Environ. Sci. Technol. 1991,25,678-683
with this latter assumption. In order to verify the above hypotheses, model experiments are currently being performed in our laboratories, which will allow a decision on which of these two microbiological degradation pathways discussed above gives rise to the observed enantiomeric excess of tu-HCH. Furthermore, representative water samples from all parts of the North Sea have to be analyzed, in order to determine whether high concentrations of a-HCH and/or y-HCH are vital for the enantioselective microbial decomposition and which factors cause the enantiomeric excess. In conclusion, a modification of the enantioselective capillary gas chromatography method by Konig et al. (3, 4 ) renders it possible for the first time to determine the enantiomeric excess of chiral compounds a t low concentrations as encountered in environmental samples. Thus, a new experimental approach can be suggested that allows a discrimination between microbiological decomposition (enantioselective) and nonenzymatic processes (nonenantioselective). Acknowledgments
The investigation reported herein could not have been accomplished without the dedicated assistance of numerous collegues. During the cruise, M. Gonzales-Davila, Dr. Lange, and Dr. Lutz assisted in taking the water samples, Dr. Brockmann, Dr. Huber, Dr. Kattner, and Dr. Schmidt took care of the overall logistics of the campaign. During the chemical analysis in the laboratory and the
evaluation of the data sets, valuable assistance by H. Dannhauer and H. Nommsen was provided. This is gratefully acknowledged. Thanks are also due to M. Richters, who prepared the chiral Pyrex glass capillary column. Registry No. (-)-cu-HCH, 119911-70-5; (+)-a-HCH, 11991169-2; water, 7732-18-5.
Literature Cited (1) Weber, K.; Balint, U.; Huhnerfuss, H. Mar. Ecol. Prog. Ser., submitted for publication. (2) Huhnerfuss, H.; Weber, K. Mar. Ecol. Prog. Ser., submitted for publication. (3) Konig, W. A.; Krebber, R.; Mischnick, P. J . High Res. Chromatogr. 1989, 12, 732. (4) Konig, W. A. Nachr. Chem., Tech. Lab. 1989, 37, 471. (5) Ernst, W.; Schaefer, R. G.; Goerke, H.; Eder, G. Z. Anal. Chem. 1974, 272, 358. (6) Gaul, H.; Ziebarth, U. Dtsch. Hydrogr. Z. 1983, 36, 191. ( 7 ) Cristol, S.J. J. Am. Chem. Sac. 1949, 71, 1894. (8) Benezet, H. J.; Matsumura, F. Nature 1973, 243, 480. (9) Huhnerfuss, H.; Weber, K. J. Geophys. Res., submitted for publication. (10) Malaiyandi, M.; Shah, S.M. J . Environ. Sci. Health 1984, A19, 887. Received for review September 6, 1990. Accepted October 29, 1990. This work has been supported by the Ministry of Science and Technology of the Federal Republic of Germany ( B M F T projects M F U 0545 Zirkulation und Schadstoffumsatz in der Nordsee and MFU,, 0620 Prozesse i m Schadstoffkreislauf Meer-Atmosphare; Okosystem Deutsche Bucht).
Measurement and Prediction of Copper Ion Activity in Lake Orta, Italy Marina Camusso and Gianni Tartari Istituto di Ricerca sulle Acque, C.N.R., 20047 Brugherio, Italy
Albert0 Zirino" Naval Ocean System Center, Code 522, San Diego, California 92152
A commercial Cu ion selective electrode (ISE) mounted on a field conductivity, temperature, depth probe (CTD) equipped with pH and oxygen sensors was used to measure a profile of Cu ion activity [a(Cu2+)]in Lake Orta, Italy. Lake Orta water contains approximately 32-34 pg L-' Cu from anthropogenic sources. Below the mixed layer, a(Cu2+)was directly related to the pH of the lake water. In the body of the hypolimnion, measurements of a(Cu2+) were in good agreement with estimates of a(Cu2+)obtained from total Cu concentrations. The pH dependence of the activity/concentration of free Cu2+was modeled with a simple ion association model of the lake water. The results of the model were verified by a potentiometric titration of a sample of lake water using Cu, pH, and NH3 ISEs. The titration simulated a forthcoming chemical treatment now in progress. Introduction
Lake Orta, the seventh largest lake in Italy, occupies the southwestern part of the Lake Maggiore drainage basin. Like other glacially carved, subalpine lakes in northern Italy, it is long and narrow, extending 12.6 km north to south, with a maximum width of 1.9 km and an average depth of 70 m (Figure 1). Its important morphological characteristics are given in Table I. 678
Environ. Sci. Technol., Vol. 25, No. 4, 1991
Table I. Main Morphometric Features of Lake Orta
watershed area, km2 lake surface, km2 mean lake level altitude, mas1 mean lake length, km maximum width, km maximum depth, m mean depth, m volume, m3 theoretical renewal time, year mean water residence time, year
116.0
18.2 290 12.6 1.9
144 70 1.3 x 109
8.9 10.7
From 1926 to 1980, Lake Orta received industrial discharge containing large quantities of ammonium and copper sulfate from a plant engaged in the manufacture of rayon fiber. The discharge eliminated the floral and faunal populations of the lake and caused similar drastic changes in its chemical composition. These environmental alterations along with occasional reports on the chemical and biological conditions of the lake have been documented in many publications (1-6). In 1958, the copper load of the plant's effluent was effectively reduced from 40-80 t year-l to 4-5 t year-' and the ammonium load was reduced to 1'7'0 of its former value (2.0 X lo3 t year-') in 1981 (2). However, from the 1950s, numerous small electroplating factories distributed around the lake have continued to discharge heavy metals, al-
0013-936X/91/0925-0678$02.50/0
0 1991 American Chemical Society
Rayon factory o u t l e t
\"
Figure 1. Lake Orta drainage basin.
though the quantities so disposed are much smaller than the original copper discharge. A lasting legacy of the original discharge is the inordinately low pH of the body of the lake (pH 4.3-4.5 between June 1986 and August 1987) caused by the bacterial oxidation of ammonia to nitrate ( 4 ) . A t the spring overturn of 1987, the average copper concentration of the lake was approximately 5 X M (32-34 pg L-l). The combination of high copper and low pH results in high copper activity, which, in turn, may be directly toxic to algae, bacteria, and larval fauna (7-9). For a number of years, copper ion selective electrodes (Cu ISEs) made from Ag,S/CuS have been used to estimate copper ion activity [a(Cuz+)]in aquatic media (IO). In general, these measurements have been made in the laboratory, under controlled conditions (25 "C and atmospheric pressure). We have used a commercial Cu ISE in situ in Lake Orta waters in order to estimate a(Cu2+) and to determine its vertical distribution. We also studied the relationship between a(Cu2+)and the pH of the lake water in order to predict the effect of a forthcoming chemical treatment (the addition of finely dispersed CaCO,) on a(Cu2+). For this purpose, we developed an equilibrium model of the interactions between the major inorganic components of the lake and "titrated it" by increasing the carbonate concentration in our computations. The calculated relationship between pH, Cu2+,and NH, was verified in the laboratory by using the appropriate ISEs. Experimental Section
Apparatus. An Ocean Seven (Idronaut Srl., Brugherio (MI), Italy) Model 401 conductivity, temperature, depth probe (CTD) was used for all in situ measurements. The Idronaut CTD measures depth, temperature, conductivity, dissolved oxygen, and pH and can be fitted with a number of ISEs. An Orion Model 94-29 Cu ISE was used in the field with the CTD and in the laboratory. A Radiometer Model K701 double-junction standard calomel electrode (SCE) was used as reference in the laboratory. An Idro-
naut, solid-gel Ag/AgCl reference was used in the field. An Orion Model 95-12 NH3 ISE was used to measure ammonia. Electrode potentials were determined with an Orion Model 407 meter. In the laboratory, pH measurements were performed with a Radiometer Model GK2401C combination glass-reference electrode and a Radiometer Model 80 portable pH meter. A specially designed manifold (11)was used to calibrate the Cu ISE, solid-gel reference electrode pair for temperature. The equipment was identical with that used to calibrate pH electrodes (12). A pressure chamber charged with compressed nitrogen was used to determine the pressure dependency of the electrode pair. Total dissolved copper concentrations of the lake water were determined by atomic absorption measurements using a Perkin-Elmer Model 5000 spectrophotometer equipped with graphite furnace (HGA 500). Standard copper solutions were measured directly with a J Y 38 inductively coupled plasma (ICP) instrument. Calibration of Electrodes. The temperature and pressure calibrations of the electrode were performed a t the San Diego Laboratory of the Naval Ocean Systems Center. All the other measurements were made a t the Brugherio laboratory of the Istituto di Ricerca sulle Acque or, in the field, at Lake Orta. The temperature dependency of the electrode was determined by using a standard solution of Cu2+in lo-, M KNO,. The solution was recirculated past the electrodes at a velocity of 0.1 m s-l in order to simulate the flow of water as the CTD was lowered in the lake. After a stabilization period of several hours, the electrode potential was recorded along with temperature from 5 to 25 "C during both heating and cooling of the solution. The temperature response of the electrode was measured to be 0.3 f 0.1 mV "C-'. No significant pressure dependency was recorded to a pressure of 1.3 MPa. The response of the Cu ISE was calibrated for a(Cu2+) a t room temperature by adding CuSO, solution in increments to a standard phosphate pH buffer (Beckman-Altex, pH 7.42 a t 25 "C) made with high-purity deionized water. The final concentration of Cu2+was 6.32 X M. The solution is both a pH buffer and a metal buffer (13). a(Cuz+)was varied over 4 orders of magnitude by adding Cu incrementally and finally by titrating the buffer solution with dilute HNO, to pH 5.71. a(Cu2+)at each pH was computed from the dissociation constants of phosphoric acid and the Cu-phosphate complex (14) by using MICRCQL, a computer program in BASIC for calculating chemical equilibria (15). Strictly speaking, individual ion activities are unmeasurable and thermodynamic activity is unitless. However, for the sake of usefulness in this application, we have chosen to define a(Cu2+),the response of the Cu ISE, as a practical actiuity in terms of molarity. Figure 2 shows the relationship obtained between computed a(Cu2+)and the electrode potential. The calibration of the ammonia electrode was conducted according to its instruction manual. A nearly Nernstian slope (58 mV decade-l) was obtained both with lake water and with a laboratory solution. The pH electrode was calibrated with standard buffer solutions in accordance with Idronaut specifications. Field Protocol. Electrode calibrations were carried out in the laboratory on the morning of, or the evening prior to, the day during which field sampling was scheduled. The entire electrode end of the CTD was immersed in the buffer solution used for calibration and transported to the lake site where it was transferred to an open boat equipped with a davit and a winch. Once on station, the CTD was Environ. Sci. Technol., Vol. 25, No. 4, 1991
679
Table I:. Concentrations (mol L-') of Components and Equilibrium Constants in the DISORTA .Model
components cu Ca Mg
Na K 3 "
species
V
I
30
105
80
55
130
180
155
E ( W
Flgure 2. Cu ISE calibration plot.
rinsed profusely with lake water, suspended on the hydrowire, and allowed to equilibrate with surface water for 30 min. The electrode-equipped CTD was then lowered to depth a t a velocity of 10 m min-'. A display monitor (Idronaut Model 401) aboard the boat both registered instantaneous values and printed recorded average values per meter as a function of depth. Titration of a Lake Orta Sample. In order to simulate the forthcoming chemical treatment, a surface water sample from Lake Orta, collected in August 1986 (station A, Figure l),was titrated with 5.0 x M Na2C03to a pH of 8.6. Cu2+,pH, and NH, activities were recorded as described above. (Na2C03was chosen over CaCO, in order to avoid solubility problems). Model DISORTA. A chemical equilibrium model DISORTA (DIssolved Species in Lake ORTA) was developed to study the relationship between the copper activity and the other ionic species present in the lake water. The various species, components, and equilibrium constants are reported in Table 11. Where possible, equilibrium constants a t room temperature and extrapolated to zero ionic strength were chosen. Activity coefficients were assumed equal to unity. The numerical program MICROQL was used for computing chemical equilibria (15). Total Dissolved Copper. Total dissolved copper was determined on water samples collected with a precleaned hydrographic bottle, filtered (0.4 km) in the field shortly after collection and acidified to p H 2.
Results and Discussion Vertical Profile. Vertical profiles of temperature, pH (at 20 "C), dissolved oxygen, conductivity, and (practical) a(Cu2+) (corrected to 20 " C ) recorded at station A on TEMPERATURE ('C) 6
10
14
16
22
PH 4.2 4.6 5.0 5.4 5.8
-log
OXYGEN (mg 1.5 4.5 7.5 10.513.5
A
60
En. 90
120
150
Flgure 3. Vertical profiles of parameters measured in Lake Orta in August 1987. Environ. Sci. Technol., Vol. 25, No. 4, 1991
1.0x 10-4 1.64 x 10-4 keq
components H
c1 so4
NO, PO4 CO3
species OHCu(OH)+ Cu(OH),O HS04CuSO4' HPOd2H,PO,H,PO? CuHPO,' Ca(HCOJ+ CaCO,'
concn 1.9 x 10-4 7.1 x 10-5
3.23 x 10-4 3.21 x 10-4 8.1x lo-@ 1.15 x 10-4 -1%
k,q
-14.0 -8.0
-13.7 2.0 2.3 12.4 19.6 21.7 16.6 1.26 9.9
August 26, 1987 (Figure 1)are shown in Figures 3 and 4. Several features that appear in the profiles of the measured variables are worth noting. A pronounced thermocline between 10 and 20 m separates the epilimnion from the deeper waters. Below the thermocline, temperature decreases slightly with depth, from approximately 5 to 4.8 "C at the bottom (144 m). In the epilimnion, pH increases from -4.6 to approximately 5.9, probably in response to algal photosynthesis and the alkalinity of inflowing river waters ( 3 ) . Just below the thermocline, a t approximately 20 m, a pronounced minimum is observed. Thereafter, pH decreases smoothly to approx 110 m, where a small maximum is observed. Finally, from 130 m to the bottom, pH increases substantially, from -4.4 to 5.8. From nearsurface to bottom, conductivity is seen to mirror pH, indicating that the changes in conductivity are related to changes in H+ activity caused by production, respiration, and nitrate formation from ammonium ion, e.g. NH4+ + 2 0 2 = NO3- + HzO + 2HS The oxygen concentration is near saturation at the surface, M or 12.5 mg L-l) becoming supersaturated (to 3.9 X in the region of the thermocline, and then decreasing sharply to near-anoxic levels in the last 14 m. While it appears reasonable to suggest that microbial and infaunal respiration reduces the oxygen concentration, the concomitant increase in pH suggests that other processes may also be invovled. Figure 4 shows that (practical) a(Cu2+)at the surface M and increases to 3.7 X is approximately 2 x
30
680
5.0 x 10-7 1.62 x 10-4 6.2 x 10-5 4.08 x 10-4
NH4+ 9.3 4.1 Cu[NH3I2+ C U [ N H ~ ] ~ ~ + 7.6 C U [ N H ~ ] ~ ~ + 10.5 CU[NH,],~+ 13.0 cuc1+ 0.5 CUC1,O 0.3 HC0,10.3 16.6 H2C03: 6.75 cuco, 3.2 Cu(HCO?)+
0
5
concn
C(WDucnVITY(pScm-')
93 W 105 111 117
-
OCL2*
0
? ,A
0 ' 3-
I'd,.
A
3
3
,
.
I
I
~
P
*',-I)
A
, , , : ,
,
0-
+=
, , : ,
I
-*.I.
T
i
3 . - .. .*
+ f, 2 4:
I
0
e'
.
-
4 0
Figure 4. Vertical profile of (practical) copper ion activity [a(Cu2+)] in Lake Orta in August 1987.
Table 111. Results and Analaytical Precision of the Total Copper Determinations depth, m
1 5 10 15 30 75 120 140
total dissolved Cu concns mol L-' mg m-3 4.8 5.0 5.2 6.4 6.1 5.9 5.5 2.5
mg of Cu m-3
n
16 33
12 11 12
42
31 32 33 41 39 38 35 16
cv,
7 0
8.3 6.3 5.7
M a t 10 m. It decreases again a t the foot of the thermocline to form a small maximum and then increases to a nearly constant value of 4.7 X M from 50 to 110 m. From 130 m, a(Cu2+)decreases to the bottom t o approximately 9.0 X M. Except at the very surface, the copper profile is the mirror image of the pH profile and directly follows the conductivity profile. Overall, a(Cu2+) increases by a factor of -30 from the surface to 130 m. For comparison, values of copper concentration, measured by atomic absorption, are presented in Table 111. The total dissolved copper concentration ([CUI,) increases slightly from the surface, 4.8 X to 6.5 X M a t 15 m (31 to 41 pg L-l). Near the bottom, [CUI, decreases by a factor of 2 (to 2.5 M) while a(Cu2+)decreases by a factor of 5. The low values of [CUI, are associated with high value of pH and pCu. Figure 5 shows a plot of pCu against pH. The data fall essentially into four zones. A surface layer, from 0 to 10 m, where pCu is relatively independent of pH, a transition zone, where both pCu and pH decrease rapidly to the foot of the thermocline, the body of the lake, from 11to 135 m, where pCu and pH still covary despite the small range of their values ( R = 0.65, n = 121), and the bottom (136-144 m) where pH and pCu both increase. Below 10 m, a(Cu2+)and pH profiles are very similar, and we conclude that a(Cu2+)responds to pH. Because (1)carbonate activity in the lake is low ( 2 ) ,(2) the ion activity product
8 I I , ' "> ' />
' "
4.40
4.60
4.80
5.00
5.20
5 40
5.60
5.80
6.00
of Cu(OH),O lies considerably below the thermodynamic solubility product of Cu(OH),,,, [approximately 10-lg-lO-m, (14)],and (3) [cu2+]Tdecreases with increasing pH, we suggest that a(Cu2+)in situ is regulated indirectly by the pH via adsorption on particles. [This suggestion is strengthened by work done since the first writing of this paper. Monthly profiles of a(Cu2+)and pH collected a t this station for a year (1988-1989) show a high degree of correlation between these variables. Similarly, values of total Cu measured over the same period are inversely correlated to pH (16)l. In the upper 10 m, pCu decreases by almost 1 order of magnitude while pH remains relatively constant a t approximately 5.8 (with a small amount of structure). Because this is the euphotic zone, it is possible that here a(Cu2+)is lowered by a different mechanism, perhaps one related to algal primary production. Because pH is relatively constant, the change in pCu may be caused by differing amounts of Cu-complexing ligands present in the water. However, we did not and have not investigatsd this topic further. The ionic strength of Lake Orta water is approximately Thus, we can estimate an activity coefficient for 3X Cu2+ of approximately 0.8 (17). Concentrations of dissolved copper can be estimated by dividing a(Cu2+)by 0.8. This procedure yields a maximum [CUI, of 5.9 X M, in good agreement with the maximum concentration of 6.4 X M (41 pg L-l) obtained by spectrophotometric analysis of the filtered lake water. This implies that copper in the body of the hypolimnion, a t pH 4.6, is almost fully ionized and that complexation or adsorption onto colloidal particles reduces a(Cu2+)at the surface and at the bottom. Because the total concentration of copper near the bottom is less than half that of the water column above it, the large reduction of a(Cu2+)at the bottom indicates the presence of a process that simultaneously results in a net removal of copper from the water, a pH increase, and an oxygen decrease. The oxidation of sedimentary Fez+to Fe3+and subsequent adsorption of Cu2+onto the resulting Fe(OH)3 is suggested (18). This process is discussed further in a separate paper (19). The high degree of correlation between pCu and pH below the mixed layer strongly suggests that, for the body of the lake, the equilibration time between the Cu ISE and the lake water is sufficient to yield meaningful results, since the response time of the pH electrode is known to be rapid (20). Similarly, the good agreement between measured [CUI, and [CUI, computed from practical activity is also a good indication that the response time of the Cu ISE is sufficiently rapid to allow a lowering rate of 5-10 cm s-l. Environ. Sci. Technol., Vol. 25,
No. 4, 1991 681
Table IV. Species and Their Activities at Different pHs Computed by the DISORTA Model Simulating the Chemical Treatment of Lake Orta Watersd
PH 4.0
6.0
7.0
4.7 x 10-7 8.2 X 1.6 x 10-4
4.6 x 10-7 7.4 x 10-8 1.6 x 10-4 4.3 x 10-10 1.0 x 10-10 4.1 x 10-9 7.3 x 10-9 5.3 x 10-10 3.0 x 10-8
1.7 x 10-7 X lo-? 1.6 x 10-4 1.6 x 10-9
species
cu2+ 3"
NH4+ Cu[NH3]*+ CUCl+ C u (OH)+ Cu(OH),O CU(CO3)O cuso40 [CUIr = 5.0 X
-
1.1 x
10-10
-
3.0
M; ["I,
X
= 1.64 X
mil hval t s
5000
10000
9.0
8,O
7.7
3.1 x 10-9
-
7.0 X lo4 1.6 x 10-4
5.9 x 10-5 1.0x 10-4
2.8
-
X
-
-
1.6 X 3.0 X 8.5 x 1.1 x
lo-@ lo-$
10-9 10-8
2.8 x 10-9 4.9 x 10-7 2.6 x 10-9 2.0 x 10-10
2.2 x 10-10 5.0 x 10-7 6.0
X
-
M; -, 7). Obviously,the results of the titration experiment on lake water should be treated with caution because the stirring applied during the experiment was higher than that which occurs in the lake. On the other hand, the actual scavenging of Cu2+by suspended particles may reduce a(Cu2+) observed during the titration is somewhat less than the change observed in situ. pH-dependent substances present in natural waters (such as colloidal hydrated iron oxide) and not explicitly included in models may be indirectly included in other stability constants and may account for the fact that the larger stability constant for Cu(OH), is a better predictor of our experimental results with Lake Orta water. The copper (ISE) showed itself to be a suitable probe for in situ measurements yielding detailed profiles of (practical) Cu ion activity, both for assessing toxicity and possible regulatory mechanisms governing its concentration. Although the concentration of Cu2+in Lake Orta is relatively high, activity levels a t the surface were much lower, and there is no apparent reason why the electrode should not be able to detect still lower levels. We believe that electrode sensitivity is not an issue because the electrode is well-known to measure a(Cu2+) in buffered solutions to extremely low levels (ref 13, and note also the linearity of Figure 2). Presumably, Cu in the lake is in equilibrium with the Lewis bases, the dissolved, colloidal, and particulate substances that bind it. For trace metals in the natural waters, these substances are, in the main, present in much larger concentrations than that of the metals they bind. Thus, low levels of a(Cu2+)are stabilized by the ligand field (buffered) and therefore measurable. Furthermore, any contribution to the electrode potential by Cu2+coming from the electrode itself, a problem when low-level measurements are made in closed systems, is also minimized by the flow nature of the measurement as well as the natural ligand field. As far as we known, this work presents the first detailed copper ion activity profiles obtained in situ in a lake. The continued use of the Cu ISE in the field and in the laboratory, combined with the judicious use of equilibrium models, should prove useful for further studies of lakes and other freshwater environments.
Acknowledgments We thank Prof. R. Marchetti and the Consiglio Nazionale delle Ricerche for making this collaboration possible by awarding travel grants to the participants. Registry No. Cu, 15158-11-9; Ca, 7440-70-2; Na, 7440-23-5; K, 7440-09-7; "3, 7664-41-7; NH4+, 14798-03-9; H', 12408-02-5;
Cu[NH3I2+, 18616-93-8; Cu[NH3]zZ+,21646-42-4; C U [ N H ~ ] ~ ' + , 28101-94-2; C U [ N H ~ ] ~16828-95-8; ~+, CUCl', 15697-17-3; CUCl,, 7447-39-4; HC03-, 71-52-3; HzC03, 463-79-6; CuCO,, 1184-64-1; Cu(HC03).+, 114572-22-4; Cu(OH)+, 19650-79-4; Cu(OH)Z, 20427-59-2; HS04-, 14996-02-2; CuSO,, 12400-75-8; HP042-, 29505-79-1; H2P04-, 57540-25-7; H3P04, 7664-38-2; CuHPO,, 13587-24-1; Ca(HC03)+,52409-16-2; CaCO,, 1184-64-1.
Literature Cited (1) Calamari, D.; Marchetti, R. Prog. Water Tech. 1975, 7, 569-577. (2) Bonacina, C.; Bonomi, G.; Mosello, R. Mem. Zst. Ital. Idrobiol. 1986, 44, 97-115. (3) Mosello, R.; Baudo, R.; Tartari, G. A. Mem. Ist. Ital. Idrobiol. 1986, 44, 73-96. (4) Mosello, R.; Bonacina, C.; Carollo, A.; Libera, V.; Tartari, G. A. Mem. Ist. Ital. Idrobiol. 1986, 44, 47-71. ( 5 ) Provini, A.; Gaggino, G. F. In Sediments and Water Interactions; Sly, P. G., Ed.; Springer-Verlag: New York, 1986. (6) Bonacina, C.; Bonomi, G.; Barbanti, L.; Mosello, R.; Ruggiu, D.; Tartari, G. In Toxic Contamination i n Large Lakes; Impact of Toxic Contaminants of Fisheries Management, Vol. 11; Schmidtke, N. W., Ed.; Lewis: Chelsea, MI, 1988. (7) Sunda, W. G.; Guillard, R. L. L. J . Mar. Res. 1976, 34, 511-529. (8) Sunda, W. G.; Gillespie, P. A. J. Mar. Res. 1979,37,761-777. (9) Sunda, W. G.; Ferguson, R. L. In Trace Metals i n Seawater; Wong, C. S., e t al., Eds.; Plenum: New York, 1983. (10) Bailey, P. Analysis with Ion Selective Electrodes; Heyden and Son Ltd.: London, 1978. (11) Zirino, A.; Seligman, P. F. Mar. Chem. 1981, 10, 249-255. (12) Fuhrmann, R.; Zirino, A. Deep-sea Res. 1988,35,197-208. (13) Hansen, E. M.; Lamm, C. G.; Ruzica, J. L. Anal. Chirn. Acta 1972,59, 403-426. (14) Sillen, L. G.; Martell, A. E.; Stability Constants of Metal-ion Complexes; Spec. Pub1.-Chem. SOC.1971, No. 25. (15) Westall, J.C. MICROQL. I. A chemical equilibrium program in BASIC. Department of Chemistry, Oregon State University, Corvallis, OR, Report 85-02; 1986. (16) Camusso, M.; Tartari, G., Zirino, A. Proceedings, Heavy Metals in the Environment Conference, Geneva, Switzerland, September 1989. (17) Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley & Sons: New York, 1981. (18) Sigg, L. In Chemical Processes i n Lakes; Stumm, W., Ed.; Wiley & Sons: New York, 1985. (19) Camusso, M.; Tartari, G.; Cappelletti, E. Sci. Total Environ. 1989, 87/88, 59-75. (20) Operations Manual for Ocean Seven Model 401, Idronaut Srl., Brugherio (MI), Italy. (21) Paulson, A. J.; Kester, D. R. J. Solution Chem. 1980, 9, 269-277. (22) Vuceta, J.; Morgan, J. J. Limnol. Oceanogr. 1977, 22, 742-745. (23) Emerson, K.; Russo, R.; Lund, R. E.; Thurston, R. V. J . Fish. Res. Board Can. 1975, 32, 2379-2383. Received for review M a y 14, 1990. Accepted October 23, 1990. Part of this work was funded by the Naval Ocean System Center under a Developmental Training Program for A.Z. at the Scripps Institution of Oceanography (University of California,Sun Diego) under the supervision of Prof. P. Niiler.
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