and the magnitude of P-uptake drastically decreases while the lag period sharply increases. Based upon the visual observations of the algal cells at various degrees of P-starvation, the appropriate color notations are made on Figure 12. This can serve as a qualitative means of visually determining the degree of P-starvation and, therefore, assist in evaluating the potential for P-uptake and growth. Discussion The results presented in this paper would suggest that the chemical composition of a grab sample of the standing crop from a lake or river might easily range anywhere from less than 0.5 to more than 20% PO, by weight--a 40-fold variation ! Limiting the possibilities to the most common natural situations, this span might be narrowed to about 1 to 15%, depending upon the environment and the previous physiological condition of the algae. If we assume the existence of an excess of phosphorus, composition of algae should be around 10% PO, (or 3 % P) by weight under circumstances corresponding to those in this study. As the supply of phosphorus in the environment decreases, the algae will contain less phosphorus without showing any effect on its growth up to the point called the critical concentration, CCP (about 3 POc by weight in this research). Hence, ‘normal’ algae under similar conditions may generally contain 3 to 10% PO4 by weight.
Literature Cited APHA, AWWA, WPCF, “Standard Methods for the Examination of Water and Wastewater,” 12th ed., Water Pollution Control Federation, Washington, D.C., 1965. Azad, H. S., Doctoral Dissertation, University of Michigan, Ann Arbor, Michigan, 1968. Azad, H. S., Borchardt, J. A., Proc. 23rd Ind. Waste Conf., Purdue Univ., Ext. Ser. 132, pp. 325-42, 1969a. Azad, H. S., Borchardt, J. A., J . Water Pollut. Contr. Fed. 41 (ll), R392-404 (1969b). Borchardt, J. A., Azad, H. S., J . Water PolIut. Contr. Fed. 40 (lo), 1739-54 (1968). Gales, M. E., Jr., Julian, E. C., Kroner, R. C., J . Amer. Water W O ~ASS. ~ S58 (lo), 1363-68 (1966). Gates. W. E.. Borchardt. J. A,. J . Water Pollut. Contr. Fed. 36 (4), 443-62 (1964). ‘ Giese, A. C., Ed., “Photophysiology,” Vol. 1, Chap. 10, Academic Press. New York. 1966. Kamen, M. D., “Primary Processes in Photosynthesis,” “Advan. Biochem. Series,” Academic Press, New York, 1963, p. 30. Kandler, O., Annu. Rer. PIcrntP/zysiol. 11, 34 (1960). Krauss, R . W., “Algal Culture from Laboratory to Pilot Plant,” J. S. Burlew, Ed., Publ. 600, Carnegie Institution of Washington, Washington, D.C., 1963. Mahler, H. R., Cordes, E. H., “Biological Chemistry,” Chap. 18, Harper and Row, New York, 1966. Marre, E., “Physiology and Biochemistry of Algae,” R. A. Lewin, Ed., Academic Press, New York, 1962, pp. 541-50. Yoshida, A., J . Biochei~i.(Tokyo) 42, 165 (1055). Rewired f o r wrie\t, December 6 , 1968. Accejifed April 2, 1970
Water-Clay Interactions in North Carolina’s Pamlico Estuary David A. Dobbins,’ Paul C. Ragland, and J. Donald Johnson Dept. of Geology and Dept. of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill 27514
The pore to bottom water ratio of cationic equivalents measured at 20 stations increases systematically downstream from about 0.6 to 1.2 along a 50-mile profile in North Carolina’s Pamlico River estuary. This trend is apparently controlled by entrapment of relatively saline water after northeasterly storms as the suspended load settles. Cationic concentrations generally decrease in the pore waters with depth, from a maximum 10-20 cm. beneath the sediment-water interface. Because top portions of the cores are from a zone of active sediment-water mixing and cores were taken at a time of near maximum salinity in bottom waters, pore water from deeper portions of the cores may reflect long-term mean salinities. An increase of K equivalent ionic fraction in the pore waters with depth may be related to H- ion exchange with the sediments. A strong correlation between exchangeable Mg and chlorite, as well as exchangeable K and illite, was noted.
1 Present address: Dept. of Geology, Northwestern State College of Louisiana, Natchitoches, La. 71457
T
his paper presents information on the chemical relationships between the bottom water, pore water, and adsorbed cations on clay from sediments in a natural estuarine environment. The Pamlico estuary a*ffords a n opportunity to study interactions between these phases over a wide salinity range. Freshwater enters the estuary through the Tar River with headwaters in the Piedmont near Durham, N. C., and flows eastward across the Coastal Plain through the cities of Tarboro and Greenville before becoming the Pamlico River a t Washington, N. C. (Figure 1). Marine water flows into the sound through Hatteras Inlet and Ocracoke Inlet in the Outer Banks of North Carolina. The phosphate mine of the Texas Gulf Sulphur Corp., located near Aurora on the Pamlico River, was under construction during the period of collection of the samples used for this study; consequently, the samples were obtained before possible contamination of the estuary by phosphate rock tailings, although municipal waste i s disposed of in the Tar River. Contamination from municipal waste, however, apparently does not invalidate the results of this study for overall trends in the data can be explained by systematic changes in the natural system unrelated to contamination. Volume 4, Number 9, September 1970 743
0
5
29
I?
30
MILES
Figure 1. Map of the Pamlico River and Sound showing sampling Stations 1-20
Procedures
Bottom water samples and 25-35-cm.-long core samples of the bottom sediments were collected at 20 stations at intervals of about 2.5 miles beginning 2 miles southeast of Washington and continuing in the same direction down the middle of the river and on out into Pamlico Sound (Figure 1). They were collected during November 14-16, 1965. By staying in the middle of the river, we hoped that principally clay-sized sediments could be obtained by the Phleger corer. Sediment cores 50 mm. in diameter were extruded from the plastic core liners in the boat, sectioned, and placed in Desicoted glass bottles. Toluene was added to prevent bacterial metabolism; little or no toluene was observed to penetrate the cores. Immediately after returning to the laboratory the water was passed through a ~[(Si,Al>,Ol~l(OH), .nHsO * From Deer, er a / . (1966). Although all these formulas should be taken as approximate, this particularly holds for montmorillonite. e Summarized from Rich (1968).
Clay Mineralogy
Clay mineral and quartz variations within the Pamlico Sound and estuary are shown in Figure 4, along with the Caexchange capacity for the total sediment. Chemical compositions of relevant clay minerals are given in Table 111. The method of moving averages used for calculating the data points in Figures 4 (exclusive of quartz percentage and Caexchange capacity) and 5 is justified because of the semiquantitative nature of the clay mineral determinations. The trends are in general agreement with those reported by Allen (1964) in the Tar-Pamlico system, by Griffin and Ingram (1955) in the Neuse River estuary of North Carolina, and by Nelson (1963) in the Rappahannock estuary of Vir746 Environmental Science & Technology
Table IV. Characteristic Settling Rates of Clay Minerals (meters/day)'
Salinity ( %) 0.9 1.8 3.6 10.9 21.7 32.5
Kaolinite 11.5 11.7 11.8 11.8 11.8 11.8
Illite 12.9 14.3 15.1 15.8 15.8 15.8
Montmorillonite 0.03 0.05 0.11 0.58 1.1 1.3
Chlorite 12 7 12 8 12.8 12.8 12.8 12.8
Data from Whitehouse. et al. (1958); Fig. 12, p. 33, and Table 13, p. 34; pH ranged from 7.5 to 8.5. temperature at 26' C.
‘a 40
rK/Mg+Na +K
\ a
-
t 2o
aMg/Mg+Na+K
I I
EXCH CAP
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i”.. 1
to
INW I
O
O
4
the sound
5
--+
4
I5
IO
L
L
---d--
IO
20
RIVER K
L-
30
-4-
-
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SE
6
5
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__
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LES
a
‘,a
.
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T-i
Figure 5. Theoretical summation of exchangeable cations and measured summation of exchangeable cations
’\-
Figure 6. K + and Mga+ equivalent ionic fractions in the 1 N NH,Ac leachate
The numbers plotted a r e moving averages calculated in each case from three adjacent values
Water-Chy lnteracrions
At the outset of this discussion it should be pointed out that although Ca concentrations were determined in all phases, they are omitted from consideration in this study. There are small but varying amounts of skeletal C a C 0 3 detritus in all these sediments that would affect the exchangeable Ca concentrations and possible pore and bottom water concentrations as well. The effect of dissolution of CaC03 on the Ca concentrations in the aqueous phases is somewhat conjectural, as it is dependent upon varying conditions of pH, but the solubility of C a C 0 3 in neutral 1 N NHIAc is ample reason for excluding Ca from the exchangeable data. Inasmuch as one of the purposes of this study is to compare equivalent ionic fractions of the various phases, it was decided to exclude Ca from all the data. Relationships between exchangeable cations and clay mineralogy are summarized in Figures 5 and 6 and Table V. Figure 5 is a plot of theoretical summation of exchangeable cations, as estimated from the clay mineral percentages and average exchange capacities for different clay minerals (Weed and Leonard, 1963) and the measured summation of major cations excluding Ca. It is not surprising that the theoretical exchange capacity curve does not coincide with the measured exchange capacity curve. The theoretical curve is based upon semiquantitative clay mineral determinations and exchange capacities for the vilrious clay minerals estimated from the literature (Weed and Leonard, 1963). Indeed, the fact that both curves reach a maximum near Station 13 is encouraging. The equivalent ionic fraction of K increases systematically downstream while Mg decreases, although the Mg trend is more diffuse (Figure 6). Inasmuch as the summation of the cations is recalculated to 1.000, the law of constant sums holds and the inverse relationship between Mg and K is not surprising; however, the fact that these antithitetic trends correlate with position in the estuary is significant. The principal hlg-bearing clay mineral in the estuary and sound is chlorite, while the main K-bearing mineral is illite (Table 111). As shown in Figure 4, illite content increases downstream while chlorite decreases. The relative change in K equivalent ionic fraction downstream is approximately equal to the change in the percentage of illite, considering the limits of error in the semiquantitative clay mineral analyses; likewise the relative change in the Mg equivalent ionic fraction is
Table V. Mean Equivalent Ionic Fractions for All Twenty Stations4 Sol ut ion
Bottom H 2 0 Pore H 2 0 2-6 cm. 8-14 cm. 17-22 cm. 29-34 cm. Sea H,O Exchangeables
Na 0.777 i 0.004
K Mg 0.022 i 0,001 0.200 =k 0.004
0.779 =t 0.010 0.791 i 0.011 0.791 f 0.008 0.787 I 0 . 0 1 4 0.801
0.027 f 0.002 0.028 f 0.003 0.027 =t 0.003 0.032 =t0.004 0.017
0.191-
0.288 0.096-
0.194 k 0.011 0.180 f 0.010 0.181 9: 0.006 0.181 3: 0.013 0.182
0.177 0.698-
0.557
Plus or minus Yalues are expressed as one standard deviation. T h e equivalent ionic fraction for Na, for example, as calculatcd in this paper K Mg, all concentrations being i n equivalents. D a t a is NaiNa on exchangeables are from the 2-6-cm. cuts and arc given as the range, rather than meail and standard deviation, since they varv systematically with position in thc estuary (Figure 6 ) . The pore water :quivalent ionic fractions, however, kary randomly with position in the estuary.
+
+
similar to the change in chlorite content. It would appear, then, that the correlation between exchangeable Mg and chlorite. as well as K and illite, must involve removal of these cations from exchange sites or possibly actual dissolution of these minerals during N H A c extraction. Probably no sharp line demarcates “exchangeable” and “nonexchangeable” ions in the clays of an estuary. A more appropriate term might be “readily leachable” ions, those that are available for any given extraction technique. Although considerably more erratic, the N a equivalent ionic fraction reaches a maximum toward the center of the profile, corresponding to the montmorillonite maximum, the principal Na-bearing clay (Table 111).
The effect of amorphous or organic material in the sediments is impossible to ascertain here and may be quite significant with regard to the exchangeable fraction. McCrone (1967), for example, has shown that 75$ of the exchangeable capacity of the sediments in the Hudson estuary can be accounted for by the organic fraction. However, the correlation between clay mineral distributions and “exchangeable” ions in the Pamlico estuary indicates that clay mineralogy exerts the primary influence upon the chemistry of the NHIAc leachate. The fact that K may have been preferentially leached relative to the other cations during the sediment’s residence time in the estuary is suggested from the data on average equivalent Volume 4, Number 9 , September 1970 747
ionic fractions shown in Table V. The K equivalent ionic fraction in the pore water is distinctly higher than that in the bottom water and still higher than the value for normal marine water. This fact seems to be evidence against Allen’s (1964) suggestion that montmorillonite may be collapsing in situ and extracting K from the pore water to make illite. Weaver (1967) has suggested that mildly weathered stripped illite (e.g., degraded illite) may satisfy its demand for K in the rivers on the way to the ocean. The higher K equivalent ionic fractions in the pore waters indicate that this process is not operative in interstitial waters of the Pamlico system. Siever, Beck, et al. (1965) found lower N a : K ratios in interstitial waters from recent deep sea sediments than normal marine ratios, which is in general agreement with this study. They attributed these lower ratios to the fact that Na does not become incorporated in silicate minerals on the ocean floor to any significant extent, but K is liberated by hydrolysis of detrital K-feldspar (KAISi308)in situ. They felt that clay minerals would tend to adsorb K rather than desorb it but that the ubiquitous K-feldspar would be out of its stability field and tend to dissolve. Minute quantities of K-feldspar were occasionally noted in the coarser fractions of the Pamlico sediments, and this explanation is a possibility. Powers (1954) also reported an increase in K relative to the other ions with depth in the sediments of the James River estuary, Va. He attributed this to the desorption of K from the clays rather than dissolution of K-feldspar. Like Siever, Beck, et al. (19651, however, we feel that clays would tend to adsorb K rather than desorb it. He also found a decrease in Mg with depth; a hint that this may occur in pore waters from the Pamlico system is offered in the data of Table V. He found no systematic trends in ionic ratios as a function of position in the estuary, nor were any observed in this study. Deviations around the means for the equivalent ionic fraction data of the aqueous phases are quite small (Table V). Perhaps another explanation for the increase in the K equivalent ionic fraction with depth may be found in the work of ZoBeU (1946). He found minimum pH values 15-20 cm. beneath the sediment-water interface. This has been later substantiated in studies that have found lower average values for pH in the muds close to the interface than in the overlying water (for example, McCrone, 1967, Table 2). Tentative results are similar for the Pamlico system. The kinetics of exchange reactions involving clays and ionic species in water may be sufficiently rapid so that equilibrium exists between a clay particle in the suspended load of the river and cations in the water. When the clay particle settles to the bottom it will eventually encounter a lower pH environment. The mass action effect then will be such that there could be an exchange reaction set up between K+ ions in the interlayer positions and H+ ions in the interstitial waters until equilibrium is reached. The result wouid be an increase in K in the interstitial waters compared to ions less available for exchange. The Na+ ions would undoubtedly be involved in this exchange as well, but because of the high equivalent ionic fraction of Na, the effect is less obvious. The decrease in the equivalent ionic fraction of Mg with depth in the cores might simply be explained by the fact that the Mg2+ ion is less available for exchange. Presumably the pH will increase in the sediments at some depth lower than the cores taken for this study (ZoBell, 1946) and the compositional trends in the interstitial water seen in Table V will reverse. It should be pointed out that the above implications of cause and effect are quite speculative and further research is required in these areas. As stated earlier, in the text, there is additional evidence to 748 Environmental Science & Technology
suggest that the uppermost portions of the cores are in an active zone of sediment-water mixing. This is shown by the data in Table V, which indicate that Mg equivalent ionic fraction in the 2-6-cm. cuts of the cores is much more similar to bottom water values than those from deeper cuts. It is interesting to note that the Mg equivalent ionic fraction in the pore water from the deeper cuts is closer to seawater values than are those from the bottom waters. This may demonstrate the importance of the role clay-water cation exchange reactions play in affecting the ultimate composition of seawater. Acknowledgment The authors wish to acknowledge the helpful suggestions and discussions of Gale K. Billings and Robert C. Harriss during the latter phases of the work. The constructive criticisms of Bruce W. Nelson, Roy L. Ingram, Ray E. Ferrell, Jr., and Daniel A. Textoris are appreciated. Lit era t ure Cired Allen, D. W., M.S. Thesis, University of North Carolina, Chapel Hill, 1964. Biscaye, P. E., Amer. Mineral. 49, 1281-1289 (1964). Deer, W. A., Howie, R. A., Zussman, J., “An Introduction to the Rock-Forming Minerals,” Longsmans, Green, and Co., London, 1966, pp. 231-260. Duane, D. B., Ph.D. Dissertation, University of Kansas, Lawrence, 1962. Emery, K. O., Stevenson, R. E., Geol. SOC.Amer. Mem. 67, Vol. I, 729-750 (1957). Freas, D. H., BuN.‘Geoi SOC.Amer. 78, 1341-1363 (1962). Gibbs, R. J., Amer. Mineral. 50, 741-751 (1965). Griffin. G. M.. Inmam. R. L.. J. Sediment. Petrolonv 25. 194200 (1955). Hill, L. O., M.S. Thesis, University of North Carolina, ChaDel Hill. 1966. Jacks&, M. ’L,, “Soil Chemical Analysis,” Prentice-Hall, Englewood Cliffs, N. J., 1958, pp. 62-86. Kolthoff, I. M., Stenger, V. A., “Volumetric Analysis,” 2nd ed., pp. 334 and 335, Interscience, New York, N.Y., 1947. McCrone, A. W., J . Sediment. Petrol. 37, 475-486 (1967). Nelson, B. W., C[ays Clay Miner. 11, 210 (1963). Powers, M. C., Ph.D. Dissertation, University of North Carolina, Chapel Hill, 1954. Rich, C. I., Clays Clay Miner. 16, 15-30 (1968). SheDard. F. P., Moore, D. G.. Bull. Amer. Ass. Petrol. Geol. 39, 1463-1593 (1955). Siever, R., Beck, K. C., Berner, R. A., J . Geol. 73, 39-73 (1965). Taylor, H. F., “Survey of Marine Fisheries of North Carolina.” Universitv of North Carolina Press, ChaDel Hill. 1951,’pp. 36-50.Weaver, C. E., Geochim. Cosmochim. Acta 31, 2181-2196 (1967). Weed. S. B.. Leonard. R. A.. Soil Sci. SOC.Amer. Proc. 27, 474’and 475 (1963). Whitehouse, U. G., Jeffrey, L. M., Debbrecht, J. D., Clays Clav Miner. 5. 1-80 (1958). ZoBeil,C. E., Bull. Amer. Ass. Petrol. Geol. 30,477-513 (1946). 1 .
,
’
Receiced for review July 25, 1967. Resubmitted May 20, 1969. Accepted April 22, 1970. Presented in part at the Division of Water, Air, and Waste Chemistry, 152nd Meeting, ACS, New York, N . Y., September 1966. Also presented at a symposium on the geochemistry of sediments held at the annual meeting of the Geological Society of America, Mexico City, November 1968. This study was supported in part by the North Carolina Diu. of Mineral Resources, Dept. of Conservation and Development, and the U.S. P.H.S. The atomic absorption unit was purchased through a grant by the North Carolina Board of Science and Technology. The data in this paper are from a dissertation submitted by David A . Dobbins in partial fulfillment of the requirements for the Ph. D. degree, University of North Carolina at Chapel Hill. ESE Pub. NO. 214.
