Environ. Sci. Technol. 1997, 31, 1968-1972
Extending the Use of the Stable Lead Isotope Ratios as a Tracer in Bioavailability Studies Y V E S G EÄ L I N A S * A N D JEAN-PIERRE SCHMIT Chemistry Department, Universite´ du Que´bec a` Montre´al, C.P. 8888, Succ. Centre-Ville, Montre´al, Que´bec, Canada, H3C 3P8
In this study, the 206/207Pb stable isotope ratio tracer was used to decipher the biogeochemical pathway leading to the Pb contamination of freshwater macrophytes and periphyton in a small agricultural watershed of southern Quebec (Canada). Lead ratio and concentration were measured in atmospheric bulk deposition, soils, freshwater (dissolved and particulate) as well as in different plant materials. The 206/207Pb ratio measured for the two main sources of lead contamination, bulk deposition and agricultural soils, was significantly different. This allowed a direct apportionment of lead found in plant materials throughout the watershed. In the agricultural streams, a preferential uptake of atmospheric Pb over eroded soil Pb was observed, suggesting a higher degree of mobility and bioavailability for atmospheric Pb. This preferential uptake was affected by the suspended solids concentration. In contrast, >95% of the Pb tissue content in an urban stream with a low suspended solids concentration was from anthropic origin. Apportionment calculated for two freshwater invertebrate species were more variable. However, results obtained for minnows showed that the isotopic Pb signature is preserved during trophic transfer of Pb from phyton to nekton. Therefore, this natural tracer could become a powerful tool for managers of ecoregion or hydrologic units in determining the extent of responsibility for up to four distinct sources of Pb contamination and in addressing priority remedial actions.
Introduction In most countries, legal requirements for the management of aquatic resources include the identification and quantification of the sources of pollution (1). Determining the extent of contamination and its impact from the individual sources also implies a reasonable knowledge of the biogeochemical pathways involved. However, when dealing with multiple point sources and nonpoint sources, reliable and specific tracers of contamination must be used. Since the pioneering work of Sturges and Barrie in 1987 (2), lead became one of these tracers. Because it retains the isotopic composition of the ore it originated from when released into the environment (3), the sources of natural and anthropogenic lead often have different isotopic compositions. The contrasted 206/207Pb ratio for anthropogenic aerosol lead originating from distinct regions has thus been exten* Corresponding author present address: Department of Chemistry, Lederle Graduate Research Center, University of Massachusetts, Amherst, MA 01003. Telephone: (413)549-3035; Fax: (413)549-4490; e-mail:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 7, 1997
FIGURE 1. Map of the Des Hurons River watershed depicting the atmospheric bulk deposition sampling sites. sively exploited in environmental studies. However, until now, this tracer has only been used either to pinpoint the origin of atmospheric Pb (4-7) or to evaluate the long-term importance of atmospheric Pb deposition (8-11). While a few studies focus on the stable Pb isotope ratios in water/ suspended solids (12-16) and in biological tissues (17-20), none report the use of this unique tracer to determine contaminant pathways for short time scales and small spatial scales, which are the most relevant for managers of ecoregion or hydrologic units in determining the extent of responsibility for Pb contamination and in addressing priority remedial actions. In spite of the phasing out of alkyl-leaded gasoline in North America during the past decades, anthropogenic Pb contamination remains important in many surface water environments such as the Des Hurons River watershed (Quebec, Canada). In this work, the 206/207Pb isotope ratio was determined in atmospheric bulk deposition, soils, freshwater (dissolved and particulate) as well as in different plant materials in an attempt to decipher the biogeochemical pathways leading to Pb accumulation in freshwater macrophytes and periphyton and to apportion the contaminant Pb into the main sources of contamination.
Methods Study Site. The Des Hurons River watershed is located at 30 km east of Montreal in the St. Lawrence Valley plain, between 73°02.5′-73°16.4′ W and 45°21.4′-45°38.3′ N (Figure 1). About 48% of the catchment area (334 km2) is occupied by agricultural activities, mostly corn crops. The Des Hurons River is a typical agricultural stream (mean annual discharge of 4.76 m3 s-1), with high peak runoff rates and massive losses of suspended solids due to improved drainage of crop lands throughout the watershed. It is divided in two streams of similar flow rate at the confluence of the St. Louis Brook, a mostly urban stream that flows through the main town of the watershed, Marieville. Both streams and their tributaries are
S0013-936X(96)00791-2 CCC: $14.00
1997 American Chemical Society
highly mineralized with a pH ranging from 7.4 to 8.2. The suspended solids concentration is much lower in the urban part of the St. Louis Brook except following snowmelt and heavy rainfall. Material. Polypropylene collection bottles were decontaminated by soaking in 2 N HCl for at least 1 week after washing with acetone in a clean room and kept in 0.5 N HNO3covered baths until they were used. They were then rinsed thoroughly with Milli-Q water (Millipore Corp., Bedford, MA), doubled bagged in acid-cleaned polyethylene bags, and stored in a plastic cooler box for transportation to the field. All the material used for filtering, leaching, and mineralizing was decontaminated in the same way as for bottles. The polycarbonate filters (0.45 µm, Millipore Corp.) were leached in 2 N HNO3 for at least 48 h and kept in a 0.5 N HNO3-covered bath in a clean room until they were used. They were then rinsed with Milli-Q water, dried in a class-100 laminar flow hood installed in a clean room, weighed, and stored individually in preleached Petri dishes. Sampling. Bulk deposition was collected at five background sites and at 14 other locations covering the entire surface of the watershed following the method of Harrison et al. (21) between May 1994 and August 1995. Because the Des Hurons River watershed is located in the industrialized St. Lawrence Valley plain, five different sites were selected to obtain the mean background bulk deposition fluxes over the area (Figure 1). Each background site was fully exposed, covered with grass, and located at >1 km from local point sources of contamination (e.g., road, town, or tilled farm land). Triplicate preleached polypropylene sampling containers were left in the field for 8 weeks in the winter and 4-8 weeks when the snow cover had disappeared. Working in a class100 clean room, the collected containers were completed to 50 mL using ultrapure nitric acid and Milli-Q water to obtain a final normality of 2 N. The samples were then leached for 2 h at 60 °C, filtered (0.45 µm polycarbonate membranes), and preconcentrated using an immobilized 8-hydroxyquinoline (I-8-OHQ) column (22). Two field blanks were included in each collection and were processed in the same way as samples. Lead contamination from the entire method was negligible. Surficial soils from agricultural sites were sampled at >1 km from local sources of contamination. The top 0.5 cm was considered representative of the layer eroded by wind and rain or during snowmelt. The samples were collected in preleached polypropylene tubes and ground to
