Environ. Sci. Technol. 2010, 44, 6576–6582
Tracing Natural and Anthropogenic Pb in Sediments along the Mediterranean Coast of Israel Using Pb Isotopes Y E H U D I T H A R L A V A N , * ,† AHUVA ALMOGI-LABIN,† AND BARAK HERUT‡ Israel Geological Survey, Jerusalem 95501, Israel, and Israel Oceanographic and Limnological Researches, Haifa, Israel
Received December 23, 2009. Revised manuscript received July 13, 2010. Accepted July 14, 2010.
The natural and anthropogenic sources of Pb in surface sediments offshore the Israeli Mediterranean coast were studied using the isotopic composition of Pb in diluted acid sediment extracts. Surface sediments were collected at the lower reaches of coastal streams, along a south-north offshore transect and at selected monitoring stations of the Dan Region Wastewater Plant (DRWP) outfall pipe. The background values of the Pb isotopic composition were determined from the deepest part of two representative cores collected offshore and were found to have a narrow range dominated mainly by clays derived from both inland soils and the Nilotic cell and to a lesser extent from the Saharan dust. The impact of the DRWP activated sludge can be traced to a distance of ca. 2 km from the outfall pipe. Enrichment factors of Zn, Cu, and Pb were up to 25 and are strongly correlated with each other and with the Pb isotopic composition, thus demonstrating the sludge to be their common source. The isotopic compositions of Pb in stream sediments have the widest range of values and indicate a strong anthropogenic contribution, probably from both post1992 aerosols and point sources. However the impact of stream sediments on marine sediments could not be clearly detected.
Introduction When considering metal pollution, it is important to locate and estimate the potential sources for pollution. Lead, which is one of the most widely used metals in industry, is present naturally in rocks, soils, and in the hydrosphere. However, up until the industrial revolution, Pb was not present in the atmosphere except as part of natural airborne particles. The introduction of anthropogenic Pb to the atmosphere is due to the extensive use of Pb in industry, predominantly as an additive to gasoline in the form of tetra-ethyl lead (TEL; (CH3CH2)4Pb). As TEL burns, elemental Pb, PbS, PbSO4, and PbO are emitted to the atmosphere. Lead particles adhere to aerosols typically smaller than 10 µm which have longrange transport. The isotopic composition of Pb is utilized in many cases to trace Pb in the environment. Lead has four stable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb, and whereas the first is a primordial nuclide, the others are decay products * Corresponding author phone: (972)2-3514295; fax: (972)2-314330; e-mail:
[email protected]. † Geological Survey of Israel. ‡ Israel Oceanographic and Limnological Researches. 6576
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of 238U, 235U, and 232Th, respectively. Thus the isotopic composition of Pb in any given material (i.e., natural or anthropogenic) depends on the U/Pb and Th/Pb ratios and time. Indeed, the isotopic composition of Pb in gasoline around the world is shown to reflect the source of TEL used in the refinery at that time. In many studies the distinct Pb isotopic composition of Pb in ores facilitate its use to distinguish the natural versus anthropogenic components and also to track its source, route, and fluxes (e.g. refs 1-9). The growing urban population around the Mediterranean Sea leads to increasing inputs of Pb to the marine environment via sewage, solid waste, industrial, and air pollution. It is estimated that about half of the total external input of Pb to the Mediterranean Sea was deposited between 1950 and 1992 (10). In contrast to several other ocean waters, some trace metals in the Mediterranean Sea have high concentrations in surface water and are low and constant in their vertical profile (e.g. refs 11 and 12). It was suggested that this nonsteady-state is due to dissolved atmospheric and terrestrial inputs to surface water. For example, it was calculated that from 1960 to 1985 concentrations of Zn, Pb, Co, and Cd in surface water increased annually by 6, 2, 2, and 2%, respectively (13). Nonetheless, since then, Pb concentrations have decreased in the Western Basin due to Pb consumption regulations regarding the western European countries. Indeed, a decrease in Pb concentrations is observed in the northwestern and western parts of the Mediterranean in both surface seawater (i.e., dissolved Pb) and in the atmosphere (i.e., aerosol and rainwater) from ca. 1986 to 1995 (14, 15). All the same, it was postulated that if Pb consumption is not limited in the Eastern Mediterranean, Pb concentrations will increase and reach critical concentration for marine organisms by the middle of the 21st century (10). Thus the Mediterranean Sea, which had acted as a sink for Pb prior to 1980, has become a source of Pb for the Atlantic Ocean. In Israel, as a result of growing development pressures, the sea is subjected to contamination via atmospheric fallout, direct discharge, or via streams. Since natural Pb concentrations in coastal seawater are low (0.05-0.25 ppb (16)) any atmospheric addition should significantly influence the coastal environment. The major source of atmospheric Pb in Israel is derived from gasoline combustion, which increased from ca. 0.5 in 1970 to over 2 million tons in 1997 ((17) and references therein). The use of unleaded petrol in Israel began in 1991, and by the end of 2005 it comprised ca. 100% of the petrol used. Overall, the calculated mean annual atmospheric mass flux for the eastern Mediterranean basin is 36-72 g m2 y-1 (18). As a consequence, offshore sediments consist of up to 80% of eolian deposition which in turn is responsible for more than half of the Pb input to the Mediterranean (19). It is estimated that along the Israeli coast, the dry atmospheric input of Pb to marine sediments decreases from south to north from ca. 2.5 to 1.1 g m-2 y-1 (20), and it contributes about twice the amount of Pb compared to that of the riverine (1600 vs 850 tones y-1, 9). The aerosol solubility in seawater depends on whether the aerosol is of urban- or crustal-rich origin. This is because ca. 20 to 80% of the Pb in urban-rich aerosols (anthropogenic Pb) resides in the exchangeable sites compared to 30% in crustal-rich (19, 21). An additional source of anthropogenic metals to sediments off the Israeli shore is the direct discharge of sewage sludge by the Dan Region Wastewater Project (DRWP), a plant which treats the sewage of ca. 1.5 million inhabitants of the Tel Aviv metropolitan area since 1987 (22). The activated sludge is discharged through a single outfall, 5 km offshore. The percent of particulate matter, bacteria and heavy metals 10.1021/es9039055
2010 American Chemical Society
Published on Web 08/06/2010
FIGURE 1. Sample location map. A. Sampling map showing locations of sediments taken from the lower reaches of the coastal streams (full dots), north-south transect (gray dots). B. The relevant Dan Region Wastewater Project (DRWP) monitoring stations at which samples were taken (PL). in the sludge is ca. 1%. The impact of the DRWP sludge on the sediments in its vicinity has been monitored since 1992 (22, 23). The other potential sources for contaminants are the coastal streams (24), which, except for a few, have no natural flow to the sea except during winter storm/flood events. Stream sediments are affected by point sources that are situated along their course. Shallow sediments along the coastline were found to be enriched in Hg, Cu, Zn, and Cd at point sources (1988 to 1992) (25, 26). Nevertheless, the lack of Pb enrichment close to these point sources suggests that most of the Pb in marine sediments originates from the atmosphere (26). In most cases, Pb levels in marine sediments offshore Israel are below the threshold of a contaminate guideline (375-1000 ppm) (24-27). Thus in this study, we tested the concept that Pb isotopes are a more sensitive tool than Pb concentration in distinguishing anthropogenic from natural sources in Mediterranean coastal sediments offshore Israel. In order to study the potential sources of Pb, sediments were collected from stream reaches, the DRWP monitoring stations and coastal sediments along a south-north (S-N) transect as well as reference samples taken from deep parts of two marine cores.
Methods Sampling and Sample Preparation. In order to study the different sources of Pb in the marine sediments offshore Israel’s coast, four sets of sediment samples were collected (Table S1, Figure 1). (1) Reference cores: samples were taken from two short cores located ∼27 km apart, offshore the Israeli coast (PL-29, AS-1). Cores were subsampled from a box corer (BX700 AL Ocean Instruments) using Perspex pipes. The three deepest samples (28-30 cm) from each core were analyzed. (2) DRWP monitoring stations: surface sediments (1 cm thick) were collected from 10 monitoring stations which are situated along the 38 m water depth to the north and south of the outfall station (Figure 1). It should be noted that in this study, stations PL-29 and AS-1 were studied for their reference values (deepest samples) as well as two of the surface sample monitoring stations. In addition, in stations AS-1, PL-29, and PL-3 surface sediments to a 4 cm depth were also studied.
(3) Fifteen sediment samples were collected at the mouths of various coastal streams, at distances of 50 and 150 m upstream. Since point sources of pollution can vary along a stream, samples were taken at the reaches and thus reflect an integrated contribution to the marine sediments. These sediments were previously studied for their metal concentrations (Figure 1b 24, 25). (4) Surface sediments collected along a south-north transect in the shallow continental shelf (Figure 1, Table S1). These sediments were previously studied for their metal concentrations (26). Samples were lyophilized, and for the most part the
