Environ. Sci. Technol. 1997, 31, 1985-1991
Discriminating between Sources of Arsenic in the Sediments of a Tidal Waterway, Tacoma, Washington A N D Y D A V I S , * ,† P . D E C U R N O U , † A N D L. EDMOND EARY‡ Geomega, 2995 Baseline Road, Suite 202, Boulder, Colorado 80303, and Shepherd Miller, Inc., 3801 Automation Way, Fort Collins, Colorado 80525
Various industries located along the Hylebos Waterway in Tacoma, WA, and runoff from slag used for road ballast are potential sources of elevated arsenic concentrations found in the waterway sediments. To discriminate between specific sources of As to the waterway, the history of arsenic deposition was reconstructed using 137Cs sediment age-dating combined with characterization of arsenic solid phases. In a sediment core collected near a former powdered metals facility, As occurred primarily in metal sulfides, reaching peak concentrations in the mid-1950s, consistent with the operational history of that facility. In a second core collected near a pesticide plant that historically discharged soluble arsenic, total arsenic was present predominantly as surface-bound species in sediment predating the mid-1980s, decreasing in recent sediments coincident with initiation of remedial measures in 1981. Ferroalloy and smelter slags were recent and optically distinguishable. The combination of optical microscopy and electron and laser-ion microprobe techniques in conjunction with historical information and radioisotopic analysis provides a powerful tool to differentiate between metal sources to sediments.
Introduction Elevated concentrations of both inorganic and organic compounds have been found in subtidal and intertidal sediments of the Hylebos Waterway (Table 1), a dredged, deep-water channel near Tacoma, WA (Figure 1). A variety of industries have operated along the waterway since it was first dredged in 1931, a subset of which are known to have produced or handled As-containing products. A comprehensive investigation of the waterway bathymetry, sedimentation rate, species diversity, and toxicity effects has been conducted (1), while a subsequent study (2) demonstrated that releases from these facilities adjacent to the waterway resulted in bulk As sediment concentrations ranging from 0.38 to 1260 mg/kg. The major industrial user of As was a pesticide manufacturing plant that disposed of highly alkaline, As-containing aqueous wastes to lagoons located adjacent to the waterway (1939-1974) (3) and, also between 1939 and 1980, directly to the waterway (4). Other potential sources of As include a rock-wool plant that produced slag from the feedstock of a local smelter (1959-1973) and steel slags (1973-present); a powdered metal plant that generated a variety of metal * To whom correspondence should be addressed; phone: 303442-2549; fax: 303-442-8123. † Geomega. ‡ Shepherd Miller, Inc.
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1997 American Chemical Society
products (1946-1958); run-off from slags used for road ballast in log-sort yards (1975-1983); shipbuilding facilities that used slag as a sand-blasting medium; marinas that stripped arsenical paints from boat hulls; and regionally the Tacoma Smelter, which operated between 1921 and 1986 (5). To date, methods to distinguish between multiple sources of metals to sediments have focused on chemometric analysis (6). Here we show that the deportment of As in sediments can be used to differentiate between industrial sources. In this study, accurate sediment dating using 137Cs was necessary to put As loading into historical perspective. The presence of 137Cs in the environment is a result of fallout from atmospheric testing of nuclear weapons that began in 1945, initially occurring in sediments at measurable concentrations in 1954 (7). The rate of 137Cs fallout and, consequently, sediment concentrations reached a maximum in 1963, generally decreasing after the Test Ban Treaty of 1963, although there were minor increases in 1971 and 1974 (7). 137Cs is an excellent tracer in sediments because it sorbs strongly to clay and organic particles and is essentially nonexchangeable (8). The Hylebos Waterway was dredged from the mouth as far as the Middle Turning Basin in 1931, and the waterway was extended to the Upper Turning Basin during 1965-1967 (Figure 1). Dredging also occurred in 1938 when some middle parts of the waterway were deepened and widened. Because the channel has been continuously used for ship passage, all sampling locations were located in the intertidal or nearshore subtidal zone to minimize the effect of prop-wash and wave generation from passing freighters.
Methods Intertidal Sediment Samples. Twelve sediment samples were obtained from the intertidal zone, defined as the area between tidal elevations of 0 and +1.7 m, referenced to the U.S. Army Corps of Engineers’ local mean lower low water level (Figure 1). Cores were collected using a stainless steel push-corer and sectioned at 10-cm intervals. Sediments from each interval were individually homogenized within 4 h after collection and then archived at 4 °C. Subtidal Sediment Samples. Two subtidal core samples approximately 2 km apart were collected from sites 2111 and 5114 (Figure 1), selected based on dredging history and location in the waterway adjacent to industrial sites. At each site, two cores were collected to a depth approximately 0.5 m below the anticipated historic dredging horizon using a pneumatic vibracorer to drive a 10-cm o.d. aluminum tube into the sediment. The coring device was equipped with an exterior acoustic transducer, with a second transducer mounted above the core tube inside the corer to detect the elevation of the retrieved sediment within the core tube. The ratio of the two elevations determined the vertical compaction (15-20%) of the core. Compaction does not affect source identification because the relative locations of As and 137Cs peaks are unaffected. After retrieval, the cores were capped and transported to the laboratory for processing for a variety of analytical tests (Figure 1, inset). Approximately 50 mL of porewater was collected from the top 10 cm of each core in a glovebox under an N2(g) atmosphere, filtered at 0.45 µm, and preserved by acidification with HNO3 to pH < 2.0 for dissolved metal determination. Prior to sectioning, the sediment inside the core tube was extruded, and the smear zone (sediment in contact with the interior wall of the core tube) was removed and discarded. The pH and Eh of the sediment were measured at 10-cm intervals down the length of the cores by inserting electrodes directly into the sediments. The cores were then
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FIGURE 1. Sampling locations, As concentrations, slag percentage, and potential As sources around the Hylebos Waterway. Insert shows core sectioning strategy. sectioned at discrete intervals from the surface down to the observed dredging horizon, identified visually by a distinct granulometric change from fine sands to subjacent clay. In core 5114, the 1931 dredging horizon was clearly apparent at 170 cm, while in core 2111 the 1938 dredge horizon occurred at 122 cm. Sediments from each core section were individually homogenized prior to chemical and mineralogical analyses. Analytical Methods. Samples for 137Cs determination were dried at 110 °C and then pulverized with a porcelain mortar and pestle. A 10-g subsample was placed in a stainless steel planchet for γ-spectral analysis of the 137Cs content counted on germanium detectors connected to Nuclear Data 6620 and 6700 data acquisition systems (9). 137Cs concentrations ranged from
