Environ. Sci. Technol. 1999, 33, 1996-2000
Assessment of Sewage Treatment Plant Effluents Impact on River Water Quality Using Dissolved Rb/Sr Ratio PASCALE M. NIREL* AND ROGER REVACLIER Service Cantonal d’Ecotoxicologie, 23 Avenue Sainte-Clotilde, CP 78, 1211 Gene`ve 8, Switzerland
We present a new approach to elucidate and manage the impact of sewage effluents on river water quality using the dissolved Rb/Sr ratio. Rubidium is present in larger quantities than strontium in biological matrices (such as feces and urine), so that the ratio of these two elements represents an effective tracer. This is especially true in regions where the natural Rb/Sr ratio is low (calcareous regions). For a given water quality, the Rb/Sr ratio will remain constant, regardless of the river flow. Since both elements are easy to measure, this ratio has already been successfully used for pollution assessment in Geneva. The Rb/Sr ratio can also be used to model, for a given watershed, the capacity to withstand the effect of sewage effluent and to recover its natural quality.
Introduction The goal of this work is to develop a procedure to assess and possibly control the impact of sewage treatment plants on natural running waters. At this time, most of the methods in use rely on the measurement of water quality parameters, such as chemical indicators (ammonium, biological oxygen demand, dissolved organic carbon, etc.), and/or the study of living organisms used as water quality monitors. However, the concentrations of these chemical parameters are not conservative, and they intrinsically provide little insight into the dynamics of the water mixing downstream from the influent. As for the biological indicators, their determination can be fairly cumbersome not to mention timeconsuming. The dissolved rubidium/strontium ratio (Rb/Sr) represents a promising tool. In biological materials, Rb is enriched as compared to Sr. This effect is observed because rubidium follows potassium assimilation, whereas strontium follows calcium assimilation. As a result, biological matrices can present very high Rb/Sr ratios. Typically, human serum and urine yield Rb/Sr ratios of 6.1715 (with 1.691 µg/g of Rb and 0.2740 µg/g of Sr) (1) and 5.8371 (2), respectively. The Rb/Sr isotopic ratio is widely used by the geochemists for dating purposes. The Rb/Sr ratio varies according to the mineral or rock considered; for example, limestone has a ratio of 0.0050, whereas in sandstone Rb/Sr reaches a value of 3.0000 with * Corresponding author phone: (41 22)781 01 03; fax: (41 22) 320 67 25; e-mail:
[email protected]. 1996
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TABLE 1. Sites and Years of Sampling river
sampling site
1994 1995 1996 1997 1998
Naza * Flies St Genis (downstream the STP) Allondon Moulin de Fabry * Allondon mouth * Lion Vesegnin Lion mouth (downstream the STP) Allemogne mouth
Allondon Allondon Allondon
a
* *
* *
*
*
* *
* *
* *
* * * *
*
*
*
Results grouped with the ones from Flies.
60 ppm of Rb and 20 ppm of Sr (3). To our knowledge, however, the dissolved Rb/Sr ratio has never been used. Also, the concentrations of these two elements in surface waters are rarely measured together. Data from Bruland (4) permit the estimation of a ratio of 0.01556 (with dissolved Rb ) 0.12 mg/L and dissolved Sr ) 7.89 mg/L) in average seawater. In the surroundings of Geneva, measured dissolved Rb/Sr ratios are low (for instance, the dissolved Rb/Sr averaged 0.00117 ( 0.00005 in the pristine part of the Allondon River between 1994 and 1998; 54 measurements). This is due to the fact that the Geneva area is predominantly calcareous. Therefore, the input of municipal wastewater, with a higher Rb/Sr ratio because of the biological waste it carries, can affect the Rb/ Sr ratio in receiving waters. The extent of this effect will depend on the mixing and dilution capacity of the receiving medium. Typical Rb/Sr ratios in sewage treatment plant effluents are in the 0.01 range (from five samples, from two different STPs at different times of the day and during different seasons). The major advantages of using these two elements come from the fact that they are fairly conservative as long as there are no major redox changes (5). Contamination of the samples is easily avoidable as compared to transition trace metals; moreover, the use of a ratio renders flow considerations less critical. We present here the first results obtained by using the Rb/Sr ratio as a tool to assess the impact of a sewage treatment plant on a small river in Switzerland and in France.
Experimental Section Study Area and Sampling Strategy (Figure 1). The Allondon River has its source in France and flows into the Rhoˆne about 10 km downstream from the city of Geneva, Switzerland. The total subsidence and length are 306 m and 18 km, respectively. The river has several small tributaries, of which we considered the major ones: the Lion, located shortly before the Swiss border, and the Allemogne in the border area. The river catchment basin has a total surface of 148 km2. The average flow of the river near its mouth is 3.3 m3/s, with centennial floods of 100 m3/s. The drought flow, which can be reached or exceeded 347 days per year, is 0.5 m3/s. At the mouth of the river, annual peak flows occur between November and March and can reach more than 10 m3/s. The minimum flows are as low as 1 m3/s from July to September. The Allondon River is of major importance for the conservation of the local fauna and flora (6). Since 1977, the 10.1021/es981097g CCC: $18.00
1999 American Chemical Society Published on Web 05/06/1999
FIGURE 1. Study area and sampling locations. valley has been listed on the Swiss federal list of landscapes, sites, and monuments of national importance. Despite this measure, the river quality clearly has been degrading over the last years because of the effluents of the domestic sewage treatment plants (STP) (7). The river receives the effluents of four sewage treatment plants: the Journans STP (equivalent to 16 000 inhabitants), the St. Genis STP (equivalent to 20 000 inhabitants), the St-Jean de Gonville STP (equi-
valent to 2500 inhabitants), and the STP from a camping location (variable output). The area is urban and agricultural with no industrial activity. The St Genis STP represents the major source of pollution to the Allondon River, even though its capacity together with the flow of the river at the effluent is equivalent to the STP of similar size at Journans on the Lion tributary (7). Local authorities are considering improving this STP in order to reduce the impact on the VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Spatial evolution of mean Rb/Sr ratio along the Allondon River. For each sampling site, this mean is determined on the basis of 49 samples for the Flies sampling site, 24 for the St Genis sampling site, and 54 for both the Fabry and the mouth sampling sites. receiving waters. In this context, it is of major importance to accurately assess the extent of the impact of the plant on the river water quality together with the river dilution capacity. We focused our attention mostly on the Fabry sampling site, which integrates contamination from the two major STPs along the river: the St Genis STP and the Journans STP (Figure 1). We also sampled the Lion River in 1998 upstream and downstream from the Journans STP as well as the Allemogne River in 1995, 1996, and 1998. The Allemogne River presents pristine conditions. Water samples were taken on a monthly basis. Sampling locations are presented Figure 1. Sampling sites and strategy are summarized Table 1. Samples Collection and Treatment. Individual water samples were hand-collected monthly in polyethylene vials previously soaked in 10% (v/v) suprapur nitric acid (Merck) and thoroughly rinsed with Milli Ro-Milli Q water (Millipore). The samples were then filtered through 0.45-µm filters (Millex Durapore, Millipore) in the hours following the sampling and acidified (2% v/v suprapure nitric acid, Merck) prior to analysis. Analytical Method. Elements were measured by inductively coupled plasma mass spectrometry (ICP-MS) on a VG-PQ2 instrument in its standard configuration, with a Meinhard nebulizer. The Rb and Sr concentrations were determined in a quantitative mode (with external standards calibration). Analytical uncertainty on Rb and Sr measurements were estimated as 2% for both elements. Blank values were determined for each sampling. They averaged 0.008 µg/L for Rb and 0.08 µg/L for Sr (Rb concentrations vary between 0.1 and 5 µg/L, and Sr concentrations vary between 80 and 290 µg/L).
Results and Discussion Rb/Sr as a Diagnostic Tool. The evolution of the mean Rb/ Sr ratios along the river together with the minimal and maximal values in each sampling site are presented Figure 2. We observe that the St Genis STP effluents cause a dramatic increase in the Rb/Sr ratio, which then never recovers its initial value downstream. On the other hand, the Journans STP input does not affect (and even it improves) the Rb/Sr ratio downstream from the St Genis STP input (average Rb/ Sr at the mouth of the Lion tributary: 0.00457, n ) 7). These results confirm the previous ones obtained by measuring physicochemical parameters (temperature, pH, conductivity, 1998
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dissolved O2, biological oxygen demand, dissolved organic carbon, ammonium, nitrates, nitrites, orthophosphates, totalphosphorus, sulfates, chlorides) and biological parameters (total germs, coliforms, and benthic macroinvertebrates determination) (7). The Rb/Sr ratio therefore appears to be an easy tool for river pollution/impact diagnostics and is more efficient than measuring tens of parameters. The conclusion of De Souza’s study (7) is: “... it would be necessary to define the maximum anthropogenic load admissible by the Allondon as equal to 50% of the one quantified during the 1987-1988 period ...”, suggesting that an improvement of the St Genis STP efficiency would considerably improve the Allondon water quality. We reconsidered this recommendation on the basis of the information provided by the Rb/Sr ratio in the context of decision management. Rb/Sr as a Decision Tool. Figure 3a-d shows the relationship of the Rb/Sr ratio with the reciprocal of the flow. At the Flies sampling site (Figure 3a), in pristine conditions, no correlation is observed. This makes perfect sense due to the fact that, for a given water quality, the Rb/Sr ratio is not affected by the river flow. At SaintGenis (Figure 3b), downstream from the St Genis STP, however, a striking correlation is observed (r ) 0.901). This correlation results from the fact that the input from the STP, with respect to the river flow, dramatically changes the water quality. Consequently, the Rb/Sr ratio increases. At significantly higher river flows, this effect would not be as obvious. The same situation is also observed to a lesser extent at the Fabry sampling site (Figure 3c, r ) 0.774). This abatement is due to the contribution of the cleaner waters from the Lion River. At the mouth of the Allondon River (Figure 3d), no relationship is observed (r ) 0.066) mainly because the Allemogne tributary injects clean waters into the Allondon. However, the Rb/Sr ratio does not recover its initial value. This shows that the dilution by the cleaner tributary waters and the autoepurative capacity of the river do not counteract the STPs impact. Due to the inaccessibility of the flow values at the Saint Genis sampling site, we studied the Fabry sampling site. At this site, to maintain a desired water quality, one can set a target Rb/Sr value. Expecting a pristine Rb/Sr value in the 0.001 range is somewhat unrealistic. From the values of biological indexes measured in Genevan rivers, however, we estimate that the Rb/Sr ratio must be lower than 0.007 if biological diversity is to be maintained (8). Referring to Figure 3b, such a Rb/Sr ratio corresponds to
FIGURE 3. Rb/Sr ratio as a function of the reciprocal of the flow during the 1994-mid 1998 period: (a) at the Flies sampling site, (b) at the Saint Genis sampling site, (c) at the Fabry sampling site, (d) at the Allondon mouth sampling site. a river flow of 0.68 m3/s. Classified flows at the Fabry sampling site are presented Figure 4. From this figure, we see that the river flow at the Fabry sampling site was inferior
to 0.68 m3/s 105 days peryear in period from 1989 to 1997, or 29% of the time. This situation occurs each year during the drought period (mid-July to late September). During this VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Classified flows at the Fabry sampling site during the 1989-1997 period. summer period, the effluents of the St Genis and Journans STPs can be estimated to be 150 L/s on average (9). This represents as much as 60% of the river flow, whereas on an annual mean basis, the sewage effluents represent only 19% (9). From this study, we can conclude that the dramatic impact of the STPs located on the Allondon River is related to the low flow of the river, mostly during the summer period. In this context, an improvement in water quality would require both an increase in the lowest water level as well as an improvement of the St Genis STP efficiency. In conclusion, the use of the dissolved Rb/Sr ratio as a diagnostic tool is very efficient. It confirms results obtained on the basis of hundreds of measurements of physicochemical and biological parameters in areas with low Rb/Sr background values (i.e., calcareous regions). It is useful in preliminary studies, before expensive and timeconsuming analyses are undertaken, even if it does not replace these to measure the consequences of pollution by a STP. Moreover, it represents a basis for modeling procedures that compute the maximum tolerable STPs effluents for a given river.
Acknowledgments We want to express our indebtedness to Theophile Lutz without whom this work would never have been conceived. We also want to thank the following people who participated in this work: Dr. M. Biber who corrected our English and made fruitful comments and Gilbert Pfister who performed
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ICP-MS analysis of our samples. And, last but not least, we would like to thank the anonymous reviewers for their helpful comments and for their patience, in reviewing our English which was thoroughly tested!
Literature Cited (1) Vandecasteele, C.; Block, C. B. Modern methods for trace element determination; John Wiley & Sons: New York, 1994. (2) Nirel, P. M.; Revaclier, R.; Pfister, G. In Contaminated Soils: Third International Conference on the Biogeochemistry of Trace Elements; Prost, R., Ed.; Inra: Paris, 1995. (3) Martin, J. M.; Meybeck, M. Mar. Chem. 1979, 7, 173. (4) Bruland, K. W. In Chemical Oceanography; Riley, J. P., Chester, R., Eds.; Academic Press: London, 1983; Vol. 8. (5) Andersson, P. S.; Wasserburg, G. J.; Ingri, J.; Stordal, M. C. Earth Planet. Sci. Lett. 1994, 124, 195. (6) Theurillat, J. P.; Legier, P.; Champeau, A. Allondon, Moulin de Vert, Verbois: Situation, e´volution, protection, gestion; WWW, Section de Gene`ve; 1989. (7) De Sousa, J.; Dethier, M.; Revaclier, R. Arch. Sci. 1992, 45, Fasc.1, 1. (8) Nirel, P. M.; Landry, J.-C.; Pfister, G.; Revaclier, R. Presented at Contaminated Soils: Fourth International Conference on the Biogeochemistry of Trace Elements, 1997. (9) Rapin, F. Rapp. Comm. Int. Prot. Le´man Contre Pollut. 1995, 1996, 247.
Received for review October 23, 1998. Revised manuscript received March 16, 1999. Accepted March 31, 1999. ES981097G
