depleted.

Fig. 5 e The response of JO2 and water-side parameters influencing diffusive flux (a; diffusive boundary layer thickness (dDBL) and CSWI) to turning off the HOx corresponds directly to variations in near-sediment mixing (b) as characterized by current velocity at 2 m above the sediment (obtained via ADCP), turbulence (defined by energy dissipation rate (e) estimated as a function of waterside dDBL), and temperature at the SWI (obtained via MP4 temperature microsensor). Results shown are based on in situ data obtained during the August 2008 campaign at CCR-6; similar results were obtained from the June CCR-2 campaign. Data in Fig. 5 are average bi-daily values (standard deviation data provided in Table 1). The period during which the HOx was turned off is designated by the shaded region.

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becomes negligible when the sediment and overlying water are anoxic, JO2 goes to zero. Analogous to O2 data presented in Figs. 2c and 4a, JO2 and CSWI increased and dDBL decreased briefly after the HOx was turned back on (Fig. 5a, Table 1). However, JO2 and CSWI remained at approximately zero until August 29 when a steady-state flow pattern and oxic conditions at the SWI returned. As a well-defined and thinner DBL was re-established and diffusive O2 transport to the sediment resumed by August 29, CSWI increased to w70 mmol L
1 and JO2 increased to w15 mmol m
2 d
1 (Fig. 5a, Table 1). Average bidaily values are shown in Fig. 5 with standard deviations provided in Table 1 (variation in e not provided as this parameter is directly related to dDBL). The response of JO2 and the water-side vertical O2 distribution to HOx operations (Fig. 5a) closely parallels variation in near-sediment current velocity (as measured by ADCP), estimated turbulence levels (as characterized by e), and temperature at the SWI (as measured by temperature microsensor; Fig. 5b). Velocities at 2 m above the sediment dropped sharply soon after the HOx was turned off. After the HOx was turned back on, near-sediment velocities are observed to increase though they remained quite variable. Velocities returned to pre-campaign levels (w5 cm s
1; summer average was 6.2 cm s
1) on August 29, which corresponds with the timing of the re-established vertical O2 structure at the SWI (Figs. 2 and 4). Near-sediment velocity is shown to strongly correlate

with e, both of which decreased considerably when the HOx was turned off (Fig. 5b). The correlation between ADCP velocity data and estimated e values, which are based on O2 microprofile data, supports the evaluation of e as a function of dDBL and highlights the influence of mixing on dDBL. Estimates of dDBL-based e for this study are within the same range as ADV-based e values obtained by Bryant et al. (2010a) for windinduced mixing (i.e., seiching) in a freshwater lake. HOxinduced mixing, which has been shown to result in elevated temperatures in the hypolimnion (Gantzer et al., 2009b; Liboriussen et al., 2009), is further confirmed by variations in temperature at the SWI that closely follow trends in e and current velocity (Fig. 5b). A peak in current velocity, e, and temperature on August 21 corresponds to the oxic vertical O2 distribution, decreased dDBL, and elevated JO2 observed briefly at this time (Figs. 4a and 5) and supports the occurrence of a large internal wave following HOx start-up. These results reveal the controlling influence that HOx operation can have on the degree of mixing in the hypolimnion (Fig. 5b) and corresponding sediment O2 uptake (Fig. 5a). O2 concentrations in the lower hypolimnion remained largely unaffected by halting oxygenation (Fig. 3) with O2 levels staying relatively high only w8 cm above the sediment. Despite these high O2 concentrations, as near-sediment mixing decreased, dDBL increased, JO2 and CSWI decreased to zero, and the sediment and overlying water became anoxic (Figs. 3

Table 1 e Bi-daily averages and standard deviations for sediment oxygen uptake rate (JO2 ), dissolved oxygen (O2) at the sediment-water interface (CSWI), temperature at the sediment-water interface, diffusive boundary layer thickness (dDBL), near-sediment current velocity (U; measured at w2 m above the sediment), and hypolimnetic oxygenation system (HOx) flow rate during the August 2008 CCR-6 campaign. Average values for JO2 , CSWI, temperature, and dDBL (based on microsensor data) were estimated using the number (n) of profiles obtained during the designated time period. Corresponding averages for U were estimated using acoustic Doppler current velocity (ADCP) profile data. Standard deviations based on an assumed normal distribution. Date

Flow (m3 h
1)

n

JO2 (mmol m
2 d
1)

CSWI (mmol L
1)

Temperature ( C)

dDBL (mm)

U (cm s
1)

8/12 18:00 8/18 15:00 8/19 19:00 8/20 0:00 8/20 12:00 8/21 0:00 8/21 12:00 8/22 0:00 8/22 12:00 8/23 0:00 8/23 12:00 8/24 0:00 8/24 12:00 8/25 0:00 8/25 12:00 8/26 0:00 8/26 12:00 8/27 0:00 8/27 12:00 8/28 0:00 8/28 12:00 8/29 0:00 8/29 12:00 8/30 0:00 8/30 12:00 9/14 16:00

51 51 51 to 0 0 0 0 0 to 51 51 51 51 51 51 51 51 51 51 51 51 51 51 51 51 51 51 51 59

2 2 6 7 14 13 13 12 14 13 13 13 13 8 15 15 1 12 9 8 10 9 11 11 13 2

9.0  1.3 6.9  1.5 12.5  4.8 0.0  0.0 0.0  0.0 1.6  2.5 0.0  0.0 6.6  10.3 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 9.4  10.4 0.0  0.0 12.8  10.2 14.6  4.6 16.1  2.3

114.2  11.4 78.0  7.8 68.8  6.9 23.7  2.4 0.0  0.0 3.1  0.3 0.0  0.0 18.1  1.8 0.1  0.0 0.1  0.0 0.3  0.0 0.3  0.0 0.0  0.0 0.0  0.0 1.2  0.1 1.2  0.1 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 0.0  0.0 33.8  3.4 11.4  1.1 62.6  6.3 73.2  7.3 117.0  11.5

13.4  0.0 13.1  0.0 12.7  0.1 12.5  0.1 12.1  0.1 12.1  0.2 12.0  0.1 12.3  0.1 12.1  0.1 12.0  0.1 12.1  0.1 12.0  0.0 12.2  0.2 12.2  0.2 12.0  0.0 12.1  0.0 12.2  0.1 12.1  0.2 11.9  0.0 11.9  0.0 11.8  0.2 12.0  0.1 12.0  0.1 12.3  0.1 12.5  0.2 13.2  0.0

0.8  0.1 1.8  0.2 0.6  0.2 5.0  0.0 5.0  0.0 4.3  1.2 5.0  0.0 3.5  2.1 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 5.0  0.0 2.4  2.1 5.0  0.0 2.1  2.1 1.2  0.4 0.4  0.1

3.1  0.7 3.8  0.9 4.2  0.1 3.6  0.7 1.9  0.2 1.4  0.2 2.3  0.2 2.5  0.3 2.4  0.9 2.5  0.3 2.1  0.5 2.2  0.7 1.2  0.0 2.1  0.4 1.6  0.5 2.0  0.6 3.1  0.8 3.4  0.2 2.1  1.0 2.8  0.4 2.6  0.8 1.3  0.2 3.8  0.8 4.2  0.9 5.1  1.1 4.5  0.0

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and 5). The significant role that HOx-induced turbulent mixing plays in driving oxygenated water down to the SWI and facilitating JO2 is thus clearly demonstrated. Brand et al. (2009) and Bryant et al. (2010a) similarly showed that turbulence from seiching was a controlling factor in maintaining oxic sediment conditions regardless of O2 concentrations only a few cm above the SWI. The influence of seiching on sediment O2 uptake (Bryant et al., 2010a) was expected to be minimal in CCR due to relatively mild average wind speeds and irregular basin bathymetry (Fig. 1). Wind speed (based on National Oceanographic and Atmospheric Administration (NOAA) data for the Roanoke airport, located w5 km from CCR) was assessed to determine if seiching was an influence in observed changes in current velocity (NOAA, 2008). As anticipated, a correlation between average wind speed and current velocities (nearsediment, mid-hypolimnion, and mid-epilimnion; data not shown) was not observed which confirms that seiche-induced turbulence was not a significant factor in sediment-water O2 dynamics in CCR.

3.3.

Correlation between zmax and HOx gas flow

While induced JO2 can be problematic if not properly accounted for in HOx design and operation, increased JO2 is beneficial if an enhanced sediment oxic zone inhibits the transport of reduced soluble species into the water column (Lin et al., 2003; Beutel et al., 2007; Gantzer et al., 2009a). To evaluate the relationship between HOx flow rate and zmax, additional data from summer 2007 were used to cover a broader range of flow rates. The 2008 off/on HOx experiments revealed that zmax and the sediment O2 distribution responded directly to changes in HOx operation based on the flow rates of 0 and 51 m3 h
1 (Fig. 4). These results were expanded upon by including O2 microprofile data obtained at CCR-2 and CCR-6 while the HOx was operated at five different flow rates during 2007e2008 (Fig. 6). The sediment oxic zone (quantified by zmax) was found to be linearly related to HOx flow rate in both the near field and mid-reservoir region, with zmax increasing in accordance with flow rate (Fig. 6) and a P-value of 0.04 for CCR-2 and CCR-6 data. The response of zmax to increased flow rate was similar in both regions, although zmax was slightly lower near the HOx (CCR-2) than further upstream (CCR-6). Conversely, average JO2 values were comparable but higher at CCR-2 than at CCR-6 (13.6  7.2 vs. 6.6  2.4 mmol m
2 d
1). These results suggest that increased O2 reached the near-field sediment but was then consumed more rapidly by sediment O2 consumption processes, resulting in decreased zmax (O’Connor et al., 2009). CCR near-field sediment has been found to have considerably higher levels of total organic carbon, Fe, and Mn in the bulk sediment (Bryant et al., unpubl.) which may result from enhanced oxide precipitation and sediment focusing in the deeper region where the HOx is installed (Fig. 1; Schaller and Wehrli, 1997). Thus, sediment near the HOx most likely has increased sources of electron acceptors and subsequently would have a greater capacity for O2 consumption. Variations in mixing resulting from interaction between the HOx bubble plume and CCR bottom topography (Singleton and Little, 2006; Singleton et al., 2010) also likely affected sediment O2 uptake. However, the fact that fairly similar zmax values were observed

at CCR-2 and CCR-6 indicates that oxygenation maintained a balance between sediment O2 supply and consumption processes in both the near- and far-field regions.

3.4.

Spatial variation in influence of HOx

The response of the vertical sediment-water O2 distribution to halting oxygenation was found to be similar both near the HOx at CCR-2 and mid-reservoir at CCR-6 (Fig. 4). Furthermore, ongoing HOx operations maintained a fairly uniform sediment oxic zone throughout most of CCR (Fig. 6) and average JO2 values were comparable in both the near field and midreservoir region (Section 3.3). Results do indicate, though, that proximity to the HOx influenced localized flow patterns and the time required for a vertical O2 distribution to be reestablished at the SWI after resuming oxygenation. Ideally, HOx-induced turbulence establishes a gently mixed hypolimnion (Singleton and Little, 2006) which is supported by ADCP measurements in CCR indicating relatively similar current velocities throughout the reservoir (data not shown). However, short-circuiting of plume-induced flow can occur in the region near the HOx (McGinnis et al., 2004; Singleton et al., 2010). Although the far field is usually less affected by the plume, HOx plume model results by Singleton et al. (2007) showed that the area most directly influenced by the HOx was a detrainment region between the depth of maximum plume rise and the fallback elevation of equal density. CCR-6 is located within this detrainment region and may therefore be subject to more intense turbulence, while CCR-2 is located below the fallback elevation. As shown in Fig. 4, it took w5 days for the vertical O2 distribution to be restored at CCR-2 and w8 days at CCR-6. The time required for suspended particles to settle in the near field and for HOx-induced near-sediment mixing to resume could

Fig. 6 e Variation in zmax as a function of HOx flow rate. A linear relationship between the sediment oxic zone and HOx flow is observed, with zmax increasing in response to elevated HOx flow at both locations (P-value of 0.04 for CCR-2 and CCR-6 data). On average, 2e3 measurements were obtained per flow rate, with the exception of data obtained during the 2008 multi-day campaigns for which bi-daily averages were used.

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 3 6 9 2 e3 7 0 3

have contributed to this delayed response (Figs. 4 and 5). Sediment resuspension and the introduction of reduced species into the hypolimnion, particularly near the HOx, likely occur immediately after turning on the HOx (Gantzer, 2002). Enhanced sediment resuspension at CCR-2 may have prevented the brief establishment of oxic conditions as observed at CCR-6 (Fig. 4). However, the vertical O2 distribution at the SWI was most likely restored more quickly in the near field due to local mixing effects. At CCR-6, the response was more gradual as it took longer for HOx-induced increases in O2 and near-sediment turbulence to influence sediment O2 conditions up-reservoir. Once an oxic vertical O2 distribution was established, though, O2 microprofiles measured at CCR-6 (Fig. 2) typically maintained a more stable, well-defined structure than those at CCR-2. This may be attributed to minimal sediment disturbance and enhanced turbulence due to plume detrainment mid-reservoir as compared to near the HOx where sediment resuspension and localized flow instabilities are likely more prevalent (Singleton et al., 2010). Less consistent conditions near the HOx are supported by increased variation in CCR-2 data (Fig. 6) especially at high flow rates.

3.5.

Effect on source water quality

While HOx operations were found to increase sediment O2 uptake (Figs. 5 and 6), an O2 balance performed on the CCR water column by Gantzer et al. (2009b) showed that enough O2 can be supplied by the HOx to counteract this increased demand and prevent O2 depletion within the water column. The work presented here on sediment O2 uptake supports that HOx can effectively replenish O2 in thermally stratified watersupply reservoirs such as CCR as long as HOx-induced increases in JO2 are properly accounted for. Though additional factors like external nutrient loading may be important (Ga¨chter and Wehrli, 1998; Liboriussen et al., 2009), previous studies also support the substantial benefits of HOx (Gemza, 1997; Prepas and Burke, 1997; Beutel et al., 2007). Furthermore, the sediment oxic zone was found to be enhanced considerably by oxygenation throughout most of the reservoir (Fig. 6), thereby promoting decreased transport of reduced chemical species to the water column. It has been shown that maintaining oxic conditions in the upper w1e2 mm of sediment can suppress the release of reduced chemical species from the sediment to the hypolimnion (Jørgensen and Boudreau, 2001; Beutel, 2003). By establishing that HOx can facilitate an oxic sediment zone on a reservoir-wide basis, the current study links sediment-water O2 dynamics to water-column studies showing that oxygenation can result in significantly improved water quality (McGinnis et al., 2004; Gantzer et al., 2009a,b).

4.

Conclusions

This research emphasizes the viability of using HOx to maintain a sediment oxic zone on a reservoir-wide scale in order to minimize reduced chemical fluxes from the sediment to the overlying water. Understanding how HOx-induced variations in near-sediment mixing and O2 concentrations affect diffusive transport at the SWI is crucial for accurately quantifying JO2 and other sediment-water fluxes, optimizing

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water quality, and effectively managing lakes and reservoirs. Significant conclusions include: 1. The vertical distribution of O2 on both sides of the SWI was strongly controlled by HOx operations. Oxic conditions at the SWI were maintained in both the near field and midreservoir during continuous oxygenation. The influence of the HOx on sediment O2 uptake was emphasized by the onset of anoxia in the sediment and overlying water in response to turning the HOx off for only w48 h. 2. Decreased dDBL and increased JO2 and zmax were observed during oxygenation. While HOx operation increased both near-sediment O2 concentrations and mixing, sediment O2 uptake was more strongly correlated to mixing as opposed to O2 levels in the lower hypolimnion. Regardless of O2 concentration several cm above the sediment, turbulent transport of oxygenated water to the sediment surface governed the vertical O2 distribution above and below the SWI and the corresponding JO2 . 3. A linear relationship between zmax and HOx oxygen-gas flow was established, with zmax increasing with escalating flow rate. 4. Sediment response time for an oxic vertical O2 distribution at the SWI to become re-established after initiating HOx operations was determined to be w1 week in both the near and far field. However, spatial variation in the influence of the HOx was observed as the response time was several days longer and the vertical sediment-water O2 distribution was more stable mid-reservoir than near the HOx. 5. Taking HOx-induced increases in JO2 into account when designing and operating HOx is critical for enhancing source water quality via oxygenation. By evaluating the effect of HOx on sediment O2 uptake, these results enable successful HOx operations that facilitate both elevated source water O2 concentrations and an oxic environment at the SWI. Furthermore, while this study focused on HOx-induced changes in near-sediment mixing and O2 concentrations, variation in these parameters can also be induced naturally (e.g., via fall overturn, wind-induced seiching, and hydraulic inputs during storm events). Results should therefore be more generally applicable.

Acknowledgments The authors thank Elizabeth Rumsey, Kevin Elam, and the staff at Western Virginia Water Authority who offered invaluable assistance in the field and with laboratory analyses. Alfred Wu¨est, Daniel McGinnis, Lorenzo Rovelli, John Petrie, and Peter Berg contributed via beneficial discussion and advice on data interpretation. Feedback from two anonymous reviewers greatly improved the manuscript. Financial support came from the National Science Foundation (NSF IGERT Program) and the Western Virginia Water Authority. The research described in this paper was also partially funded by the United States Environmental Protection Agency (EPA) under the Science to Achieve Results (STAR) Graduate Fellowship Program. EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA.

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