Environ. Sci. Technol. 1999, 33, 378-391
Sediment Transport and Hg Recovery in Lavaca Bay, as Evaluated from Radionuclide and Hg Distributions PETER H. SANTSCHI,* MEAD A. ALLISON, SHAUNNA ASBILL, AND A. BRITT PERLET Laboratory for Oceanographic and Environmental Research, Department of Oceanography, Texas A&M University, 5007 Ave U, Galveston, Texas 77551 STEVEN CAPPELLINO Parametrix, Inc., 10540 Rockley Rd., Suite 300, Houston, Texas 77099 CHARLES DOBBS AND LARRY MCSHEA Aluminum Company of America, State Highway 35, Point Comfort, Texas 77978-0101
Mercury was released in the late 1960s from a chloralkali facility managed by ALCOA and deposited into sediments of Lavaca Bay, TX. Sediments have recorded this event as a well-defined subsurface concentration maximum. Radionuclide, mercury, X-radiography, and grain size data from sediment cores taken in 1997 at 15 stations in Lavaca Bay were used to assess sediment and Hg movements in the bay. Sediment accumulation rates were calculated from bomb fallout nuclide (137Cs, 239,240Pu) peaks in 1963 and from the steady-state delivery of 210Pb from the atmosphere. Sedimentation rates are highest (∼2 cm/yr) at near-shore sites near the ALCOA facility and generally decrease away from shore. Sedimentation rates in some areas are likely influenced by anthropogenic activities such as dredging. Particle reworking, as assessed from 7Be measurements, is generally restricted to the upper 2-7 cm of sediments. Numerical simulations of Hg profiles using measured sedimentation and mixing parameters indicate that at most sites high remnant mercury concentrations at 15-60 cm depth cannot supply substantial amounts of Hg to surface sediments. Assuming no future Hg supplies, Hg concentrations in surface sediments are predicted to decrease exponentially with a recovery halftime of 4 ( 2 years.
Introduction Natural and anthropogenic radionuclides are powerful geochemical tracers and geochronological tools (e.g., ref 1). Natural radionuclides include primordial (e.g., U and Th decay series nuclides such as 210Pb) and cosmogenic radionuclides (e.g., 7Be). The latter nuclides are delivered to the earth surface through atmospheric processes, while the former are ubiquitous and are also produced in situ. Geochemical fractionation processes can cause radioactive * Corresponding author phone: (409)740-4476; fax (409)740-4786; e-mail
[email protected]. 378
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disequilibria between parent and daughter nuclides of a decay chain (e.g., 210Pb in the atmosphere produced from 222Rn escaping from soils). Anthropogenic nuclides include the isotopes produced from bomb tests in the 1950s and 1960s (e.g., 137Cs and 239,240Pu). Since the production rates in each of the earth’s reservoirs is known for many of these nuclides, they can, for example, be used to date sediments and define transport parameters, i.e., calculate sedimentation and sediment mixing rates. 7Be and 210Pb fluxes from the atmosphere have been measured in nearby Galveston and College Station, TX, over a two year period and have been found to be controlled primarily by the amount of rainfall (2). 137Cs and 239,240Pu nuclides have been shown to be useful dating tools in Gulf Coast estuaries (e.g., refs 3 and 4). Studies of geochemical transport mechanisms of natural radionuclides in these estuaries show short residence times (days to weeks) in the water column for particle-reactive nuclides 210Pb, 7Be, and 234Th (5). It is commonly assumed that concentrations in a particular reservoir do not change in time (i.e., steady state), and thus, inputs equal outputs. In cases of transient inputs, this assumption cannot be made, and the time dependent inputs need to be explicitly known to model sedimentation and mixing parameters. To constrain transport rates and pathways, it is often advantageous to have several radionuclides available with distinctly different input functions but similar transport pathways. Mercury was introduced into Lavaca Bay from a chloralkali facility managed by ALCOA, mainly between 1966 and 1970, as a byproduct of the production of NaOH which is used for the extraction of Al from bauxite ore. This Hg contamination caused a portion of the bay to be closed to the taking of finfish and crabs (“closed area” in Figure 1). In this radiochemical investigation of Hg transport in Lavaca Bay, a number of radionuclides have been applied: 7Be (for deriving sediment mixing rates in surface sediments), 137Cs, and 239,240 Pu (for deriving sedimentation and sediment accumulation rates as well as verifying sediment mixing rates) (e.g., ref 1). Natural 210Pb is useful as an independent geochronometer (e.g., ref 1) and also for testing if sediment and mercury accumulation has been continuous or episodic. Most importantly, using a numerical sedimentation and mixing model, it is possible to simulate selected radionuclide and Hg profiles. This allows for predictions about sediment recovery rates for Hg, provided that all inputs of Hg either cease or significantly decrease. Many recent radiochemical studies of sediment accumulation and mixing in coastal and marine environments (e.g., ref 6) incorporate geological information to improve interpretation of results. Nittrouer and Sternberg (7) first showed that the ratio of accumulation rate to mixing rate can be used to predict the extent that physical sedimentary structures (e.g., laminations and beds) are altered by biological mixing. High ratios are indicated by the preservation of these primary features, while low ratios show a massive, homogenized stratigraphy. Stratigraphy of modern sediments can be recognized in X-radiographs and petrographic thin-sections. Grain size information also aids in interpretation of radiochemical profiles because of the strong affinity of tracers for fine-grained particles (e.g., refs 3 and 4). Detailed granulometry has also been used (summarized in ref 8) to identify the sediment source and delivery mechanisms of sediment to the depositional site. The main objectives of our investigation were to evaluate (1) the potential for transfer of mercury to aquatic ecosystems through a mechanism by which subsurface particle reworking 10.1021/es980378l CCC: $18.00
1999 American Chemical Society Published on Web 12/18/1998
FIGURE 1. Sampling sites in Lavaca Bay for radiochemistry and X-radiography core collection. “Y” indicates the location of the Y ship channel. is coupled to sediment resuspension, (2) the impact of an for concentrations of Hg in surface sediments to decrease to anthropogenically altered sedimentation regime on burial one-half of their initial value. Radiochemical methods, rates of Hg in impacted sediments, and (3) the recovery halfcoupled to a numerical simulation approach, provide a means times of Hg contaminated sediments after all external inputs to determine the sedimentation history of the bay and to cease. Recovery half-time is defined here as the time it takes resolve if surface sediment concentrations of a contaminant VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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such as Hg are supported by mixing processes. Identification of sources of Hg to the surface sediments is a critical element in the overall understanding of the site as the surface sediments play a key role both in the conversion of inorganic Hg to methyl mercury and subsequent uptake of methylmercury into the food chain.
Approach and Methods Site Description and Sample Collections. Sampling locations for radiochemistry, grain size, and X-radiography (Figure 1), can be divided into four different geographical regions, which also coincide with distinctive sedimentation regimes, consisting of three anthropogenically impacted regions and the less impacted open bay: (1) area north and west of Dredge Island: cores 128(A,B), 134, 142, 201, (2) CAPA shoreline: cores 924, 926, 928, 8721, 8827, (3) Cox Bay: cores 285(A,B), 286, 296(A,B), and (4) sites in South and West Lavaca Bay and Keller Bay, located beyond the closed area of Lavaca Bay: cores 300, 8826, 4293. Areas for sediment coring were identified based on both surface and subsurface sediment Hg concentrations. Coring sites were then selected for a high percentage of fine grained sediments (grain size maps of ref 9) and because of the strategic importance of the closed area (Figure 1). Open bay coring sites are therefore not necessarily representative for the bay as a whole, as they are biased toward areas with higher clay content. Additional cores (Figure 1) were collected for X-radiography to provide wider spatial coverage of stratigraphic trends. Samples were collected by a scuba diver inserting a 4′′ ID acrylic core tube into the sediments to the desired depth, capping the core tube with an airtight pressure cap, and removal with the aid of an electric winch mounted on a 24foot aluminum research boat. Once on board the boat, core tubes were sealed for transport to the laboratory. Analytical Methods. Core Sectioning and Porosity Determination for Radiochemical Analysis. After delivery to the lab in Galveston, TX, cores were sectioned in 1 cm intervals (except core 128, where it was 5 cm intervals below 10 cm) and homogenized. Each sample was weighed wet and dry to determine porosity. Dried core material was ground for radiochemical and elemental analysis. X-Radiography and Grain Size. X-Radiography and grain size investigations were used to (1) provide insight into sedimentation patterns by defining the vertical stratigraphy, (2) normalize radiochemical and elemental concentration variations, (3) interpret radiochemical profiles because of strong affinity of tracers for fine-grained particles, and (4) act as tracers of sediment source and delivery mechanisms. Plexiglas (10 × 4 × 90 cm) tray cores were collected from the Lavaca site and returned to the LOER lab for X-raying on the same day to minimize disturbance and drying of the cores. Trays were X-rayed with a Kramex PX-20N machine at 15mA/70 kV. Fuji sheet negatives were developed in house and scanned into digital form at 300 dpi resolution. Individual X-ray sections of cores were merged digitally to form a single image of each core. Sediment samples for grain size analysis were collected from the tray cores following X-radiography. Basic grain size information (% sand, silt, clay) was obtained for 12-16 one cm depth intervals from 15 cores, with detailed granulometry from 2 to 4 intervals/core. Samples were analyzed for sand: silt:clay ratios by a combination of sieve and pipet analysis. Detailed granulometric analysis was conducted using Sedigraph/RSA analysis. The mud fraction was analyzed with a Sedigraph Model 5100 automated X-ray particle analyzer as a mud/deionized water slurry. Samples were disaggregated prior to analysis by ultrasonification and the addition of the deflocculant sodium hexametaphosphate. The sand fraction was analyzed with a 180-cm-long Rapid Settling Analyzer 380
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(RSA). Analyses were integrated by the pipet method (10). Data are reported in phi size classes that are a standard geological simplification (10) of grain size information into a dimensionless number. The phi (φ) scale is related to particle sizes in mm by
phi size ) -log2d where d ) the particle size in mm
(1)
Mean grain size and sorting characteristics of a sediment are frequently reduced to a single number using a graphical formula (11). This is a standard method of calculating
graphic mean ) (φ16 + φ50 + φ84) ÷ 3
(2)
where φ16 is the grain size in phi units for the 16th percentile from a cumulative frequency curve. Graphic Inclusive Standard Deviation (GISD) is a sorting parameter about this mean calculated as
GISD ) ((φ84 - φ16) ÷ 4) + ((φ95 - φ5) ÷ 6.6)
(3)
GISD of 4 is poorly sorted (11). Radiochemical Analysis. Sediment samples were homogenized when wet. Wet sediment (∼2 g) was weighed, freezedried, or oven-dried for 24 h at 80 oC. Samples were then reweighed when dry and homogenized through pulverization. Aliquots of the oven dried sediments were divided as follows: (1) about 1 g for 210Pb analysis by wet chemistry and alpha counting and (2) about 10 g for gamma counting, and on selected cores, subsequent wet chemistry followed by alpha counting for 239,240Pu. 210Pb (t 226Ra (t 1/2 ) 22.4 yr, Eγ ) 46 keV), 1/2 ) 1600 yr, Eγ ) 351keV), 137Cs (t1/2 ) 30 yr, Eγ ) 662 keV) and 7Be (t1/2 ) 53 d, Eγ ) 478 keV) were measured by nondestructive gamma counting (13, 14). 239,240Pu (239: t1/2 ) 2.41 × 104 yr; 240: 6.57 × 103 yr) in selected samples was measured by alpha counting, after digestion of 10 g aliquots in concentrated HCl and HNO3, followed by addition of 242Pu tracer, chemical separation of Pu and electroplating onto stainless steel disks (12, 15, 16). 210Pb in most cores was determined more accurately by alpha counting of a fraction after complete digestion (concentrated HNO3/HF), addition of 209Po tracer, chemical separation, and spontaneous deposition of Po nuclides on Ag disks (12, 4). Excess-210Pb (210Pbxs) was calculated as the difference between total and supported activities. Supported 210Pb activities were calculated from constant 210Pb activities at depth and/or 226Ra activities. Mercury and TOC Analysis. Mercury analyses were carried out by Cold Vapor Atomic Absorption (CVAA), according to EPA method SW7471A. Total organic carbon (TOC) analyses were carried performed using IR spectrometry according to EPA method SW9012. Laboratory quality assurance procedures included the use of duplicates, matrix spikes, and matrix spike duplicates for Hg, and triplicate samples for TOC. Modeling of Particle Reworking and Sedimentation Accumulation Rates. Determination of Sedimentation Rates. Sedimentation rates (in cm/yr) can be calculated from time markers or from profiles of chemicals with a known decay rate (1). Methods applied to date Lavaca Bay sediments include the following methods. (1) From Bomb Fallout Nuclide (e.g., 137Cs, 239,240Pu) Peak Concentration in 1963. This method could be successfully applied for many (but not all) Lavaca Bay sediment cores, by plotting activity concentrations vs depth (or mass depth in g cm-2). Sedimentation rates are then calculated from the depth (or mass depth) divided by the time difference to 1963. In some of the cores, there were no well-developed peaks of these nuclides. However, the presence of significant amounts of 137Cs at a given depth can be used to constrain the
sedimentation rate, i.e., put an upper limit on low sedimentation rates. (2) From the 210Pbxs () [210Pb] - [226Ra]) Profile. Sedimentation rates can be calculated, under steady-state conditions, and when the sediment inventory of unsupported 210Pb (210Pbxs) is close to that expected from atmospheric fallout from the Constant Initial Concentration (CIC) model (17, 18), given in eq 4
[210Pbxs(z)] ) [210Pbxs(o)] exp(-Rz)
(4a)
R ) (λ/S)
(4b)
with z ) depth in cm (or mass depth in g cm-2) and S ) sedimentation rate in cm/yr (or sediment accumulation rate in g cm-2 yr-1). This method assumes that particle reworking rates are negligible over the depth interval used to calculate sedimentation rates, which can be verified by numerical modeling of profiles of radioisotopic and Hg time markers, as described below. In most Lavaca Bay cores, sediment mixing depths were only of the order of a few centimeters, and thus, the CIC model can be utilized below the mixed layer to calculate sedimentation rates (i.e., eq 4). This model is applicable for sediments where sedimentation rates are either constant or, if variable, initial concentrations vary in proportion to the sediment accumulation rate. Determination of Particle Reworking (Bioturbation) Rates from 7Be Penetration. Particle reworking (bioturbation) rates can be calculated, under steady-state conditions, by eq 5
[7Be(z)] ) [7Be(o)] exp(-(λ/Db)1/2z)
(5)
where Db is the particle reworking (or benthic mixing or bioturbation) rate, in cm2/yr, and λ is the decay constant of 7Be () 4.8 y-1). Db was assumed to be constant, even though it might be better described as exponentially decreasing with depth in Lavaca Bay sediments. This approach results in average mixing rates and maximum mixed layer depths. For the case of Db decreasing with depth, there is no exact analytical solution, since porosity also decreases with depth. However, numerical model fits (see below) did not substantially improve our estimates of mixed layer depths and mixing coefficients. Model Simulations. Model simulations of bomb fallout nuclide and Hg profiles were carried out using a numerical mixing/sedimentation model written in MATLAB (19), which is based on earlier models (12, 20). The same model was also used for predictive recovery calculations where the resulting concentrations in the top 0.1 cm were plotted as a function of time. The numerical transport model allows for inputs of tracer flux, sediment accumulation rates (g cm-2 yr-1), and particle mixing rates (in g cm-2 yr-1), in two adjacent mixed layers (only one was used), which can also vary with depth or time (not used). Proper algorithms for depth-dependent advective (sedimentation) and diffusive (particle mixing and pore water diffusion) transport across boxes of defined thickness are specified numerically. The model allows box thickness to be specified (assumed to be 0.1 cm, to minimize numerical mixing effects), which defines the time step. Model runs are not sensitive to box thicknesses smaller than 0.1 cm. Tracers are assumed to equilibrate instantaneously between sediment and pore water, using a sediment-water partition coefficient (Kd) and molecular diffusion coefficient (Dm) to define relative mobility through pore water diffusion. Even though it is possible to vary Kd with depth, Kd was assumed to be constant, and usually given a high value to minimize pore water diffusion. The input function for bomb fallout nuclides was
taken from measurements of 90Sr in Houston after 1960 and before 1960 proportional to that measured in New York City (3, 4); Hg input into Lavaca Bay was assumed to have taken place from 1966 to 1969, with a maximum in 1968. Resulting concentrations were then scaled to the measured concentration profile. Sediment accumulation rates (in g cm-2 yr-1) were assumed to be constant with time and depth in all model runs. The most important constraint is that the same mixing and sedimentation parameters which define the time and depth variable bomb fallout nuclide and Hg concentration profiles also have to fit the profiles of the steady-state tracer 210Pb, assuming a constant delivery rate.
Results and Discussion X-Radiography and Shallow Stratigraphy. X-Radiographs were collected from tray cores at the sample sites to determine the relative importance of biological and hydrodynamic processes on strata emplacement and preservation in the upper ∼70 cm of the sediment. Despite differences in the grain size of the cores, an overall pattern of primary physical stratification is apparent. The horizontal layers observed in the X-radiographs (Figure 2a) represent variations in grain size (Figure 2b) that influence penetration of X-rays through the sample. In these negative images, coarse-grained layers show as white and fine-grained are black. These layers are emplaced at the time of deposition (e.g., primary sedimentary structures) by a combination of temporal variations in bed stress that serve to sort sediment and by temporal variations in sediment source such as the seasonality of riverine sediment input. Replicate X-ray cores show that major layers are continuous over at least meter scales and remain undisturbed below the area of surface mixed layer on seasonal time scales. The preservation of layering in the cores emplaced at the time of deposition is indicative of limited vertical mixing of stratigraphy by biological and physical processes. Preservation of primary stratification in productive environments such as estuaries can be best explained as a function of high sediment accumulation rates (e.g., cm-scale), where rapid burial of primary features limits the period of time they remain in the surface mixed layer. Bioturbation is a secondary (i.e., postdepositional) process that produces disruption and homogenization of laminations giving the X-radiograph a massive appearance that was not observed in the areas examined. Some burrow traces are observed in X-radiograph (Figure 2a) and are visible because they tend (1) to backfill with sediment that differs from the surrounding material or (2) they contain fecal pellets or other wall linings produced by the organism. The observed burrow traces are mainly polychaetes and meiofauna that tend to burrow from the surface to
