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Distribution, Residence Time, and Fluxes of Tetrachloroethylene and 1,4-Dichlorobenzene in Lake Zurich, Switzerland Rene P. Schwarzenbach", Eva Molnar-Kubica, Walter Giger, and Stuart G. Wakeham' Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-8600 Dubendorf, Switzerland ~~
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A 1-year study of tetrachloroethylene and l,4-dichlorobenzene in Lake Zurich, Switzerland, has been conducted. The seasonal distribution of the compounds reflects the circulation in the lake and is compatible with the assumption derived from laboratory studies that mass transfer to the atmosphere is the predominant elimination mechanism. A mass balance for 1,4-dichlorobenzenehas been established for the central basin of the lake, and an average residence time of approximately 5 months has been found. By applying a simple steady-state model, an average annual mass transfer coefficient of 1 cm/h is obtained for the transport of 1,4-dichlorobenzene and tetrachloroethylene from the lake to the atmowhere. Large quantities of volatile halogenated hydrocarbons (e.g., halomethanes, chlorinated ethylenes, and benzenes) are introduced continuously t o the environment by man ( I , 2). It is, therefore, not surprising that chlorinated hydrocarbons are ubiquitous trace contaminants in natural waters (3-6). Since many of these chemicals may damage human health and the environment (7-9), there is great interest in assessing their sources, transports, and sinks in the aquatic environment. Laboratory studies and model calculations have been conducted to allow prediction of the fate of organic pollutants in the hydrosphere (10-19). Some of these studies (2,10,12, 16) have indicated that the atmosphere is the major "sink" for many of the volatile chlorinated hydrocarbons occurring in surface waters. However, t o verify whether such laboratory studies are applicable t o processes occurring in nature, a comprehensive program of field measurements is necessary. Furthermore, with such measurements the relationship between the distribution of compounds and the natural transport phenomena in a given water body can be evaluated. In this paper we discuss the seasonal variations in concentration of two selected chlorinated hydrocarbons, tetrachloroethylene (also perchloroethylene, PER) and l,.l-dichlorobenzene (DCB), in the water column of Lake Zurich, Switzerland. Estimates of the annual fluxes of these compounds from the lake to the atmosphere are given. By applying a simple steady-state box model, an average mass transfer coefficient (19) for the transport of DCB and PER to the atmosphere is determined. Experimental
Description of the Lake. Lake Zurich consists of two basins connected by a shallow channel a t Rapperswil (R, Figure Present address, Department of Chemistry, h'oods Hole Oceanographic Institution, Woods Hole, Mass. 02543.
1). The central basin, to which this study was confined, is about 29 km long and 2-3 km wide. The surface area is 68 km2 and the mean depth 50 m. The water flows from the upper basin to the central basin; the major outlet of the central basin is the river Limmat. The average residence time of the water in the central basin is 1.2 years. During summer the lake is stratified, while in winter the waters are mixed (20). The central basin receives effluents from 12 sewage treatment plants serving about 120 000 inhabitants. Presently the lake is the source of about 75% (-75 X lo6 m'j per year) of the drinking water supply of the city of Zurich and surroundings (population about 1 million). Sampling. Six vertical concentration profiles of volatile organic compounds were determined over the course of 1year a t the deepest point in the lake (136 m, middle of the lake, off the town of Thalwil (T),Figure 1).Triplicate samples were taken a t various depths in November 19'77, February 1978, and May 1978, duplicate samples in August 1978. and single samples in March and November 1978. Additional profiles were determined a t two other points in the lake (60 m, off Zollikon (Z); 15 m, off Rapperswil (R, inlet)) in March and August 1978. Numerous samples from the outlet of the lake (river Limmat), the effluents of three sewage treatment plants, and from four creeks entering the lake were analyzed throughout the year. In order to minimize contamination and losses during sampling, all samples (except for deep waters) were collected in the same bottle (1L)from which the volatile organic compounds were subsequently enriched. The bottles and their contained air were cleaned in the laboratory by a stripping procedure (see below) and were sealed for transport to the sampling site. Samples were taken by lowering the bottle to the appropriate depth where the stopper was removed by a sharp pull on a cord attached to the stopper. T h e bottle was brought t o the surface and closed immediately with a glass stopper. Deep water samples (30-130 m) were taken with a 5-L metal sampler suspended from a hydrographic wire. The water was carefully transferred to the glass bottle, which was filled completely and closed without headspace. Samples were stored in a cold room (4 "C) within 6 h after collection and were analyzed within 48 h. It was found that no significant losses of volatile halogenated hydrocarbons occurred during this storage period. Additional parameters, e.g.. temperature and dissolved oxygen, were measured by the laboratory of the Zurich Waterworks (Wasserversorgung Zurich), which routinely surveys the lake waters (20). Analytical Procedure. The volatile organic compounds in the lake water were concentrated by a closed-loop gaseous stripping/adsorption/elution procedure developed by Grob (21,22).This method permits the analysis of low boiling trace
0013-936X/79/0913-1367$01 .OO/O @ 1979 American Chemical Society
Volume 13, Number 11, November 1979
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Figure 1. Map and schematic length profile of Lake Zurich, Switzerland. Sampling locations: R, Rapperswil;T, Thalwil; Z, Zollikon
components in water at the nanogramhiter level (3-5,23). For this study, 25 ng each of 1-chlorohexane, 1-chlorodecane, and 1-chlorotetradecane internal standards were added to the I-L water sample, which was then stripped for 90 min a t 30 "C. The volatile compounds thus purged from the water were trapped by adsorption onto a small charcoal filter (1.5 mg of activated charcoal). The filter was then extracted with a total of 15 FL of carbon disulfide, and the extract analyzed by gas chromatography. Gas chromatographic analyses were performed on a Carlo Erba gas chromatograph (Model Fractovap 2400 T) equipped with a 50 m X 0.3 mm i.d. glass capillary column coated with Pluronics 121, according to the barium carbonate procedure described by Grob (24, 2 5 ) . Aliquots (2 pL) of the charcoal extracts were injected onto the column a t ambient temperature using the Grob splitless injection technique (26). The effluent of the column was split to allow a dual flame ionization/electron capture detection. The conditions were: carrier 0.95 atm, oven temperature 5 min a t ambient, then gas (H2) 3 "C/min to 180 "C. For quantification the response from the flame ionization detector was used. Peak heights were measured by a digital integrator (Supergrator-3, Columbia Scientific Inc.). The concentration of a specific compound was calculated by comparing its peak height with the peak height of the internal standard eluting closest (e.g., 1-chlorohexane for PER, 1-chlorodecane for DCB) and by applying the appropriate predetermined response factor. Peak heights were selected for quantification because the integrator does not always determine proper peak areas in a complex chromatogram.
Results and Discussion The volatile chlorinated hydrocarbons perchloroethylene (PER) and 1,4-dichlorobenzene (DCB) have been found in numerous studies of surface waters in the Zurich area (3-5). In groundwaters PER is often the most abundant volatile organic contaminant detected (5, 2 7 ) , while DCB is rarely found ( 2 7 ) .Whereas PER appears to enter the aquatic environment by various pathways (e.g., spills, rain, wastewater effluents, etc.), DCB is introduced to surface waters primarily by domestic sewage effluents ( 4 , 2 7 ) .Thus, both chemicals are introduced into Lake Zurich continuously a t various locations. We therefore selected PER and DCB as marker compounds for this study. Vertical Concentration Profiles. The vertical concentration profiles determined for PER and DCB a t the deepest point in the lake (T,Figure 1)are presented in Figure 2. Error bars indicating the relative standard deviations obtained from 1368
Environmental Science & Technology
three replicate analyses a t each depth are given for profiles 1 , 2 , and 4. I t should be noted that these errors were due to a combination of factors: sampling error, analytical error, and natural inhomogeneity of the lake at the time of sampling. Good replication was generally obtained (better than 10% relative standard deviations). Only a few samples taken at the thermocline, where the concentration gradient was greatest, showed relative standard deviations of up to 30%.For profiles 3,5, and 6, where only duplicate or single measurements were made, it seems reasonable to assume a similar reproducibility as found for profiles 1 , 2 , and 4. Several interesting observations can be made from the profiles of PER and DCB: (1)Although some significant concentration changes occurred for both compounds in the water column during the year, virtually identical profiles were obtained in November 1977 and November 1978 (profiles 1 and 6). (2) During times of stratification, concentrations of PER and DCB were lower in the epilimnion than in the hypolimnion, with a significant change in concentration a t the thermocline (profiles 1,4, 5, and 6). (3) During winter (profiles 2 and 3) very uniform distributions of both compounds were obtained down to depths to which rapid mixing within the water column can be assumed (28) (cf. oxygen profiles). (4) As the stratification of the lake broke down (profiles 1-3), there was a progressive decrease in concentration of PER and DCB in the hypolimnion, whereas an increase was observed after stratification built up again (profiles 4-6). (5) During the stagnation period, a more or less uniform vertical distribution of the compounds was obtained in the epilimnion at times of calm weather (profiles 1,4, and 6), while a concentration gradient was detected during a heavy storm (profile 5). Inputs, Outputs, Residence Times. Sewage effluents are probably the most important input sources for volatile chlorinated hydrocarbons in Lake Zurich. This is particularly true for DCB, which is used in a variety of household products, e.g., toilet deodorizers, and therefore is always present in effluent waters of the treatment plants ( 4 , 2 7 ) . Effluent samples of three different treatment plants showed a very uniform concentration of DCB among the plants and throughout the year (Table I). We estimate the total annual amount of DCB discharged to the central basin by treatment plants (62 kg of DCB/year) by multiplying this average concentration by the total amount of sewage effluent. In addition to the treatment plants, we believe that the waters from the upper basin contribute the only other significant input of DCB to the central basin (Table I). The concentrations of DCB and PER in the water flowing from the upper to the central basin corresponded generally to the concentrations found in the epilimnion of the central basin. This is not surprising, since the upper basin receives about the same amount of treated wastewater (relative to its total volume) as the central basin. I t is much more difficult to obtain a good estimate of the total input of PER to the central basin of the lake. Owing to large fluctuations of PER concentrations in the effluents, it was only possible to place a lower limit on the total input from treatment plants (Table I). Since this chemical is widely used as a solvent, e.g., dry cleaning, it may be introduced to the lake by additional pathways. For the determination of the average residence time of a compound in a water body, it is necessary first to calculate the total amount of the compound present in the water body a t a given time. For this purpose, we consider, as a first approximation, our vertical concentration profiles from the deepest point (T) as representative of the whole lake, Le., we assume that a t a given depth PER and DCB are evenly distributed
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Figure 2. Vertical profiles of 1,.l-dichlorobenzene(DCB), tetrachloroethylene (PER), temperature,and dissolved oxygen at the deepest point of Lake Zurich (T, Figure 1). The concentrations of DCB and PER at the bottom of the lake (136 m) were always equal to those found at 130 m
horizontally throughout the lake. According to Imboden and Lerman (28), horizontal mixing in most Swiss lakes is fast enough to produce a homogeneous distribution of a conservative pollutant over the whole area of the lake. This is particularly true for the upper layers, where the apparent horizontal eddy diffusion coefficient is large because of the wind. Our measurements support this assumption, since vertical profiles of PER and DCB taken a t three different locations (R, T, Z, Figure 1)on the same days (March 6 and August 8, 1978) showed equal concentrations a t corresponding depths. Close to the outfall of a sewage treatment plant, of course, a horizontal gradient in the concentrations of chlorinated hydrocarbons present in the wastewater can be observed. Figure 3 shows vertical profiles of DCB taken in July 1978 a t stations located 20, 150, and 450 m from the outfall of the treatment plant at Thalwil. Similar profiles were obtained for other chlorinated hydrocarbons present in high concentrations in the effluent, e.g., PER and 1,2,3- and 1,2,4-trichlorobenzene.
On that day, the effluent water, which is discharged at a depth of 6 m, appeared to sink and disperse a t a depth between 10 and 15 m (cf. concentration maxima of DCB, Figure 3), which corresponded to the depth of the thermocline. This is in agreement with findings of Buhrer and Ambuhl(29),who have shown that the effluent water flows to the depth of its own density in a stratified lake. I t should be noted that observations similar to those shown in Figure 3 were made in the summer of 1977 during a preliminary study ( 5 ) .Thus, during the summer, when the temperature of the sewage effluent is generally lower than the temperature in the epilimnion of the lake, the effluent waters will probably disperse a t or near the thermocline. During the rest of the year, when the temperature of the effluent is a few degrees higher than the temperature of the lake water, the wastewater probably disperses within the epilimnion where fast horizontal and vertical mixing leads to homogeneous distributions as reflected by profiles 2 and 3 (Figure 2). If we neglect the areas close to the outfalls of the treatment Volume 13, Number 11, November 1979
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Table 1. Estimates of Annual Inputs of 1,4-Dichlorobenzene (DCB) and Tetrachloroethylene (PER) to the Central Basin of Lake Zurich total amount 01 water input to the central basin a ( l o 6 m’/year)
source
total input lo the central basin, kglyear DCB PER
av concn, ng/L PER DCB
upper basin 2500 creeks 100 rain 100 sewage effluents 28 other sources (Le.,spills, groundwaters, etc.) ? a Reference 44. Reference 27. 2200 f 700 ng/L, 24 measurements.
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