Environ. Sci. Technol. 1994, 28, 1674-1685
Input and Dynamic Behavior of the Organic Pollutants Tetrachloroethene, Atrazine, and NTA in a Lake: A Study Combining Mathematical Modeling and Field Measurements Markus M. Ulrich, Stephan R. Muller, Heinz P. Singer, Dleter M. Imboden, and Ren6 P. Schwarzenbach'
Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 Dubendorf, Switzerland
The dynamic behavior of tetrachloroethene (PER), atrazine, and nitrilotriacetate (NTA) in a small lake in Switzerland has been evaluated quantitatively using field data and a simulation software (MASAS) for modeling organic pollutants in lakes. For PER, atrazine and NTA, the input and the seasonal variation in the vertical distribution could be described successfully by applying simple box models as well as a one-dimensional vertical model. In the case of PER, it was possible to reconstruct the input history for a long time period (9 years), and it was found that the stagnant boundary layer model was well suited for estimating the average gas exchange rate of this volatile compound between the lake and the atmosphere. Atrazine showed, except for a short time period during the summer, a very conservative behavior in the lake. For NTA, an average in situ rate constant of 0.035 d-l was determined for its elimination from the water column. The elimination was attributed primarily to biodegradation. The results of this study demonstrate nicely the power of the combined use of mathematical models, including computer simulations, and field measurements for assessing the environmental behavior of organic pollutants.
Introduction To date, the majority of field investigations of organic pollutants in natural waters have been confined primarily to determining concentration levels of specific compounds in various compartments of the aquatic environment. These monitoring-type studies have provided important information on the occurrence and distribution of a great number of anthropogenic organic compounds in groundwaters, lakes, rivers, estuaries, and oceans. However, the data collected in such studies rarely provide quantitative information on the various processes that determine the fate of a given compound in the system considered. Investigations focusing on the determination of in situ rates of transport, mixing, and transformation processes as well as on quantification of the inputs of specific organic chemicals to natural waters are, although becoming more common, still rather scarce. Some illustrative examples of recent studies of organic pollutants in surface waters include investigations of PCBs in the Great Lakes (1,2), and of pesticides ( 3 , 4 ) ,complexing agents (5),and volatile halogenated hydrocarbons (6) in streams and rivers. In this paper, we present the results of a study aimed at quantifying the inputs and the processes determining the spatial and temporal distribution of three quite different organic pollutants [tetrachloroethene (PER), atrazine, and nitrilotriacetate (NTA)] in a lake. To this end, experimental data derived from several field investigations conducted at Greifensee, Switzerland, were '
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quantitatively evaluated using literature data, simple "back-of-the-envelope" calculations, and computer simulations. Each of the three chemicals treated in this paper has its own characteristic input and elimination patterns in the lake and raises specific concerns with respect to environmental pollution. With a global annual consumption of approximately lo6 t, PER can be considered to be one of the most important organic solvents worldwide. In Switzerland, PER is used primarily for metal degreasing and for dry cleaning. Over the past 10 years, the consumption of PER in Switzerland has decreased by about a factor of 5, and in addition, measures have been taken to diminish losses of this compound to the environment, particularly to natural waters. The major objectives of the PER study were to evaluate the suitability of the stagnant boundary layer model for predicting average gasexchange rates of such a volatile organic compound between lakes and the atmosphere and to assess the longterm trends in the inputs and concentrations. In addition, the response of Greifensee to an accidental significant input of PER was studied. Atrazine is one of the most widely used herbicides. Although there is a vast literature concerning the fate of atrazine in soils and aquifers, rather little is known about its behavior in surface waters, particularly in lakes. In a recent study, Buser (7) measured the concentrations of atrazine and other triazine herbicides in various Swiss lakes. From this data, he concluded that atrazine is quite persistent in the water column of the lakes investigated, which at first glance seems contradictory to a recent study in which significant elimination of atrazine was observed in a small stream in Iowa ( 4 ) . The study presented here was aimed at providing more detailed information regarding the fate of atrazine in lakes. In addition, the seasonal variation of the atrazine input was of great interest, because in Switzerland the application of atrazine is confined to the months of May and June. NTA is widely used in detergents as complexing agents. It enters the aquatic environment primarily through the sewage system. A total of 75-99% of the NTA is eliminated in wastewater treatment plants (WWTPs) primarily by microbial degradation (5). Therefore, the input of NTA to natural waters is highly variable, due to varying operation conditions of the treatment plants and due to overflow of untreated sewage during high-water events. The major goal of this study was to determine in situ degradation rates for NTA in the water column of the lake. Also, this last example serves to illustrate how simple mass balance calculations combined with more sophisticated computer simulations can be used to derive quantitative process information even in cases in which more than one parameter is unknown (i.e., input, degradation rate). 0013-936X/94/0928-1674$04.50/0
0 1994 American Chemlcal Soclety
Swltzerland
Table 1. Characteristic Data of Greifensee and Its Catchment Area Lake Morpholofl volume (m9) area surface (m2) 10 m (m2) 20 m (m2) 30 m (m2) max depth (m) mean depth (m)
lkm
2km
Figure 1. Map of Greifensee, Switzerland, and its catchment area showing major inflows, outflow, residential areas (W), and the sampling locations: 1, sampllng site in the lake at the deepest polnt; 2-6, streams: 2, Aa (Uster); 3, Aabach (Monchaltorf); 4, Tufenbach; 5, Werrikerbach; 6, Dorfbach Maur; 7-9, effluents of wastewater treatment plants: 7, Monchaltorf; 8, Uster; 9, Maur; 10, sampling site for rain samples.
Experimental Section Description of Field Site: Greifensee a n d Its Catchment Area, Greifensee is a small eutrophic lake located 10 km east of Zurich, Switzerland (see Figure 1). Data on the morphology and hydraulics of the lake are summarized in Table 1. Greifensee is a holomictic lake with regular overturn in winter (December-March). Regular successions of oxic (spring) and anoxic conditions (summer and fall) are observed in the hypolimnion of the lake. The lake has been investigated intensively for more than 20 years by a limnological research group a t EAWAG. Thus, for the time period of this study, all the data needed for physical and chemical characterization of the lake were available. The catchment area of Greifensee comprises about 160 km2 with approximately 100 000 inhabitants. The high population density and the intense agricultural activities (see Table 1)cause a significant input of anthropogenic chemicals into the lake. The effluents of eight WWTPs are discharged into Greifensee either directly or indirectly via the two major tributaries, Aa and Aabach (Figure 1). The only outflow of the lake is the River Glatt. , An overview of the various sampling locations 'i presented in Figure 1. In the lake, samples were c o l l e c t e k \ a t the deepest point a t various depths (location 1in Figure 1). Vertical concentration profiles at this location were assumed to be representative for the whole lake, because in general, horizontal mixing in Greifensee is fast as compared to elimination processes (see discussion following). Note that close to point sources (e.g., inflows of streams or WWTPs), pollutant concentrations may have been somewhat higher; these parts of the lake were, however, not the focus of this study. Sampling Program a n d Analytical Procedure f o r PER. Over a time period of 9 years (1982-1990), a total
8.49 X 106 6.55 X lo6 3.51 X lo6 1.02 x 106 32 17.8
Lake Hydraulic@ 408 mean residence time of water (d) 3.7 X 106 av throughflow of water, Q (m3 d-1) 5 av epilimnion depth (summer),hepi(m) 4.2 x 107 av epilimnion volume (summer), Vepi (m3) Other Parametersb 5Xlo-B particle concn in water column, [SI ( m a value) (kg, L-1) 0.5-2.5 range of av settling velocity of particles, uB(m d-l) 0.4 fraction of organic carbon in particles, fOc (max value) (kg, kg;l) av wind speed 10 m above the lake 1.6 summer (ms-1) 2.0 winter (ms-1) Catchment AreaC 10 corn fields (km2) 25 arable land (km2) 104 total agricultural area (including meadows, pastures, etc.) (km2) forest (km2) 32 24 sealed area (streets, houses, etc.) (km2) total catchment area (km2) 160 inhabitants in catchment area 100 000
\
Okm
1.51 X 10s
a
Ref 33. b Ref 34. c Ref 35.
of 26 vertical sample profiles (8-12 depths) were collected a t the deepest point of the lake. Eighteen profiles were taken between October 1982and November 1985,and eight profiles were taken a t monthly intervals in 1990. In addition, in 1983and in 1990,a total of 30 discrete samples were taken periodically in the two main tributaries and from the effluents of the WWTPs a t Maur and Monchaltorf (locations 2, 3, 7, and 9). The water samples were collected in 1-or 2.5-L glass bottles (containing precleaned air) without leaving a head space (8). The samples were stored a t 4 "C in the dark and analyzed within 2 days of collection. Volatile organic compounds including PER were concentrated by the closed-loopgaseous strippingladsorptionl extraction procedure developed by Grob and Zurcher (9). The organic solvent extract was subsequently analyzed by capillary gas chromatography (GC)using a dual electron capturelflame ionization detector. The procedure is described in detail elsewhere (8). The detection limit for PER was 10 ng L-l. In a previous study, it was determined that the total error of the method was always below 10% (relative standard deviation, RSD) (8). Sampling Program a n d Analytical Procedure for Atrazine. Between January and November 1991, an intensive sampling program for atrazine was conducted. In monthly intervals, lake water samples were collected from location 1a t 10 different depths using a Niskin bottle, from which the water was transferred to l-L glass bottles that were later sealed with aluminum caps. The two main Envlron. Scl. Technol., Vol. 28, No. 9, 1994 1676
tributaries Aa and Aabach as well as the effluents of the three WWTPs (locations 2, 3, and 7-9) were sampled continuously using flow-proportional fixed sampling devices. The smaller streams (locations 4-6) were sampled with time-proportional portable samplers (4900 portable priority contaminant sampler, Manning Products, Texas). Based on water gauge recordings flow-proportionalsamples were then obtained by appropriate mixing of the timeproportional samples. Flow-proportional 7-d averaged samples were analyzed during and right after the application period of atrazine (May-August). For the remainder of the year, the sampling intervals were extended to 14 d. Finally, eight rainwater samples were collected between March and June at a field station near the lake (location 10). After collection, the samples were brought to the laboratory where they were spiked with 200 ng of pentadeuterioatrazine (in toluene) as the internal standard. The samples were then stored at 4 “C in the dark until analysis. Atrazine was analyzed using the method developed by Buser (7). The method includes a solid-phase preconcentration step followed by GCiMS analysis of the eluent fraction containing the triazines. The GC/MS analyses were carried out with two different systems: Carlo Erba GC/MS system (GC, MFC 500 Mega Series; MS, QMD 1000; auto sampler, A200S) and Hewlett Packard GC/MS system (GC, 5890 Series 11; MSD, 5971A; auto sampler 7673). Both systems were operated in the electron-impact mode (70 CV). Separation of the compounds was performed on a 17 m X 0.32 mm glass capillary column coated with PS089 (film thickness: 0.2 ym) using helium as the carrier gas and splitless injection. Single-ion monitoring was used for quantification. The detection limit was 10 ng L-1. By using pentadeuterioatrazine as the internal standard, a high analytical precision (& 2.5% RSD) was obtained. Sampling Program and Analytical Procedure for NTA. Lake water samples (location 1)and 24-h averaged samples from the main tributaries (locations 2 and 3) and from the effluents of three WWTPs (locations 7-9) were collected in monthly intervals. Between March 1990 and January 1991, the lake was sampled a t 7-10 different depths using a Niskin bottle. River water samples were collected with a time-proportional portable sampler. Based on water gauge recordings, 24-h averaged samples were then obtained by appropriate mixing of the timeproportional samples. Effluent samples from the three WWTPs were collected during 24 h with flow-proportional fixed sampling devices that were maintained at 5 “C. In addition to the monthly sampling program, 13 discrete samples were collected during high-water events at the stormwater overflow after the primary clarifier at two WWTPs (locations 7 and 9). All samples were collected in or transferred to 250-mL PE bottles, containing either 1mL of concentrated HC1 (lake samples) or 2.5 mL of a 37 % formaldehyde solution (river and WWTP samples) to prevent biodegradation. After collection, the samples were brought to the laboratory where they were stored at 4 “C until analysis. NTA was determined by using a slightly modified esterification method described by Schaffner and Giger (10). This method determines the total concentration and does not distinguish among various species of the chemicals (e.g., different complexes, particle-bound species, etc.). Briefly, 25-50 mL of a water sample was spiked with an 1676 Envlron. Scl. Technol., Vol. 28, No. 9, 1994
internal standard (propylenediaminetetraacetic acid), evaporated to dryness, and redissolved in 5 mL of 50% formic acid. After evaporation to dryness again, 1.5 mL of an acetyl chloride/propanol mixture (1:9) was added, and the reaction mixture was heated to 90 “C for 1h. After addition of a second internal standard (octadecanoic acid nitrile), 25 mL of water was added, and the tetrapropyl esters of NTA were extracted twice with 2 mL of CHC13. The combined extracts were dried with Na2S04, evaporated to dryness, and redissolved in 0.2 mL of toluene. The toluene extract was then analyzed by capillary gas chromatography using a nitrogeniphosphorous detector. The detection limit was 0.2 yg L-l. For concentrations >1 yg L-l, the analytical error was smaller than 20 % ,for lower concentrations it was rt0.2 yg L-l. Chemicals. Atrazine (99% ’ purity) was kindly donated by Ciba Geigy AG, Base1 Switzerland; pentadeuterioatrazine (atrazine-& 98 % purity) was purchased from Icon, New York; all other chemicals were obtained from Fluka Chemie AG, Buchs, Switzerland, or Merck, Darmstadt, Germany. Computer Software. The computer simulations were performed using MASAS (Modelling of Anthropogenic Substances in Aquatic Systems), which is a user-friendly, versatile simulation tool that allows the development of mathematical models for describing the dynamic behavior of organic chemicals in lakes. A lake is described by models with different spatial resolutions (1-box, 2-box, “combibox”, continuous model). The most complex model implemented is the one-dimensional vertical transportreaction model encompassing the water column as well as the sediment (11). In this model, complete horizontal mixing is assumed at each depth. In contrast, vertical mixing is explicitly described as a turbulent diffusion process with time- and depth-dependent diffusion coefficients. For the numerical calculation, the continuous depth dimension is approximated by an appropriate number of boxes. The model is suited for deep lakes having simple horizontal structure and for shallow lakes that are vertically and horizontally well mixed. The program was developed for Apple Macintosh personal computers, using the Dialogmachine, a development system for menu- and window-oriented software (12). A detailed description of MASAS is given in refs 13 and 14. B a c k - o f - t h e - E n v e l o p e Calculations
Before evaluating and interpreting field data, it is useful to start out with some simple “back-of-the-envelope calculations’’ in order to get an idea of the relative importance of the various processes that determine a compound’sbehavior in a lake (Le., elimination by flushing; gas exchange; sorption and sedimentationiresuspension; chemical, photochemical, and biological transformation). Such calculations employ simple mathematical models and are based on literature data and on theoretical considerations. They frequently allow one to make simplifying assumptions, which aid in choosing an appropriate, more complex model for a given compound and question. Because in lakes some of the most important processes occur at or near the surface (e.g., gas exchange, direct and indirect photolysis), the epilimnion is the most obvious lake compartment to be used for preliminary calculations. Considering the magnitude of the horizontal and vertical mixing rates in lakes such as Greifensee (15), as a first approximation, the epilimnion can be looked at as a well-
estimated for the three model compounds. Comments on how these values were obtained follow. Sorption and Sedimentation. The sedimentation of an organic compound out of a well-mixed box can be described by a characteristic rate constant, ksed, which is defined as follows (ref 17; Chapter 15):
where
Ct C,, J,, k
= total concentration of compound in the epiiimnion = average input concentratian at compound (defined as Jin/ 0 ) = average input of compound (mass per time) = characteristic first-order rate constant lor a given process (i&
export by water (flushing), gas exchange, dimentation, and M i c a i , @Q!QchemicaI and biological transformation) Vepl = volume of the epilimnion hepi = average depth of the epilimnion Q = average throughflow of water
Flgure 2. 1-Box model for the epilimnion of a lake. The model is used for some back-of-the-envelope calculations by making the following assumptions: (1) Total input of water and compound into the eplllmnlon, (2) no water exchange between the epilimnion and the hypolimnion, and (3) all processes can be expressed as pseudo-first-order reactions.
mixed box that is isolated from the hypolimnion during the summer (stagnation period). Furthermore, for the low pollutant concentrations typically encountered in lake waters, each of the processes may be expressed as a firstorder or a pseudo-first-order reaction exhibiting a characteristic rate constant (11,16). By doing so, the relative importance of each of the processes may be assessed immediately by a direct comparison of the characteristic (pseudo-) first-order rate constants (see Figure 2). Because elimination by flushing is equally important for all compounds (dissolved and particulate species), the characteristic rate constant, k,, for this process can be considered as a reference point:
where Q is the average flow through the epilimnion and Vepiis the volume of the epilimnion. Assuming that, during stratification in Greifensee, the total water input occurs into the epilimnion, a k, value of 0.009 d-l is obtained (Table 1). Table 2 summarizes the characteristic rate constants for the other processes as far as they can be
where usis the average particle settling velocity, hepiis the average depth of the epilimnion, and (1- f e ) is the fraction of the compound in particulate form. Assuming sorption equilibrium and a linear sorption isotherm, the fraction in dissolved form, f e , can be calculated by
Kd is the particle/water distribution ratio (e.g., in L kg,-l) and [SI is the particle concentration (e.g., in kg, L-1). For compounds for which hydrophobic partitioning is the major sorption mechanism (e.g., PER, atrazine),& is given by (ref 17; Chapter 11) Kd
(4)
=fosoc
where foc is the fraction of particulate organic carbon (e.g., kg,, kg;l) and KO, is the natural organic matter/water partition constant of the compound (e.g., in L kg,,-'). If a maximum particle concentration of 5 X lo4 kg, L-l, an average foc value of 0.4 kg,, kg,-l), and a maximum particle settling velocity of 2.5 m d-l are assumed for the epilimnion of Greifensee (see Table 11,then for both PER and atrazine, sedimentation is predicted to be unimportant (see Table 2). Based on the available literature data, one arrives a t a similar conclusion for NTA. For this compound, adsorption by surface complexation to iron oxide surfaces is postulated to be the major sorption mechanism (18). However, the reported adsorption constants are much too small to make sedimentation a relevant removal process for NTA from the water column. Gas Exchange. The flux of a compound by gas exchange between the lake and the atmosphere can be expressed as (ref 17; Chapter 10):
Table 2. Characteristic First-Order Rate Constants for Elimination of PER, Atrazine, and NTA from Epilimnion of Greifensee Estimated from Literature Data and Model Calculations (See Text)*
-
PER
atrazine
NTA
nondimensional Henry's law constant, KH* (-) lb ,10-7c nr octanol/water partition constant, KO,(-) 750b 350b nr natural organic mattedwater partition constant, Koc (L kgw-l) 400b 200c nr particle/water distribution ratio, Kd (L kga-l) 150 80 nr fraction in particulate form, 1- fe (-) (8 x 10-4
