Transport mechanism for Pb-210, Cs-137 and Pu fallout radionuclides through fluvial-marine systems

0016-7037/82/060941-14503.00/0

Geochimica CI Cosmochimica Acra Vol. 46, pp. 941-954 0 Pcrgamon Press Ltd. 1982. Printed in U.S.A.

Transport mechanism

for Pb-210, Cs-137 and Pu fallout radionuclides through fluvial-marine systems J. N. SMITH and K. M. ELLIS

Atlantic Oceanographic

Laboratory, Bedford Institute of Oceanography,

Dartmouth, Nova Scotia B2Y 4A2

(Received July 20, 198 1; accepted in revised form January 25, 1982) Abstract-Pb-210, Cs-137 and Pu-239,240 sediment-depth profiles in an anoxic, unbioturbated, estuarine depositional regime at the head of the Saguenay Fjord, Que. exhibit a seasonally-modulated component caused by pulsed inputs of silts and sands during high energy, spring river discharge events superimposed on an ambient depositional pattern of finer grained clays and organic matter. A precise sediment timestratigraphy has been determined by the inverse correlation of the Pb-210 activity with the rate of river discharge during the period, 1963- 1976. The historical record of Cs- 137 and Pu-239,240 sediment fluxes has been reconstructed through the normalization of fallout radionuclide activities to the excess Pb-210 activity profile. Radionuclide flux geochronologies have been interpreted on the basis of a fluvial-marine transport model which distinguishes between inputs due to direct adsorption of radionuclides onto particles in the water column and inputs resulting from the erosion of particle-associated radionuclides from the drainage basin. Rate constants corresponding to residence times of one year for Cs-137 and Pu239,240 in the water column and 1500 years for each radionuclide in the drainage basin provide reasonable agreement between the model and experimental results, although there is some evidence for a slightly longer drainage basin residence time for plutonium. Both the threshold for the initial appearance of Pu-238, derived from the atmospheric burnup of a SNAP-9A satellite reactor in 1964, and the magnitude of its isotopic dilution by drainage basin inputs of Pu-239,240 are also in agreement with model predictions. 1.0 INTRODUCTION RATES of particle erosion, transport and deposition in north temporate estuarine systems can exhibit a seasonally-modulated component due to climatological forcing. Increased flow velocities during the spring river freshet promote an erosional phase within the drainage basin and increased sediment deposition in the downstream estuarine environment. Correlation of the frequency and amplitude of eposodic depositional signals in estuarine sediments with modulations in hydrological parameters (river discharge, precipitation, etc.) can elucidate the mechanism governing particle transport fluxes from continents into the oceans (Allen, 1974; Schumm, 1977). Ubiquitous radioactive particle tracers, such as Pb210 and the fallout radionuclides, Cs-137, Pu-238 and Pu-239,240, can be used to resolve the annual cyclicity in estuarine sedimentation patterns and to identify the source function for sediment inputs. Measurements of the annual periodicity in particle tracer input functions, requires either continuous measurements over a several year time period (Alberts et al.. 1979), or analysis of a recent (post- 1954) well resolved, sedimentary record characterized by minor, post-depositional redistribution of sediment. These latter constraints are satisfied by an anoxic, unbioturbated, high sedimentation regime at the head of the Saguenay Fjord, Quebec (Smith and Walton, 1980). Annual variations in the sedimentdepth profile for Pb-210, caused by pulsed inputs of Pb-210-deficient silts and sands during each spring river discharge, provide a precise sediment time-stratigraphy. Fallout radionuclide profiles in the sedi941

ment can then be interpreted on the basis of a timedependent, particle transport model which is used to estimate rate constants governing fluxes of radionuclides along different pathways through the drainage basin. The approach presented in this study provides a new method for estimating short term changes in fundamental geological parameters of the fluvial system, such as soil erosion rates and particle transit times. 1.1 Analytical methods A 155 cm Leheigh gravity core (10 cm I.D.) was collected from a water depth of 40 m at the head of the Saguenay Fjord (Sta. 18; Fig. 1) in April, 1976. The core was stored upright at 4 deg. C; it was subsequently split longitudinally into two halves, a working half (core 3111) and an archive half (core 3111-A) and X-radiographed. Each core half was subsampled twice at 1 cm intervals using a modified 10 cc syringe and sediment samples were analysed for Pb-210 and Ra-226 by methods outlined in Smith and Walton (1980). Readily oxidizable organic matter was measured by the wet oxidation method (cold H2S0~) described bv Lorina and Rantala (1977). Sediment-particle size distributions-were determined both by standard sieving techniques and by Coulter counter methods (Kranck and Milligan, 1979). Details of the microfossil assemblages, Hg sediment-depth profiles and sediment stratigraphy have been reported in Schafer et al. (1980) and Smith et 01. (1980). Cesium- 137 measurements were conducted by non-destructive analyses of dried sediment samples using a 25% efficient Ge-Li detector interfaced with a 4096 channel analyser (Smith and Ellis, 1981). Plutonium isotopes were separated from acid leached sediment samples using radiochemical methods outlined by Wong (1971), electroplated onto stainless steel discs and analysed using alpha spectrometric methods by Environmental Analysis Laboratories (Richmond, Cal.).

.I N. SMITH AND K. ELLIS

942

46’

LANmLlDE -1971 ST. JEAN -VIANNEY

54

70.S2’

50’

71.

70.

FIG. 1. Location of.Station 18 in the Saguenay Fjord drainage basin. Inset illustrates the bathymetry in the vicinity of Statron 18.

I.2 Environmental setting

The Saguenay Fjord occupies a deep (270 m), elongated, submarine valley incised into the crystalline rocks of the Canadian Shield in eastern Quebec (Fig. 1). Water circulation at the head of the fjord, in the vicinity of Sta. 18, has a two layered, estuarine component characterized by an average surface salinity of 3% and a bottom (40 m) salinity of 27%. Greater than 90% of the freshwater input to the fjord is derived from the Saguenay River which itself drains the Lac St.-Jean watershed. Despite the fact that freshwater discharge through the Saguenay River is regulated by a series of hydroelectric facilities, mean monthly

DATE

discharge rates during the spring runoff have attained values greater than four times the mean monthly discharge rate for the 1960 to 1975 period of 1580 m’ s-’ (Annuaire Hydrologique, 1976). Elevated discharge rates during the spring season cause enhanced erosion and resuspension of sediments from the Saguenay River channel and Lac St.Jean and subsequent transport of this material into the fjord resulting in a pronounced seasonal variability in suspended particulate matter (SPM) levels in the water column at the head of the fjord (Sundby and Loring, 1978; Schafer et al., 1980). Sedimentation in the fjord is controlled principally by the influx of particles from the Saguenay River resulting in an

LRIVER

DISCHARGE)

w

isn^ 1.0 2

Q

w-” 8 b

1.5

5

2.0

2 (L

“E

2.5

6.0

DEPTH

(Cm)

FIG. 2. Pb-210 activities as a function of depth in core 3 I Il. The depth scale between 90 and 150 cm has been expanded relative to the 0 to 90 cm portion of the core. An inverse correlation (r = 0.80, n = 60) has been drawn between the river discharge rates (three month running average; 1963-71) and Pb-210 activities using a variable, annual sedimentation rate. This provides a geochronology (upper axis) with a time-stratigraphic resolution of better than one year.

TRANSPORT

MECHANISM

in river flow promotes the settling of particulate material. Much of the sediment load at Sta. 18 consists of terrigeneous organic matter, derived from upstream pulp and paper mill and sawmill outfalls. The high flux of organic matter has depleted the oxygen reserves of the sediment and produced a depauperate benthic life zone almost totally devoid of bioturbating organisms. The absence of these organisms has resulted in the excellent preservation of sediment stratigraphy. The ambient sedimentation pattern throughout the fjord has been periodically interrupted during the past 100 years by puked inputs of material which have produced anomalies in radionuclide, Hg and pollen sediment-depth profiles (Smith and Walton, 1980; Smith and Loring, 1981). The largest perturbation in the sedimentation regime occurred as the result of a massive landslide at Saint Jean-Vianney (Fig. 1) in May, 1971 when 25 million tonnes of Champlain Sea, marine sands and clays of post-Wisconsin age were displaced into a tributary of the Saguenay River. Subsequent transport of much of this material through the Saguenay River followed by its deposition in the fjord produced a well-defined, gray, sandy mud layer of sediment between 68 and 90 cm depth in core 3 111 (Fig. 2) (Smith and Walton, 1980; Schafer et al., 1980). Two additional gray sandy mud layers that occur within the 8 to 23 cm and 33 to 45 cm intervals are the result of resuspension of landslide-displaced sediment from the Saguenay River bed followed by river transport into the fjord during high, spring river discharge events in 1973 and 1974.

2.0 RESULTS AND DISCUSSION

943

FOR ““Pb

layers of finer grained, organic rich (5-155 by wt.) material deposited during periods of low river discharge are comparatively enriched in Pb-210 (4.07.0 dpm g-r). Consequently sedimentation at Sta. I8 appears to have a bi-modal component with pulsed inputs of Pb-210-deficient, silts and muddy sands, introduced during spring runoff conditions, super-imposed on an ambient depositional pattern of finergrained clays. The annual modulations produced in the Pb-210 profile by this deposition mechanism are most pronounced in sediment strata (O-70 cm) deposited subsequent to the landslide in 1971, but are also evident in the detailed Pb-210 profile (90-150 cm) for sediment deposited prior to the landslide event (Fig. 2). A precise sediment geochronology can be constructed for core 3 111 using a river discharge-sedimentation model based upon the following assumptions; (1) the sediment accumulation rate, o, is directly proportional to the suspended load, Q, of the river, which, in turn, is a power function, s, of the rate of river discharge, D, i.e., w N Q = kD: where k is a constant, and (2) excess Pb-210 is associated with the clay-size component of the suspended load whose input flux to the sediments remains constant [for a review of the Pb-210 constant flux and related models, see Robbins (1978)], while the seasonal variability in w is predominantly due to changes in the silt-sand components of the suspended load. It follows from Eqn. 1 and the above assumptions that;

2.1 Fluvial sedimentation mechanism APb-210,&)

Atmospherically-derived Pb-210 (t112= 22.3 yr) is rapidly scavenged by particles within the water column and may accumulate as an unsupported excess in the sediments. In the absence of post-depositional redistribution processes, the excess Pb-210 activity (J&.z~& is equal to the total Pb-210 activity (&,_rIO) minus the Ra-226 supported component which in sediment deposited t years before present (BP.) is given by: APb-2IO,(t)

=

F(t)e-A”/w(t)

(1)

where F(t) is the flux (dpm cm-’ yr-‘) of excess Pb210 to the sediment-water interface, w(t) is the sediment accumulation rate (g cm-’ yr-‘) and A, is the radioactive decay constant (0.031 yr-‘) for Pb-210. The Pb-210 activity profile for core 3 111 (Fig. 2) illustrates the effect of a seasonally-modulated, sediment transport mechanism on the depositional regime at Sta. 18. Three layers of coarser grained (>45% sand), landslide-derived sediment which occur between the 0 and 90 cm intervals, are associated with material transported through the Saguenay River during the high spring river discharges of 1971, 1973 and 1974. These sediments have low Pb-210 activities, only slightly greater than the mean Ra226 supported activity of 0.75 + 0.12 dpm g-’ for core 3111 (Smith and Walton. 1980). Alternatine ho

m

w(t)-‘e-xL’ = k,D(t)-se-X1’

(2)

where k, is the product of several constants. Measurements of suspended load, Q, as a function of river discharge on a daily basis indicate that s generally lies in the range of 2 to 3, although there is usually considerable scatter in these correlations, in addition to hysteresis (out of phase) effects (Allen, 1974). Measurements of suspended particulate material (>0.45 cc) carried out by Sundby and Loring (1978) in 1974 near Sta. 18 indicate that Q increased by a factor of 5 during the spring (May) period of high river discharge (3500 m3 s-r) compared to the SPM load measured during the lower (1300 m3 s-r) discharge regime prevailing during September. Their measurements of relatively low water column inventories of SPM at Sta. 18 (2-10 mg cm-‘), overlying an extremely high sedimentation rate regime (4 g cm-* yr-‘; Smith and Walton, 1980) suggest that sediment deposition is dominated by rapidly settling particulate material at this location, with possible additional inputs due to material transported as bedload. The dominance of the silt-sand component of the particle flux in controlling the time dependence of w is discussed in the following section. The inferred, inverse relationship between AP~-ZIO_ and river discharge (Eqn. 2) has been used to establish a sediment geochronology between 90 cm, the lower boundary of landslide-derived sediment

944

1. N. SMITH AND li. EL.L.LS DATE 1970 ~~~~~~~~-~~~~-~~.

1968

1966

1964 L.._-_..L__._.___L.-.. _ __L

*P”-239,240,2

(dpm g-l

)

APu-238

.”

-

(dpm 9-l) 0

’

90

100

l-l IIO

120 DEPTH (cm)

130

nnll

140

150

FIG. 3. Pb-210 activity (dpm g-‘) profile between 90 and 150 cm in core 3111-A (archive half). Pu239,240 and Pu-238 activity (dpm g-‘) profiles given in lower portion of figure. Arrows denote Pu-238 activity below the detection limit of 0.002 dpm g-‘. Sediment interval denoted by arrow between 136 and 143 cm was subsampled for particle size measurements (Fig. 4).

deposited in May 1971, and the bottom of the core at 150 cm, by maximizing the correlation coefficient for a least squares correlation between AP&210_and D-” (three month running average). In this procedure the monthly time scale for river discharge was adjusted to fit the sediment-depth scale for APb_210_X by using a variable, annual sedimentation rate which constrained each one year portion of the river discharge record to fit a specific, sediment-depth interval. For an arbitrary set of annual sedimentation rates ( 1962- 197 1) the correlation coefficient between A pb-210, and D-” was determined by linear interpolation between Pb-210 activities in adjacent 1 cm intervals of sediment. The correlation coefficients for the fit illustrated in Fig. 2, were r = 0.80 (s = I), r = 0.83 (s = 2) and r = 0.72 (s = 3) for the data (n = 60) between 90 and 150 cm. The correlation coefficient decreases for higher (s > 3) and lower (s < 1) power functions while the response of the model to changes in s is comparatively uniform through the range, 1 5 s 5 2. The correlation coefficient is significantly improved (r = 0.90, s = 1) if the entire Pb-210 distribution (O-150 cm) is compared to the 1962-1976 river discharge record, assumming that the sediment in the 68 to 90 cm interval was deposited instantaneously during the May 197 1 landslide event (Fig. 2). This dating application of Pb-210 as a fine particle tracer represents a significant departure from previous dating applications in which the radioactive decay properties of Pb-2 10 have been exploited (Robbins, 1978). According to the geochronology established for core 3111, the lowermost stratum of the

core at 150 cm was deposited during the winter of 1962-1963. This date is in good agreement with the date of 1962 previously estimated for this level on the basis of the average decrease, by radioactive decay, in the excess Pb-210 activity with depth in a second suite of subsamples from core 3111 (Smith and Walton, 1980). The highest annual sedimentation rates occurred in 1971 (dominated by landslidederived inputs), 1973 (20 cm yr-‘) and 1974 (22 cm yr-‘). The unusually high deposition rates recorded CORE3llI-A --------

.2

SIl_T_SAND---

----CLAY-

I

,....

142 -143c.m 140-141 cm 138 - 139 cm 137 -139 cm 136-13, Cm

2

50

5 PAR&

i%.METER

loo

200

500

(pm1

FIG. 4. Particle size distributions for samples from 1 cm intervals between 136 cm and 143 cm levels in core 311 lA. Increase in the sand-size component corresponding to a decrease in the Pb-210 activity (see Fig. 3) is due to increased deposition of sands associated with elevated river discharge rates during the spring and summer months.

TRANSPORT

MECHANISM

in the latter two years are due to the large quantity of landslide-displaced silts and sands, originally deposited in the Saguenay River channel and subsequently resuspended and transported into the fjord during periods of high, spring river discharge. Prior to 1971, the highest annual sedimentation rates, ranging from 7.5 cm yr-’ to 9 cm yr-‘, occurred in the three years, 1964, 1966 and 1970 of highest river discharge and the smallest annual increment of sediment (4 cm) was deposited in 1968, the year of lowest river discharge. 2.2 Particle size distributions Evidence supporting the application of a bi-modal sedimentation model is provided by measurements of the particle size distributions in sediment from core 311 l-A, the archive half of core 3111. The Pb210 activity profile between 90 cm and 150 cm in core 3 111-A (Fig. 3) exhibits a modulated signal, almost identical to that measured in core 3 111, indicating that the geochronology determined for core 3111 is applicable to both cores. According to this geochronology (upper axis, Fig. 3), sediment between the 143 cm and 136 cm strata is composed of material progressively deposited during the winter months of 1963-1964 through to the spring and summer months of 1964. This change from a lower energy hydrodynamic regime, typical of low river discharge conditions, to higher energy conditions during the spring and summer months is reflected in the particle size distributions for inorganic material from successive 1 cm intervals of sediment (Fig. 4). Sediment from the 142-143 cm interval consists mainly of poorly sorted clays and silts whose comparatively linear size distribution between 1 pm and 60 pm indicates that this material has been deposited from suspension as floes (Kranck, 1979, 1980). Flocculation in the saltwater-freshwater mixing zone in the vicinity of Sta. 18 leads to the formation of particle aggregates having comparatively high settling velocities, each composed of fines having a wide range of particle sizes. However, the extension of the size distribution function into the sand (>63 pm) portion of the size spectrum provides evidence for simultaneous, single particle deposition of sands having a modal size of approximately 80 pm. The particle size distribution for the 140- 141 cm sediment interval, exhibits a more pronounced sand peak that shifts to a higher (90 pm) modal size in addition to a silt size peak having a modal size of 30 pm. The appearance of a silt peak indicates that the transition from a lower to higher energy hydrodynamic regime is accompanied by an increase in the rate of deposition of individual silt particles compared to that of silt bearing floes. This phenomenon can develop during high river discharge conditions because of increased fluvial erosional velocities or by downstream displacement of the freshwater-saltwater mixing zone with a consequent decrease in the rate of floe formation at Sta. 18. The sand size peak

FOR

945

““Pb

subsequently dominates both the silt peak and the &-tail portion of the particle size spectrum in the 136-137 cm (modal size = 15Opm) and 137-138 cm (modal size = 130 pm) intervals which correspond to sediment deposited under the highest energy river discharge conditions. Surface active, particle tracers such as Pb-2 10 are apparently transported to the sediments by the fine grained portion of the sediment load (Robbins, 1978; Megumi, 1978). A direct correlation (r = 0.82, n = 7, p 5 0.05) between the excess Pb-210 activity and the total surface area of unit weight of particles >l pm in diameter (calculated assuming particles are spheres of uniform density) for each sediment interval supports the contention that the excess Pb210 is primarily associated with the flocculated clay component of the sediment. Pulsed inputs of sands and silts during high spring-summer river discharge conditions produce a large increase in the sediment accumulation rate but contribute a proportionally smaller amount to the excess Pb-2 10 inventory in the sediments. Despite the seasonal variability in the sediment accumulation rate, a constant flux (Robbins, 1978; Oldfield et al., 1978) Pb-210 depositional model appears to be applicable to this sedimentary regime. The geochronology estimated for core 3111 using the Pb-210 constant flux model, and an excess Pb210 flux of 20.0 dpm crne2 yr-’ (Smith and Walton, 1980), generally differs from the river discharge geochronology (Fig. 2) by less than 1 year, although there is some evidence for reduced Pb-210 fluxes prior to the landslide which occurred in 1971 and elevated fluxes subsequent to this event. The Pb-210river discharge geochronology is judged to be more accurate because it is based on annually resolved horizons whose timing is not subject to cumulative analytical uncertainties or anomalous depositional or resuspension effects which can introduce a systematic error into the constant flux geochronology. The measured inventory of excess Pb-210 in each annually deposited increment of sediment, based upon the timing provided by the Pb-210-river discharge geochronology, gives a Pb-210 flux of 20.8 & 7.8 dpm cm-* yr-‘. Both the Pb-210 flux and the standard deviation in the flux are reduced to 19.7 & 5.1 dpm cm-* yr-’ if the anomalously high 197 1 (40 dpm cm-‘) and low 1968 ( 10.5 dpm cmd2) inventories are neglected. The anomalously high input of excess Pb-210 in 197 1 is related to the large quantity of sediment pulsed into the fjord during the landslide event and the low inventory for 1968 probably stems from the unusually low river discharge conditions prevailing during this year. 2.3 Cesium-137

activity

profile

Cesium- 137 is a fission product initially introduced into the environment in significant quantities as a result of atmospheric nuclear weapons tests conducted in the early 1950’s. Cesium- 137 is rapidly and

946

J. N. SMITH AND K. ELLIS

19?4 F------i--

DATE 1972

1970 .‘.’ - 1--_-r--

b-___

1966

r_

1964 r_ -.-.._-.i

..__

DEPTH (Cm,

FIG. 5. Cs-137 activity (dpm g-l) and organic matter concentration (wt. %) as a function of depth in core 3 111.Geochronology for sediment deposition determined from river discharge-Pb-2 10 correlation is indicated on upper axis.

almost irreversibly adsorbed onto clay minerals (Tamura, 1964; Alberts and Muller, 1979) and possibly organic matter (Tahir and Stewart, 1975) in freshwater systems and is transported with eroded soils into freshwater catchment basins (Ritchie and McHenry, 1978; Edgington et al., 1976) or into the marine environment in association with suspended particulate matter (Simpson et al., 1976). The Cs-137 activity profile for core 3111 (Fig. 5) exhibits pronounced annual peaks within the 90 cm to 150 cm interval that denote sediment deposited between 1962 and 1971. This corresponds to the period of most extensive nuclear weapons testing (Carter and Moghissi, 1977) which resulted in a fallout deposition record typical of the bi-monthly record for New York City (inset, Fig. 6; HASL, 1977). Yearly maxima in the fallout record result from enhanced downward mixing of Cs- 137 of stratospheric origin into the troposphere during the spring months of each year. Both the timing and magnitude of the Cs-137 maxima observed in the core are in good agreement with those noted in the fallout record. However, peaks in the Cs- 137 sediment-depth profile are also synchronous with peaks in the Pb-210 activity ~diment-depth profile, and these latter maxima have been previously identified as artifacts of the seasonally-modulated sedimentation rates at Sta. 18. It follows that the yearly variability in the Cs-137 profile is also an artifact of this bi-modal sedimentation mechanism. The minima in this profile (in addition to those in the % organic matter profile, Fig. 5) are caused by dilution of clay sized particles and organic material, the presumed ‘carrier phase’ (Ben-

ninger and Krishnaswami, 198 1) with which the Cs137 is associated, by seasonaliy-pulsed inputs of silts and sands during high, spring river discharge conditions. However, in contrast to the Pb-210 activity profile, the G-137 activities are maximized in sediment deposited in 1963-i 964, synchronous with the period of highest fallout de~sition rates. This relationship indicates that changes in the Cs-137 input function are reflected in the sediment-depth profile. The dependence of sedimentary Cs-137 activities, ACs_,,,, on the time, t, of sediment deposition can be expressed by an equation equivalent to Eqn. I; (3) where Fcr_137(f)is the flux of Cs-137 to the sediments and X2 is the radioactive decay constant (=0.023 yr-‘) for Cs-137. In order to interpret the es-137 activity profile it is necessary to eliminate the effect of the seasonally-m~uiat~, sediment accumulation rate (w(t)). This can be accomplished through normalization of Cs-137 activities to excess Pb-210 activities, i.e. by dividing Eqn. 2 by Eqn. 3 and solving for Fcc13, (t) for each 1 cm interval of sediment;

term in Eqn. 4, a comparatively slowly varying function of t over the period, 19631976, can be calculated as a step function using annual increments in t derived from the Pb-210-river discharge g~hronoIogy. The value for the excess Pb-210 flux (20 dpm cm-’ yr-‘1 can be used in Eqn.

The exponential

TRANSPORT

MECHANISM

947

FOR *“‘Pb

DATE 1974

1972

1970

1966

1969

1964

I’

YEARS

I u)

2 2.0

DEPTH km) FIG. 6. Cs-137/Pb-210, activity ratios (decay corrected to date of deposition) versus sediment depth (lower axis) and date of deposition (upper axis) for core 3 111. The product of this ratio and Fpc2,,,, (=20 dpm cm-* yr-‘) equals the sediment Cs-137 flux (Fc,_,&r)) indicated on right hand axis. Inset illustrates bi-monthly Cs-137 deposition record (1.5 X Sr-90 deposition fluxes) for New York City. Model results for Fc,_,&)) correspond to total Cs-137 flux @) and contribution due to soil erosion from drainage basin (A) for rate constant values given in text.

4 to derive Fc,_,~,(~) as a function of sediment and time as illustrated in Fig. 6.

depth

The steady-state atmospheric flux of Pb-210 at the latitude of the Saguenay Fjord is approximately 1.O dpm cm-’ yr-’ (Turekian et al., 1977). The much higher flux to the sediment of 20 dpm cm-’ yr-’ reflects the high deposition rate of fine-grained material at Sta. 18 which causes ‘focusing’ of atmospherically-derived Pb-210 in these sediments. Similarly, the sedimentary Cs-137 fluxes estimated from the core analysis are generally an order of magnitude greater than the atmospheric fluxes (inset, Fig. 6) at the time of sediment deposition. The Cs-137 flux geochronology (Fig. 6) is based on a constant flux depositional model for Pb-210. Variations in the Cs137/Pb-210 flux ratio between adjacent intervals of sediment may reflect small seasonal variations in the Pb-210 flux but the average annual Cs-137 fluxes calculated using this normalization procedure should be a good approximation to the actual values for deposition at Sta. 18. The Cs- 137 flux distribution in core 3 111 lags the atmospheric input function because of the residence time of particle-associated Cs-137 within the water column and soil compartments of the Saguenay drainage basin. The radionuclide transport model,

outlined below, provides a basis for estimating the time constants associated with the passage of fallout radionuclides through this fluvial-marine system. 3.0 RADIONUCLIDE TRANSPORT MODEL

3.1 Cesium-137 model results The mechanism for the introduction of fallout Cs137 into the sediments can be divided into two components; (1) immediate adsorption of atmospherically-derived Cs-137 onto particles in the water column followed by deposition in the sediments and (2) the erosion of soil particles and their inventory of sorbed radionuclides from the drainage basin with subsequent river transport of these particles through the water column to the sediments. The geochronology for Cs-137 fluxes to the sediments can be interpreted within the context of a two component, box model, consisting of the combined Lac St.-Jean, Saguenay River and upper Saguenay Fjord water system, having a surface area, A,, of 1000 km*, and the Saguenay drainage basin, having a surface area Ans, of 78,000 km*. The time-dependent flux of Cs137 into unit area of the drainage basin is equal to the atmospheric fallout flux, Zti.,~,(t)(dpm cm-* yr-‘). The flux, Sc,_137(t),into unit area of the water

94x

i. N. SMITH AND K. El.I.lS

h-137 A

OUT

SOlL

EROSION

(h-137

FLUX

SAGUE~AY

MODEL FOR

DRAINAGE BASIN

FLUX

FLUX

REMOVAL

FLUX

FIG. 7. Schematic of G-137 fluxes through Saguenay Drainage Basin. The magnitude of the Cs-137 fluxes is governed by equations 6 to 10 given in the text.

system (Fig. 7) includes the fallout flux and an additional term due to the erosion of particle-associated Cs-137 from the drainage basin;

Here, [Acr_137jsoi&f) (dpm cm-‘) is the time dependent inventory of es-137 in the drainage basin and Xna is a first order rate constant for erosional transport of Cs- 137 from unit area of the drainage basin into unit area of the water column, The residence time, ToB, for Cs-137 in the soil phase of the drainage basin is inversely proportional to &a. i.e., TDB= In Z/Xna. The soil inventory of es-1 37 can be calculated as a function of time from the atmospheric input function if it is assumed that losses of Cs- 137 by particle erosion and leaching by ground water are negligible compared to losses by radioactive decay. This is a reasonable assumption in view of the small losses of Cs-137 (less than 3% of the total inventory by 1972) reported for a small, forested watershed in the Mississippi River drainage basin by Ritchie et al. (1974). By 1976, the Cs- I37 soil inventory calculated in this manner had attained a value of 26 dpm cm-*. Inputs of Cs-137 from the water column to the sediments will be delayed by a factor pro~rtional to the residence time, T.,,, of es-137 in the water column. The time-dependent inventory, [Ac~_~~~]~(~), of Cs-137 in the water column is governed by the equation; soil

=

&s.df)

-

(Xw

+

~zfIdC,.~37lw(~~

(6)

where Sc,,37(t) is the input flux (Eqn. 5) and A, (=ln 2/T,) is the rate constant for removal of Cs137 from the water column. This rate constant is equal to the sum of the rate constants, X,0, for advection of Cs- 137 from the confines to the box model system, and, XP, for sediment deposition of Cs-137, both of which are assumed to be first order processes. It is implicitly assumed in this model that both the soil erosion and direct atmospheric com~nents of the es-1 37 input follow the same transport pathway in the water column, Equation 6 is a linear, first order equation whose solution can be expressed, using an integrating factor, e(hr+X2)1, as;

for the boundary condition, [dcS.,371w= 0 at t = 0. The above integration cannot be performed directly because neither fcs_137(t)nor [Ac~_~~~]~~~(~) are analytical functions of t. However, equation (7) can be approximated by the following discrete summation using a monthly running index, to (At0 = 1 month), the monthly fallout record for Ic+,37(f), calculated monthly values for [Acrl,t]soir(t) and assumed values for the rate constants X~Band A*;

This approximate soiution to equation 7 is valid for Cs-137 residence times (T,) in the water column

TRANSPORT MECHANISM FOR *“Pb

several times greater than a month, which is also the level of resolution of the Cs- 13’7input function data. The actual es-137 flux to the sediments at an arbitrary depositional site (Sta. 18) is then given by; F,,,&t)

=

L[&.,,,lw(~)

(9)

where X, is a site specific, first order rate constant for deposition of Cs-137 in the sediments at Sta. 18. Note that A, is one of an ensemble of site specific rate constants whose average value for the entire s~imentation regime of the box model corresponds to the Xp component of X, in Eqn. 6. The value of X, reflects particle settling velocities, water circulation and other physical and chemical parameters which govern the deposition of G-137 at Sta. 18. Fcrl&t) can be determined numerically from equations 8 and 9 using an atmospheric input function, Zcr.& t) (==1.5 X Is&t)) calculated using Sr90 fallout data for New York City (Health and Safety Laboratory, 1977). A set of F~s_,~~(f)values computed using the rate constants, X, = 0.693 yr-’ (Z’, = 1 yr), Xna = 4.6 X 10m4yr-’ (7’na = 1SO0 yr), and h, = lo,0 yr-’ is compared in Fig. 6 (denoted by circled triangles) to the experimental results. The lower set of computed Cs-137 flux values (triangles) corresponds to the soil erosion component of the total Cs- 137 flux to the sediments. The choice of the above rate constant values, which give good agreement between the computed and experimental values for F,,,,,(r), is based on the following considerations. The soil inventory of Cs-137, [ACs_137]roii(f)t is a more slowly varying function of f through the 1960’s compared to the atmospheric input function, Zcs_,&l), and, consequently, it is the direct atmospheric component of the input function in Eqn. 8 which governs the short term (annual) variability in the computed es-l37 flux geochronology. If the residence time of Cs-137 in the water column is short (less than a month), then a steady-state inventory of Cs- 137 would be maintained in the water column which would be proportional to the atmospheric input function, Ze&t). However, Z,.,,,(r) attains a maximum in 1963 (inset, Fig. 6) and the measured maximum in the G-137 flux to the sediments occurs in 1964. Therefore, the sedimentary record at Sta. 18 indicates that there is a delay of the order of 1 year, in the delivery of atmospherically derived Cs-137 to the sediments and it is this observation which dictates the choice of X, = 0.693 yr-’ (‘Z’, = 1 yr) in the transport model. The use of this rate constant value results in a computed maximum for FCs-&f) in 1964 which is in agreement with the experimental results. Most of the surface area of the water component of the transport model is occupied by Lac St.-Jean, whose water residence time (estimated from the ratio of total water mass/average river discharge rate) is approximately three months (Ouellet, 1979). The residence time of particles in the Saguenay River and in the water column at the extreme head of the Sa-

949

guenay Fjord is of the order of days to weeks. The one year residence time for Cs- 137 in the water column, inferred from the experimental data, probably corresponds to its residence time in Lac St.-Jean. The larger G-137 residence time compared to the average water residence time in the lake would be consistent with rapid sorption of Cs-137 on to particles (Stanners and Aston, 1981) that were temporarily trapped within the catchment basin of Lac St.-Jean prior to their transport into the fjord. Periodic resus~nsion of Cs- 137 rich, bottom sediments during lake overturn or during storm events could also contribute to the water column inventory of Q-1 37. Similarly, tidal exchange between the fjord and the St. Lawrence estuary (Sundby and Loring, 1978) could result in an additional advective component to the G-1 37 flux (and Pb-210 flux; Carpenter et af., 1981) at Sta. 18, although the low Pu-239,240/G137 activity ratios measured in the sediments, and discussed below, indicate that radionuclide inputs from the marine component of the box model are small compared to inputs from the freshwater component. However, both of the above processes could result in an increase in the ‘apparent’ water residence time for Cs-l37 and the value of ‘Z’,may be partially determined by the rate of Cs-137 exchange at the boundaries of the idealized, box model system. A second effect of the one year residence time for G-1 37 in the water column is to average out or damp the annual m~ulations in the atmospheric input function. This averaging effect results in a comparatively smooth, computed, sediment flux of Cs-137 as a function of time, in agreement with the experimental results. However, there is some experimental evidence for elevated Cs- 137 fluxes in the coarser grained, landslide-derive sediment layers observed in the upper portion of the core. This effect may be due to post-depositional migration of es-137 from the finer grained, mud layers into the adjacent, Cs137 deficient, landslide-derived layers of material. The value for X, of 10 yr-‘, used to fit the experimental and model results, is an order of magnitude greater than the rate constant, X, = 0.693 yr-‘, for removal of Cs-137 from the water column by processes other than radioactive decay. The ratio, &,I X,, is defined as the focusing factor,f. The high value for the focusing factor of 14.4 for Cs-137 reflects the relatively high depositional flux of Cs- 137 across the sediment boundary at Sta. 18, compared to the average removal flux, both by advection and sedimentation, across all other boundaries of the box model system. An important feature of the experimental results which is predicted by the transport model is the comparatively large Cs-I 37 Aux to the sediments during the late 1960s and 1970s. During this period the atmospheric input flux was small compared to its magnitude during the early 1960s. This feature is interpreted on the basis of the model results to be due to delayed inputs of Cs-137 associated with soil parti-

9.50

J. N. SMITH AND K. ELLIS DATE 1970

1968

1966

1964 ‘7

SNAP - SA EVENT

+

CiJ Ii-l

TURFACE

DEPTH

AIR

(cm)

FIG. 8. Upper panel illustrates measured Pu-239,240 flux dependence on sediment depth (lower axis) and date of sediment deposition (upper axis) for core 3111-A. Transport model results correspond to; (A) Pu-239,240 atmospheric input function equal to 0.018 X Is..&), (Harley, 1975) and same rate constant values used for 0-137, (0) same as above, except T us = 3ooO yr, and (X) Pu-239,240 input function of 0.012 X Is,+&), (Thomas and Perkins, 1975) and same rate constant values used for Cs137. Lower panel illustrates measured Pu-238/Pu-239,240 activity ratio versus sediment depth and deposition date. Crosses denote average surface air activity ratios (Richland, Wash.) used to estimate the Pu-238 input function. Circles denote sediment Pu-238/Pu-239,240 activity ratios predicted by transport model for rate constant values noted above (Tna = 3000 yr).

cles eroded from the Saguenay drainage basin. Note, that during the 1960’s the soil erosion component was small compared to the total Cs-137 flux to the sediments (Fig. 6) but by the 1970’s 80% of the Cs137 flux was derived from particle erosion from the drainage basin. The physical significance of XDBis discussed in greater detail in the following sections of this report. 3.2 Plutonium-239,240

results

Plutonium-239 and Pu-240 are particle reactive, fallout radionuc~ides whose atm~pheric deposition function since 1962 has been proportional to that of Cs-137 (Thomas and Perkins, 1975; Harley, 1975). The Pu-239,240 sediment depth profile (analytical uncertainties average + 10%) between 90 cm and 150 cm in core 3111-A (Fig. 3) exhibits the same general features as the Cs-137 profile in core 3111, with the highest plutonium activities occurring in sediments deposited during 1963 and 1964. The Pu239,240 activities have been normalized to the excess Pb-2 10 activities for core 3 111 -A using a generalized version of Eqn. 4, giving the Pu-239,240 flux profile illustrated in Fig. 8. The Pu-239,240/C+ 137 flux ratios for corresponding (synchronous) sediment intervals from each half

of the core average O.Oi I between 1963 and 1970. This ratio is in good agreement with the fallout activity ratio (Pu-239,24O/Cs-137) of 0.012 estimated from measurements conducted on stratospheric particulate samples collected during the early 1960s (Harley, 1975). However, Thomas and Perkins (1975) have estimated the Pu-239,24O/Cs-137 ratio in fallout debris to be 0.008 based on a series of measurements conducted on atmospheric particulates collected at Richland, Washington (46 deg. N.) between 1962 and 1973. Pu-239,24O/Cs-137 ratios in soil tend to be close to fallout debris ratios (Hardy et al., 1973), while elevated ratios (0.01-0.05) have been measured in freshwater sediments (Edgington and Robbins, 1975a,b). In contrast, Pu239,24O/Cs- 137 ratios in marine sediments can be greater by an order of magnitude (Livingston and Bowen, 1979) as a result of reduced particle/water distribution coefficients for Cs in higher ionic strength media such as seawater. The low Pu-239,24OfCs-137 flux ratios measured in sediments at Sta. 18 indicate that particle inputs at this location are almost exclusively derived from freshwater environments and that negligible desorption of Cs- 137 from suspended particles has occurred prior to deposition from the marine waters at the head of the fjord. The radionuciide transport model previousiy des-

TRANSPORT MECHANISM FOR 2’@Pb

cribed for Cs- 137 can also be applied to determining the time-dependent fluxes for Pu-239,240. The discrete summation approximation (Eqn. 8) and the same set of rate constant values for ADB,Xw,and &,, used in the transport mode1 for Cs-137, have been employed in fitting the Pu-239,240 data (note; X2 N 0 for Pu-239,240). The plutonium, atrn~pho~c input function can be estimated from the Sr-90 fallout data for New York City (Health and Safety Laboratory, 1977). The upper set of model results (triangles; Fig. 8) corresponds to a Pu-239,240 atmospheric input function &239,&t) = 0.018 Zsr.90 (t)) based upon the results of Harley (1975) and the lower set of results (X) is derived from an input function (=0.,012 &,0(t)) based upon the results of Thomas and Perkins (1975). The third set of model results (0) corr~~nds to a plutonium, drainage basin residence time, Tas = 3000 yr, again computed using the Pu input function from Harley (1975). In general, changes in Xua have a proportionally greater effect on the post-1967 portion of the modeled Pu flux distribution, while changes in X, affect the entire distribution by the same factor and changes in X, affect both the shape (position of the peak) and magnitude of the entire distribution. Comparison of the model results (Fig. 8) for drainage basin residence times for Pu of 1500 yr and 3000 yr indicates that the latter provides better agreement (generally within the ?lS% analytical uncertainties in the measured fluxes) with the experimental results. For the 3000 yr drainage basin residence time for plutonium 50% of the flux to the sediments by 1971 would have stemmed from the erosion of soils from the drainage basin. However, in view of the more limited data set available for the Pu-239,240 sediment fluxes, uncertainties in the atmospheric input function for plutonium and the simplicity of the transport model itself, con~rmation of a significant difference in Cs137 and plutonium drainage basin residence times would require further analyses of longer sediment cores, containing the complete, post-1950 fallout record. In contrast to the results of core 3111, fallout radionuclide profiles have been measured in other unbioturbated estuarine or coastal sediments which increase, almost continuously, upwards in the sediment column through recently deposited (post-1967) material (e.g., Koide et al., 1975, 1980). It may be possible to explain these profiles, using the present model, by a smaller residence time for the radionuclide in the drainage basin, by a smaller ratio, A,/ ADB,of the water surface area to the drainage basin area, or by a proportionally longer residence time for the radionuciide in the water column. 3.3 Plutonium-23% remIts The Pu-238 sediment~epth profile (analyti~l uncertainties average + 40%) between 90 and 150 cm (Fig. 3) is markedly different from that of Pu239,240 and reflects the different characteristics of

951

the Pu-238 atmospheric input function. Prior to 1964 the ratio Pu-238/Pu-239,240 in nuclear weapons fallout debris was estimated to be 0.024 based on measurements conducted on a large number of soil samples collected in northern latitudes (Hardy et al., 1973). In April, 1964, a navigational satellite carrying a SNAP-9A nuclear power generator containing 17 kCi of Pu-238 was burned up, upon re-entry into the southern hemisphere stratosphere, resulting in a tripling of the global inventory of Pu-238 by 1970. SNAP-9A derived Pu-238 was observed in surface air particulate samples in 1966 at Richland, Washington (Thomas and Perkins, 1975) evidenced by Pu-238/Pu-239,240 ratios greater than levels typical of nuclear weapons tests debris. The Pu-2381 Pu-239,240 ratio in this material attained values averaging 0.4 (with considerable month to month variability) during the spring of 1967. It gradually decreased to 0.2 in 1970 and then decreased rapidly to pre-SNAP-9A levels in 1971 as illustrated by the averaged set of surface air ratios denoted (crosses) in Fig. 8. The atmospheric input function for Pu-238, Zru_238 (r), has been estimated on the basis of surface air Pu-238 /Pu-239,240 ratio data of Thomas and Perkins (1975) and a pre-SNAP-9A fallout ratio of 0.024 (Hardy et ol., 1973). Using the same set of rate constants noted previously for Pu-239,240 (TDe = 3000 yr; XZ= 0.693/87 yr-’ for Pu-238), the radionuclide transport model has been used to predict sediment Pu-238/Pu-239,240 ratios as a function of time and these values are compared to the measured sediment ratios for core 3111-A in Fig. 8. Sediment deposited prior to 1967 is characterized by Pu-238/ Pu-239,240 ratios averaging 0.03, excluding a high value of 0.17 measured in the 149- 150 cm interval which may represent contamination introduced at the bottom of the core during the coring operation by downward transport of more recently deposited sediment trapped in the core catcher. Although the SUPface air ratios increased sharply in 1966, the model predicts a measurable threshold increase in the scdiments in 1967 as a result of delayed inputs to the fjord sediments associated with a one year residence time of fallout radionuclides in the water column. Measured sediment ratios of Pu-238/Pu-239,240 through 1966 and 1967 are near or below the detection limit, but elevated ratios are evident in sediment deposited in 1968. Although the surface air activity ratios were in the range of 0.15-0.30 between 1968 and 1971, the transport model predicts sediment ratios of 0.10 during this time period for TDB= 3000 yr (Fig. 8) and a reduced ratio of 0.08 for TDB= 1SO0 yr. The difference in surface air and sediment ratios is due to isotope dilution of the Pu-238 atmospheric inputs by partide inputs from the drainage basin characterized by pre-SNAP-9A activity ratios of 0.024, in addition to averaging effects associated with the one year residence time of Pu in the water column.

952

I. N. SMITH AND K. ELLIS

The measured Pu-238/Pu-239,240 activity ratios, averaging 0.13 + 0.05 in sediment deposited subsequent to 1967 are in better agreement with the model predictions for Ton = 3000 yr compared to those for shorter drainage basin residence times. However, analytical uncertainties in the experimental results and the uncertainty in the Pu-238 atmospheric input function for the Saguenay region, inferred from the considerable spatial and temporal variability in Pu238 concentrations measured in surface air during the 1960’s and 1970’s (Health and Safety Laboratory, 1976), are greater than the 20% difference in Pu-238/Pu-239,240 ratios predicted by the model for the two drainage basin residence times noted above. The threshold horizon for SNAP-9A-derjved, Pu-238 inputs has been previously detected in permanent ice sheets in Greenland (Koide et al., 1977), in Antaractica (Koide et al., 1979; Cutter et RI., 1979), in Rocky Flats, Colorado, lake sediments (Hardy et al., 1980) and in coastal marine sediments (Koide et al., 1975). It has not been previously observed, to our knowledge, in a clearly dated, estuarine sedimentary record. 3.4 Radionuclide

transport model discussion

The key features of the measured Cs- 137 and Pu239,240 flux geochronologies and Pu-238/Pu-239,240 activity ratio profiles are predicted by the transport model for a radionuclide, drainage basin residence time, of 1500 yr, although slightly better agreement is obtained for a residence time of 3000 yr for plutonium. These residence times correspond to the removal of 0.046%( 7’na = 1500 yr) and O.O23%(Toa = 3000 yr) of the total fallout inventories each year. Cesium and Pu have markedly different characteristics in the nature of their chemical association with soil particles. Plutonium is associated with a reductant-soluble hydrous oxide phase of the soil matrix (Muller, 1978) while Cs exhibits cation-exchange characteristics (Tamura, 1964). From their measurements of a strong correlation between Cs-137 and Pu-239,240 throughout the soil and sediment phases of two dissimilar midwestern watersheds Muller et al. (1978) concluded that both radionuclides are strongly attached to soil particles and that there is minor fractionation or sorting between the different soil components with which each is associated during transport through aquatic systems. The low drainage basin removal rates estimated for these radionuclides in the present study are also consistent with their strong association with soil particles, although there is some evidence for preferential transport of Cs compared to Pu, Annual transport of Pu-239,240 by soil erosion from a 1,401 km2 flat, agricultural watershed in Ohio was estimated by Sprugel and Bartelt (1978) to be 0.05% of the total steady-state inventory on the basis of plutonium measurements conducted on riverborne suspended material. The lower removal rate

inferred from the dynamic, time-dependent model employed in the present study may be a consequence of the forested nature of the Saguenay watershed and its greater size (78,000 km2), characteristics which tend to result in lower soil erosion rates on an area basis (Glymph, 195 1). Similarly, Cs- 137 removal rates from a much smaller (1.2 km’) forested watershed in Mississippi are almost an order of magnitude greater (~0.3% of the total inventory each year; Ritchie ef al., 1974) than the rate estimated in the present study. The radionuclide transport model can also be applied to the case of Pb-210. Since the atmospheric input function for Pb-210, IP,,_~,~~~, is constant (1.0 dpm cm-’ yr-’ f as a function of time, Eqns. 7 and 9 (expressed in terms of Pb-210 activity) can be solved analytically for the steady-state excess Pb-2 10 flux to the sediments giving;

Here, the steady-state soil inventory of excess Pb210 is calculated from the ratio zrb.zt&z/& (=32.3 dpm cmV2). For the rate constant values previously used to fit the Cs-137 sediment flux data, Eqn. 10 gives a Pb-210 flux of 29.9 dpm cmw2yr-I, a factor of 50% greater than the average flux of 20 dpm cm-* yr-’ estimated for core 3 111. Better agreement with the measured Pb-210 flux is obtained if the 3000 yr drainage basin residence time (Xun = 2.3 X 10e4 yr-I), consistent with the plutonium results, is used in the above equation, in which case the calculated Pb-210 flux is 2 1.8 dpm cme2 yr-‘. Lewis ( 1977) estimated the first order rate constant for the removal of Pb-210 from the watershed of the West Branch of the Susquehanna River to be 5 X lO-4 yr-‘. A lower rate constant for Pb-210 removai from the Saguenay River drainage basin may reflect differences in characteristics of the respective drainage basin systems such as geomorphology and soil erodibility. Despite the different atmospheric input functions for Cs-137, Pu-239,240, Pu-238 and Pb-210, the same focusing factor, j(= x,/x,) of 14.4 for deposition at Sta. 18 provides reasonable agreement between the model and experimental results. This result indicates that minor fractionation occurs between these radionuclides during transport through the water column to the sediments at the head of the fjord. 4.0 CONCLUSIONS 1. Annual modulations in the Pb-210 sedimentdepth profile at the head of the Saguenay Fjord are caused by a bi-modal, sediment deposition mechanism that is characterized by pulsed inputs of coarser-grained silts and sands during high, spring river discharge events superimposed on the ambient

TRANSPORT

MECHANISM

sedimentation pattern of finer grained clays and organic matter. 2. An inverse correlation between the historical record of river discharge and the sediment-depth profile for Pb-210 has been used to determine a precise sediment geochronology, having a time-stratigraphic resolution of better than one year between 1963 and 1976. 3. Despite large seasonal fluctuations in sedimentation rates, Cs-137 and Pu-239,240 fluxes to the sediments can be estimated as a function of time by the normalization of sediment Cs-137 and Pu239,240 activities to excess Pb-210 activities and by the assumption of a constant flux Pb-210 deposition model. 4. The geochronology for Cs-137 and Pu-239,240 fluxes to the sediments can be simulated by a transport model which distinguishes between particle-associated radionuclide inputs due to soil erosion from the drainage basin and inputs resulting from direct adsorption of atmospherically-derived radionuclides onto particles in the water column. Agreement between predicted and measured fallout radionuclide fluxes is obtained for (1) a residence time of 1 year for each radionuclide in the water column (or catchment basin of Lac St.-Jean) prior to deposition in the sediments at the head of the fjord, and (2) a radionuclide residence time of 1500 years in the soils of the drainage basin, although there is some evidence for a longer (3000 yr) drainage basin residence time for plutonium. 5. Both the threshold for the initial appearance of SNAP-9A derived Pu-238 in sediment strata deposited in 1967-1968 and the magnitude of its isotopic dilution by soil erosion inputs of Pu-239,240 are in agreement with the transport model predictions. Acknowledgements-The authors wish to express their thanks to Dr. C. T. Schafer, Dr. J. M. Bewers, Dr. J. Syvitski and Dr. P. Yeats for their helpful discussions and review of this manuscript and to J. Barron for his computational efforts. We also extend our gratitude to Mr. J. Bishop and Ms. M. Huh who performed many of the analyses reported in this paper and to the Environmental Analysis Laboratories (Richmond, Cal.) where the plutonium analyses were performed. We are particularly indebted to Dr. K. Kranck who provided the particle size analyses.

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FOR *?b

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excess ‘u’Pb and 239*2”‘Pu.Earth Planer. Sci. Lett. 53, 158-174. Carpenter R., Bennett J. T. and Peterson M. L. (1981) 2’aPb activities in and fluxes to sediments to the Washington continental slope and shelf. Geochim. Cosmochim. Acra 45, 1155-1172. Carter M. W. and Moghissi A. A. (1977) Three decades of nuclear testing. Health Phys. 33, 55-71. Cutter G. A., Bruland K. W. and Risebrough R. W. (1979) Deposition and accumulation of plutonium isotopes in Antarctica. Nature 279, 628-629. Edgington D. N. and Robbins J. A. (1975a) The behaviour of plutonium and other long-lived radionuclides in Lake Michigan. II. Patterns of deposition in sediments. In Impacts of Nuclear Release into the Aquatic Environment. IAEA, Vienna, pp. 245-260. Edgington D. N. and Robbins J. A. (1975b) Patterns of deposition of natural and fallout radionuclides in the sediments of Lake Michigan and their relation to limnological processes. In Environmental Biogeochemisrry Vol. 2 (ed. J. 0. Nriagu) Ann Arbor Sci. Pub]., Ann Arbor, Mich., pp. 705-729. Edgington D. N., Alberts J. J., Wahlgren M. A., Karttunen J. 0. and Reeve C. A. (1976) Plutonium and americium in Lake Michigan sediments. Transuranium Nuclides in the Environment. IAEA-SM-199147. IAEA. Vienna 493-516. Glymph L. M. (1951) Relation of sedimentation to accelerated erosion in the Missouri River Basin. USDA, SCSTP-102, 6 p. Hardy E. P., Krey P. W. and Volchok H. L. (1973) Global inventory and distribution of fallout plutonium. Nuture 241.444-445. Hardy E. P., Volchok H. L., Livingston H. D. and Burke J. C. (1980) Time pattern of off-site plutonium deposition from Rocky Flats Plant by lake sediment analyses. Environ. Int. 4, 21-30. Harley J. H. (1975) Transuranium elements on land. Health Safety Lob. Q. Rep. HASL-291. l-104. Health and Safety Laboratory Fallout Program (1976) U.S.A.E.C., HASL-308 (Appendix) Bll8-B126. Health and Safety Laboratory Fallout Program (1977) U.S.A.E.C., HASL-318 (Appendix) A73-A75. Koide M., Griffin J. J. and Goldberg E. D. (1975) Records of plutonium fallout in marine and terrestrial samples. J. Geophys. Res. 80, 4153-4162. Koide M., Goldberg E. D., Herron M. M. and Langway C. C. Jr. (1977) Transuranic depositional history in South Greenland firn layers. Nuture 269, 137-139. Koide M., Michel R., Goldberg E. D., Herron M. M. and Langway C. C. Jr. (1979) Depositional history of artificial radionuclides in the Ross Ice Shelf, Antartica. Earth Planet. Sci. Lett. 44, 205-223. Koide M., Goldberg E. D. and Hodge V. F. (1980) *“Pu and 24’Am in sediments from coastal basins off California and Mexico. Earth Planet. Sci. Lett. 48, 250-256. Kranck K. (1979) Dynamics and distribution of suspended particulate matter in the St. Lawrence Estuary. Nururuliste Can. 106, 163-173. Kranck K. (1980) Experiments on the significance of flocculation in the settling of fine-grained sediment in still water. Can. J. Eurth Sci. 17, 1517-1526. Kranck K. and Milligan T. (1979) The use of the Coulter counter in studies of particle size distributions in aquatic environments. Bedford Institute of Oceanography, Report Series BI-R-79-7. Lewis D. M. (1977) The use of “‘Pb as a heavy metal tracer in the Susquehanna River system. Geochim. Cosmochim. Acta 41, 1557-1564. Livingston H. D. and Bowen V. T. (1979) Pu and ‘r’Cs in coastal sediments. Earth Planet. Sci. Len. 43, 29-45. Loring D. H. and Rantala R. T. T. (1977) Geochemical analysis of marine sediments and suspended particulate

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AND K. I-.LLIS Smith J. N., Schafer C. T. and Loring D. H. (1980) Depositional processes in an anoxic, high sedimentation regime in the Saguenay Fjord. In Fjord Oceanogruphy (eds. H. J. Freeland, D. M. Farmer. C. D. Levings), 625 63 I, Plenum. Smith J. N. and Ellis K. ( 1981) Transport mechanism for fallout Cs- I37 to estuarine sediments. Impacts of Radionuclide Releases into the Marine Environment. IAEASM-248/118, IAEA, Vienna. Smith J. N. and Loring D. H. (1981) Geochronology for mercury pollution in the sediments of the Saguenay Fjord, Quebec. Environ. Sci. Tech. 15, 944-95 I, Sprugel D. G. and Bartelt G. E. (1978) Erosional removal of fallout plutonium from a large midwestern watershed. J. Environ. Qual. 7, 175- 177. Stanners D. A. and Aston S. R. ( 198 I ) Factors controlling the interactions of “‘Cs with suspended and deposited sediments in estuarine and coastal environments. Impacts of Radionuciide Releases into the Marine Environment. IAEA-SM-248/141, IAEA. Vienna. Sundby B. and Loring D. H. (1978) Geochemistry of suspended particulate matter in the Saguenay Fjord. Can. J. Earth Sci. 15, 1002- 101 I. Tahir M. and Stewart J. W. B. (1975) Effect of organic matter incorporation into soils on “‘Cesium uptake by wheat plants. Radiat. Bot. 15, 323-328. Tamura T. (1964) Selective sorption reactions with mineral soil. Nucl. Su/: 5, 262-268.

of cesium

Thomas C. W. and Perkins R. W. (1975) Transuranium elements in the atmosphere. Health Safety Lab. Q. Rep. HASL-291, I-80. Turekian K. K., Nozaki Y. and Benninger L. K. (1977) Geochemistry of atmospheric radon and radon products. Annu. Rev. Earth Planet. Sci. 5, 227-255. Wong K. M. ( 1971) Radiochemical determination of plutonium in seawater, sediments, and organisms. Anal. Chim. Acta 56, 355-364.