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Flocculation of dissolved Pb, Cu, Zn and Mn during estuarine mixing of river water with the Caspian Sea
Environmental
PII:
SO269-7491(96)00047-4
Pollution, Vol. 93, No. 3, pp. 257-260, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0269-7491/96 $15.00+0.00
ELSEVIER
FLOCCULATION DURING ESTUARINE
OF DISSOLVED Pb, Cu, Zn AND Mn MIXING OF RIVER WATER WITH THE CASPIAN SEA
A. R. Karbassi”* & Sh. Nadjafpo& “Faculty of Environment, Tehran University, PO Box 14155-6135, Tehran, Iran bFisheries Research Center, PO Box 961, Sari, Iran
(Received 10 May 1995; accepted 4 April 1996)
25, 780 and 1035 m in the northern, central and southern parts of the lake, respectively. Average water temperatures at the surface and in the deepest parts of the Caspian Sea are about 13 ’ and 5.5 “C, respectively. The water volume of the Caspian Sea is about 78 100 million m3. The Volga and Ural Rivers contribute 59% and 11% of the water inflow into the Caspian Sea, respectively. The Sulak, Samur, Kura and Aras Rivers, which flow into the Caspian Sea via its western coast, account for 7%, and rivers on the southern coast (e.g. the Sefidrud and the Haraz) and on the eastern coast (e.g. the Atrak and the Amba) account for 23% of the water inflow. The salinity of the water ranges from 4%0 in the northern part of the sea to 13%0in the southern part. Many Iranian rivers flowing into the Caspian Sea are used as transport agents for the disposal of industrial, agricultural and urban waste. This also holds for rivers flowing into the Caspian Sea via its northern, western and eastern coasts. Therefore, it is essential to closely investigate the overall geochemical cycle of trace metals in this region. In the present investigation, we examine a part of the geochemical cycle through flocculation studies. While this method is useful in quantifying the removal of dissolved substances by this one mechanism, it does not estimate the overall behavior of any substance for which other mechanisms, such as sedimentwater interactions, are important (Day et al., 1989).
Abstract This is the first
study of the jocculation of dissolved Pb, Cu, Zn and Mn during mixing of river water with the largest lake in the world (the Caspian Sea). Flocculation of dissolved metals was investigated on a series of mixtures with salinities ranging from 3.2 to 7.4%0. TheJEocculation rates (Cu (74%) > Pb (61%) > Mn (58%) > Zn (34%)) are indicative of the non-conservative behavior of metals during estuarine mixing. Statistical analysis indicates that the flocculation of Pb and Mn is governed by salinity. The flocculation rates reveal that the overall metal pollution loads may decline by about 57% during estuarine mixing. Copyright 0 1996 Elsevier Science Ltd Keywords: flocculation, chemical mass balance, estuarine mixing, dissolved metals.
INTRODUCTION The flocculation of dissolved elements during estuarine mixing significantly influences the chemical mass balance between rivers and the sea. Many investigations have been carried out on estuarine mixing of dissolved substances in relation to colloidal stability, surface properties, humic acids, salinity and pH (Sholkovitz, 1978; Sholkovitz et al., 1978; Eckert & Sholkovitz, 1976; Forstner & Wittmann, 198 1, 1984; Fox & Wofsy, 1983; Zhiqing et al., 1987; Hunter, 1983). However, not much information is available on the flocculation in large lakes. In an attempt to study floes of dissolved elements, we have carried out a laboratory experiment on water samples originating from the Shirud River and the Caspian Sea. The Caspian Sea is rather unique, in that the salinity of the water is much higher than that of freshwater lakes and lower than that of sea water. The Caspian Sea lies below sea level in the Caucasus mountains of southwestern Russia and northern Iran. It covers an area of 371000 km2, is 1030 km long and ranges from 196 to 435 km in width. Its depth varies between about
METHODOLOGY River water (salinity=0.22’&) and lake water (salinity = 12.5%0) samples were collected from the Shirud River and the Caspian Sea on 15 and 16 January 1994, respectively (the sampling locations are presented in Fig. 1). The samples were filtered through 0.22 pm Millipore AP and HA filters on the same day. Mixing experiments were conducted by adding appropriate volumes of lake water to a constant volume of river water (500 ml) to obtain a series of mixtures of different salinities (Shankar & Karbassi, 1992; Karbassi, 1989). These were held for 24 h with occasional stirring. The resulting flocculates were collected on 0.22 pm
*To whom correspondence should be addressed. 257
A. R. Karbassi, Sh. Nadjafpour
258
concentrations of river water, are given in Table 1. Maximal flocculation of Cu and Zn takes place at salinities of 5.8 and 7.4%0, while flocculation of Mn and Pb is maximal at 6.1 and 7.4%0. Zinc, Pb, Cu and Mn are non-conservative trace elements. However, Cu and Zn are known for their nutrient-like behavior and Mn and Pb belong to a group of elements that are easily scavenged on particles. This statement may justify their different behavior. It should be pointed out that during natural estuarine mixing, flocculation processes may not occur as shown in Table 1. In fact, at the very first stages of mixing of river water with lake water, some of the dissolved metals may ooze out of the fresh water in the form of flocculates. Thus, at the later stage of mixing (i.e. at higher salinities) fresh water is impoverished in base metals and fewer flocculates form (Table 2). The values presented in Table 2 are actually derived from Table 1 by subtracting the concentrations of flocculates at each salinity from the prior step. In this way, the floccular quantity is not calibrated to the very first concentrations of the metals
Millipore membrane filters and the concentrations of Pb, Cu, Zn and Mn were determined by thin film X-ray fluorescence (Shankar & Karbassi, 1992). Procedural blanks and duplicates were run with samples in a similar way. The accuracy of analysis was *4% for all elements except for Pb, where the accuracy was better than f 7%. Of the existing clustering techniques (Lance & Williams, 1966; Anderson, 1971; Davis, 1973), the weighted-pair group method (Davis, 1973) was used in this study because of its merits. It uses the linear correlation coefficient as a similarity measure. The highest similarities are clustered/linked first, and variables connected only if they are highly correlated. After two variables are clustered, their correlations with all the other variables are averaged. The results of clustering are displayed in the form of a dendrogram (Fig. 2). RESULTS
AND DISCUSSION
The base metal (Cu, Zn, Mn and Pb) concentrations of flocculates at various salinities, as well as the metal
I
I
I VolgaR.
TTrnl R
,
/
50
v
samnle River water ____ --___r__
---.--
Sea water sample
Sefidrud
R.
4
Haraz R
Fig. 1. Map showing location of sampling sites.
Flocculation
Pb Mn 1-Q -
!
.
z
C
Sample
II.8 _
River water I 2 3 4 5 6
0.6 . COEFFICIENT
Table 1. Metal concentrations in river water and in flocctdants at various salinities
0.9 _
D-7 . SIMILARITY
0.5 .
Y
A
CU (tig I-‘)
82
27 8.3 13.3 19.2 14.2 2.5 20.0
Zn (/lg 1-l)
(30)* (49) (71) (52) (09) (74)
11.7 (14) 9.2 (11) 27.5 (33) 13.3 (16) 12.5 (15) 20.0 (24)
(p$) 16 4.2 6.7 4.2 7.5 3.3 9.2
(26) (42) (26) (47) (20) (57)
(,$)
0.22
0.8 (03) 4.2 (16) 0.0 (00) 15.5 (60) 0.0 (00) 10.0 (38)
3.20 4.20 5.80 6.10 6.70 7.40
*Percentile of removal is given within parentheses.
cl.3 .
Table 2. Actual metal concentrations in Sbirud water flocculants
Sample
0.1 Dendrogram of cluster analysis showing similarity coefficients among metals and salinity.
in the river water. The maximum removal of Cu and Zn occurs between salinities of 3.2 and 5.8%. A major part of the Pb flocculates at a salinity of 6.1%. In the case of Mn, the maximum removal is at a salinity of 3.2%. It is worth noting that flocculates of Cu, Zn, Mn and Pb do not form at a salinity of 6.7% (Table 2). Based on this assumption, the flocculation rates of the studied metals are in the following order: {Cu (74%) > Pb (61 O/o> > Mn (58%) > Zn (34%)}. The data suggest that flocculation occurs rather late in the mixing process (at about 3.25.8% for Cu, Zn and Mn and at about 3.2-6.lo/oo for Pb). Many researchers have reported rapid flocculation in the earlier stages (at about 2%0) of mixing (Duinker & Nolting, 1976; Burton, 1976; Bewers et al., 1974). The variation in the maximal removal of the studied metals may be due to destabilization of dissolved metals, corresponding to the different stages of mixing with sea water, and a decrease in their negative net charge. We investigated the relationship between metals and salinity through cluster analysis (Fig. 2). Lead and Mn, which form cluster A with salinity, join each other at a very high similarity coefficient (0.8273). It seems that the flocculation of the dissolved Pb and Mn is governed by salinity. Copper and Zn, which form cluster B, also join each other at high similarity coefficient (0.6657). The fact that cluster B joins cluster A at a very low similarity coefficient (0.2964) may indicate that the flocculation of Cu and Zn is not controlled by salinity. Considering the concentrations of dissolved Cu, Zn, Mn and Pb (27, 82, 16 and 26 ppb, respectively) in the Shirud River, the mean annual discharge rates of the river load of dissolved Cu, Zn, Mn and Pb entering the Caspian Sea via this river are 3660, 11 120, 2170 and 3525 kg year-‘, respectively. If 74, 34, 58 and 61% of the Cu, Zn, Mn and Pb flocculate during estuarine mixing, only 950, 7340, 910 and 1374 kg year-’ of Cu, Zn, Mn and Pb enter the Caspian Sea by the Shirud River in
1 2 3 4 5 6
CU (CLg1-I) 8.3 5.0 5.9 0.0 0.0 0.8
Total removal
Zn (pgJ_‘)
(30)* 11.7 (14) 0.0 (00) (19) (22) 15.8 (19) 0.0 (00) (00) 0.0 (00) (00) 0.0 (00) (03)
20.0 (74)
27.5 (34)
&
26
0.4 .
0.2 -
Fig. 2.
259
of dissolved metals
River water-sea
Mn (pug]-‘)
Pb (I.LgJ-‘)
(&
4.2 (26) 2.5 (16) 0.0 (00) 0.8 (05) 0.0 (00) 1.7 (IO)
0.8 (03) 3.4 (13) 0.0 (00) 11.6 (44) 0.0 (00) 0.0 (00)
3.20 4.20 5.80 6.10 6.70 7.40
9.2 (58)
15.8 (61)
*Percentile of natural removal is given within parentheses.
dissolved form. Hence, the investigated process may reduce overall metal pollution loads by 57%. These assumptions hold if the flocculation rates of the studied metals remain constant throughout the year. REFERENCES Anderson, A. J. B. (1971). Numerical examination of multivariate of soil samples. Math. Geol., 3, I-14. Bewers, J. M., MacAulay, I. D. & Sundby, B. (1974). Trace metals in the waters of the Gulf of St Lawrence. Ccm. J. Earrh sci., 11, 939-950. Burton, J. D. (1976). Basic Properties and Processes in Estuarine Chemistry. Academic Press, London. Davis, J. C. (1973). Statistics and Data Analysis in Geology. Wiley International, pp. 456473. Day, J. W., Hall, C. A. S., Kemp, W. M. & Yanez-Arancibia, A. (1989). Estuarine Ecology. John Wiley, N.Y., USA. Duinker, J. C. & Nolting, R. F. (1976). Distribution model for particulate trace metals in the Rhine estuary, Southern Bight and Dutch Wadden Sea. Netherlands J. Sea Res., 10, 71-102.
Eckert, J. M. & Sholkovitz, E. R. (1976). The flocculation of Fe, Al and humates from river water by electrolytes. Geochim. Cosmochim. Acta, 40, 847-848.
G. T. W. (1981). Metal Pollution in the Aquatic Environment. Springer, Berlin, Germany. Forstner, U. & Wittmann, G. T. W. (1984). Metals in Hydrocycle. Springer, Berlin, Germany. Forstner, U. & Wittmann,
FOX, L. E. & Wofsy, S. C. (1983). Kinetics of removal of iron colloids from estuaries. Geochim. Cosmochim. Acta, 47, 21 I216.
Hunter, K. A. (1983). On the estuarine mixing of dissolved substances in relation to colloidal stability and surface properties. Geochim. Cosmochim. Acta, 47,467473.
A. R. Karbassi, Sh. Nadjafpour Karbassi, A. R. (1989). Some geochemical and magnetic studies of marine, estuarine and riverine sediments near Mulki (Karnataka). Ph.D. Thesis, Mangalore University, India. Lance, G. N. & Williams, W. T. (1966). A generalized sorting for computer classifications. Nature, 212, 218. Shankar, R. & Karbassi, A. R. (1992). Flocculation of Cu, Zn, Ni and Fe during mixing of Mulki river water and Arabian Sea water, west coast of India. The 7th Int. Symp. WaterRock Interaction, WRI-7/UTAH/USA, pp. 565-568.
Sholkovitz, E. R. (1978). The flocculation of dissolved Fe, Mn, Al, Cu, Ni, Co and Cd during estuarine mixing. Earth Planet. Sci. Lett., 41, 77-86.
Sholkovitz, E. R., Boyle, E. A. & Price, N. B. (1978). The removal of dissolved humic acids and iron during estuarine mixing. Earth Plant. Sci. Lett., 40, 13&136. Zhiqing, L., Jianhu, Z. & Jinsi, C. (1987). Flocculation of dissolved Fe, Al, Mn, Si, Cu, Pb and Zn during estuarine mixing. Acta Oceanologica Sinica, 6, 567-576.
PII:
SO269-7491(96)00047-4
Pollution, Vol. 93, No. 3, pp. 257-260, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0269-7491/96 $15.00+0.00
ELSEVIER
FLOCCULATION DURING ESTUARINE
OF DISSOLVED Pb, Cu, Zn AND Mn MIXING OF RIVER WATER WITH THE CASPIAN SEA
A. R. Karbassi”* & Sh. Nadjafpo& “Faculty of Environment, Tehran University, PO Box 14155-6135, Tehran, Iran bFisheries Research Center, PO Box 961, Sari, Iran
(Received 10 May 1995; accepted 4 April 1996)
25, 780 and 1035 m in the northern, central and southern parts of the lake, respectively. Average water temperatures at the surface and in the deepest parts of the Caspian Sea are about 13 ’ and 5.5 “C, respectively. The water volume of the Caspian Sea is about 78 100 million m3. The Volga and Ural Rivers contribute 59% and 11% of the water inflow into the Caspian Sea, respectively. The Sulak, Samur, Kura and Aras Rivers, which flow into the Caspian Sea via its western coast, account for 7%, and rivers on the southern coast (e.g. the Sefidrud and the Haraz) and on the eastern coast (e.g. the Atrak and the Amba) account for 23% of the water inflow. The salinity of the water ranges from 4%0 in the northern part of the sea to 13%0in the southern part. Many Iranian rivers flowing into the Caspian Sea are used as transport agents for the disposal of industrial, agricultural and urban waste. This also holds for rivers flowing into the Caspian Sea via its northern, western and eastern coasts. Therefore, it is essential to closely investigate the overall geochemical cycle of trace metals in this region. In the present investigation, we examine a part of the geochemical cycle through flocculation studies. While this method is useful in quantifying the removal of dissolved substances by this one mechanism, it does not estimate the overall behavior of any substance for which other mechanisms, such as sedimentwater interactions, are important (Day et al., 1989).
Abstract This is the first
study of the jocculation of dissolved Pb, Cu, Zn and Mn during mixing of river water with the largest lake in the world (the Caspian Sea). Flocculation of dissolved metals was investigated on a series of mixtures with salinities ranging from 3.2 to 7.4%0. TheJEocculation rates (Cu (74%) > Pb (61%) > Mn (58%) > Zn (34%)) are indicative of the non-conservative behavior of metals during estuarine mixing. Statistical analysis indicates that the flocculation of Pb and Mn is governed by salinity. The flocculation rates reveal that the overall metal pollution loads may decline by about 57% during estuarine mixing. Copyright 0 1996 Elsevier Science Ltd Keywords: flocculation, chemical mass balance, estuarine mixing, dissolved metals.
INTRODUCTION The flocculation of dissolved elements during estuarine mixing significantly influences the chemical mass balance between rivers and the sea. Many investigations have been carried out on estuarine mixing of dissolved substances in relation to colloidal stability, surface properties, humic acids, salinity and pH (Sholkovitz, 1978; Sholkovitz et al., 1978; Eckert & Sholkovitz, 1976; Forstner & Wittmann, 198 1, 1984; Fox & Wofsy, 1983; Zhiqing et al., 1987; Hunter, 1983). However, not much information is available on the flocculation in large lakes. In an attempt to study floes of dissolved elements, we have carried out a laboratory experiment on water samples originating from the Shirud River and the Caspian Sea. The Caspian Sea is rather unique, in that the salinity of the water is much higher than that of freshwater lakes and lower than that of sea water. The Caspian Sea lies below sea level in the Caucasus mountains of southwestern Russia and northern Iran. It covers an area of 371000 km2, is 1030 km long and ranges from 196 to 435 km in width. Its depth varies between about
METHODOLOGY River water (salinity=0.22’&) and lake water (salinity = 12.5%0) samples were collected from the Shirud River and the Caspian Sea on 15 and 16 January 1994, respectively (the sampling locations are presented in Fig. 1). The samples were filtered through 0.22 pm Millipore AP and HA filters on the same day. Mixing experiments were conducted by adding appropriate volumes of lake water to a constant volume of river water (500 ml) to obtain a series of mixtures of different salinities (Shankar & Karbassi, 1992; Karbassi, 1989). These were held for 24 h with occasional stirring. The resulting flocculates were collected on 0.22 pm
*To whom correspondence should be addressed. 257
A. R. Karbassi, Sh. Nadjafpour
258
concentrations of river water, are given in Table 1. Maximal flocculation of Cu and Zn takes place at salinities of 5.8 and 7.4%0, while flocculation of Mn and Pb is maximal at 6.1 and 7.4%0. Zinc, Pb, Cu and Mn are non-conservative trace elements. However, Cu and Zn are known for their nutrient-like behavior and Mn and Pb belong to a group of elements that are easily scavenged on particles. This statement may justify their different behavior. It should be pointed out that during natural estuarine mixing, flocculation processes may not occur as shown in Table 1. In fact, at the very first stages of mixing of river water with lake water, some of the dissolved metals may ooze out of the fresh water in the form of flocculates. Thus, at the later stage of mixing (i.e. at higher salinities) fresh water is impoverished in base metals and fewer flocculates form (Table 2). The values presented in Table 2 are actually derived from Table 1 by subtracting the concentrations of flocculates at each salinity from the prior step. In this way, the floccular quantity is not calibrated to the very first concentrations of the metals
Millipore membrane filters and the concentrations of Pb, Cu, Zn and Mn were determined by thin film X-ray fluorescence (Shankar & Karbassi, 1992). Procedural blanks and duplicates were run with samples in a similar way. The accuracy of analysis was *4% for all elements except for Pb, where the accuracy was better than f 7%. Of the existing clustering techniques (Lance & Williams, 1966; Anderson, 1971; Davis, 1973), the weighted-pair group method (Davis, 1973) was used in this study because of its merits. It uses the linear correlation coefficient as a similarity measure. The highest similarities are clustered/linked first, and variables connected only if they are highly correlated. After two variables are clustered, their correlations with all the other variables are averaged. The results of clustering are displayed in the form of a dendrogram (Fig. 2). RESULTS
AND DISCUSSION
The base metal (Cu, Zn, Mn and Pb) concentrations of flocculates at various salinities, as well as the metal
I
I
I VolgaR.
TTrnl R
,
/
50
v
samnle River water ____ --___r__
---.--
Sea water sample
Sefidrud
R.
4
Haraz R
Fig. 1. Map showing location of sampling sites.
Flocculation
Pb Mn 1-Q -
!
.
z
C
Sample
II.8 _
River water I 2 3 4 5 6
0.6 . COEFFICIENT
Table 1. Metal concentrations in river water and in flocctdants at various salinities
0.9 _
D-7 . SIMILARITY
0.5 .
Y
A
CU (tig I-‘)
82
27 8.3 13.3 19.2 14.2 2.5 20.0
Zn (/lg 1-l)
(30)* (49) (71) (52) (09) (74)
11.7 (14) 9.2 (11) 27.5 (33) 13.3 (16) 12.5 (15) 20.0 (24)
(p$) 16 4.2 6.7 4.2 7.5 3.3 9.2
(26) (42) (26) (47) (20) (57)
(,$)
0.22
0.8 (03) 4.2 (16) 0.0 (00) 15.5 (60) 0.0 (00) 10.0 (38)
3.20 4.20 5.80 6.10 6.70 7.40
*Percentile of removal is given within parentheses.
cl.3 .
Table 2. Actual metal concentrations in Sbirud water flocculants
Sample
0.1 Dendrogram of cluster analysis showing similarity coefficients among metals and salinity.
in the river water. The maximum removal of Cu and Zn occurs between salinities of 3.2 and 5.8%. A major part of the Pb flocculates at a salinity of 6.1%. In the case of Mn, the maximum removal is at a salinity of 3.2%. It is worth noting that flocculates of Cu, Zn, Mn and Pb do not form at a salinity of 6.7% (Table 2). Based on this assumption, the flocculation rates of the studied metals are in the following order: {Cu (74%) > Pb (61 O/o> > Mn (58%) > Zn (34%)}. The data suggest that flocculation occurs rather late in the mixing process (at about 3.25.8% for Cu, Zn and Mn and at about 3.2-6.lo/oo for Pb). Many researchers have reported rapid flocculation in the earlier stages (at about 2%0) of mixing (Duinker & Nolting, 1976; Burton, 1976; Bewers et al., 1974). The variation in the maximal removal of the studied metals may be due to destabilization of dissolved metals, corresponding to the different stages of mixing with sea water, and a decrease in their negative net charge. We investigated the relationship between metals and salinity through cluster analysis (Fig. 2). Lead and Mn, which form cluster A with salinity, join each other at a very high similarity coefficient (0.8273). It seems that the flocculation of the dissolved Pb and Mn is governed by salinity. Copper and Zn, which form cluster B, also join each other at high similarity coefficient (0.6657). The fact that cluster B joins cluster A at a very low similarity coefficient (0.2964) may indicate that the flocculation of Cu and Zn is not controlled by salinity. Considering the concentrations of dissolved Cu, Zn, Mn and Pb (27, 82, 16 and 26 ppb, respectively) in the Shirud River, the mean annual discharge rates of the river load of dissolved Cu, Zn, Mn and Pb entering the Caspian Sea via this river are 3660, 11 120, 2170 and 3525 kg year-‘, respectively. If 74, 34, 58 and 61% of the Cu, Zn, Mn and Pb flocculate during estuarine mixing, only 950, 7340, 910 and 1374 kg year-’ of Cu, Zn, Mn and Pb enter the Caspian Sea by the Shirud River in
1 2 3 4 5 6
CU (CLg1-I) 8.3 5.0 5.9 0.0 0.0 0.8
Total removal
Zn (pgJ_‘)
(30)* 11.7 (14) 0.0 (00) (19) (22) 15.8 (19) 0.0 (00) (00) 0.0 (00) (00) 0.0 (00) (03)
20.0 (74)
27.5 (34)
&
26
0.4 .
0.2 -
Fig. 2.
259
of dissolved metals
River water-sea
Mn (pug]-‘)
Pb (I.LgJ-‘)
(&
4.2 (26) 2.5 (16) 0.0 (00) 0.8 (05) 0.0 (00) 1.7 (IO)
0.8 (03) 3.4 (13) 0.0 (00) 11.6 (44) 0.0 (00) 0.0 (00)
3.20 4.20 5.80 6.10 6.70 7.40
9.2 (58)
15.8 (61)
*Percentile of natural removal is given within parentheses.
dissolved form. Hence, the investigated process may reduce overall metal pollution loads by 57%. These assumptions hold if the flocculation rates of the studied metals remain constant throughout the year. REFERENCES Anderson, A. J. B. (1971). Numerical examination of multivariate of soil samples. Math. Geol., 3, I-14. Bewers, J. M., MacAulay, I. D. & Sundby, B. (1974). Trace metals in the waters of the Gulf of St Lawrence. Ccm. J. Earrh sci., 11, 939-950. Burton, J. D. (1976). Basic Properties and Processes in Estuarine Chemistry. Academic Press, London. Davis, J. C. (1973). Statistics and Data Analysis in Geology. Wiley International, pp. 456473. Day, J. W., Hall, C. A. S., Kemp, W. M. & Yanez-Arancibia, A. (1989). Estuarine Ecology. John Wiley, N.Y., USA. Duinker, J. C. & Nolting, R. F. (1976). Distribution model for particulate trace metals in the Rhine estuary, Southern Bight and Dutch Wadden Sea. Netherlands J. Sea Res., 10, 71-102.
Eckert, J. M. & Sholkovitz, E. R. (1976). The flocculation of Fe, Al and humates from river water by electrolytes. Geochim. Cosmochim. Acta, 40, 847-848.
G. T. W. (1981). Metal Pollution in the Aquatic Environment. Springer, Berlin, Germany. Forstner, U. & Wittmann, G. T. W. (1984). Metals in Hydrocycle. Springer, Berlin, Germany. Forstner, U. & Wittmann,
FOX, L. E. & Wofsy, S. C. (1983). Kinetics of removal of iron colloids from estuaries. Geochim. Cosmochim. Acta, 47, 21 I216.
Hunter, K. A. (1983). On the estuarine mixing of dissolved substances in relation to colloidal stability and surface properties. Geochim. Cosmochim. Acta, 47,467473.
A. R. Karbassi, Sh. Nadjafpour Karbassi, A. R. (1989). Some geochemical and magnetic studies of marine, estuarine and riverine sediments near Mulki (Karnataka). Ph.D. Thesis, Mangalore University, India. Lance, G. N. & Williams, W. T. (1966). A generalized sorting for computer classifications. Nature, 212, 218. Shankar, R. & Karbassi, A. R. (1992). Flocculation of Cu, Zn, Ni and Fe during mixing of Mulki river water and Arabian Sea water, west coast of India. The 7th Int. Symp. WaterRock Interaction, WRI-7/UTAH/USA, pp. 565-568.
Sholkovitz, E. R. (1978). The flocculation of dissolved Fe, Mn, Al, Cu, Ni, Co and Cd during estuarine mixing. Earth Planet. Sci. Lett., 41, 77-86.
Sholkovitz, E. R., Boyle, E. A. & Price, N. B. (1978). The removal of dissolved humic acids and iron during estuarine mixing. Earth Plant. Sci. Lett., 40, 13&136. Zhiqing, L., Jianhu, Z. & Jinsi, C. (1987). Flocculation of dissolved Fe, Al, Mn, Si, Cu, Pb and Zn during estuarine mixing. Acta Oceanologica Sinica, 6, 567-576.