Environ. Sci. Technol. 2001, 35, 4675-4679
Application of Strontium Isotopes for Tracing Landfill Leachate Plumes in Groundwater J. D. VILOMET,† B. ANGELETTI,† S. MOUSTIER,† J. P. AMBROSI,† M . W I E S N E R , ‡ J . Y . B O T T E R O , * ,† A N D L. CHATELET-SNIDARO§ CEREGE, U.M.R. 6635 CNRS/Universite´ Aix-Marseille 3, B.P. 80, 13545 Aix en Provence Cedex 4, France, Department of Environmental Science and Engineering, Rice University, MS 317 6100 Main, Houston, Texas 77005-1892, and SITA Group, 132, rue des Trois Fontanot, 92758 Nanterre Cedex, France
We are evaluating strontium isotopes as alternative tracers of landfill leachate in groundwater. The municipal landfill studied here is located in southeastern France. This landfill has no bottom liner, and wastes are placed directly on the ground. Based on the evaluation of chloride concentration, the plume extends a maximum of 4600 m. Strontium isotopic composition characterizes two sources: natural groundwater (87Sr/86Sr ) 0.708175) and landfill leachate contamination (87Sr/86Sr ) 0.708457). The evolution of mixing ratios obtained with strontium reveals a second source of groundwater contamination: fertilizers (87Sr/ 86Sr ) 0.707859). These results suggest that isotopic signatures can be used to provide useful information on sources of groundwater contamination where conventional water quality parameters may yield ambiguous results.
Introduction Old sanitary landfills are generally lacking a leachate treatment system. Leachates from these landfills contain a large variety of organic and inorganic pollutants that can migrate into the groundwater and lead to the development of a contamination plume (1). Organics and Anthropogenic Specific Organic Compounds (ASOCs) seem to constitute the most significant group of municipal waste landfill leachate pollutants. Attenuation mechanisms which reduce the concentrations of pollutants during the transport to and in the aquifer create a sequence of redox zones downgradient of the waste disposal (2-4) showing methanogenic conditions near the landfill and oxidizing conditions in the vincinity of the plume. The extent of leachate plume must be clearly defined in order to determine attenuation processes and to evaluate the potential risk of deterioration of aquifers. Previous studies usually trace contamination plumes using chloride concentration, dissolved organic matter, a specific compound concentration like BTEX, or tritium (5-7). However chloride concentration provides no information on the source of contamination. The use of δ34S, δ18O (8, 7), δ13C (7, 9, 10), and * Corresponding author phone: (33) 4 42 97 15 15; fax: (33) 4 42 97 15 59; e-mail:
[email protected]. † CEREGE. ‡ Rice University. § SITA Group. 10.1021/es000169c CCC: $20.00 Published on Web 11/01/2001
2001 American Chemical Society
δ11B (11) provides a better understanding of the leachate production and attenuation mechanisms: stable carbon isotope ratios are useful for a better understanding of the generation of methane and carbon dioxyde in landfills, whereas boron and sulfur isotopes can be used to identify leachate contamination in groundwater. Limits on these methods used for a contaminant source identification are isotope fractionation during biological and physicochemical processes and, for the same element, different isotope signatures in the landfill. Moreover, these physicochemical tracers do not explain interactions between aquifer materials, atmosphere, unpolluted water, and other contamination sources. Radiogenic isotopes such as lead (Pb) or strontium (Sr) isotopes, which are end products of radioactive decay, can be used as geochemical and environmental tracers. Siegel et al. (12) showed that strontium isotopic analyze could be used to identify seawater or leachate contaminated groundwater. Strontium has four naturally occurring isotopes: 88Sr, 87Sr, 86 Sr, and 84Sr. Their relative abundances are approximately 82.53%, 7.04%, 9.87%, and 0.56%, respectively. The isotopic abundance of strontium is variable because of the formation of radiogenic 87Sr by the decay of naturally occurring 87Rb. For this reason, the precise isotopic composition of strontium in rock or mineral that contains rubidium depends on the age and the Rb/Sr ratio of the rock or mineral (13, 14). There is a low biological or geological fractionation of its stable isotopes in the environment (15), and different ratios of stable strontium isotopes are characteristic of different Sr sources. Therefore, strontium and its stable isotopes can be effective tracers of transport processes in the environment (16-18). The ionic radius of strontium is slightly larger than that of calcium, which it can replace in many compounds. Thus, there are many wastes containing strontium in a landfill, e.g. plastics, cements, plasters, or others manufactured products. In the landfill leachate plume, strontium will be attenuated and take part in sorption or cation exchange properties. This phenomena can affect its transport but will not change the isotopic signature of strontium sources because fractionation of its stable isotopes in the environment is low. Consequently, strontium signatures in water are a promising tool to understand transfer and transport processes in the leachate plume. In the present study, hydrologic and chemical data from a shallow groundwater are used to monitor leachate migration downgradient of a landfill. Then, we compare these results with those obtained with strontium isotopes to evaluate the relevance of these tracers.
Site Characterization The municipal landfill studied here is located in southeastern France. It is isolated from local industrial influences. About 8 × 109 kg of domestic and municipal waste have been landfilled since 1900. The still active site covers about 70 ha, and its average height is 20 m (Figure 1). This landfill has no bottom liner, and wastes are placed directly on the ground. Landfilling occurs on an aquifer which flows through a phreatic outcropping constituting the , La Crau . alluvial plain (500 km2). , La Crau . is the old alluvial plain of the Durance river. The part impacted by the landfill, named Crau du Luquier, is constituted of quaternary alluvions which contain calcareous, metamorphic, and endogenous crushed stone. Its thickness is variable and always less than 15 m. The clayey sand matrix is abundant with illite, chlorite, vermiculite, and mixed layer clay (19). The alluvium substratum includes marl of Miocene age. VOL. 35, NO. 23, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4675
FIGURE 1. Site presentation. This municipal landfill is located in southeastern France where only domestic and municipal wastes have been landfilled since 1900; there is no bottom liner, and wastes are placed directly on the ground. The groundwater quality was mapped by one piezometer (#0) located 1 km upgradient the waste disposal and two transects downgradient of the landfill. The first transect (900 m) including piezometers #1, #2, and #3 allows the determination of the main pollution source. The second transect (4.6 km) is constituted by piezometers #3, #5, #6, and #7 and one water well (#8). The overall flow direction is NE-SW.
FIGURE 2. Schematic representation of a vertical section along the first transect including boreholes #1, #2, and #3. This cut reveals a lateral shift of facies from crushed stone (#1) to sandy clay (#3): the east side of the landfill is placed on a low permeability zone. Subsurface investigations revealed a lateral shift of facies from crushed stone (8 m thickness) to sandy clay (4.5 m thickness). The landfill seems to be on a shoal caused by a paleochannel structure (Figure 2). The east side of the landfill may be a low transmissivity zone. About 200 m downgradient of the landfill, we have investigated the hydraulic properties 4676
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 23, 2001
of the aquifer by a pumping test. We found a transmissivity of 2.2 × 10-2 m2/s corresponding to a derived permeability of 8.5 × 10-3 m/s. This test reveals a permeability discontinuity as a lateral limit: the east side of the landfill is placed in a low permeability zone, estimated at 10-6 m/s. Thus, the flow path structure is a paleochannel with variable transmissivities. Groundwater is supplied by irrigation (70%) from the Durance river and rainwater infiltration (30% - infiltration rate 60%). The overall flow direction is NE-SW, and the hydraulic gradient is 3‰ (20). Annually, the water table fluctuation is approximately 1 m. Near the landfill, the water table is located 1-3 m below the ground surface. Methodology. Five sampling campaigns have been carried out between 1998 and 2000. Sampling Methods. The groundwater quality was mapped by one piezometer (#0) located 1 km upgradient of the waste disposal site and two transects downgradient of the landfill. The first transect (900 m), piezometers #1, #2, and #3, allows the determination of the main leachate source; the second transect (4.6 km), piezometers #3, #5, #6, #7, and #8, was investigated in order to determine the extend of the contamination plume (Figure 1). Water quality (pH, dissolved oxygen, temperature and conductivity) was determined with depth (0.5 m increments) and measured in-situ using electrodes by WTW Profiline oxy 197 equipped with a self-stirring dissolved oxygen probe, pH 197 and LF 197. Groundwater samples have been collected with a special Grundfos MP1 pump after a 15 min flush when pH, conductivity, and dissolved O2 content are constant.
TABLE 1. Groundwater Composition Obtained by Five Sampling Campaigns in Terms of Average Values and Ranges for Parameters between Maximal and Minimal Values piezometers 0
Fe (µg/L) min
