Environ. Sci. Techno/. 1995, 29, 470-477
N a m o n aml Elemetttal Sedimelrt C-ation in the Coastal United States K O S T A S D . DASKALAKIS* A N D THOMAS P. O'CONNOR National Status and Trends Program, NOAAlNOSIORcA21, Silver Spring, Maryland 20910
Elemental contamination has been examined in sediments collected from 350 sites along the coast of the United States by NOAAs National Status and Trends (NS&T) Program. To isolate natural from man-made factors, data from 60 sites with fewer than 10 000 people living within 20 km were used to test correlations between concentrations of trace elements and four candidate normalizing factors: iron, aluminum, grain size, and total organic carbon. At least 50% of the concentration variations for the elements As, Cr, Cu, Ni, Pb, Sn, and Zn can be accounted for by covariation with Fe or AI, and either of those major elements can be used for normalization. For Ag, Cd, Hg, Sb, and Se, on the other hand, no candidate normalizers accounted for more than even 30% of the variation. Based on these results, the spatial extent of contamination has been examined, and it was found that severe contamination is mostly limited to small areas near large population centers.
Introduction Chemicals added to river waters or directly discharged to the estuarine environments are usually adsorbed onto particle surfaces and are removed as the particles settle to the bottom, providing a pool of contaminants that can be released to the overlaying waters through natural or anthropogenic processes, such as redox cycling, bioturbation, flooding, or dredging. To determine sediment contamination, that is, concentrations above what would naturally occur, it is necessary to calculate the natural component of the total concentration. This can, in principle, be done by defining the ratio of natural concentrations to that of some normalizing factor whose concentration is unaffected by human activity. There is no consensus on the appropriate sediment constituent to be used for normalization. Among those used have been aluminum, iron, lithium, total organic carbon, and grain size (1-7). Aluminum has often been used to normalize trace element concentrations because of its high natural concentration and minimal anthropogenic contamination and because it is a structural element of clays. Since trace elements tend to be adsorbed onto surfaces of particles,their concentrations naturally increase as particle sizes decrease. As a central component of clay, the smallest sized particles, aluminum, can serve as a measure of clay content of a sediment sample. The pollutionhistory of the Savannah River Estuarywas studied (I), and aluminum was chosen as the normalizing parameter. Windom et al. (2) also used aluminum normalization in a study of estuarine and coastal sediments from areas distant from known sources of Contamination in the southeastern United States. They also tested for correlations with total organic carbon (TOC)and found, with the exception of Cd, that they were weaker than correlations with aluminum. Use ofparticle-bound TOC for normalizing trace elements is based on the assumption that it is serving as a matrix on particle surfacesfor complex formation. One weakness of TOC as a normalizer, however, is the fact that TOC itself is subject to considerable augmentation by human activity and is often a contaminant. In a study of Galveston Bay sediments, Morse et al. (3) normalized trace element concentrations (1 M HCl extractable pyrite metal) with total reactive iron. The ratio of l0Be/Fehas been used as an indicator of sediment origin in central Chesapeake Bay ( 4 ) ,where it was found that l0Be and Fe varied by more than 1order of magnitude while the ratio of 'OBe/Fe varied by an amount comparable to analytical error. Iron is not a matrix element, like aluminum, but is like trace elements in being associated with surfaces. Iron geochemistry is similarto that of many trace metals both in oxic and anoxic environments. That direct associationwith trace elements and the fact that its natural concentrations in sediments are more uniform than Al (1, 8) and beyond the influence of humans (9), argue for its use as a normalizer. Lithium was used as the normalizing parameter for sediments derived from glacial erosion of rocks from the high latitude eastern shore of North America (5). Lithium
+
* To whom correspondence should be addressed Fax: (301) 7134388;e-mail address:
[email protected].
470 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 2,1995
This article not subject to U.S.copyright. Published 1995 bv the American Chemical Sociehr.
correlations with trace elements in fine-grain sediments were equal or greater than those with Al. Recently, results were reported for sediments from the southeast United States and Gulf of Mexico which were analyzed as part of the N O M National Status and Trends (NS&T) Program Benthic SurveillanceProgram (6‘). Baseline sites were selected as those thought to be distant from human activities, and linear correlations of trace metals with Al were calculated. A baseline model was considered acceptable when the covariation with aluminum was statistically significant, and the linear regression intercept was close to zero. Data for As, Cu, Ni, Sn, and Zn revealed good correlations with aluminum, while lower coefficients of determination (s) for Ag, Cd, Hg, Mn, and Se and significant positive intercepts for Ag, Cd, Cr, and Pb suggested deviations from the model for these elements. The N O M NS&T Program has been collecting and analyzingsurface grab sedimentsat approximately350 sites along the U.S. coast. The objective of the NS&T Program is to quantify contamination over large spatial scales, and known point sources of contamination were avoided. The NS&T data constitute a comprehensive and internally consistent database on chemical contamination of the coastal and estuarine United States. Initial results from the NS&T sediment analyses for Benthic Surveillance until 1986 and Mussel Watch program until 1989 have been presented (7). In that report, data were excluded from comparisonsif the sediment was more than 80%sand, and the remaining data were adjusted for the fine-grained fraction of the sediment, which is that which consisted of particles less than 0.64 pm in diameter. This adjustment is not a normalizationprocedure in the sense that there are natural baseline concentrations of trace chemicals in the fine fraction of sediment. It does, however, account for the natural diluting effect of sand that forces measured concentrations to differ among sites. In this paper, we will use the NS&T data to test normalization procedures and then, using normalization examine the contaminationalong the coastal United States.
Experimental Methods Sample Collection. The NS&T sampling and analysis protocol has been documented (10). Briefly,three sediment sampleswere collected at each station, using box corers or grab samplers, and three stations were sampled at each site. Composite samples were made from surface sediment of three grabs or cores at each station. Sediment analyses for a site consisted of organic analyses of three composites (one from each station), inorganic analyses of three composites, and grain size and other ancillary measurements on a third set of three composites. Since 1988, sediment collection and analysis have been limited to sites not previously sampled. Analytical Methods. Several laboratories have been analyzing sediments for the NS&T program, and analytical methods varied among laboratories and over time (10). Variability among data sets is minimized through quality assurance(QA) protocols, which include analytical methods documentation, required participation in interlaboratory comparisons exercises, and use of Standard Reference Materials and Interim Reference Materials. The quality and consistency of the analytical data in the NS&T database provides us with a unique opportunityto explore differences in concentration levels and to investigate the relationships between trace and major element concentrations.
TABLE 1
Coefficients of Determination (rz) for Metal Correlations with Aluminum or Iron Gulf (n = 230)
East (n = l W ) Ag AI As
Cd Cr
Cu Fe Hg Ni Pb Sb
Se
Sn Zn
West (n = 18)
AI
Fs
AI
Fe
AI
Fe
0.15
0.18 0.75 0.64 0.06 0.47 0.58
0.16
0.05 0.76 0.71 0.02 0.64 0.39
0.03
0.30 0.39 0.72 0.08 0.02 0.78
0.59 0.12 0.42 0.64 0.75 0.27 0.67 0.76 0.03 0.55 0.59 0.62
0.14 0.66 0.64 0.00 0.62 0.53 0.71
0.53 0.06 0.56 0.53 0.76 0.36 0.61 0.66 0.37 0.06 0.49 0.61
0.45 0.56 0.72 0.29 0.07 0.57 0.52
0.10 0.00 0.01 0.15 0.39 0.02 0.01 0.10 0.38 0.54 0.09 0.24
0.52 0.57 0.43 0.45 0.41 0.15 0.90
The elements measured in NS&Tsediment samples are Ag, Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, Se, Si, Sn, and Zn. For inorganic analysis, 0.10-0.45 g of dried sediment was added in Teflon (PFA) vials and digested using “OBHF, HNO3-HC1-HFor HN03-HC104-HFby conventionalor microwave heating. The digestates were transferred to polyethylene screw cap bottles, and boric acid solution was added to dissolve the fluorides. All solutions were subsequently analyzed using graphite furnace atomic absorption spectrometry (GFAAS),inductive coupled plasma-mass spectrometry (ICP-MS), or cold vapor spectrometry (for Hg). X-ray fluorescence (XRF) and instrumental neutron activation analysis were employed for some sediment analyses, too.
Results and Discussion Data Evaluation. The percentage of the data below the detection limit varied depending on the particular metal, with Al, As, Cd, Cr, Cu, Fe, Ni, Pb, and Zn in the range of 1-5%, Ag 7%, Sn 12%,Hg 16%,Sb 23%, and Se 25%. No attempt has been made to substitute estimated concentrations for below the detection limit results, and such cases have not been included in this analysis. Determination of natural ratios requires selection of uncontaminated sites. To date, that has been subjectively done based on knowledge of each samplingsite (2,s). With the NS&T data, we can objectively establish an a priori criterion for selecting uncontaminated sites. We have used 1990 U.S.Census data (B. Davis, TIGER Systems staff, U.S. Census Bureau) to determine which sites have fewer than 10 000 people residing within 20 km and assumed that sediments from such sites are free of human influence (see Figure 1). The number of samples (n)used in the linear regressions varied between 300 and 315 for most metals, with the exception of Cr (2401, Sb (1801, and Se (200).Data analysis was performed with SASIPC version 6.08 (SAS Institute Inc., Cary, NC) and Origin version 3.5 (MicroCal, Northampton, MA). Geographic Dependence of Correlations. Because of regional differences in mineralogy, natural ratios of trace to major elementconcentrationratios can vary on a national scale. This effect is shown in Table 1 where coefficients of determination are shown separatelyfor correlations of P20 < 10 000 data from the East Coast, Gulf of Mexico Coast, and West Coast. Interestingly, the West Coast shows no VOL. 29, NO. 2, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY m 471
h
FIGURE 1. S i r with total populaioma less (empty circles) and mora (filled circlerllhan 10 WO database.
a distance of74 kn included in the N U T
TABLE 2
Coefficients of Determination (#) for Linear Relations of Trace Metal9 Eaat and Gulf Ot Mexico sles'lor P20 < lOwD (n= 360) TOC grain size AI Ag
.A..I
As Cd Cr Cu Fe
Hg Ni Pb Sb Se Sn Zn
0.01
0.15
n nn
# A7 l
0.04 0.14 0.10 0.13 0.06 0.06 0.06 0.07 0.03 0.10 0.01 0.07
0.19 0.23
-...
0.28 0.47 0.37 0.20 0.45 0.36 0.10 0.14 0.26 0.47
Georgia and South Carolina' (n = 340)
floridec(n = 100)
East and Gulf ot Mexicod(n = 149)
AI
AI
AI
F0
0.11
0.06 0.75*
0.52* 0.08 0.50* 0.59* 0.75* 0.34 0.63" 0.65' 0.16 0.17 0.48 0.63'
0.68* 0.03 0.58* 0.44
0.38 0.59* 0.66' 0.10 0.17 0.54' 0.57*
0.13 0.77" 0.64* 0.91*
0.81* 0.61* 0.88*
0.53* 0.62*
0.68. 0.69*
0.20
0.83.
0.70*
a An asterisk (*) indicates that coefficient of determination is greater than 0.5. bThis work.
good correlations with Al and very good correlations with Fe, but since there were only 24 samples from five sites, little should be concluded. Correlations with grain size were significant only for the West Coast, while in general metals and TOC did not co-vary on any coast.
Forthevastmajorityofthedatathereisgoodagreement in +values between the East and Gulf Coasts. The linear regression models for As, Cd, Cr, Hg, Pb, Sb, Sn, and Zn, with Fe as the independent variable for the East and Gulf of Mexico coasts, are statistically indistinguishable (95% confidence limit). When Al was used as the independent variable, As, Cd, Cr, Cu, Hg, Ni, Sb, Sn, and Zn produced statistically indistinguishable linear models for East and Gulf of Mexico Coasts. Given the general similarity, these data were combined to a single database for assessing the relative merit of normalization procedures. Baseline Models. Results from the linear regression models are compared with previously published values in 472 m ENVIRONMENTALSCIENCE &TECHNOLOGY I VOL. 29, NO. 2.1995
0.61* 0.14 0.64* 0.76* 0.82*
+
0.63* 0.76* 0.44 0.28 0.44 0.72*
Reference 2. "Reference 6.
Table 2. The values derived here for correlation of P20 < 10 000 sites from the East and Gulf of M 4 c o Coasts with Alcomparewellwithresultsfrompreviousworks(2,s). For As, Cr, Cu, Pb, Ni, Sn, and Zn at least 50% of the concentration variations (+ =- 0.5) are accounted for by either Fe or Al. For three of those elements, was higher withAlthanwithFe,whiletheFecorrelationswerestronger for the other four. There is no particularly compelling reason to choose one over the other. but if only Fe or Al data were available, either would do. We tested the possibility that multiple correlationswith Fe and Al would account for more trace element variability than did either Fe or Al alone. However, we found a small improvement in the combined coefficient of variation. The intercepts and slopes for the linear relations between trace metals and Al or Fe are presented in Table 3. Concentrations of fiveelements CAS, Cd, Hg, Sb, and Se) are not well correlated with either Fe or Al. It has been
~
TABLE 3
TABLE 5
Linear Correlations of Seven Metals and Fe and AI for East and Gulf of Mexico Coasts with P20 < 10 oov
Effects of Normalization to Within-Site RSD
correlation with Fe a slope 6 intercept AS
Cr
cu Ni Pb
Sn Zn
3.25 x 10-4 1.41 x 3-41 10-4 5.43 x 5.06 x 5.97 x 10-5 2.01 x
0.59 18.20 4.19 4.64 5.76 0.23 16.16
correlation with AI a slope 6 intercept
1.68 x 7.63 x 2.32 x 3.35 x 2.95 x 3.40 x 1.25 x
-0.54 13.13 0.12 -0.05 2.25 -0.13 -1.62
8All elemental concentrations in units of Fglg (ppm) dry wt.
TABLE 4
Coefficients of Determination of Metals and Fe and AI for East and Gulf of Mexico Coasts with P20 < 10 000 and P20 > 10 000 correlation with Fe Pzo
