Environ. Sci. Technol. 1999, 33, 4522-4527
Use of Elemental Tracers to Source Apportion Mercury in South Florida Precipitation J. TIMOTHY DVONCH,† J O S E P H R . G R A N E Y , †,‡ G E R A L D J . K E E L E R , * ,† A N D ROBERT K. STEVENS§ The University of Michigan Air Quality Laboratory, Ann Arbor, Michigan 48109-2029, and Florida Department of Environmental Protection at U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711
Source-receptor relationships for mercury (Hg) and other trace elements wet deposited in south Florida were investigated using daily event precipitation samples collected concurrently at 17 sites from August 6 to September 6, 1995. A multivariate receptor modeling approach found municipal waste incineration and oil combustion sources to account for 71 ( 8% of the Hg wet deposited at five Florida Everglades sites. A similar analysis of a year-long record of event samples (June 22, 1995-June 21, 1996) collected at Davie, FL, found 73 ( 6% of the Hg wet deposited to be accounted for by local anthropogenic sources. Receptor modeling results closely agreed with stack measurements made at local point sources during the study. An emissions reconciliation found that local medical waste incineration sources, which emitted Hg primarily in the reactive form (Hg(II)), could account for the Hg wet deposition left unexplained by the multivariate receptor model. The above findings suggest that emissions from local urban point sources have played the dominant role in the wet deposition of Hg to south Florida and the Everglades. Additional speciated Hg emissions data are needed not only in south Florida but also nationally and globally to reduce uncertainties in modeled Hg loadings to aquatic ecosystems.
Introduction Mercury (Hg) is a pollutant of considerable concern in aquatic ecosystems due to its strong tendency to bioaccumulate up the food chain and its demonstrated link to human health effects. The discovery of elevated levels of Hg in freshwater fish in the Florida Everglades (1), the deaths of several endangered Florida panthers (believed to be a result of Hg toxicosis), and the linking of Hg to the decline of wading bird populations in this sensitive ecosystem has prompted State and Federal agencies to investigate the sources of Hg to this region. The atmosphere has been determined to be a major pathway for the transport and deposition of Hg to south Florida and the Everglades (2, 3) as well as other areas of North America and Europe (4-8). Less certain, however, is the source of this atmospheric Hg that is deposited. * Corresponding author phone: (734)936-1836; fax: (734)764-9424. † The University of Michigan Air Quality Laboratory. ‡ Present address: Department of Geological Sciences & Environmental Studies, State University of New York at Binghamton, Binghamton, NY 13902-6000. § Florida Department of Environmental Protection at U.S. Environmental Protection Agency. 4522
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Anthropogenic emissions of Hg are currently estimated at between 60 and 80% of total Hg emissions on a global scale (9-14). Global and regional models suggest that about 50% of anthropogenic mercury emissions contribute to the global cycle, while the remaining 50% deposit on a local/ regional scale (11, 15). However, since the relative contribution of local, regional, and global sources is location-specific (11), there is a continued need for more quantitative assessments of the amount of Hg that is locally deposited to specific ecosystems and geographical areas (16). This is especially true in locations adjacent to large urban/industrial areas since an important fraction of the total Hg from point sources may be emitted as soluble Hg(II) (11, 17-19), which is expected to be removed from the atmosphere on a local scale (within 100 km of the source) (11). Geographically, the urban point sources in southeast Florida (Broward and Dade Counties) are located between the Atlantic Ocean coast and the eastern boundary of the Everglades, only 8-35 km from the Everglades proper (see Figure 1). The 1995 South Florida Atmospheric Mercury Monitoring Study (SoFAMMS) was conducted to quantitatively assess the sources contributing to the Hg found in precipitation. A preliminary analysis of precipitation data collected during SoFAMMS suggested that local anthropogenic sources may play an important role in the atmospheric wet deposition of Hg to south Florida (20). This analysis revealed large variations in the Hg concentration measured in precipitation, both spatially across sites on individual days as well as temporally at each individual site over the duration of the study period, that were suggestive of large impacts from local sources. The analysis also indicated that source-receptor relationships for Hg could be established in south Florida by analyzing the elemental composition of each precipitation event and quantifying the relationship between specific elements or ratios of elements and elevated levels of Hg in precipitation. Since many sources emit trace elements that are characteristic to their specific source type, relationships may be established by linking emissions with receptor conditions through specific source “fingerprints” or tracers of opportunity (21). The results presented here include analysis of the elemental composition of event precipitation samples collected in south Florida. A multivariate receptor modeling approach is used to source apportion the Hg wet deposited at Everglades sites during SoFAMMS as well as Hg wet deposited over a 1-year period at a site in Davie, FL.
Methodologies Precipitation Collection. The SoFAMMS was conducted from August 6 to September 6, 1995. Daily event precipitation samples were collected at 17 sites located throughout Dade and Broward Counties in the extreme southeast portion of Florida (see Figure 1). Samples were collected manually at each of the 17 sites on each day throughout the 1-month study period. After oxidation with concentrated BrCl (to 1% v/v solutions), samples were analyzed for total Hg using cold vapor atomic fluorescence (22). Samples collected for quantification of additional trace elements were acidified with concentrated HNO3 (to 0.2% v/v solutions) prior to analysis by inductively coupled plasma-mass spectrometry (Perkin-Elmer ELAN 5000). A complete description of the sampling and analysis methods deployed is described by Dvonch et al. (20). In addition to the precipitation measurements made during SoFAMMS, samples were also collected at the site in Davie, FL (26.10° N, 80.23° W, FLG site in Figure 1) for an entire 1-year period, June 22, 1995-June 21, 1996. Daily event wet-only precipitation samples were collected at 10.1021/es9903678 CCC: $18.00
1999 American Chemical Society Published on Web 11/09/1999
FIGURE 1. SoFAMMS field sampling sites. the Davie site using a modified MIC-B automatic precipitation collector that included a custom acrylic insert, which replaced the standard MIC-B Teflon-coated steel collection funnel (23). The collection funnel and bottle configurations employed as well as sample handling, processing, and analysis methods used for the Davie samples were identical to those used for SoFAMMS samples. Source Sampling. As part of SoFAMMS, the U.S. EPA conducted concurrent stack testing in order to construct a more robust database of emissions from various anthropogenic source types in south Florida (24). Sampling was performed at three different source types in Dade County during the 1-month SoFAMMS period. These three source types included (i) a municipal waste incineration facility, (ii) a coal-fired cement kiln operation, and (iii) a medical waste incineration facility. Trace element samples were obtained from each source via a dilution sampling apparatus (24). Fine particle samples were collected onto 47 mm, 2-µm pore Teflon (PTFE) membrane filters (Gelman) and analyzed using both X-ray fluorescence (XRF) (25) and instrumental neutron activation analysis (INAA) (26). Speciated Hg emission measurements were obtained using U.S. EPA Method 29 (27). Three to eight days of sampling was performed at each of the source types.
Results and Discussion Speciation of Hg Emitted during Source Sampling. Significant amounts of soluble Hg(II) species were measured in the emissions from waste incineration point sources in south Florida during this study with the Hg(II)/Hg(total) ratio ranging from 0.75 to 0.95. The results from these source tests are presented in Figure 2. The testing performed at the cement kiln found approximately 25% of the total Hg emitted to be the form of Hg(II). This result has previously been observed for other coal combustion facilities (17). However, the municipal waste incinerator tested during SoFAMMS was found to emit roughly 75% of its total Hg emissions as Hg(II),
while the medical waste incinerator emitted nearly 95% of its emissions as Hg(II). The Hg(II) species, being very soluble, are readily available for incorporation into cloud droplets with subsequent removal from the atmosphere via precipitation. Source Apportionment of Hg in South Florida Precipitation Using a Multiple-Tracer Receptor Modeling Approach. A multiple-tracer receptor modeling approach was utilized to assess Hg source contributions to wet deposition. The interrelationships among the trace elements and Hg measured in precipitation were quantified using principle component factor analysis (PCA). The basic purpose of PCA is to reduce the dimensionality of a data set of interrelated variables so that a minimum number of factors can explain the maximum amount of variance in the data (28). When PCA is applied to a set of atmospheric measurements, these factors often represent different sources impacting the receptor site. The method of PCA and those similar to it have been previously applied to other atmospheric data sets, as summarized by Henry (28). However, only recently have these methods been used to source apportion atmospheric Hg (29). Following the application of PCA, the contributions of individual source types were estimated using a multiple linear regression (MLR) analysis. This was done by regressing the absolute factor scores produced by the PCA (for each sample) against the corresponding measured Hg values (30). Principal component factor analysis was applied to the SoFAMMS Everglades wet deposition data (collected at sites AND, EVC, EVN, EVS, and THP in Figure 1). The elemental factor loadings produced by the PCA for the Everglades data set (N ) 81) are shown in Table 1. The data clustered into four significant factors or source types. The waste incineration factor (factor 1) was identified by loadings of Pb and Sb. Source testing performed during this project revealed both Pb and Sb to be associated with waste incineration emissions (24). The second factor explained much of the Sr and Mg variance and was interpreted as a marine component. Magnesium is a known tracer of marine aerosols (31). The oil combustion factor (factor 3) was identified by strong loadings of V and Ni, which are both known to be associated with oil combustion sources (32). The last factor explained much of the Rb and Ti variance and represented a crustal component. In this analysis, Hg clearly loaded strongest onto the waste incineration factor. The PCA was also applied to the 1 year of precipitation data collected at Davie (N ) 112). The elemental factor loadings for the Davie data (Table 2) are similar to those for the SoFAMMS Everglades data. The Davie data clustered into six significant factors. The first factor explained much of the Mn, Rb, and Ti variance and represented the crustal component. The waste incineration factor (factor 2) was identified by strong loadings of Pb and Sb. The oil combustion factor (factor 3) was identified by loadings of V, Ni, and Mo. The fourth factor had strong loadings of Cu and Zn and represented an anthropogenic source. The fifth factor explained much of the Sr and Mg variance and represented the marine component, while the sixth factor explained much of the As variance and also represented an anthropogenic source. Mercury loaded strongest onto the waste incineration factor, as was observed with the Everglades sites PCA. The PCA for the SoFAMMS Everglades data and the yearlong data record from Davie was re-run without Hg in order to calculate the Hg source contributions. For each data set (Everglades and Davie), significant relationships that existed between Hg and these factors, each of which represented a specific source type, were quantified through a multiple linear regression analysis of the absolute factor scores against the corresponding Hg values. The relationships (coefficients) quantified using the MLR analysis were then used to estimate the contributions from the individual source types for which VOL. 33, NO. 24, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Speciation of Hg measured in emissions from three point source types during SoFAMMS (24).
TABLE 1. Varimax Rotated Factor Matrix for Wet Deposition at SoFAMMS Everglades Sites (N ) 81) factor 1 (waste incineration)
factor 2 (marine)
factor 3 (oil combustion)
factor 4 (crustal)
0.39 0.25 -0.12 0.01 0.76 0.01 0.21 0.77 0.91
0.04 0.19 0.58 0.91 0.02 0.97 0.23 -0.18 0.07
0.91 0.90 0.08 0.09 0.48 0.05 0.14 0.52 0.15
-0.02 0.26 0.74 0.36 0.26 0.13 0.90 0.01 0.01
Eigenvalue
2.28
2.24
2.19
1.64
cumm. %
46
77
element V Ni Rb Sr Pb Mg Ti Sb Hg
85
93
TABLE 2. Varimax Rotated Factor Matrix for Wet Deposition at Davie, FL (N ) 112) element
factor 1 factor 2 factor 3 factor 4 factor 5 factor 6 (crustal) (incin.) (oil) (Cu, Zn) (marine) (As)
V Mn Ni Cu Zn As Rb Sr Pb Mg Ti Mo Sb Hg
0.21 0.92 0.30 0.17 0.10 0.24 0.87 0.35 0.22 0.24 0.91 0.17 0.10 0.20
0.34 0.03 0.35 0.35 0.36 0.41 0.28 0.16 0.78 0.15 0.21 0.46 0.72 0.86
0.82 0.11 0.77 0.28 0.40 0.35 0.20 0.17 0.30 0.22 0.17 0.59 0.31 0.23
0.21 0.20 0.30 0.72 0.68 0.20 0.07 0.47 0.23 0.15 0.01 0.35 0.38 0.14
0.19 0.02 0.20 0.35 0.06 0.22 0.27 0.57 0.20 0.89 0.21 0.29 0.13 0.06
0.21 0.09 0.09 0.01 0.30 0.70 0.11 0.41 0.01 0.09 0.07 0.24 0.28 0.19
Eigenvalue
2.96
2.90
2.38
1.78
1.62
1.00
60
73
80
84
87
90
cumm. %
the factors represented. For the Everglades data, statistically significant relationships with Hg were found only for the waste incineration and oil combustion factors. The waste incineration factor was found to account for 56.7 ( 7.0% of 4524
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TABLE 3. Summary of Hg Source Apportionment at Davie and Everglades Sites
source type
% Hg deposition explained at Davie
% Hg deposition explained at Everglades sites
waste incineration (Pb & Sb) oil combustion (V & Ni) As Cu & Zn crustal (Ti & Rb) marine (Mg & Sr)
27 ( 2 17 ( 3 22 ( 4 8(2 4(1 0
57 ( 7 14 ( 5
total
77 ( 6
71 ( 8
0 0
the Hg wet deposited at the five Everglades sites, while the oil combustion factor accounted for 14.4 ( 4.8% of the Hg measured. The uncertainties in these estimates were calculated using the standard error of each coefficient in the MLR. Overall, 71% of the Hg wet deposited at these Everglades sites could be accounted for by both waste incineration and oil combustion combined. For the Davie data, significant relationships with Hg were found for the waste incineration, oil combustion, Cu and Zn, crustal, and As factors. For the 1-year Davie analysis, the waste incineration factor was found to account for 26.8 ( 2.4% of the Hg wet deposited at the Davie site. The oil combustion factor accounted for 17.0 ( 2.6% of the Hg, while the As factor, the Cu and Zn factor, and the crustal factor were found to account for 21.9 ( 4.3%, 7.6 ( 1.8%, and 3.9 ( 1.0%, respectively, of the Hg wet deposited. These source apportionment results are summarized in Table 3. Overall, 77% of the Hg wet deposited at Davie could be explained, with 73% being accounted for by local anthropogenic source emissions. Additional analyses were performed in an effort to gain a better understanding of the Hg contributions from the anthropogenic sources that the Cu and Zn factor and the As factor represented for the Davie analysis. One analysis performed included additional PCA/MLR of the Davie data by season. This included independent analyses of the wet season data (May-October) and the dry season data (November-April). These analyses found the Hg contributions from the Cu and Zn source to occur throughout the year, while the Hg contributions
from the As source only occurred during the wet season. Since the Cu and Zn source and the As source both contributed Hg to the Davie site during the wet season, a subsequent PCA/MLR analysis was performed on the wet deposition data collected during SoFAMMS at the urban Broward County sites (BCN, FLG, SBI, and SOT) located nearest to the Davie site in order to determine the spatial impact of these two sources of Hg. This analysis found no Hg contributions from the As factor or the Cu and Zn factor at these sites. Therefore, although the As source and the Cu and Zn source both significantly contributed to Hg wet deposition at the Davie site, these anthropogenic sources were likely not major point sources. Instead, these sources were likely relatively small sources of Hg located very near (
