Environ. Sci. Techno/. 1995, 29, 1495- 1503
History of Trace Metal Pollution in Sabine-Neches Estuary, Beaumont, Texas MAHALINGAM R A V I C H A N D R A N , * , t j t MAHALINGAM B A S K A R A N , I P E T E R H . SANTSCHI,+A N D T H O M A S S . BIANCHIg Department of Marine Sciences, Texas A M University, Galveston, Texas 77553, and Department of Ecology, Evolution and Organismal Biology, Tulane University, New Orleans, Louisiana 70118
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Introduction The distribution of metals in sediment cores sampled from industrialized estuaries has been used as an indicator of past and present pollution events (1,Z). Such reconstruction studies are useful to improve management strategies as well as to assess the success of recent pollution controls. Trace metal contamination in sediments has received more attention in the last two decades, and a recent issue of Estuaries (Vol. 16 issue 3B, September 1993) has been dedicated to the study of dated sediment cores for environmental research. Recently, Valette-Silver (3) reviewed the availableliterature on the reconstruction studies and concluded that, in general, heavy metal pollution of sediments began in the early 1800s, became more prominent in the 19OOs,and increased sharply between 1940 and 1970. In the last two decades, generally a decreasing trend in heavy metal inputs into the environment is observed (2, '
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Sabine-Neches Estuary, near Beaumont,TX, receives wastewater effluents from over 160 industrial and municipal treatment plants. The concentrations of trace metals (Co, Cr, Cu, Ni, Pb, and Zn), AI, Fe, Mn, and organic carbon were determined in four dated sediment cores. A reliable geochronology and reconstruction of the history of trace metal inputs of these sediments was possible because the 239,240Pu profiles closely trace the bomb fallout history into the environment. Down-core variations of aluminumnormalized enrichment factors for these metals demonstrate that the sediments of this estuary have remained relatively 'pristine' with respect to trace metal contamination since 1860. While the concentrations of Pb and Zn at various depths in the sediment column are slightly enriched, Co, Cr, Cu, and Ni are depleted. The sedimentary and biogenic particles that are presently being deposited are also depleted in trace metals. Lack of strong enrichment for trace metals like Cu can be attributed to the short residence time of water, low salinity conditions, and possibly strong complexation of these metals with organic matter.
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The most commonly used methods of dating recent sediments is based on the concentration gradient of natural radionuclides such as 210Pband the occurrence of bomb fallout radioisotopes such as 137Csand239,240P~ (4). Because of its 22.3 yr half-life, 210Pbhas been widely used to determine sedimentation rates over the last 100-150 yr (5, 6). Establishing reliable sediment chronology in shallow estuarine environments using 210Pbis complicated by the post-depositional mixing of particles by physical and biological (bioturbation) processes. Hence, a second particle-reactive tracer, such as 239,240Pu, is used in conjunction with 210Pbto deconvolute mixing from sedimentation (refs 4 and 7 and references cited therein). 239~240Pu is a byproduct of atmospheric testing of nuclear weapons, introduced into the environment around 1952 with a maximum fallout in 1963. Plutonium is an excellent analogue for studies of heavy metal transport through the environment, because it is particle reactive, it is effectively retained in aquatic and marine sediments, and it has a well-defined input function (4, 7, 8). The Sabine-Neches Estuary is strategically located in the golden triangle area of Beaumont, Port-Arthur, and Orange, which constitute the principal industrial areas in the region. Since the beginning of petroleum related industry in the study area in 1901 (91,the industrial activity has steadily grown, and now almost all of the major oil and chemical companies have plants in the vicinity of the estuary. The combined ports of these three cities comprise the fourth largest port facility in the nation. As of 1980, there were over 160 permitted wastewater dischargers into the Neches and Sabine Rivers, apart from other diffuse sources such as urban runoff (10). Despite the concentration of industries and the consequent discharges of wastewater effluents into the estuary, very few studies have been carried out on the levels of heavy metal concentrations in the sediments of the SabineNeches Estuary. The Texas Department ofwater Resources t Texas A&M University. 4 Present address: Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, Campus Box 428, Boulder, CO 80309-0428; Fax: (303) 492-7317; e-mail address:
[email protected]. Tulane University.
0013-936W95/0929-1495$09.00/0
0 1995 American Chemical Society
VOL. 29, NO. 6, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1495
TABLE 1
Selected Metal Concentrations in Average Shale (37),Average Soil (38),and Screening Criteria (IO)for Metal Pollution in Sediments elements
av shale av shale concn [metal/Al]
ev soil concn
av soil [metal/Al]
screening criteria a C
is the smallest (surface area = 259 km2),well mixed, and the freshest estuary (average rainfall = 140 cm). It has the highest annual freshwater inflow (1.63 x 1O1O m3/yr) with an average salinity of just 2 . 3 0 ~(14). Due to the large freshwater inflow, it has the highest areal loadingof carbon, nitrogen, and phosphorus. Short residence time of water (-10 days) and high freshwaterinflow have resulted in the decline in productivity of both primary producers and fisheries (15).
Trace Metals
Co (ppm) Cr (ppm) Cu (ppm) Ni (ppm) Pb(ppm) Zn (ppm)
19.0 90.0 45.0 68.0 20.0 95.0
1.26 5.95 2.98 4.50 1.32 6.28
9.00 70.0 30.0 50.0 19.0 90.0
NA
NA 100
0.672 5.22 2.24 3.73 1.42 6.72
60 52 44 110 170
50 50 50 75
1.00 0.427 41.0
NA NA NA
NA NA NA
Major Elements
A1203 (%) 15.1 FezOB(%) 6.75 Mn (ppm) 850 a
1.00 0.446 56.2
13.4 5.72 550
National 85th percentile (70). Dredge disposal criteria ( 1 0 ) .
(TDWR) conducted screening-type studies in the Sabine and Neches Rivers in 1980 to idenufy areas of organic and inorganic pollution in surface sediments and their results indicated that the concentrations of Cr and Cu were significantly enriched above the national 85th percentile screening criteria and that Pb concentration was highest below the Mobil effluent discharge site (10). The followup work at selected effluent sites in 1987 revealed that Cr, Cu, Pb, Ni, and Zn concentrations increased temporally between 1980 and 1987 in most of the effluent sites (11). During 1974- 1978,TDWR also collected surface sediments at six study sites within the estuary (12)for trace metal analyses, and they found that only Zn was slightly enriched above the dredge disposal criteria (ca. 79 ppm vs Table 11, while the measured sediment concentrations of Co, Cu, Pb, and Zn were always below average soil concentrations. Apart from the above-mentioned reports, no known prior work has been done on the trace metal concentrations in the sediments of the Sabine-Neches Estuary. The major objective of this study was to reconstruct the historical variation of trace metal concentrations in sediment cores since the beginning of industrial activity in the early 1900s. We have measured the concentrations of both 210Pband 239,240Pu in four sediment cores, and it was found that 239,240Pu was a reliable geochronometer in these estuarine sediments (13). For the first time, we have established the geochronology of these estuarine sediments and measured the down-core variation of trace metal concentrations in the sediment cores of the Sabine-Neches Estuary. An attempt was made to separate the naturalvariation of trace metal concentrations from changes due to anthropogenic sources. This study is one of the very few studies that have been carried out in shallow estuaries that are also highly industrialized. The concentrationof metals in the sediment cores was compared with commonly used screening criteria, namely, dredge disposable criteria and national 85th percentile criteria (10). The possible causes for the observed concentrations are also discussed.
Setting of the Study Area The Sabhe-Neches Estuary, situated on the border ofTexas and Louisiana (Figure l),is the northernmost estuary of Texas. Of the seven estuarine systems on the Texas coast, the Sabine-Neches Estuary is unique in several ways: it 1496 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6,1995
Materials and Methods Collectionof Samples. Four sediment cores were collected across the upper, mid, and lower regions of Sabine-Neches Estuary (Sabine Lake region) over a period of 10 months in 1992-1993. The locations of the sampling sites of sediment cores are shown in Figure 1. The mean water depth for the four sampling stations are 1.5 m at station 2, 2.0 m at station 4, and 2.2 mat stations 6 and 7, respectively. Sediment cores were collected using hand-held corers and transported vertically to the lab, where they were sectioned, bagged, and frozen until chemical analyses. The sections were shaved off to avoid possible smearing of near-wall parts of the cores. A portion of the sediment was weighed and dried at 105 "C for 24 h to determine the water content and porosity. Parts of the sediment sampleswere also wetsieved with an ASTM 63-pm sieve to determine the mass of the 163-pm size fraction (Le., silt plus clay). To ensure quantitative size separation, the portion retained on the sieve was placed in an ultrasonic bath and sieved again. Sediment Digestion. In order to measure the concentrations of trace metals in the bulk sediments, the dried sedimentsampleshad to be completelydissolved. Recently, microwave digestion has been shown to be an effective method for the complete dissolution of sediment samples for the analyses of most of the trace metals and major elements (16). This method is advantageous in several ways: it needs only one-third of the time and amount of acids required by any open digestion method; it is much easier and safer to use than alkali-fusion methods; and it results in very low blank levels and low yields of total dissolved solids (TDS), enabling low limits of quantitation. Our method is a slight modificationof that used by Totland et al. (16). For this study, a commercial microwave digestion system, the CEM-MDS-81D, was used. About 250 mg of dry, pulverized sediment was transferred into microwave digestion vessels to which 5 mL of concentrated HF and 5 mL of concentrated "03 acids of trace metal grade were added. The contents were heated in the microwave at a maximum pressure of 90 psi for about 3 h. The solution was then transferred to acid-cleaned Teflon beakers with 2 mL of concentrated HCIOl and dried on a hot plate. We differed from the method outlined by Totland et al. (16)by not adding HC1O4into the microwave digestion vessels as this tends to corrode the vessels. To further increase the temperature of volatilization and completely remove fluoride ions, two 2-mL aliquots of HC104 were added to the dried residue and evaporated to incipient dryness. The resulting residue was dissolved in 25 mL of 0.8 M trace metal grade "03, which was further diluted before measurement. For calibration purposes, the USGS sediment standard, Cody Shale (SCo-11, was digested along with the samples. Analytical blanks were also prepared with each digestion. Analysis of Trace Metals and Major Elements. Trace metals (Co, Cr, Cu, Ni, Pb, Zn) and major elements (A, Fe,
I 94'00'
PORT ARTHUR b
k-
Gulf of Mexico
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KM FIGURE 1. Map of the Sabine-Neches Estuary, in southeast Texas, showing the location of the sampling stations.
Mn) were measured using a Sciex Elan-500 inductively coupled plasma mass spectrometer (ICP-MS). The blank levels were very small, often below detection limits, and were subtracted from the samples. The concentrations of most of the metals in sediment standard (SCo-1) were accurately determeed (17 ) within the reference values (16, 18). The exceptionswere Zn,which was consistentlyhigher by about 20%, and Mn, which was consistently lower by about 10%than the reference values. The relative standard deviation was
