Environ. Sci. Technol. 2007, 41, 5252-5258
Characterizing Water Circulation and Contaminant Transport in Lake Geneva Using Bacteriophage Tracer Experiments and Limnological Methods NICO GOLDSCHEIDER,† L A U R E N C E H A L L E R , ‡ J O H N P O T EÄ , ‡ W A L T E R W I L D I , ‡ A N D J A K O B Z O P F I * ,§ Center of Hydrogeology (CHYN) and Institute of Biology, Laboratory of Microbiology (LAMUN), University of Neuchaˆtel, Rue Emile-Argand 11, 2009 Neuchaˆtel, Switzerland, and Institute FA Forel, Route de Suisse 10, 1290 Versoix, Switzerland
Multi-tracer tests with three types of marine bacteriophages (H4/4, H6/1, and H40/1), together with various limnological methods, including physicochemical depth profiling, surface drifters, deep current measurements, and fecal indicator bacteria analyses, have been applied to characterize water circulation and pathogen transport in the Bay of Vidy (Lake Geneva, Switzerland). The experimental program was carried out twice, first in November 2005, when the lake was stratified, and a second time during holomixis in February 2006. The bacteriophages were injected at three points at different depths, where contaminated waters enter the lake, including the outlet pipe of a wastewater treatment plant, a river, and a stormwater outlet. Thereafter, water samples were collected in the lake at 2 m depth during a 48 h sampling campaign. The results demonstrate that (i) contaminated river water spreads rapidly in the bay; (ii) a well-developed thermocline is highly effective in preventing contamination from the depth to rise up to the surface; (iii) rapid vertical mixing and pathogen transport occur under thermally homogeneous conditions; and (iv) repeated multi-tracer tests with bacteriophages are a powerful technique to assess water circulation and contaminant transport in lakes where high dilution occurs.
Introduction Lakes are important drinking water resources in many parts of the world, although only 0.3% of the global freshwater stored in lakes and rivers (1). At the same time, untreated or only partly purified wastewaters from households, agriculture, and industry are frequently directly or indirectly released into lakes, so that there is a potential short circuit between wastewater and drinking water. This is particularly problematic for microbial pathogens (bacteria, protozoans, and viruses), which can be present in extremely high concentrations in wastewaters (often >108/100 mL), while already very low levels (sometimes 100 PFU/mL were measured near the mouth of the river (V20a-d) in November; even >107 PFU/mL were detected at these points during the second tracer experiment. In the following days, lower concentrations of H40/1 were detected at most points in the bay, whereby the general propagation pattern nicely corresponded with the surface currents (Figure 3). In both tracer tests, H40/1 phages also appeared at V1, the sampling point closest to the drinking water pumping station. Matching with a suspected clockwise surface water circulation during the first tracer experiment, it took a rather long 48.6 h to arrive there. In February when the surface current circulation was reversed and the traveling distance was shorter, H40/1 was detected already after 5.6 h. Also, the results of the H4/4 phages, which were injected at 10 m depth near the outlet of the Flon drainage pipe, were similar for both experiments. Although H4/4 was found at fewer points and at lower concentrations, it displayed a similar propagation pattern as H40/1. This reduced detection was partly due to the lower amount of injected H4/4 phages (Table 1) but additionally a result of a less efficient propagation of the tracer in the bay since the Flon stormwater outlet was not active during both tracing experiments. The most significant differences were found for H6/1 phages, which were injected near the WWTP outlet pipe at 30 m depth. In November, when the lake was stratified, not a single phage was detected in any of the 54 samples that were taken during the whole experiment. This finding confirms that the well-developed thermocline at 25 m depth acted as a highly effective barrier preventing vertical transport from the hypolimnion to the epilimnion. During the February experiment, when the water column was mixed, H6/1 phages appeared only 5-6 h after injection at several sampling points. The concentrations often exceeded 106 and sometimes 107 PFU/mL, indicating a rapid upwelling of water from the depth. The phages first arrived at sampling sites in the central and western part of the bay (V1, V2, V4, V14, and V20), confirming the northwestward, counter-clockwise circulation that was also recognized by the surface drifters. VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Multi-tracer results (maximum phage concentrations Cmax) and surface-drifter displacement vectors: (a) H40/1 and H4/4, November 2005, (b) H40/1 and H4/4, February 2006, and (c) H6/1, February 2006. No H6/1 phages were detected during the November 2005 experiment. 5256
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Transport Processes. Extreme differences in concentrations were observed in temporally and spatially close samples, indicating that the distribution of the phages in the bay was highly heterogeneous (Supporting Information, Tables S1 and S2). For example, during the February experiment, no H6/1 phages were observed at point V4 in the samples taken 2.2, 7.5, and 50.4 h after the injection, while high levels were detected in samples collected after 5.8 h (5.5 × 106 PFU/mL) and 26.0 h (5.53 × 106 PFU/mL). Similarly, during the third sampling campaign of this tracer test, 1.62 × 107 PFU/mL H6/1 were detected at V14, while the results from all other points were negative. These findings suggest that the propagation of the phages in the bay does not occur by homogeneous mixing processes, but in the form of plumes with sharp boundaries, which are laterally displaced with the overall water current while permanently changing their form. Therefore, it is not possible to interpolate between different measurement points (i.e., draw iso-concentration lines). For the same reason, it is not possible to interpolate in time between measured concentrations at a given sampling point. The heterogeneous transport processes established by the bacteriophage tracer test are consistent with the observed spatial and temporal variability of the FIB levels. A similar observation was made by Wanninkhof and coworkers (22), who used SF6 to trace the discharge plume of a point source in coastal waters. Within the first 20 km from the outfall, highly variable SF6 surface concentrations were measured, while they decreased monotonically with distance afterward. Consequences for Water Protection. Although no phages were detected at the drinking water pumping station, it is important to note that five of the six bacteriophage injections that were performed during the two multi-tracer tests resulted in positive detections at the westernmost sampling point (V1), at only 1.5 km distance from the pumping station. During the February experiment, the H6/1 phages arrived at V1 at high concentrations of 1.60 × 106 PFU/mL after only 5.6 h. These results indicate that it is possible that the continuous input of partly persistent pathogens via the known contamination sources can actually reach the area of the drinking water pumping station. The results of the tracer tests further confirm that contaminated waters, mainly from the WWTP outlet, can rapidly reach the surface when the lake is thermally homogeneous, spread in the bay, and also impact its tourist beaches, although this impact is highly variable in time and space. Advantages and Drawbacks of the Methodology. Tracer tests provide direct evidence and quantitative information on the propagation of contaminants. In the present case, the combination of tracer tests and limnological methods made it possible to obtain a more complete picture of water circulation and contaminant transport and to identify the influence of the thermal state of the lake on these processes. The advantages of using marine bacteriophages as tracers include their absence in freshwater, their low detection limits, the simplicity of producing large quantities, and their invisibility and non-harmfulness to all organisms other than their specific bacterial host (18). There is no significant interference with other phage types and tracer substances such as fluorescent dyes. The stability of phages in the environment is limited; thus, they do not accumulate and the background level remains negligible, which makes repeated tracing experiments at the same site possible. On the other hand, the limited persistence also restricts the duration of sampling campaigns to a few days or weeks, although this was not a critical issue for this study. Furthermore, the detection of phages relies on discrete water sampling and subsequent analysis in the laboratory within 24 h, while methods of continuous on-site detection are not available. In conclusion, marine bacteriophages can be
recommended as tracers to study the transport of contaminants, particularly viruses and other pathogens. Their properties make them favorable for multi-tracer tests in freshwater systems with high dilutions, such as lakes, and when visible coloring and other environmental impacts must be avoided.
Acknowledgments We thank Vanessa di Marzo (LAMUN) for phage analysis and Dr. Pierre Rossi (EPFL) for helpful advise. Philippe Arpagaus and Vincent Sastre (Institute FA Forel) are thanked for navigating R/V La Licorne, and the Municipality of Lausanne is thanked for financial support.
Supporting Information Available Stability tests of bacteriophages in lake water as well as complete data from the two tracer tests illustrating the temporal and spatial variability in the distribution of phages in the Bay of Vidy. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review February 13, 2007. Revised manuscript received May 10, 2007. Accepted May 18, 2007. ES070369P
