226Ra behavior in the Pee Dee River-Winyah Bay estuary

Earth and Planetary Science Letters, 48 (1980) 239-249 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

239

[61

226Ra BEHAVIOR IN THE PEE DEE RIVER-WINYAH BAY ESTUARY ROBERT J. ELSINGER and WILLARD S. MOORE Geology Department, University o f South Carolina, Columbia, SC 29208 (U.S.A.)

Received December 20, 1979 Revised version received March 26, 1980

Concentrations of dissolved 226 Ra in Winyah Bay, South Carolina, and in the adjacent Atlantic Ocean are augmented by the desorption of radium from sediments in the low-salinity area of the estuary and diffusion from bottom sediments. Desorption of 226 Ra is reflected by lower concentrations in suspended sediments from highersalinity regions of the estuary. Bottom sediments from the high-salinity region have lower 226 Ra/230Th activity ratios than those from the low-salinity end. The shape of the dissolved 226 Ra vs. salinity profile is influenced by the river discharge. During average-discharge conditions, desorption of 226 Ra from suspended and bottom sediments increases the dissolved 226 Ra concentrations by a factor of 3.5 as the water passes through Winyah Bay. High river discharge produces an initial increase of dissolved 226Ra by a factor of 2 to 3 and apparently reflects only desorption from suspended sediments. By driving the salt wedge down the estuary and reducing the zone of contact of salt water with bottom sediments, the highflow conditions sharply reduce the flux of 226 Ra from bottom sediments.

I. Introduction 226Ra (tl/2 = 1622 years) has been used by marine geochemists to study various oceanic mixing processes. There have been studies o f 226Ra throughout the world's oceans [ 1 - 4 ] and in some rivers [ 5 - 7 ] , but very little work has been done in the mixing zone between river and ocean water. Blanchard and Oakes [8] collected water samples from 14 coastal locations o f the United States and determined that the dissolved 226Ra concentrations were higher than values reported for open ocean or river waters. They explained the high concentrations by the diffusion o f 226Ra from coastal sediments. Moore [9] proposed this mechanism to support the high 22SRa concentrations in coastal waters. Li et al. [10] were the first to study 226Ra in the mixing zone o f river and ocean water. They observed an excess o f 226Ra within the Hudson estuary that was clearly above a mixing line connecting fresh and ocean waters and proposed that desorption o f 226Ra from the river-borne sediments in the estuarine environment contributed to the observed excess. Li and

Chan [11 ] calculated that desorption from estuarine sediments accounted for 1 7 - 4 3 % o f the total 226Ra flux from coastal sediments. Desorption could thus augment the total global river flux o f 226Ra to the ocean by a factor o f nine. Hanor and Chan [12] observed a strong non-conservative increase o f dissolved Ba during mixing o f fresh and marine water in the lower Mississippi River. They attributed the increase to desorption of Ba from the suspended clays by exchange with major cations in seawater and concluded that river concentrations o f dissolved Ba in the absence o f exchange data are inadequate to estimate the continental supply o f dissolved Ba to the oceans. Similar desorption properties were found in laboratory experiments by Kharkar et al. [13] for Co, Se, and Ag. Their estimates o f trace metal supply to the oceans due to desorption from particles show an increase o f 200% for Co and 10% for Ag and Se. Macay and Leathedand [14] observed a non-conservative behavior for Zn in the Firth o f Clyde, Scotland, during both low- and high-flow fresh water discharge. The concentrations in the estuary during high-flow

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241 conditions were 2 - 3 times lower than during lowflow conditions. Diffusive fluxes of radium isotopes from bottom sediments in Long Island Sound have been studied by Thomson et al. [15] and Cochran [16]. 22SRa fluxes were determined to be 0.3 dpm/cm 2 yr I15] and about 2 dpm/cm 2 yr [16] while the 226Ra flux was 0.01 dpm/cm 2 yr [16]. The mass balance model for the Hudson River estuary by Li and Chan [11 ] determined a flux of 226Ra of 2 dpm/cm 2 yr if the only source of the non-conservative 226Ra were the bottom sediments. If the diffusive flux [16] from Long Island Sound is the same in the Hudson estuary then desorption of 226Ra from suspended sediments must contribute the major portion of the 226Ra to the water. In this paper we describe the behavior of 226Ra in the Pee Dee River-Winyah Bay estuary in South Carolina. In particular, we attempt to answer the following questions: (1) How does the behavior of dissolved 226Ra compare with th'e results from the Hudson River estuary [10] ? (2) What is the behavior of 226Ra on the riverborne suspended sediments during mixing with hisher ionic strength water? (3) What effect, if any, do different discharge rates otr fresh water have on the exchange of 226Ra with the suspended and bottom sediments? To answer these questions we analyzed 226Ra in the suspended and dissolved states during average and high river stages to provide data on the physiochemical processes which occur during fresh and salt water mixing. Analyses of 226Ra from bottom sediment cores provided supplemental 226Ra data for this study.

2. The Pee Dee River-Winyah Bay system The Pee Dee River is one of the largest riverestuary systems in the Eastern United States (average discharge = 425 m3/s [17]). It originates on the eastern slope of the Blue Ridge Mountains of North Carolina crosses the Piedmont Province and meanders through the Atlantic Coastal Plain before discharge into Winyah Bay (Fig. 1). Four dams are located in the Piedmont section of North Carolina but there are

no dams within South Carolina. There are several tributaries to the Pee Dee River which originate in coastal plain sediments and supply approximately 40% of the average discharge entering Winyah Bay. Thus about half of the source water to Winyah Bay is derived from waters draining the Piedmont and half is derived from the Coastal Plain. Winyah Bay (Fig. l) extends 24 km from the Atlantic Ocean to the river entrance. Salt water intrusion can extend from about km 15 within the Bay to 25 km upstream from the river entrance, depending on river discharge. This combination of river flow and tidal influence produces a partially mixed estuary during most of the year [18]. The rural setting of the basin, mostly small towns and farms, provides minimal impact from point sources upon water quality; however we recognize that agriculture, especially the use of fertilizers and pesticides may have significant effects on the chemistry of the water and the suspended material. The Pee Dee River-Winyah Bay system offers an opportunity to study a major southeastern river-estuary system that has not been altered by large metropolitan and industrial uses.

3. Field collection and experimental techniques Water samples of approximately 20 liters were taken at the surface and stored in plastic collapsible water containers or taken at depth by suspending a hose to the desired level and filling an evacuated glass bottle. The water was filtered through 0.4-/~m Nuclepore filter paper. Radium was extracted by passing the water through an exchange column packed with approximately 3 - 5 grams of manganeseimpregnated acrylic fiber (Mn-fiber) [17] at a flow rate of 5 - 1 0 ml/minute. Moore and Reid [20] and Reid et al. [21 ] have shown that this technique quantitatively removes radium from natural waters. The Mn-fibers were leached with hot 6N HC1 and rinsed with deionized water. The leach and rinse solutions were combined in glass equilibrators for 226Ra analysis by the 222Rn emanation technique [22]. The Mn-flbers from the March 1979 samples were not leached prior to 222Rn extraction. 222Rn emanation from the adsorbed 226Ra was measured directly by placing the Mn-flber into a glass equilibrator and

242 using the same technique used for liquid samples. Laboratory tests show that radon emanation from the Mn-fibers was over 95% (W.S. Moore, in preparation). The suspended sediments were dried, weighed, and leached in concentrated HC1 and HNO3. The solution was then transferred to an equilibrator and 226Ra was analyzed using the same method used on the water fractions. A series of bottom sediment cores were taken at various positions within the estuary (Fig. 1). The cores were taken by pushing a 10-cm diameter PVC pipe into the sediment and subcored with a 10-ml plastic syringe through previously drilled holes in the side of the core tube. Approximately 1 gram (dry weight) of sediment was leached in an HC1, HNO3, and HC104 mixture. The residue was dissolved with HF and all the solutions combined and split. One split was spiked with 2a2U in equilibrium with 228Th for chemical yield tracers and analyzed for 226Ra and uranium and thorium isotopes. The other split was used to determine natural 2~STh/232Th activity ratios. The uranium and thorium analyses followed the procedures described by Bhat [23] with minor alterations. The activities of the various uranium and thorium isotopes were determined by alpha spectrometry.

4. Results and discussion 4.1. 226Ra during average river discharge (June 1978) Table I shows that the dissolved 226Ra content from various river discharges of the Pee Dee River and some of its major tributaries are comparable with reported values for 226Ra in other rivers of the world. Fig. 2a shows the dissolved 226Ra vs. salinity for June 1978 sampling. The discharge of fresh water into Winyah Bay was 350 + 41 m3/s as estimated by the gauging station on the Pee Dee River at Pee Dee, South Carolina, and multiplying by 1.66 to correct for tributary input downstream [18]. The 226Ra value at 36.6%0 (13.6 dpm/1001) is the average of five ocean samples taken 11 and 18 km of Winyah Bay in November 1978 (Table 3). This value is almost twice the average Atlantic surface concentration of 7.6 dpm/100 1 (normalized to 35%~ and zero silicate reported by Broecker et al. [3]). Li et al. [10]

TABLE 1 Dissolved 226Ra in rivers River Hudson Hudson St. Lawrence Mississippi Amazon lndian Rivers Ganges Godavari Krishna Sabarmati South Carolina rivers Pee Dee Little Pee Dee Waccamaw Black

226 Ra (dpm/1 O0 I) 7 1-3 5 7 2 20 5 5 9

Reference [5 ] [10] [5 ] [6 ] [6]

}

3- 6 8 J 6 ] 5

[7 ]

this study

reported 226Raconcentrations from 7.9 to 10.5 d l ~ 1001 with an average of 9.5 dpm/100 1 for 11 n e u . shore surface coastal waters off the northentern United States. The four values at 0%0 repremnt four river samples, the lowest (4.2 dpm/lO01) boing from the Pee Dee River. Between salinities 0 m d 8%o the dissolved 226Ra concentration increases from 4 to 16 dpm/100 1. Between 8 and 26.6%0 there is another increase from 16 to 20 dpm/1001. Using 13.6 dpm/ 100 1 as the ocean value of 226Ra off Winyah Bay d of the points in Fig. 2a lie above a mixing line between fresh Pee Dee River water and ocean surface waters. Similar enrichments of 226Ra have been observed in the Hudson River estuary by Li et al. [10]. Fig. 2b shows the 226Ra on the suspended sediments vs. salinity from the June samples. The Pee Dee River sample has the second highest 226Ra concentration (3.21 dpm/g). Between 0 and 8%o salinity there is a decrease in 226Ra concentration on the particulate matter which corresponds well with the fourfold increase in dissolved 226Ra in this salinity range (Fig. 2a). Between 8 and 28% salinity the 226Ra concentration on suspended sediments averages 1.17 +- 0.43 dpm/g. The value at 36.6%0 represents the 226Ra concentration of the combined suspended sediments from all five ocean water samples collected in November (Table 3). 226Ra apparently desorbs from

243

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SALINITY %0 Fig. 2. Plots of 226 Ra (dissolved, on suspended sediment, and total) vs. salinity for average and high river discharge (see also Table 2). The values at 36.6~'~ in all plots represent the average (dissolved and total) or combined total (suspended sediment) of five samples taken 11 and 18 km offshore from Winyah Bay. the suspended particles when they contact the higher ionic strength water. If these data are representative of riverine and oceanic components, the suspended

sediments lose approximately 2 dpm/g by desorption. Li et al. [10] found a difference in 226Ra between a fdtered and non-f'dtered fresh water sample

244 TABLE 2 226Ra concentrations in the Pee Dee River-Winyah Bay estuary Sample No. - depth * (Kilometer point - meters)

Temperature (°C)

Salinity (%0)

226Ra ** 226Ra *** dissolved suspended (dpm/100 1) sediments (dpm/g)

Suspended sediments (g/100 1)

Total 226Ra *** (dpm/100 1)

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

28 28 28 28 28 28 28 28 28 28 28 28 28 29 27 28 27 29 28 27 26 26

0.0 0.0 0.0 0.0 1.1 2.5 4.7 5.2 5.4 6.5 6.2 7.1 7.7 8.2 13.8 15.3 16.7 17.4 24.8 26.6 26.6 25.2

4.2 8.4 6.7 5.4 7.4 9.9 15.0 13.5 15.3 16.5 16.8 15.3 17,9 15.6 19,6 19,7 21.0 21,6 23.4 20.8 21.2 18.7

3.21 1.89 3.46 1.60 2.30 2.21 1.37

7.1 1.4 1.6 1.5 2.3 2.3 1.5

27.0 14,1 9.3 8.2 12.7 15.1 17.0

1.09

3.5

20.3

1.19 1.12

1.9 3.7

17.6 22.1

0.82 0.65 1.69

5.5 5.4 1.1

24.1 23.3 22.9

1.19 0.79 1.68 1.38

2.5 3.3 2.2 1.4

26.5 20.9 25.1 20.6

0 4 0 0 4 4 4 4 3 4

11.0 11.0 11.0 10.8 10.8 10.5 10.0 9.9 9.3 9.1

0.0 0.0 0,0 1,0 3,0 4.4 10,7 19.0 23,3 30.3

4.3 6.1 3.1 5.6 11.9 10.9 7.8 11.9 10.7 12.2

3.53 2.40 3.73 2.08 1.52 1.64 1.41 0.75 0.74 0.55

2,5 1.8 2.4 8.1 3.6 3.2 3.3 3.8 3.5 5.7

13.3 10.6 12.0 22.6 17.5 16.3 12.2 14.8 13.3 15.4

June 26-28, 1978 (average river discharge) Pee Dee River Little Pee Dee River Waccamaw River Black River Pee Dee River Pee Dee River Pee Dee River Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay

(127) (97) (94) (41) 29 27 25 21 20 19 17 13 (15) (14) (3) 8(1) (11) 52.5 0 -1 -

March 3, 1979 (high river discharge) Pee Dee River Pee Dee River Waccamaw River Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay Winyah Bay

42 29 29 16 16 13 912 81-

* Sample taken in the dredged or mid-channel unless designated by name or (). (94) means approximate distance in kilometers from the Bay entrance. ** Analytical uncertainty <6%. *** Analytical uncertainty <10%.

f r o m t h e H u d s o n River o f 3.6 d p m / 1 0 0 1. In c o m p a r i -

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son, t h e d i f f e r e n c e b e t w e e n a filtered a n d n o n - f i l t e r e d sample w i t h a salinity o f 3.8%o, was o n l y 1 d p m / 1 0 0 1. T h e y r e p o r t e d t h a t t h e s u s p e n d e d s e d i m e n t s in the

data. Fig. 2c is a p l o t o f t o t a l 226Ra vs. salinity. The

H u d s o n estuarine e n v i r o n m e n t lost a p p r o x i m a t e l y 1.5

highest value at 0%0 salinity r e p r e s e n t s t h e Pee Dee River. W h e n dissolved plus particulate r a d i u m is c o n -

245

4.2. 226Ra during high river discharge (March 1979)

TABLE 3

226Ra concentrations in the Atlantic Ocean off Winyah Bay (November 24, 1978) * Sample No. Temperature Salinity Depth (kilometers (°C) (%0) (m) from Bay entrance)

226Ra ** dissolved (dpm/1001)

-18 -18 -18 -11 -11

14.1 14.4 13.2 12.5 13.6

18.5 18.4 18.4 18.1 18.1

36.6 36.6 36.6 36.4 36.2

0 8 8 0 7

Average 13.6 + 0.75 * The suspended sediments were combined for all the sampies. 226Ra = 1.04 dpm/g, suspended sediment concentration = 0.19 g/100 1, total 226Ra = 13.8 dpm/100 L ** Analytical uncertainty <10%.

sidered, the Pee Dee River could supply the total 226 Ra needed to raise the dissolved 226Ra concentration with the estuary to the measured levels if its total suspended load (measured at km 127) reached the Bay. The decrease in water velocity due to the branching and widening of the Pee Dee River starting at kilometer point 65 and continuing until Highway 17 bridge (Fig. 1) results in a loss of some of the suspended sediments. However, the sediment lost from suspension can still be an important source of dissolved 226Ra to the water column. Depending on the discharge conditions, the leading edge of the salt wedge can proceed a considerable distance up river from the Highway 17 bridge (kin 24). Johnson [18] determined that at high tide and a river discharge of 85 ma/s (low flow) the salt wedge would reach krn 49 of the Pee Dee and Waccamaw River channels. During the June sampling period the discharge of approximately 350 ma/s put the salt wedge at high tide near km 37 (Fig. 1). Salt water intrusion reaching km 37 exposed recently deposited bottom sediments within the branching channels of the river to the higher ionic strength water, leading to desorption of 226Ra and its diffusion from the sediments to the water. That supply of 226Ra combined with the 226Ra already dissolved in the water or desorbed from suspended sediments contributed to the elevated 226Ra concentrations found within the estuary.

Fig. 2d shows the dissolved 226Ra vs. salinity from samples collected March 1979 during a major flood with a discharge of about 3300 ma/s (10 times the flow in June). The fresh water values for dissolved 226Ra are comparable to fresh water values from June (Fig. 2a). Between 0 and 4% salinity, the 226Ra concentration increases by a factor of 2 - 3 ; beyond 4%0 the values remain constant at 10.7 -+ 1.7 dpm/ 100 1. Most of the increase of 226Ra occurred in areas having salinities lower than 4%~ in March. Fig. 2e shows 226Ra on the suspended sediments as a function of salinity in March. The values at 0%0 are similar to the June values (Fig. 2b). There is a rapid decrease between 0 and 4%0 salinity and only a slight decrease between 4 and 30%o salinity. The average 226Ra activity between 10.7 and 30%0 is 1.01 1.01 + 0.47 dpm/g, not significantly different than 1 .I 7 __-0.43 dpm/g found in the June samples. Fig. 2f is a plot of total 226Ra vs. salinity for the March samples. The 0%0 samples are lower in total 226Ra than the Pee Dee River sample from June but they contained only 35% as much suspended sediments and were taken closer to Winyah Bay. The 1%o salinity sample had the highest total 226Ra concentration because it contained two and one half times the amount of suspended sediments as the other samples. Perhaps the swirling water had entrained some bottom sediments. The total 226Ra concentration at 0%0 are high enough to support the total 226Ra values within the estuary. It thus appears that the amount of 226Ra desorbed from the suspended sediments contributed almost all the 226Ra needed to produce the elevated dissolved 226Ra values in the estuary.

4.3. Sediment cores Fig. 1 shows the location of a series of sediment cores taken during June 1978 in the estuary, The analyses of these sediments are reported in Tables 4 and 5 and plotted in Fig. 3. In each sediment core the top sample, which represents a thin oxidized layer (approximately 2 mm thick), has a lower 226Ra concentration than the rest of the core (Table 4 and Fig. 3). The 2~6Ra concentration of this surface layer is more representative of the suspended sediments than the bottom sediments.

246 TABLE 4 226Ra in sediment cores in Winyah Bay (June 1978) * Depth (cm)

28.5 km up bay

20.5 km up bay

14.0 km up bay

3.0 km up bay

Top oxidized 0 . 2 - 1.5 3 - 4.5 6 - 7.5 9 - 10.5 12 - 13.5 15 - 16.5 18 - 19.5 21 - 22.5 24 - 25.5 27 - 28.5 30 - 31.5 33 - 34.5 36 - 37.5 39 - 40.5

2.48 3.71 3.50 4.24 4.27 3.63 3.70 4.32

1.95 2.07 2.20 2.08 2.87 3.07 2.68 2.17 2.33 2.39 2.42

1.26 2.36 2.03 2.13 2.41 1.72 2.36 2.64 2.66 1.72 2.50 2.32 1.87 2.38 2.08

0.54 1.28 1.32 1.73 1.64 1.93 1.69 1.67 1.82 1.69 1.85 1.39 1.82 1.53

Mean **

3.83 ± 0.54

2.38 ± 0.35

2.22 -+0.31

1.64 ± 0.20

* dpm/g; analytical uncertainty <10%. ** Does not include the "top oxidized" value.

RADIUM-226 IN SEDIMENT CORES (dpmlgm) 2D . 3 . 0 4 Z. 5 .~ .ll. 1~ . 2.0 . 5.0 4~ . tl.

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20.5 krn UP BAY

14.0 km UP BAY

3.0 km UP BAY

Fig. 3. Plots of 226 Ra concentrations down sediment cores taken from Winyah Bay and the Pee Dee River during June 1973. The mean (X) does not include the top sample in the oxidized layer.

The average 226Ra concentration of the suspended sediments in the Bay water when the cores were taken is 1.17 -+ 0.43 dpm/g. The 226Ra concentrations for this top layer o f the three cores within the bay averages 1.24 + 0.70 dpm/g. The 226Ra in the top layer o f the core at 28.5 km is 2.48 dpm/g while the 226Ra of the suspended sediments from two water samples taken at km 29 and km 27 is 2.25 -+ 0.05 dpm/g. These results suggest that the top layer represents present deposition o f suspended sediments. There is a striking decrease in 226Ra concentration in the sediments the closer the core is to the bay entrance (Table 4 and Fig. 3). The values from the reducing section o f the upper bay sediment core (28.5 km) are very near the 226Ra concentration of the suspended sediments supplied by the rivers. The reducing section of the lower bay sediment core (3.0 kin) is 50% higher than the averages o f the suspended sediments concentration between 12 and 30% from June and March. These sediments thus reflect the same trend as the suspended matter, meaning that the bulk of the sediments in Winyah Bay have lost 226Ra before sedimentation or during low flow conditions when they are exposed to high ionic strength waters.

247 Table 5 shows uranium and thorium data from the upper and lower bay cores. The very top oxidized layer in the lower bay core shows a lower value than the rest o f the core for 23aU and 2a°Th as well as 226Ra. The higher values o f 238U and 23°Th in the upper bay core compared to the lower values in the lower bay core suggest either that uranium and to some extent, thorium, are being removed from the suspended material within the estuary or that material from the river is being diluted by another component lower in uranium and thorium. A grain size analysis o f the two cores revealed the 3.0-km core contained 30% quartz by weight, primarily in the >63-/~m fraction, while the 28.5-km core was only 2% quartz by weight. The higher quartz content o f the 3.0-km core probably represents eolian input from the sand dunes located on the peninsula between the lower Winyah Bay and the Atlantic Ocean (Fig. 1). Dilution o f the sediment by quartz in the 3.0-km core probably accounts for the lower uranium and thorium concentrations compared to the 28.5-km core. The 226Ra/ 2a°Th activity ratios (excluding the t o p sample) are 1.15 _+0.18 for the upper bay core whereas in the lower bay core the ratio is 0.71 -+ 0.07. This suggests that if the sediments are river-derived, they have lost about 38% o f their 226Ra before reaching the lower

estuary. If the lower bay sediments are being diluted by low-23°Thmaterials in equilibrium with 226Ra, the losses calculated may be even greater. The top samples from both cores have about the same activity ratios (0.64 and 0.61). At the time o f sampling (June 1978) the water above the upper bay core was high ionic strength during part o f the tidal cycle. This could explain the lower activity ratio in the top sample compared to the rest o f the core.

5.

226Raresponse

to different discharge conditions

Comparing the dissolved 22~Ra vs. salinity (Fig. 2a and d) for the two sampling periods, the June samples show the most non-conservative behavior. During flood conditions in March (Fig. 2d), dissolved 226Ra concentrations increased 2 - 3 times between 0 and 4% salinity but stayed constant above 4%0. During average fresh water discharge (Fig. 2a) there was a 3to 4-fold increase in dissolved 22~Ra extended over the salinity range 0-12%o. During average discharge conditions about 75% o f the total non-conservative fraction o f 226Ra is supplied at the low-salinity end o f the estuary; the remaining 25% is added in intermediate-salinity water ( 6 - 1 2 % ) . During high fresh

TABLE 5 Uranium and thorium isotopes in sediment cores in Winyah Bay (June 1978) Sample core and depth (cm)

238U (dpm/g)

230Th (dpm/g)

232Th (dpm/g)

226Ra (dpm/g)

226Ra/230Th * 234U[238 U *

3.41 4.01 3.53 3.62 4.80

± 0.19 ± 0.15 ± 0.20 ± 0.14 ± 0.20

3.88 3.68 4.51 2.63 3.77

± 0.18 ± 0.11 ± 0.18 ± 0.12 ± 0.12

4.85 4.01 4.58 3.06 3.10

± 0.15 ± 0.09 ± 0.13 ± 0.09 ± 0.09

2.48 4.20 4.24 3.63 4.32

± 0.12 ± 0.30 ± 0.18 ± 0.15 ± 0.48

0.64 1.14 0.94 1.38 1.14

± 0.04 ± 0.08 ± 0.05 ± 0.08 ± 0.13

1.10 1.07 1.12 1.00 1.00

0.63 ± 0.04 1.48 ± 0.08

0.88 1.72 2.24 2.10 2.53

± 0.04 -+ 0.08 ± 0.11 ± 0.07 ± 0.11

0.71 1.95 2.20 2.02 2.76

± 0.05 ± 0.07 ± 0.08 ± 0.06 ± 0.08

0.54 1.28 1.64 1.69 1.69 1.85

± 0.02 ± 0.04 ± 0.03 ± 0.15 ± 0.04 ± 0.10

0.61 0.74 0.73 0.80 0.67

+ 0.03 -+0.04 -+0.03 ± 0.07 ± 0.03

1.32 ~ 0.05 1.15 + 0.08

28.5 krn

Top oxidized 0.2- 1.5 6 - 7.5 12 -13.5 18 -19.5

± 0.08 ± 0.05 ± 0.09 ± 0.05 ± 0.05

3.0 k m

Top oxidized 0.2- 1.5 9 -10.5 15 -16.5 21 -22.5 27 -28.5 * Activity ratio.

2.04 ± 0.08 2.27 ± 0.08 1.99 ± 0.06

1.17 + 0.07 1.06 + 0.05 1.03 + 0.04

248 water discharge almost all of the non-conservative fraction is added at low salinities. The shape of the dissolved 226Ra vs. salinity curve strongly reflects the discharge of fresh water into Winyah Bay. River discharge, tides, and winds control the upstream advance of the salt wedge. During average flow conditions (June) the leading edge of the salt wedge reached km 37 att high tide. The fresh water discharge during March drove the leading edge of the salt wedge to approximately km 24 at high tide (field observations). During average flow conditions higher ionic strength water moved further upstream and increased the areal extent of desorption from bottom sediments. During June 1978 the 3- to 4-fold increase in dissolved 226Ra over the 0 - 1 2 ~ salinity range included the upper half of the bay (above km 13). There was nosignificant 226Ra addition from suspended sediments present in waters having salinities greater than 6%0 (km 17). Desorption of 226Ra from bottom sediments and diffusion into the water must have occurred within the Bay to account for the increase of radium below km 17. High fresh water discharge in March prevented high ionic strength water from coming in contact with the bottom sediment high in 226Ra. Desorption from the suspended sediments was the parimary mechanism for increasing the dissolved 226Ra concentrations in the estuarine waters during this period of high fresh water discharge. Thus sediments deposited between kms 24 and 37 during high-flow conditions may release 226Ra when they are covered by highersalinity waters during low-flow conditions. This source of 226Ra which in June included the upper half of Winyah Bay may further augment the concentration of dissolved 226Ra.

(2) The suspended sediments lose 226Ra as they encounter increasing salinities. This loss by desorption of approximately 2.0 dpm/g increases the dissolved 226Ra concentration. Additional 226Ra is released from bottom sediments during periods of low to average river discharge of dissolved 226Ra in the estuary. (3) Sediment core data which show decreasing 226Ra concentrations and lower 226Ra/2a°Th activity ratios down the Bay provide independent evidence of the loss of 226Ra from the suspended and bottom sediments. (4) Discharge of fresh water is very important in controlling the flux of 226Ra from bottom sediments and thus the shape of the :26Ra vs. salinity curve.

Acknowledgements We want to thank Dr. Dennis Allen for his assistance with our use of the boats, cabins, and other equipment at the Baruch Institute, Georgetown, South Carolina. Milo Meyers assisted in collecting the samples from the Atlantic Ocean. This project could not have been completed without the help of B. Benggio, S. Chapnick, H.S. Chen, K, Cole, H. Daniels, T. Harris, J. Hess, J. Michel, and M. Otten who all helped with the field collections and laboratory analyses. The typing and drafting were done by K. Albert. Support for this work was provided by the Geology Department and Baruch Institute, University of South Carolina. We thank J. Michel, Y.-H. Li, and D.F. Reid for reviewing the manuscript.

References 6. Summary (1) Dissolved 226Ra is higher in Winyah Bay than in the Pee Dee River or the adjacent Atlantic Ocean. During average and high river discharges the total amount of 226Ra (dissolved and on suspended sediments) supplied by the fresh water could account for the total 226Ra measured in the estuarine water samples. In the river more 2:6Ra is bound to the suspended sediments; in the estuary more 226Ra is dissolved in the water.

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