Task 4 - CiteSeerX

Task 4.2 – Sediment-Water Exchange by Gary Gill, Pacific Northwest National Laboratory

Introduction The major effort for this task is to conduct studies of the sediment-water exchange of mercury to determine the relative role of sediments as sources of mercury and monomethylmercury (MMHg) to the water column. This information will be used to assess the significance of sediment-water exchange in comparison to other sources into the Delta using a mass balance approach. Two basic methods were used to determine sediment-water exchange: (1) a direct assessment using deployment of benthic flux chambers and; (2) modeling the sediment-water exchange from measurements of interstitial pore water concentration gradients. The sediment-water exchange studies that we conducted in the first phase of our Calfed work (1999-2001) were focused mostly on open water areas since open water areas represent a significant portion of the surface area in the Delta. During summer months the integrated sediment-water flux of MMHg was estimated to be 6 mmol d-1 in the central Delta region, which was comparable to the riverine input into the Delta (Choe et al., 2004). The integrated flux was estimated from an average sediment-water exchange flux of MMHg measured at various sampling sites in the Delta. The studies conducted in this current work are focused on marsh areas as sites of MMHg production. The focus of marshes in this current work stems from our earlier work where we identified marshes as important sites of MMHg production (Choe et al., 2004). Specifically, we have chosen in this current work to focus on tidallydominated marshes. We hypothesize that advective processes occur in tidallydominated systems enhance the transport of MMHg by physically extracting pore water from exposed marsh sediments during a tidal cycle. Deployment of benthic flux chambers in tidal marshes are not a good means to test this hypothesis since they are not designed to capture the signal associated with pore water draining out of exposed marsh sediments during low tide. Therefore we developed a new “whole-ecosystem” level sampling approach to determine the MMHg exchange between marshes and the Delta. The whole ecosystem sampling approach involves monitoring concentrations of MMHg and other biogeochemical parameters as water flows on and off the marsh during a tidal cycle.

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Methods Sampling Locations and Dates Six sampling efforts were conducted between July 2004 and March 2006 Table 4.2.1). Five sampling sites were initially chosen to investigate the importance of tidally dominated marshes as sources of MMHg in the San Francisco Bay Delta region. (Table 4.2.1; Figure 4.2.1). Later, two marsh systems, Little Break and Mandeville Cut, were selected for detailed studies (Figure 4.2.2). Little Break is close to the confluence of San Joaquin and Sacramento Rivers. The marsh has a surface area of ~ 0.5 km2 and is separated from ambient environments by man-made levees. Mandeville Cut is located near Franks Tract and has a surface area of ~ 0.3 km2. In both systems, the water exchange between marsh and the Delta takes place only through one primary outlet. This allowed estimation of the net MMHg fluxes per unit area of the marsh by measuring time series concentrations of MMHg and water flow rates at those primary outlets. Additional details follow. Figure 4.2.1 Sample locations

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Table 4.2.1. Sample Site Locations and Sample Collection Matrix (A = Time series collection of water column samples using auto samplers; C = In situ incubation of bottom water using benthic chambers; P = Pore Water collection)

Site

Latitude Longitude

Jul. 2004

14 Mile Slough

38.00659° N 121.39224° W

C,P

Franks Tract Marsh

38.05896° N 121.61208° W

C,P

Little Break

38.01867° N 121.73989° W

Mandeville Cut

38.05954° N 121.53886° W

C,P

Dec. 2004

A,C,P

Mar. 2005

Jul. 2005

Oct. 2005

Mar. 2006

C,P*

A,C,P+

A,C,P

A,C,P

A,C,P

A,C,P

A,C,P

A,C,P

38.18859° N Suisun Bay 122.01208° W C,P * MMHg data from autosamplers deployed in Mar. 2005 was not used due to contamination. + Duplicate collection of overlying water using autosamplers.

Figure 4.2.2 Mandeville Cut (right) and Little Break (left) Sampling Sites.

Sample Collection Methods Benthic Flux Chamber Deployments Benthic flux chambers (Figure 4.2.5) were deployed to directly measure sedimentwater exchange fluxes. The measured exchange flux (FM) is based on the change in concentration within the water captured in the chamber over an incubation time, typically 3 to 5 hrs.

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FM =

ΔC V Δt A

where C is the concentration, t is the chamber deployment time, V is the internal volume of the chamber (8.1 L), and A is the sediment surface area the chamber encloses (0.0962 m2). Typically, the concentration change with time is estimated from 4-5 individual determinations of the internal concentration within the chamber. Typical sampling time points are 0, 1, 2, 3, and 4 hours. Additional details about benthic flux chamber deployments and this sampling method are described in Gill et al. (1999), Warnken et al. (2000, 2001) and Choe et al. (2004).

Figure 4.2.5. Benthic Flux Chamber.

Pore Water Diffusion Gradients MMHg concentrations were measured in sediments and pore waters extruded from intact sediment cores. Benthic diffusive fluxes of MMHg were estimated using the concentration gradient in pore water near the sediment-water interface. According to Fick’s first law, diffusive flux (FD) is modified for application to sediments in the absence of biological irrigation.

⎛ ϕD ⎞ ∂C FD = − ⎜ 2w ⎟ ⎝ θ ⎠ ∂x where ϕ is the sediment porosity, θ is the tortuosity, Dw is the diffusion coefficient of Hg in water without the presence of the sediment matrix (5 × 10-6 cm2 m-1; Gobeil and Cossa, 1993), C is the concentration of Hg in pore water, and x is the Calfed Mercury Project – Task 4.2

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sediment depth. The value of θ 2 can be estimated from porosity using the relationship θ 2 = 1 – ln (ϕ2) (Boudreau, 1996). Additional details about this method are described in Gill et al. (1999), Warnken et al. (2000, 2001) and Choe et al. (2004).

In Situ Water Samplers Automatic samplers were developed to collect water samples over the course of a full tidal cycle (Figure. 4.2.3). Each sampler can be equipped with up to ten Teflon sampling bags with maximum sample volume of 1000 mL. Typically, two samplers were deployed at each site so that a total of 20 samples could be obtained during a 24-30 hour deployment. The samplers were deployed on the sediment surface near the mouth of the marsh area and unfiltered overlying water was drawn from ~ 0.5 m above the sediment surface using an internal peristaltic pump and stored in precleaned Teflon bags. Both samplers alternatively drew water samples every ~80 minutes to collect 20 discrete water samples within 25 hours. Samples were filtered (0.45 µm) immediately upon retrieval. MMHg and trace metal samples were acidified with hydrochloric acid and nitric acid to a pH < 2, respectively.

Figure 4.2.4. Downward Looking Acoustic Doppler Current Profiler

Figure 4.2.3 In situ Water Sampler.

Determination of Water Flow Rates Water flow rates were measured with Acoustic Doppler Current Profilers (ADCP). An upward looking ADCP was placed in the deepest part of the channel to obtain velocity measurements. A downward ADCP (Figure 4.2.4) was used to collect a

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velocity profile of the entire channel. The linear relationship of the downward vs. upward data was used to correct the velocity of the upward ADCP. The downward ADCP profile was also used to develop an area/stage relationship of the channel. This relationship was used to calculate the channel area based on the upward ADCP depth. Tidal flow volumes were then calculated by multiplying area by the corresponding velocity. These flow volumes were used to calculate the fluxes of THg, MMHg, and trace metals into and out of the marshes.

Results and Discussion Benthic Flux Chamber Deployments The sediment-water exchange flux (benthic flux) of MMHg was directly measured with dual benthic chambers (Table 4.2.2). Flux measurements ranged from -58 to 120 ng m-2 d-1 with the highest values at Little Break in October of 2005. In all cases, the reproducibility between dual chambers, as defined by the relative percent difference, was low (