Phosphates in Sediments of Pamlico Estuary Joseph 8. Upchurch,l James K. Edzwald,2 and Charles R. O’Melia3 Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, N.C. 2751 4
The amount of “available” phosphorus extracted from sediment samples along the 35-mile length of the Pamlico Estuary, N.C., was observed to decrease from 1.6 mg P / g sediment in fresh water to 0.3 mg P/g sediment in water Nith a salinity of 18 ppt. A high correlation ( r = 0.99) between available phosphorus and oxalate-extractable iron was found in the upper reach of the estuary (salinity less than 1 ppt). In the lower part of the estuary the Fe-P correlation decreased ( r = 0.86). The decrease in the available P and in the Fe-P correlation along the length of the estuary are consistent with the suggestion that P is held to suspended sediments by some type of Fe-inorganic P complex of limited stability. Suspended materials entering the estuary in the fresh water inflow could lose phosphorus to solution as they are transported through waters of increasing salinity to the mouth of the estuary. During the past 20 years there have been increasing interest and research concerning the role of phosphorus in the eutrophication of natural aquatic systems. Much of this research has involved the exchange of phosphorus a t the sediment-water interface in fresh water lakes and the role of this exchange in regulating the concentration of phosphorus in lake waters. Several studies have investigated the amounts of phosphorus held by lake sediments and the mechanisms by which phosphorus is held therein. This study examines the amount of available phosphorus present in the bottom sediments of North Carolina’s Pamlico Estuary as one traverses the length of the estuary from a fresh water to an estuarine environment. The term “available phosphorus” refers to a fraction of the total phosphorus that is extracted using a procedure devised by Wentz and Lee (1969). This fraction provides an indication of the amount of phosphorus that is biologically usable and has been related to the ability of sediments to support algal growth (Porcella et al., 1970). Some of the chemical and physical factors that might influence the amount of available phosphorus held by the sediments are discussed, and the correlation between the available phosphorus and oxalateextractable iron is examined. The Pamlico Estuary (Figure 1) flows eastward some 35 miles from Washington, N.C., to Pamlico Point where the estuary enters the Pamlico Sound. The principal tributary to the estuary is the Tar River which flows about 200 miles southeast across the Piedmont and Coastal Plains of North Carolina. The suspended solids load in the Tar River is generally quite high, averaging 243 tons/day over a five-year period, and consists mainly of silt and clay. The I’amlico Estuary is a shallow, wind-mixed estuary with an average depth of about 10.5 f t and a maximum width of about eight miles. Most of the lunar tide effect is Camped out by the North Carolina Outer Banks. The salinity of the estuary varies from near zero a t the headwaters to about 15 ppt a t the mouth. Dissolved oxygen is usu-
ally present at all depths, but in the summer months can drop to less than 1 mg/l. in the bottom waters. These conditions are unstable, however, and are easily destroyed by wind, flow, or both. The Texas Gulf Co. began phosphate mining operations on the south side of the estuary near Aurora, N.C., in 1965 and uses an open dry pit to mine the mineral fluorapatite.
M e t h o d s and Material5 Bottom sediments were collected from the middle of the Pamlico River a t the 23 stations indicated in Figure 1. An Ekman dredge was used to collect all sediment samples. The sediments were frozen using Dry Ice during the return to the laboratory. In the laboratory, the sediments were thawed, mixed, dried in an oven a t 110°C for 24 hr, and ground using a mortar and pestle. A modification (Upchurch, 1972) of the HCl-H2S04 acid extraction procedure of Olsen and Dean (1965) was used to extract the available phosphorus from the sediments. After extraction, the phosphorus was measured using the vanadomolybdophosphoric acid colorimetric method (Wentz and Lee, 1969; American Public Health Association, 1971). Two methods were used to measure the iron in the sediments. First, the iron in the HCl-HzS04 extract was measured using atomic absorption. Iron was also extracted with an oxalate solution using a procedure similar to that described by Saunders (1965), and the iron again was measured by atomic absorption, The clay-sized fraction of selected sediments was examined by X-ray diffraction. The percentage of silt and clay was measured using a wet sieving analysis.
Results and Discussion The amount of available phosphorus in the sediments is plotted as a function of the distance downstream from station 1 in Figure 2. This figure includes only those samples in which the percentage of clay and silt exceeded 85%. Sediment samples containing sand and other coarsegrained materials had lower phosphorus contents. The differences in Figure 2 are therefore not due to gross changes in sediment composition. As indicated in Figure 2, the phosphorus content of the sediments decreased from about 1.60 to 0.30 mg P/g sediment along the length of the
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Present address: International Paper Co.. 1105:: Point, .\lids. SY56.3.
Present address: Deparrment of Civil Engineering. 1.niversiry Columbia. \lo. 65201, 1 Preaent address. Envirunmentel Engineering Science, California Insritute o f Technology. Pasadena. Calif. T o ivhom cvrrespondence hhould he addressed. of \Ii:wuri,
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Environmental Science & Technology
Figure 1. Map of the Pamlico Estuary Numbered points refer to sediment sample numbers and approximate locations.
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Figure 2. Amount of available phosphorus in sediments as function of distance downstream from station 1
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Figure 4. Concentrations of reactive phosphorus in surface waters of Pamlico Estuary as function of distance downstream from station 1 (after Hobbie, 1970b; Hobbie et al., 1972)
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Figure 3. Salinity of surface waters in Pamlico Estuary as function of distance downstream from station 1 (after Hobbie, 1970a; Hobbie et al., 1972, Water Resources Data for North
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estuary. The decrease in phosphorus content of the sediments is possibly due to the increasing salinity of the overlying water. Carritt and Goodgal (1954) measured the uptake of phosphorus in seawater with salinities of 17 and 34 ppt. The uptake of phosphorus was less in seawater than in freshwater, although the reduction was not in direct proportion to the salinity. In Figure 3 the average salinity of the surface water in the Pamlico River is plotted as a function of the distance downstream. The salinity values are based on bimonthly measurements made from January 1969 to December 1970 by Hobbie (Hobbie, 1970a; Hobbie et al., 1972) and from Water Resources Data for North Carolina (1965-69). The sharp decline in phosphorus content a t about mile seven coincides closely with the increase in salinity in this area. Possibly phosphorus that is adsorbed may be released or displaced by competing ions such as chloride or sulfate. From mile eight to mile 32 there is a general decrease in the phosphorus content of the sediments and a corresponding increase in the salinity of the overlying water. The concentration of reactive phosphorus in the surface waters of the Pamlico River was measured by Hobbie bimonthly during 1969 (Hobbie, 1970b; Hobbie et al., 1972) and a t Washington, N.C., by the N.C. Department of Water Resources (1965-69). The values a t each station were averaged and are plotted as a function of distance downstream in Figure 4. A plant manufacturing phosphoric acid introduces substantial amounts of phosphorus into the estuary a t mile point 21. This discharge is reflected in the surface waters (Figure 4) to a much greater extent than in the sediments (Figure 2). X-ray diffraction measurements indicated that the clay minerals in the sediments were primarily kaolinite, illite, chlorite, and
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Figure 5. Correlations between amounts of available phosphorus and oxalate-extractable iron in groups A and B sediments
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Figure 6. Correlations between amounts of available phosphorus and acid-extractable iron in groups A and B sediments
Volume 8,Number 1 , January 1974 57
14 A intergrade clay. Kaolinite decreased gradually from 60-30%, illite increased from 5-25%, and chlorite plus 14 A intergrade clay averaged about 40% along the length of the estuary. No relationship between the clay mineral composition and phosphorus content of the sediments could be established. The amounts of iron extracted by the acid and oxalate procedure were similar, with the oxalate values generally 1520% higher than the acid values. McKeague and Day (1966) have shown that oxalate extracts amorphous forms of iron almost completely but crystalline oxides only slightly. Therefore, it seems likely that both the oxalate and acid extract an amorphous form of iron and that the oxalate extracts the iron slightly better than the acid. Shulka et al. (1971) and Williams et al. (1971) studied relationships between inorganic phosphate and iron in sediments from 14 Wisconsin lakes. They found a strong correlation between the amounts of inorganic phosphate and oxalate extractable iron. The iron and phosphorus data from this study were examined to determine if a similar correlation might exist with estuarine sediments. With data for the available phosphorus and extractable iron, linear regression analyses were performed using all sediment samples. Available P exhibited correlation coefficients of 0.772 with acid-extractable iron and 0.862 with oxalate-extractable iron. Next, the sediment samples were divided into two groups. Group A included samples 1-9 (average salinity of the overlying water less than 1 ppt). Group B consisted of samples 10-23 (average salinity of the overlying water above 1 ppt and increasing). A linear regression analysis was then performed for groups A and B samples. The results of this analysis appear in Figure 5 and indicate a correlation coefficient of 0.988 for group A samples and 0.863 for group B samples. A similar analysis was performed using values for the available phosphorus apd the acid-extractable iron (Figure 6). The correlation coefficients obtained were 0.981 for group A samples and 0.771 for group B samples. The high correlation coefficients obtained for the group A samples suggest that the phosphorus may be held in an “Fe-inorganic P complex” as suggested by Williams et al. (1971). The decrease in correlation for the group B samples may be due to the increasing salinity of the water which would favor desorption of phosphorus or breakdown of the Feinorganic P complex. The results of this research are generally consistent with a transport system proposed by Carritt and Goodgal (1954). In the Tar River, low salinity and high turbidity
could favor the formation of a phosphorus-solids complex. The settling of these suspended solids would result in sediments with a high phosphorus and iron content such as those observed in the upper portion of the estuary. As the water and suspended solids move further down the estuary there is an increase in salinity and a decrease in stream velocity. These conditions would favor the coagulation and settling of suspended solids and the release of the phosphorus to solution. The suspended solids that settle under these conditions would contain less phosphorus; this was observed in the middle and lower portions of the estuary.
Literature Cited American Public Health Association, “Standard Methods for the Examination of Water and Wastewater,” APHA, WPCF, AWWA, 527-530, 13thed., 1971. Carritt, D. E., Goodgal, S., “Sorption Reactions and Some Ecological Impactions.” Deep-sea Res., 1,224-43, (1954). Hobbie, J. E., “Hydrography of the Pamlico River Estuary, N.C.,“ Rept. No. 39, Water Resources Research Institute, University of North Carolina, 69 pp, 197Oa. Hobbie, J . E., “Phosphorus Concentrations in the Pamlico River Estuary of North Carolina,” Rept. KO.33, ibid., 47 pp, 1970b. Hobbie, J. E., Copeland, B. J., Harrison, W. G., “Nutrients in the Pamlico River Estuary, N.C., 1969-1972,” Rept. No. 76, ibid.,242 pp, 1972. McKeague, J. A., Day, J. H., Canad. J . Soil Sci., 46, 13-22, 1966. Olsen, S.R., Dean, L. A,, “Methods of Soil Analysis, Part 2,” C. A. Black, Ed., pp 1035-49, Amer. SOC.Agron., Madison, Wis., 1965. Porcella, D. B., Kumagai, J. S., Middlebrooks, E . J., Proc. Amer. Soc. Cicil Eng., J . Sanit. Eng. Dic., Vol. 96, No. SA4, 911-26, 1970. Saunders, W. M., N e u Zealand J. Agr. Res., 8,30-57 (1965). Shulka, S.S., Syers, J. K., Williams, J. D. H., Armstrong, D. E., Harris, R. F., Soil Sci. Soc. Amer. Proc., 35, 244-9 (1971). Upchurch, J. B., “Sedimentory Phosphorus in the Pamlico Estuary of North Carolina,” Sea Grant Publication No. UNC-SG72-03, Univ. N. C. Sea Grant Program, Chapel Hill, K.C., 1972. Water Resources Data for ,Vorth Carolina, Part 2, Water Quality Records, 1965-69, U.S. Dept. Int., Geolog. Survey. Wentz, D. A,, Lee, G. F., Enuiron. Sci. Technol., 3, 750-4 (1969). Williams, J. D. H. Syers, J. K., Shulka, S. S., Harris, R. F., Armstrong, D. E., ibid.,5, 1113-20 (1971).
Receiced f o r reuiew April 13, 1973. Accepted September 17, 1973. This research uws supported in part by the Office of Sea Grant. National Science Foundation. and the Department of Administration, State of North Carolina. Paper ccas presented in part at the Symposium on Trace Metal and Phosphorus Interactions with Sediments, Dicision of Water, Air and Waste Chemistp, 164th National Meeting. American Chemical Society. Neu York, N.Y.. 1972.
Coagulation in Estuaries James K . Edzwald,’ Joseph B. Upchurch,’ and Charles R. O’Melia Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, N.C. 2751 4
Considerable attention has been devoted to the hydrodynamic and physical factors affecting the deposition of suspended materials in estuaries (e.g., Ippen, 1966; Krone. 1962; Partheniades, 1965; Etter et al., 1968), but little research has been concerned with the role of chemical parameters. In this paper the aggregation of colloidal Present address, Department of Civil Engineering. I-niversity of Missouri. Columbia. Mo. 65201. To whom correspondence should be addressed. Present address, International Paper Co.. Moss Point. Miss. 39563. 58
Environmental Science & Technology
suspensions in estuaries is examined with the purpose of elucidating the chemistry of this process. Estuarine sediment, both as bed material and suspended load, can be divided into two classifications: cohesionless or coarse particles such as sand and cohesive or fine particles such as silt and clays. The deposition of cohesionless particles in estuaries depends primarily on the hydrodynamics of the system since these particles remain as individual particles regardless of the flow conditions and the solution chemistry. On the other hand, cohesive particles range in size from a small fraction of one micron to several microns. Normally a large proportion are colloi-
