Environ. Sci. Technol. 1998, 32, 4031-4040
Investigation of Porewater Sampling Methods for Mercury and Methylmercury R O B E R T M A S O N , * ,† N I C O L A S B L O O M , ‡ STEVE CAPPELLINO,§ GARY GILL,| JANINA BENOIT,† AND CHARLES DOBBS⊥ Chesapeake Biological Laboratory, University of Maryland, Solomons, Maryland 20688, Frontier Geosciences, 414 Pontius Avenue N, Seattle, Washington 98109, Parametrix Inc., 10540 Rockley Road, Suite 300, Houston, Texas 77099, Marine Sciences Institute, Texas A&M University, Galveston, Texas 77553, and ALCOA Technical Center, 100 Technical Drive, Alcoa Center, Pennsylvania 15069
The feasibility of various techniques for the separation and quantification of sediment and sediment porewaters for total Hg and methylmercury (MMHg)scentrifugation, sediment filtration, whole core squeezing, and dialysis membrane techniques (peepers)swere investigated in estuarine sediments. The concentration and distribution of iron, manganese, MMHg, and total Hg in porewaters were compared to ascertain which method provided the best technique for the collection of large volume porewater samples. Direct filtration techniques did not provide sufficient sample volume. Our studies confirmed the need to filter in an inert atmosphere. Processing in an inert atmosphere is also required for centrifugation. Centrifugation was the most difficult method, requiring extensive operator training and much attention to detail during sample processing. Core squeezing is a viable alternative but suffers from sample size problems and potential artifacts due to oxidation during processing. While peepers have advantages, the investigations here show that they need to be completely degassed before they provide reliable Hg speciation measurements, especially as they were constructed from Teflon, which can store significant amounts of oxygen. The need to deoxygenate the peepers compromises their use as a routine separation method for Hg. On the basis of the need for large volumes and the large number of samples to be collected, we concluded that centrifugation was the most reliable method for the determination of Hg and MMHg in estuarine porewaters.
Introduction The prevalence and bioaccumulation of mercury (Hg), predominantly as monomethylmercury (MMHg), in aquatic food chains has led to a reevaluation of the factors controlling methylation and MMHg biogeochemical cycling (e.g., ref 1). As the sediment-water interface is often the dominant site * Corresponding author phone: (410) 326-7387; fax: (410) 3267341; e-mail:
[email protected]. † University of Maryland. ‡ Frontier Geosciences. § Parametrix Inc. | Texas A&M University. ⊥ ALCOA Technical Center. 10.1021/es980377t CCC: $15.00 Published on Web 10/24/1998
1998 American Chemical Society
for Hg methylation (e.g., refs 2 and 3), sediment-water exchange, especially in shallow dynamic systems such as estuaries (4, 5), is an important source of water column MMHg. Mechanisms for the transport of dissolved and particulate constituents from the sediment into the water column include diffusion and advection of porewater, sediment resuspension, and the biologically mediated processes of “bioturbation” and “bio-irrigation”. To understand the impact of these processes, detailed sediment and porewater distributions must be obtained. While techniques for the collection and sectioning of bulk sediment are well developed, methods for the collection of porewaters for dissolved constituents (6-9) and for metals (10-12) have only been recently been investigated and developed. Porewater extraction methods are based on three approaches: (i) sediment core sectioning followed by separation of porewater by centrifugation and/or filtration; (ii) squeezing of the core using gas pressure to extract the porewater; and (iii) use of an in situ dialysis membrane device. A dialysis device (a so-called peeper) is a structure containing one or more water-filled compartments covered with a membrane that allows the passage of dissolved constituents between the sediment porewater and the dialysis compartment. The dialysis technique relies on the equilibration of the water in the sampler with that of the porewater. While tests show that equilibration is rapid in water (hours to a day), field trials suggest that equilibration is less rapid in sediment and samplers are traditionally left in the sediment for a period of 1 week or more (e.g., refs 12 and 13). Carigan et al. (11) found that the dialysis method gave comparable results to that of centrifugation for the trace metals Co, Ni, Cr, Fe, and Mn for contaminated river sediments. Removal of oxygen from the dialysis apparatus has been suggested as necessary prior to deployment in anoxic sediments (14) as plastic materials can contain significant amounts of oxygen, and thus, if peepers are not degassed, the oxygen present could markedly impact the concentration of redox-sensitive constituents within the dialysis chamber after deployment. Montgomery et al. (15) suggest that removal of oxygen is not necessary for the measurement of total Hg in estuarine porewaters. Overall, exposure to air is a potential source of artifact for most methods (12), and thus methods that limit air exposure yield better results although they are more cumbersome than techniques that can be performed in ambient air. For the determination of Hg species, sample volume requirements are often a constraint; therefore, methods that are not as volume limitedscentrifugation or peeper samplingsare preferred. Methods that limit handling are advantageous for Hg because of potential contamination concerns. Handling also increases the possibility of redox changes. It was not apparent prior to this investigation which method would be most applicable for the collection of porewater samples for total Hg and MMHg from a substantial number of estuarine sediment cores within a short period of time.
Methods Site Location. Samples were collected from 15 sites in Lavaca Bay, a large (190 km2), shallow (average depth 1.5 m) embayment on the southeastern Texas coast (Figure 1). Lavaca Bay was the site of a historic discharge of Hg from a chlor-alkali facility between 1966 and 1970. The current study was initiated as part of an investigation focused on understanding the factors controlling continued accumulation of Hg in the bay’s biota. Most samples were collected VOL. 32, NO. 24, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Lavaca Bay, TX. The map shows the location of the former chlor-alkali plant and other related industrial sites and the location of the sampling stations. See text for details. during an intensive reconnaissance investigation in April 1996. Additionally, one site (GF-2) was monitored for 1 year to investigate seasonal trends (16). Replicate sediment cores, water column samples, and biota were collected at all sites. These sites were chosen as representative of five ecosystem types typical of Lavaca Bay: intertidal mud flats (IM), grass flats (emergent Spartina regions) (GF), open water sites (OW), 4032
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oyster reefs (OR), and the shipping channel (SC). There were three sites of each type with two of these sites within the “closed” area (the region closed for finfish consumption) and one site outside of this location but not significantly removed from the other study sites. Stringent trace metalfree protocols (e.g., refs 16-18) were adhered to throughout these investigations, both during the field effort (e.g., the
TABLE 1. Brief Description of the Pore Sampling Methods Used in This Investigation method name
brief description of method
N2 centrifugation Lavaca, method 1
extrude sediment core under nitrogen in a glovebox on site; centrifuge under nitrogen in a glovebox; filter under ambient air, acidify extrude sediment core under nitrogen in a glovebox; pack sample under nitrogen and ship overnight to a remote site; centrifuge 24-48 h later extrude sediment core under ambient air; pack sample in air and ship overnight to a remote site; centrifuge 24-48 h later remove sediment and place directly into a 0.45-µm vacuum filtratration unit and filter under ambient air extract porewater from intact sediment core into plastic syringes using nitrogen-positive pressure prepare the membrane device by removing oxygen by bubbling with nitrogen; deploy the dialysis membrane device in the sediment; retrieve 1 week later; sample porewater using a syringe
N2 centrifugation FGS, method 2 air centrifugation FGS, method 3 vacuum filtration, method 4 core squeeze, method 5 peeper, method 6
FIGURE 3. Schematic of the squeeze core setup.
FIGURE 2. Diagrammatic representation of the porewater dialysis sampler. collection of suitable field blanks) and in the laboratory. Laboratory QA procedures included analysis of certified reference materials, sample replication, and use of Hg-free reagents. Porewater Sampling Methods. The various methods tested for the separation of sediment solids and porewater at the Lavaca Bay sites are outlined in Table 1. Figure 2 shows a diagram of the squeeze core setup, and Figure 3 shows that of the peepers. The sediment cores were all collected using a precleaned acrylic core tube, similar to that used for the squeeze core method, that was pushed into the sediment by hand or by a diver at the deeper stations (19). The core was capped with a high-density polyethylene (HDPE) lid and retrieved from the sediment. Sections of the core for the centrifugation and vacuum filtration were extruded from the core tube using a plunger. All extrusions of the core were done on-site at a portable “clean container” facility. For the centrifuge separation, samples were collected in 250-mL Teflon bottles, and the headspace was purged when necessary. Samples were centrifuged for 30 min to separate the porewater, and the porewater was then filtered through 0.4 µm disposable polycarbonate filter units, which had been
acid washed prior to use, to remove any remaining particulate and acidified to 0.2% with mercury-free sulfuric acid. Direct filtration was performed using the same disposable filtration units used to filter the centrifuge supernatant. For Lavaca Bay, direct filtration of sediment was performed under ambient air. All samples from Lavaca Bay were analyzed for total Hg, MMHg, iron (Fe), manganese (Mn), and other parameters. The squeeze core method used specially designed core tubes that had inserts drilled into the tube wall to allow the connection of plastic syringes (Figure 1). Nitrogen gas pressure was applied to the top of the core through the core tube lid. Porewater was squeezed out of the sediment into precleaned polyethylene syringes. As with the direct filtration method, core squeezing provides a variable amount of pore fluid with less being obtained from the deeper, less porous sediments. After extraction, fluid was filtered using a 0.4 µm syringe cartridge filter. Thus, the filtration steps for centrifugation, direct filtration, and core squeezing provided samples filtered to the same pore size. In an attempt to determine what filter pore size was appropriate for the filtration of porewaters, porewater from both oxic and anoxic sediments was filtered through different pore size filters (50.1 µm), and the filtrate was analyzed to total Hg, MMHg, Fe, and Mn. The peeper used in this study was constructed from Teflon (Figure 2). The membrane was an Amicon 10 kDa molecular mass polysulfone membrane. There is therefore the potential that the peepers excluded colloidal material that could have passed through the other filtration devices. The membrane plate was acrylic and was held in place by plastic screws (Figure 2). To assemble, one membrane and plate were attached, and then the cavity was filled with deoxygenated DI water. The other membrane/plate was then attached. The oxygen was removed by bubbling the whole peeper with nitrogen while the device was contained in a clean, waterVOL. 32, NO. 24, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Variability in Mercury Concentrations at GF-2 from Three Cores Collected within 0.5 m Radius on April 4, 1996 mean depth (cm) sedimenta 0.5 2 4 6 porewaterb 0.5 2 4 6 a
total Hg 427 ( 23 361 ( 30 405 ( 67 436 ( 41 44.1 ( 15.3 39.9 ( 5.1 24.9 ( 8.1 8.3
Total Hg and MMHg in ng/g.
% RSD
MMHg
% RSD
5 8 17 9
4.42 ( 0.15 1.88 ( 0.31 0.78 ( 0.07 0.40 ( 0.09
3 16 9 23
35 13 33 b
46.3 ( 19.0 30.3 ( 9.9 7.6 ( 3.0 2.0 ( 0.6
41 33 39 30
Total Hg and MMHg in ng/L.
filled container. After insertion into the sediment, the peeper was left for 1 week and then retrieved. The porewater was sampled using a syringe, decanted into a Teflon bottle, and acidified immediately. The peeper cells integrate over a depth interval of 2 cm, and this was also the typical width of the core slicing for filtration and centrifugation. The core squeezing technique used 1-cm intervals between collection ports. The integration of samples over a depth that is significant as compared to the depth interval of redox changesfor example, oxygen penetration was less than 1 cm into the sediment based on our oxygen microprobe measurementsscomplicates the comparison of methods and illustrates the need to accurately determine the sample depth. Integration over small depth increments would be required if information were to be used to estimate diffusive fluxes from sediments, for example. The use of a 2-cm depth interval in this study was necessary to obtain sufficient porewater for all the required analyses. Method Comparison Benchmarks. For the comparison of methods detailed here, focus will be on the MMHg and Fe data, as these species are the most likely to be compromised by the separation techniques, and the most sensitive to oxygenation of anoxic porewaters during sampling. However, concentrations of total Hg and Mn also provide information about contamination and redox state, and the data are included where appropriate. Iron and Mn were not however measured on all of the core squeezed porewaters. For a limited number of peeper samples, major anions were measured as these would be indicative of disequilibrium in the peeperssas the peeper chambers are filled with deionized water and gain dissolved salts from the estuarine porewaters. Although only total Hg and MMHg were quantified, other studies have suggested that these are the main Hg species in porewaters (20); therefore, the lack of measurement of elemental mercury (Hg0) was not considered to introduce significant error. In porewater from a site in the Saguenay Fjord (21), Hg0 was approximately 10% of the total porewater Hg in the upper 10 cm and was undetectable at deeper depths (20). A number of samples were analyzed for DMHg, and it was not detected. Again, there is little evidence of DMHg in sediment porewaters, although there are some reports in the literature (21, 22). As the concentrations in the sediment porewaters are not homogeneoussas we will show below (Table 2)sand the exact concentration of these species was not known prior to the investigation, the method of comparison had to rely to some extent on the principle of “best agreement”, i.e., it is assumed that if a number of methods agree and one does not, then the one method is likely to be compromised. Statistical comparison of averages and standard deviation was also employed to assess if two methods were statistically different when possible. 4034
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Iron and Mn profiles provide an indication of the extent of oxidation that has occurred during processing as these metals will be soluble, and thus present in high concentration, in the reduced layers of the sediment close to the redox boundary. At deeper depths, because of the production of sulfide by sulfate-reducing bacteria, Fe and other coprecipitated metals are likely to be removed from solution by pyrite formation, resulting in a concentration decrease with depth. This type of profile is therefore indicative of suitable porewater separationsassuming that there are no particulate or colloidal Fe in the porewater fraction, i.e., the filtration/ partitioning step has removed these species. Both Hg and MMHg concentrations could be impacted by redox changes in the sediment during processing, and low Fe concentrations were therefore used as an indicator that Hg and MMHg sampling might be compromised. Additionally, large gradients at depth are possibly due to analytical artifacts as such changes in gradient with depth are unlikely to be supported by in situ production and/or destruction of MMHg or dissolved-particulate dissolution/ precipitation reactions. Description of Method Comparisons. An initial comparison of the peeper and squeeze core sampling methods (methods 5 and 6) was conducted in Galveston Bay in February 1996 (test 1). The first set of trials in Lavaca Bay focused on comparing the squeeze core technique with that of filtration (method 4) and centrifugation (methods 1-3). These trials were conducted in March 1996 (test 2). As it was thought that samples could be shipped to Frontier Geosciences, the analytical laboratory, before centrifugation, shipping to a remote location was also tested (methods 2 and 3). This comparison also investigated the impact of processing in air over processing under nitrogen. Finally, a comparison was made against the technique of sectioning the core followed by direct filtration (method 4), without centrifugation, as this approach would be simpler than that of centrifugation. The initial comparison in Lavaca Bay (test 2) was conducted at two sites: one with an expected higher concentration (GF-2) than the other (IM-3). After the first set of trials in Lavaca Bay, a further investigation was conducted in April 1996 to compare the peeper method (method 6) to that of core squeezing (method 5) (test 3). Two sites were usedsGF-2, used for the other comparisons, and a deep water site (OW-1) that was further away from the chlor-alkali plant. The peepers were deployed for 6 days and were retrieved and sampled on site in the clean facility. Processing was done on the benchtop, without employing an inert atmosphere. Analytical Methods and QC. Samples for MMHg from the squeeze core sampling during the initial study in Galveston Bay were analyzed by Frontier Geosciences, as were all the samples collected in Lavaca Bay during all the investigations. Samples from the peeper deployments besides those in Lavaca Bay were analyzed at the Chesapeake Biological Laboratory (CBL). Low-level Hg quantification was by cold vapor atomic fluorescence spectroscopy (CVAFS). Total Hg was determined after acidic oxidation of the samples by reduction to Hg0 and purging onto gold traps. Thermal desorption released the trapped Hg to the CVAFS for quantification (22, 23). The estimated method detection limit (MDL; 3 standard deviations of the blank over a 1-month period) for water samples was approximately 0.15 ng/L (n ) 40), while for the sediments the MDL was 0.4 ng/g (n ) 40). Precision, as indicated by the relative percent difference (RPD) of the duplicate digestion, was found to average 6.6% for total Hg in water (n ) 15 pairs) and 8.5% for total Hg in solids (n ) 17 pairs). Accuracy, as determined by spike recoveries and standard reference materials, averaged 99.0 ( 8.0% (n ) 94 recoveries).
Methylmercury was determined after acidic chloride distillation to liberate the MMHg from the matrix (24). The distillates were analyzed using aqueous phase ethylation, trapping on Carbotrap, isothermal GC separation, and CVAFS detection (18). At CBL, Tenax traps were used rather than Carbotrap (25). The estimated MDL for the water samples was approximately 0.021 ng/L (n ) 72), while for the sediments the MDL was 0.005 ng/g (n ) 50). Precision, based on RPD, was 8.5% for water samples (n ) 21 pairs) and 8.2% for sediments (n ) 12 pairs). Accuracy, as determined by spike recoveries and standard reference materials, averaged 98.5 ( 12.6% (n ) 147 recoveries). Because of the potential for MMHg formation from inorganic Hg in the sample during distillation (26, 27), 12 sediment MMHg analyses were duplicated using a low artifact KBr/CH2Cl2 extraction procedure (26). Briefly, this method consists of alkaline digestion of the samples, followed by methylene chloride liquid-liquid extraction, repartitioning of the MMHg into aqueous solution, followed by the standard ethylation techniques. In this study, the potential by which the distilled samples were biased was found to be 0-15%, with a mean of 3.7%. Because the effect was small, all distilled results are reported as observed. Methylmercury values for the deep cores were determined using the liquid extraction procedure as the very high inorganic Hg levels at depth (21) coupled with very low MMHg concentrations could result in errors of up to 50% for some samples. For these cores, an intercomparison of the two extraction procedures both above and below the subsurface total Hg maxima gave similar results. Dimethylmercury (DMHg) was determined on a small subset of the oxic and anoxic sediments after digestion in 25% KOH/methanol. Aliquots of the digest were analyzed using purge-and-trap onto Carbotrap, isothermal GC, and CVAFS. The estimated MDL was 0.0002 ng/g. Since DMHg was not detected in any sample, no further analysis was made for this species. Iron and manganese were determined in porewaters, after 10- or 100-fold dilution, by stabilized platform Zeemancorrected graphite furnace atomic absorption spectrometry. Matrix modification using 50 ppm palladium was employed along with the instrument manufacturer’s recommended settings. Other ancillary methods measured at CBL are detailed in the 17th edition of the Standard Methods for the Analysis of Waters and Wastewaters (28). Anions were measured using ion exchange chromatography (Method 4110B, Anions by Ion Chromatography). Sulfide was measured using fixation of samples in the field with zinc acetates to precipitate ZnSsfollowed by quantification using the methylene blue method (Standard Method 4500).
Results and Discussion The comparisons in Lavaca Bay will focus primarily on the GF-2 site as this site was common in all the comparative studies. This site was also the location of the intrasite variability test and the site at which samples were collected over the long term (16). Sample Variability. To assess the magnitude of the difference between sites and/or deployments that could be resolved, a set of multiple cores was collected and fully analyzed at one site (GF-2) to ascertain near-scale (0.5 m) and within-site (25 m) variability. The results, presented in Tables 2 and 3 are the mean, standard deviations, and relative percent deviation (% RSD) for MMHg and total Hg in sediments and porewaters extracted by centrifugation (method 1). Cores collected within 0.5 m show reasonable agreement for sediments (% RSD of
