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Article
Hydrodynamic Forcing Mobilizes Cu in Low-permeability Estuarine Sediments Minwei Xie, Ning Wang, Jean-Francois Gaillard, and Aaron I. Packman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04576 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016
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Hydrodynamic Forcing Mobilizes Cu in Low-permeability
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Estuarine Sediments
3 Minwei Xie,1,2 Ning Wang,1,2 Jean-François Gaillard,1* Aaron I. Packman1*
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1
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Road, Evanston, IL 60208-3109, USA
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2
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Chengdu, Sichuan, 610031, China
Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan
Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University,
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* Corresponding authors:
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Aaron Packman,
[email protected] Tel: 847-491-9902; Fax: 847-491-4011
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Jean-François Gaillard,
[email protected] Tel 847-467-1376; Fax 847-491-4011.
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Abstract
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Overlying hydrodynamics play critical roles in controlling surface-porewater exchanges in
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permeable sediments, but these effects have rarely been characterized in low-permeability
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sediments. We conducted a series of laboratory experiments to evaluate the effects of varied
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hydrodynamic conditions on the efflux of metals from low-permeability estuarine sediments.
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Two Cu-contaminated sediments obtained from the Piscataqua River were subject to controlled
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levels of hydrodynamic shear in Gust mesocosms, including episodic sediment resuspension.
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Overlying water and porewater samples were collected over the course of experiments and
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analyzed for metal concentrations. The two sediments had similar permeability (~10-15 m2), but
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different particle size distributions. Hydrodynamic forcing enhanced the mobilization and efflux
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of Cu from the coarser-grained sediments, but not the finer-grained sediments. Sediment
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resuspension caused additional transitory perturbations in Cu concentrations in the water column.
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Particulate metal concentrations increased significantly during resuspension, but then rapidly
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decreased to pre-resuspension levels following cessation of sediment transport. Overall, these
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results show that the mobility and efflux of metals are likely to be influenced by overlying
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hydrodynamics even in low-permeability sediments, and these effects are mediated by sediment
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heterogeneity and resuspension.
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Key words: Overlying hydrodynamics; metal mobility; metal efflux; contaminated sediments
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Introduction
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Estuaries represent transitions between riverine and marine environments.1 Estuaries receive
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inputs of fresh water, weathered materials, nutrients and contaminants from terrestrial
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environments. Estuaries are also strongly influenced by tidal currents from the ocean. At flood
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tide, saline water is forced into many estuaries, while outflows to the ocean increase at ebb tide.2
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Mixing of fresh and marine water retains and recycles particulate matter and nutrients within
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estuarine environments. Because of their dynamic and retentive characteristics, estuaries rank
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among the most productive ecosystems on earth and provide habitats for a variety of aquatic
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life.3, 4
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Metal contaminants entering into estuaries are transferred from water into sediments by many
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processes – most notably precipitation, adsorption and sedimentation. The resulting metal-
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contaminated sediments then act as potential sources to the water column, posing ongoing threats
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to the aquatic environment. Some metals such as Cu are essential elements for a variety of
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metabolic processes, but also show toxicity when overabundant.5, 6 Sub-lethal effects for
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bacterial cells, algae, aquatic invertebrates, and fishes begin to appear when the Cu concentration
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exceeds 5 µg/L.5
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The mobility of metals in sediments is controlled by a wide range of factors, including redox
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distribution, pH, and salinity.7, 8 These factors are known to vary with transport and mixing
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processes such as hyporheic exchange, bioturbation and bioirrigation.9-14 Hyporheic exchange –
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exchange of porewater with the overlying water column – directly influences mass transfer of
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both solutes and particulate matter across the sediment-water interface (SWI).15 Advective
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porewater flows influence a wide range of sedimentary biogeochemical processes, including
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metals speciation and contaminant biodegradation 16-22. However, the effects of hyporheic
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exchange and porewater flow on contaminant dynamics are frequently neglected in low-
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permeability sediments, as molecular diffusion is generally considered to be the dominant
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porewater transport process in these sediments19, 22-26. Field investigations have shown that both
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waves and currents enhance transport of solutes such as nutrients, silica, and oxygen across the
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SWI in estuaries and coastal oceans.27-29 Tidal cycles also alter redox conditions and affect the
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mobility of metals in sediments.29, 30 Metals are enriched in fine sediment particles and total
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metal concentrations in sediments are normally orders of magnitude higher than those in
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porewater and overlying water.31 Small changes in sediment geochemical conditions may thus
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exert significant effects on exchange of metals between sediments and waters. In a previous
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study, we found that sediment resuspension transiently changed the speciation of Zn in surficial
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sediments and significantly promoted the liberation of Zn to the porewater and overlying water
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column.32
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Accordingly, we hypothesize that hydrodynamic forcing enhances the mobility of metals even in
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low-permeability sediments and the overall liberation of metals to the water column. To test this
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hypothesis and improve understanding of hydrodynamic controls on metals efflux from low-
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permeability contaminated sediments, we conducted Gust chamber experiments with Cu-
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contaminated estuarine sediments. We evaluated Cu releases to porewater and overlying water
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under a range of hydrodynamic conditions, including steady flows without sediment
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resuspension and time-varying flows with episodic sediment resuspension. We then evaluated
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the mechanisms by which overlying and porewater flows mobilized metals.
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Material and Methods
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Sample collection. Metals-contaminated sediments were obtained from Portsmouth Naval
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Shipyard (PNS), a U.S. Navy shipyard located at the mouth of Portsmouth Harbor (Figure S1).
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Portsmouth harbor is predominantly saline and exhibits current velocities from 1.5-2 m/s.
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Sediments used in the laboratory experiments presented here were collected from PNS
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monitoring stations MS3 and MS4 in September of 2012. ~15 cm of surficial sediments were
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collected manually by divers and then shipped overnight in coolers to the laboratory, and
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refrigerated at 4 °C until used in the experiments.
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Sediment characterization. Bulk sediment properties, including porosity, porewater
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conductivity and salinity, particle size distribution, permeability, total organic carbon, acid
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volatile sulfide (AVS), simultaneously extracted metals (SEM), and total metal concentrations
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were measured on homogenized sediment samples. Bulk porosity was measured by drying 10
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cm3 of sediments at 70 °C for 48 hours, and then converting the weight loss of water to volume
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fraction. Porewater was extracted by centrifuging the sediments at 3414g for 20 min (Legend
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RT plus, Thermo Scientific). Conductivity of the filtered supernatant (0.2 µm nylon filter, VWR
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International) was measured with a conductivity probe (Oakton Con 11, Cole-Parmer). 10 mL of
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the supernatant was dried at 70 °C for 48 hours and the salinity was determined by measuring the
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weight of the remaining solids. Sediment grain size distribution was measured by wet-sieving
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with 45, 106, 150, 250 and 1000 µm sieves and measuring the dry weight. Permeability was
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measured in a constant-head permeameter with a test section of 2.5 cm in diameter and 5 cm in
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length (column and fittings from Omnifit Ltd.) Bulk AVS and SEM were determined by the 1M
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cold HCl extraction and 0.5 M NaOH trap technique (Supporting Information).33 Total organic
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carbon was measured by acidifying 80 mg of dry sediments with 1.5 mL of 4 M HCl followed by
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analysis with an Elemental Analyzer (ECS 4010, Costech Analytical Technologies, Inc.) Total
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bulk metal concentrations were measured using ICP-AES (Vista-MPX, Varian) after microwave-
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assisted acid digestion following USEPA method 3051A.34
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Experimental Setup. Gust-type mesocosms35 were used to subject the PNS sediments to
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controlled overlying flow conditions. The Gust chamber (Green Eyes LLC) (Figure S2, SI) uses
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a combination of a spinning disk and a central suction port to generate uniform shear stresses
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over the SWI. A DC power supply and a linear motor controller (LSC 30/2, Maxon Motor)
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precisely control the rotation rate of the spinning disk, providing calibrated shear stresses
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between 0.01-0.90 Pa. All parts of the Gust chamber were made of polycarbonate to minimize
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metal contamination. A covered 5 L plastic beaker was used as a reservoir for recirculating
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water. The inlet and outlet ports of the Gust chamber were connected to the water reservoir by
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Tygon® tubing (VWR International) and a peristaltic pump (Masterflex, Cole Parmer). Water in
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the reservoir was constantly mixed to ensure homogeneous distribution of dissolved and
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particulate material in recirculating water throughout the experiments. Rhizon in situ samplers 36-
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enable time-series sampling of porewater over the course of the experiments. Experiments were
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performed with artificial seawater (ASW) made by dissolving 32 g sea salt (Tropic Marin) into 1
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L Milli-Q water (Millipore).
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Hydrodynamic conditions. Six Gust chamber experiments were performed on PNS sediments.
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Hydrodynamic conditions for experiments (Table 1) were chosen relative to the critical shear
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(CS), which is the shear required to cause significant sediment resuspension. The critical shear of
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the sediments was first characterized by varying the shear stresses in a stepwise fashion until the
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turbidity of the overlying water significantly increased. The critical shear of both PNS sediments
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(MS3 and MS4) was 0.37 Pa (Figure S3, SI). Experiments were performed by imposing constant
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(“baseline”) shears for 14 days. Separate experiments were run with baseline conditions at 3%,
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50%, and 70% of the critical shear. Experiments run at 70% of the critical shear were also
(RISS) with 0.15 µm pores (Sunvalley Solutions Inc.) were installed into the Gust chamber to
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subject to two periods of episodic resuspension of 4 h duration each on days 1 and 8.
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Resuspension was achieved by subjecting the sediments to a constant shear of 0.90 Pa for 4 h.
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Experimental procedure. Before each experiment, all parts of the Gust chamber systems were
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cleaned by soaking in 10% HNO3 (v/v) for 24 h and then rinsing with Milli-Q water. 800 mL of
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homogenized sediments were then carefully emplaced into the Gust chamber to form a 10 cm
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deep sediment bed with a flat surface. Three RISS were inserted into the sediment at depths of 1,
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2, and 4 cm below the SWI while the sediment was introduced. The 10 cm of headspace was
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then slowly filled with ASW to avoid disturbance of the sediments. The erosion head was then
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placed onto the chamber. The system was allowed to stabilize overnight, and then 4.785 L ASW
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was continuously recirculated for 14 days under the hydrodynamic conditions listed in Table 1.
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The recirculating water was kept oxygenated during the experiments by continuously bubbling
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water-saturated air into the reservoir.
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Sampling and analysis. Dissolved oxygen (DO), pH, turbidity, and conductivity, dissolved and
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total metal concentrations in the recirculating water were measured daily. DO was measured
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directly in the water reservoir with a DO probe (HQ10, Hach). 24 mL of overlying water was
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sampled from the recirculating flow daily and split into three aliquots. One aliquot of the
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overlying water was used to measure pH (420Aplus, Thermo Orion), conductivity, and turbidity
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(2100Q, Hach) and was returned to the water reservoir following analysis. The other two aliquots
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were analyzed for dissolved and total metal concentrations and an equal volume (16 mL) of
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clean ASW was added to the water reservoir to maintain a constant reservoir volume throughout
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the experiments. Samples analyzed for dissolved metals were filtered through a 0.2 µm filter
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(nylon, VWR International) and acidified with concentrated, trace-metal grade HNO3 (Sigma
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Aldrich) to pH
