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Chronological strategies and metal fluxes in semi-arid lake sediments
Geochimica et Cosmochimica Acta. Vol. 42. pp. 1559 10 1571 8 Pergamon Press Ltd. 1978. Printed in Great Britain
0016-7037/78/1001-1559302.00/0
Chronological strategies and metal fluxes in semi-arid lake sediments K. K. Department
BERTWE*
and S. J. WALAWENDER
of Geological Sciences, San Diego State University, San Diego, CA 92182, U.S.A.
and M. KOIDE Scripps Institution of Oceanography,
La Jolla, CA 92093, U.S.A.
(Received 4 April 1978; accepted in revised form 16 June 1978)
Ahatraet-The largest flux of sediments to a semi-arid lake (Barrett Reservoir) was stream influx after the winter rains. During the times of no stream flow, resuspension of previously deposited material dominated. Direct atmospheric transport was minor but atmospheric particulates were evident in the sediments due to stream influx of particles containing a significant atmospherically derived component. Only an anthropogenic lead influx was measurable in the sedimentary record due to the high sedimentation rate. In semi-arid areas where intermittent stream flow occurs, more than one technique must be used to obtain an overall sedimentation rate.
INTRODUCTION RECENT years the historical records maintained in the sediments of lakes (BERTINE and MENDECK 1978;
IN
et al. 1976), estuaries (GOLDBERG et al. 1977, 1978) and marine sediments (BRULAND et al. 1974; BERTINEand GOLDBERG1977) have been util-
GOLDBERG
ized to arrive at anthropogenic metal fluxes through time to these areas. The intent of this study was to look at the historical record of atmospheric metal transport to a reservoir in an isolated area east of San Diego, CA, where local sources of metal contamination were minimal. The use of the recently deposited sedimentary material to reveal anthropogenic metal fluxes depends upon obtaining the sediment accumulation rate. This has normally been accomplished by the Pb-210 method (see KOIDE et al, 1973). However, Pb-210 results are open to alternate interpretations in areas such as the one studied where particle-by-particle deposition may not occur over the whole year due to seasonality of rainfall and intermittent stream flow. Therefore, a variety of additional techniques must be employed in order to obtain a valid sedimentation rate. These geochronologies involve the use of storm layers, the times of addition of algicides, Th-228F-232 radionuclides, artificial radionuclide geochronologies and measuring the fallout of suspended sediment over the period of a year. Since the mineralogy and thus the chemistry of the suspended particulates collected over the time period of a year varied with seasonal river flow, the range in metal contents of these particulates was also measured to * Present address: Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A. Contribution No. 38 from the San Diego State University Center for Marine Studies.
see if such variations would complicate the interpretation of the historical record. Whether the atmospheric fluxes of the metals could significantly affect the sediment metal contents was addressed by collecting atmospheric particulates with a bucket sampler and comparing the metal fluxes therein with the sediment metal fluxes. Finally, the historical record was sought in two cores. RESERVOIR
DESCRIPTION
Barrett Reservoir is located 33 km east of San Diego, California in the Cleveland National Forest. It was built by damming a steep sided river valley in 1921-1922 to provide flood control and storage for the municipal water supply of the City of San Diego. It is 2.3 km long, with an average width of 0.25 km and a usual depth of 20 m at the dam. The intermittent streams draining into Barrett flow after the winter rains and the occasional spring snow melt, but usually not during the summer-fall seasons. Due to the lack of extensive vegetative cover, dust derived from the surrounding hills has often been observed in the valley. Since the reservoir is free from direct entry of pollutants, the entry of any pollution is anticipated to be via the atmosphere from distant sources. METHODS Sample collection Suspended particuhtes. A funnel type suspended sediment trap (designed by A. Soutar) was suspended 5 m below the water surface and 5 m above the bottom in the water column about 100 m from the dam at Barrett Reservoir for a year beginning 13 November 1975. The top of the funnel consists of a 22.7 x 22.7 cm gridded (1 I_&) plastic. SOUTAR (personal communication) extimates that the trap efficiency is between 85 and 100%; 90% is used in the calculations. The particulates were collected at approximate monthly intervals. After extraction, they were dried at 1 10°C weighed and ground. The flux of the solids was then computed from the weight, the surface area of the trap, the time exposed and the trap efficiency.
1559
K. K. BERTINE,S. .I. WALAWENDER and M. KOIDE
1560
Atrnosphrric particulatrs. In order to measure atmospheric input to the reservoir. a bucket sampler with grid (see HODGEet al. 1978) was placed on the water surface immediately above the suspended sediment trap for the period 9 April 1976 to 18 May 1976. At the end of that period, the feathers, bees and other insects were removed. Then the solids were removed from the bucket with successive washes (totalling 150ml) of distilled water. This mixture of solids and water was rotated at 1 rps for a day at room temperature, centrifuged, and the soluble and insoluble fractions dried at 110°C and weighed. The fluxes of the soluble and insoluble materials were computed. Seditnents. Two cores were taken from Barrett Reservoir. A short core (66cm) was taken in February 1975 and a longer core (80 cm in length) in April 1976 by divers gently placing polyvinyl chloride tubes into the sediment. The two cores were immediately frozen. A 1cm thick slab was sawed from each core and X-rayed to ascertain the presence of sediment structure. The remainder of the core was then scraped to remove any contamination from the sawing. The top 32cm of the shorter core was sampled at one cm interval and the remainder according to the layering evident in the X-radiograph. The longer core was sampled only in its deeper portions according to the layering overlapping the same bottom layers of the shorter core.
Opal contents were ascertained by the method of GOLDBERG(1958). Quartz and paglioclase contents were determined by X-ray diffraction.
RESULTS Sources of
sediment
The monthly flux of particulate material to the suspended sediment trap varied over the year from 0.36 to 1.68 g/cm’/yr (Table I). The integrated flux was 0.89 g/cm’/yr. Although the two highest fluxes in the suspended sediments occurred during the months with the highest rainfall (Table I) there is no correlation between rainfall and particulate flux. Five sampling intervals of about equal duration had similar fluxes (0.3W.46g/cm2/yr) yet the rainfall over those intervals was highly variable, ranging from 0.02 to 7.19 cm. Changes in biological productivity over the time period of the year do not account for the lack of correlation. The sum of the opal, CaCO, and organic (as CH,O) contents account for between 46.0 and 62.8% of the solids (Table 2). Correcting for these Ana/.vfica/ rwthods percentages, the non-biologic fluxes in the above Elemental concentrations. The solids were totally dissamples are still very uniform, ranging between 0.16 solved using successive applications of HCI. HF, HNO, and 0.21 g/cm2/yr (Table 1). and HC104. The concentrations of Ag, Al. Cd, Co, Cr. Cu. Fe, Mn. Ni, Pb. V and Zn were determined by atomic During many of the months there was no stream absorption spectrophotometry using a Varian AA6. The flow into Barrett Reservoir, yet significant fluxes of hydrogen background corrector was used when the wavenon-biologic particulates were collected in the suslengths were under 3000A. pended sediment traps. Geochronology. Pb-210 and Th-228/Th-232 were deterThe flux of the atmospheric fallout, determined mined following the method of KOIDE and BRULAND (1975). Pu-239 and 240 and Pu-238 were determined by from one bucket sample, was 0.0035g/cm2/yr. Comthe method of KOIDE et al. (1975). 13’Cs was analyzed paring these values with the mineral flux in the susby the addition of Cs carrier, followed by a separation pended sediment trap, direct atmospheric fallout can of Cs with ammonium phosphomolybdate and further puraccount for only about 1-2’4 of the sediment flux. ification from 4”K by use of Bio-Rex-40 cation exchanger. The Cs separate was precipitated then quantified for yield The possibility that the one atmospheric particulate as Cesium chloroplatinate (Cs,PtCI,). The “‘Cs activity flux in atypical must be considered. HODGE et al. (1978) was determined with an anticoincidence low background found only slightly higher atmospheric fluxes in beta counter. nearby La Jolla, California and Ensenada, Baja CaliParriculate composition. Organic carbon and CaCO, fornia, Mexico. Considering that the sea saIt comcontents were measured using a Leco carbon analyzer.
Table I. Fluxes to Barrett Reservoir suspended sediment traps, rainfall and resultant water level rise during the period 13 November 1975 to 4 November 1976 Flux (g/cm~/year)
(8) Period Collected
No. Days
llO°C Dry Weight
Total
Opal
cacoj
Organic (CH?O)
0.10 0.15 0.62 0.11 0.03 0.08 0.27 0.50
0.04 0.05 0.00 0.02 0.11 0.12 0.13 0.19
0.05 0.05 0.15 0.10 0.05 0.06 0.13 0.17
So"Rainfall biogenic(') (cm)
GaWXl? rise (m)
y 11/13/75-12/18/?5 12118/75-l/21176 l/21/76-2127476 '/27/76-4/Y/76 4/Y/76-5/18/76 5/18/76-6129/76 6/29/76-a/5/76 a/5/76-1114176
35 34 37 41 39 42 37 91
15.76 19.50 79.03 21.54 19.66 22.80 47.15 180.00
0.36 0.46 1.68 O.i,l('f 0.40 0.42 1.00 1.56
Total 356 Bucket 4/Y/76-5118/76 (insol.) 39 4/g/76-5110176 (sol.) 39
405.44
0.89
0.1323 0.0708
0.0023 0.0012
“‘Probably low since trap was moved to dam by currents. ‘z)Approximate since water was being pumped from reservoir. c3’Computed from total flux minus organic fluxes.
0.17 0.21 0.91 0.18 0.16 0.16 0.47 0.70
7.19 0.08 17.12 7.19 4.72 0.02 1.45 7.67
0.2 0.0 7.0 2. 4 0.6(-l 0.0 0.0 3.0
1561
Semi-arid lake sediments
Table 2. ~ineralo~
%
of sus~nd~
% CH20(1)
%
11/13/77-12/U/75 12118175~01/21176
28 33
~~~f2~fJ~O2f2Jf?6 OZf2J~~6-04~0~~~6 04/09/76-05/18/76 05/18/J+06/29/76 06/29/76-08/05/76 08/05/76-L1/04/76
37 28 20 19 27 32
sediments at Barrett Reservoir
11.5 10.5 0 5
28 29.5 13 12
91
Quartz
Ka0lilIiCe(2) Biotite
90Feldspar
13.5 11.8
6 7
28 34
2y$3f 12.0 14.3 13.3 11.0
lb 3.5 7.5 8 9 6
17 23 25 30 30 20
1.29 1.69 3.58 2.74
1.10 1.43 1.51 3.16
(“Calculated from organic carbon contents. (“Ratio based on peak areas from X-ray diffraction. @)Higb leaf content.
ponent is higher there, the agreement is reasonable. The fluxes at La Jolla were constant over the year studied whereas those at Ensenada were about a fattor of 3 higher in the winter months. Even applying a factor of 1.5 to the atmospheric flux at Barrett Reservoir, it is still less than 5% of the sediment flux. During the summer months when there is no stream flow into Barrett Reservoir, the atmospheric and biogenic fluxes account for only about half the fluxes. Other processes that can put materials into the suspended sediment trap include wave erosion of the banks, resuspension of previously deposited sediment and authigenic mineral formation. X-ray diffraction of the suspended particulates indicates the presence of calcite, quartz, hornblende, biotite, feldspar and kaolinite (Table 2). No significant authigenic mineral formation other than calcite is evident. Since oxygen is frequently depleted in the bottom waters, large amounts of amorphous iron oxides are unlikely to be present. During strong wind conditions it was observed that waves with white caps were formed and subsequently the water became quite turbid and cliffs of from 30 to 6Ocm were found eroded in the banks. Turbid waters were especially noticeable around the delta areas of the streams. Two different seasonal sources for the sediment are also indicated by their mineralogies. In the months where stream transport dominates, the particulates
are characterized by higher kaolinite and lower biotite, quartz and feldspar contents (Table 2) than in the months where erosion and resuspension dominate. The question of whether the resuspended material is derived from old material or recently deposited material can be answered by looking at the radioactive nuclides Th-232, its daughter Th-228, Pu-239 + 240 and Pb-210. Because of the very short half-life of Th-228 (T, = 1.9yr), an excess of Th-228 relative to Th-232 indicates the presence of recent materials (KOIDE et al. 1972). Th-228/I&232 ratios measured on three of the suspended sediment samples range from 1.12 to 1.55 (Table 3) indicating that there is a significant recent component. Environmental Pu-239 + 240 has been generated primarily from atmospheric weapon testing and is measurable in sediments deposited in the early 1950s (KOIDE et al. 1975; GOLDBERG et al. 1976). Its environmental levels reached a maximum in about 1963 and have decreased since. The Pu-239 + 240 activities on an organic-free basis in the suspended particulates range between 79 and 190dpm/kg (Table 3). The spring-summer months have slightly lower activities than the winter months. However, there is no marked difference between the resuspension dominated months of summer and the stream transport
Table 3. Radionuclides in Barrett Reservoir suspended sediments and atmospheric fallout
Suspended Sediments 1113/?5-12tl8175 12/X3/75-1/21/76 l/21/76-2/27/7u 2/27/76-4/Y/76 4/9/76-5/18/76 5/10/76-6/29/76 6f29/76-a/3/76 815/76-U/4/76
Th22e/Th232 1.55+0.06 1.12t0.02 1.35t0.04
Organic Free pu239+240 Pb210 dpm/kS dpm!g
,aa239+240
Pb210 dpmlcm /year
14.7 13.6 9.6 14.5 10.5
0.032 0.039 0.153 0.027
2.5 2.8 a.7 2.5
0.016
147
12.0
0.024 0.069 0.103
1.7 1.1
148
10.8 11.1
357270
38.1 5.0
189 190 169 152 103 151
pb21o/p"z39+2+0 78 71 57 95
102 71
5.2 8.4
75 a2
Bucket 4/9/76-5/B/76 (Insoluble) 4/g/76-5118176 (soluble)
0.0008
0.09 0.006
107 .
K. K. BERTINE. S. J.
1562
WALAWENDER
and M.
KOIDE
19551956 1957 1959 1959 I%0 1961 1962 I%3 1964 1965 1966 1967 1966 1969 1970 I971
1972
1973
1974 1975
1976
Fig. 1. Rainfall and water height measured at the dam at Barrett Reservoir from 1955 to 1976. dominated months of winter. This supports the conclusion that the sediment trap material in summer
is probably composed of resuspended sediment that was recently transported to the delta areas or to the banks of the reservoir by streams or by slope wash during the preceding rainy season. The primary source of Pb-210 (T+ = 22.3 yr) in sediments, which is unsupported, is from atmospheric Rn-222 (T+ = 3.84 days) which enters the air from rocks and soils after formation from its parent Ra-226 (T+ = 1620 yr). Atmospheric Rn-222 decays to Pb-210 which is then removed in precipitation. The other source of Pb-210 is sedimentary Ra-226 and can enter with older sediments in which radioactive equilibrium conditions probably exist. The Pb-210 activities (on an organic-free basis) in the suspended sediment range from 8.1 to 14.5 dpm/g (Table 3). Again there is no marked decrease in the resuspension dominated months indicating that the reworked sediment had recently been brought to the reservoir. A comparison of the Pb-2lO/Pu-239 + 240 ratio in the April-May suspended sediment sample (102) with that in the insoluble atmospheric particulates (107) collected at the same time indicates that the suspended sediment is mostly composed of unsupported Pb-210. Biogenic input
The sum of the opal, calcite and organic carbon (as CHrO) contents range between 46.0 and 62.8% of the solids (Table 2). Opal accounts for between 19 and 37x, calcite between 0 and 30% and CHrO between 9.0 and 24.3%. The calcite contents and fluxes are higher in the spring-summer months (Tables 1 and 2) coinciding with the algal bloom in Barrett Reservoir. Several varieties of freshwater algae and higher plants produce encrustations of calcite (rarely aragonite) during periods of active photosynthesis when
dissolved CO1 becomes depleted and HCO; is used instead as a carbon source (see discussions by ZUCHTBAUER 1974; HUTCHIN~~N 1975; and KUZNET~OV 1970). The reaction may be written as: Ca’ + + 2HCO;
- p,antS CaCOJ + CO, + HzO.
Such a mechanism would explain the higher amounts of CaCO, in the suspended particulates during those months of the year when photosynthetic activity was greatest. The large fluctuations in the CaCOJ contents account for most of the variability in the opal patterns. On a CaCOs contents account for most of the variability in the opal patterns. On a CaCO,-free basis the percent opal is more uniform ranging between 27 and 37%. The higher fluxes of organic matter (as CH,O) resulted from two different processes. The two major storms brought a large quantity of leaves and twigs to the reservoir resulting in higher organic contents. Secondly, algal decay contributed significant organic matter during the last two sampling periods when the reservoir became eutrophic with a moderate fish kill. Affects of the difirent elemental content;
particulate sources on the
The question remains: to what extent has the elemental composition of the sediments been affected by the different sources of the particulates? Al has been used for the normalization of other elemental concentrations and fluxes for both sediments (see e.g. BRULAND et al. 1974) and atmospheric particulates (e.g. HODGE et al., 1978). The argument is made that Al represents detrital crustal material of constant composition and any deviation from constant Al content with time or depth in a sedimentary column rep
1563
Semi-arid lake sediments
wm Zn
304
I . . . I , NDJFMAMJJASON
I
,
,
1 I
c
Fig. 2. Metal contents (on an organic-free basis) in suspended sediments collected from November
1975 to November 1976. resents *dilution by biological and/or anthropogenic activities. An analysis of the Al contents (on an organic free basis) in the suspended particulates indicates that the above assumption is not valid for Barrett Reservoir sediments due to changes in mineralogy over the year (Fig 2). The months with significant stream input have higher Al contents and high kaolinite/biotite ratios (Table 2). The resuspension dominated months have much lower Al contents and lower kaolinite/biotite ratios. Since kaolinite contains about 21% Al compared with about 7.5% Al for biotite, the changes in the relative kaolinite and biotite contents will affect the Al content of the particulates. Therefore, the normalization method of metal/Al cannot be used for these sediments. Instead, a preferable way
to assess variations in elemental contents over the year is by expressing them on a CaCO,, opal and organic matter (as CHIO) free basis (Fig. 2). These results are somewhat imprecise since they reflect the cumulative errors of all analyses. Unfortunately for the interpretation of the elemental variations, the City of San Diego Water Utilities Department replaced the badly weathered steel railing atop the dam during the first few months of our field program. It was cut off by acetylene torch in late October-early November 1975. Mn and Cd contents in the suspended particulates (on an organic-free basis) co-vary over the year (Fig. 2). Both are exceptionally high during the first sampling interval when the railing was being cut down. With succeeding sampling periods their concen-
K. K. BERTINE, S. J. WALAWENDER and M. KOIDE
1564
ments may not be decipherable within the sedimennations decrease to minimum values in the summer tary column. months. The gradual decrease is interrupted by the storm of 6 February which considerably diluted the Historical record Mn and Cd contents. The X-ray pictures revealed distinct layering (Fig. 3) In late December a new railing of steel was welded which could be correlated from one core to the other. into place. During the interval when the new railings The strata possessed different colors. The light layers were being welded in place, the Fe, Cu, Zn and Ag on the X-ray were brown and characterized by lower contents are higher in the suspended sediments. These water contents (Fig. 4) whereas the dark layers were increases are most probably a result of the welding almost black with higher water contents. The light process. brown layers seen to represent periods of storm depoCuSO, was added as an algicide in late August sition 1976. The Cu content in that sample increases to The top 13 cm of the shorter core was black with 609 ppm (on an organic free basis). Mn, Fe, Co and worms followed by a light brown layer (13-19cm). Ni contents are also higher in this sediment compared Other light layers occurred at 2427, 3 l-32, 37.7-39.5, to the other months (Fig. 2). The higher Mn and Fe 5658.7 and 6&66cm depths. The remainder of the concentrations may result from a greater precipitation core was black. Small 2-3 cm worms were found to of their carbonates. The Co and Ni may either be a depth of about 30cm. They show up on the X-ray sorbed onto these carbonates or have resulted from as black dots. Their frequency decreased with depth. impurities in the CuSO,. Soon after its deployment, the 27 February 1976 to Sediment ,fiuxes 9 April 1976 suspended sediment trap was carried by In cores which were deposited under non-uniform currents to the dam. Contamination associated with its proximity to the dam is suspected to account for conditions, it is difficult to put a time frame into the sediment. Thus, five different methods (storm periodithe high Zn, Mn. Ni, Pb and Cd contents in this city, Pb-210, Th-228/Th-232, Pu isotopes and the sample. Aside from the above elemental changes ascribed .: single addition of CuSO,) were used to arrive at an rate for Barrett Reservoir to man’s activities at the dam, the contents of Fe, I overall sedimentation (keeping in mind that most of the sedimentation Cu. Zn, Cr, Co and Pb show no obvious variations occurs rapidly during storm periods as shown by the due to seasonal factors with their different sediment suspended sediment traps). sources. Thus, atmospheric anthropogenic inputs of The pattern of light and dark layers in the sedithese elements may be decipherable within the sediments was compared with rainfall data and water rise mentary column of Barrett Reservoir. On the other measured at the dam (Fig. 1). The latter is more imhand V is generally slightly higher in the winter portant since the rains vary in duration and intensity months. It does not correlate with stream input. Ni and thus in effectiveness at bringing sediment to the is slightly higher in the months with high stream reservoir. Major storm layers should have been detransport, probably reflecting the mineralogy changes. posited during January and March 1973, January Ag seems to be lower in stream transported particu1969, December 1966. March 1957 and minor storm lates. Thus, any atmospheric transport of these ele-
H
H
H
HHHH
I4
IH
so t
-
_ 40
_
--
-
_
-
-
-
I-
3
-
-30-
3
__-
3
_--
$ 0 s
-
--
-
--
-
-
--
20 -
-
-
--- -
__
-
-
.-
10 StlORT
~:: I
0
10
20
: 30
40
DEPTH
Fig. 4. Per cent dry weight of the sediments vs depth measured from the X-radiographs,
CORE
.L~___ SO
QO
iota3
CORE
~,~~~: 70
80
km)
in the core. Extent of storm are indicated by arrows.
derived
sediment,
1.565
._ 20
‘1. 1 30 :f ‘:- 40 -
CM
L 50 _:.:Y - 60 :-
;- 80 Fig. 3. X-radiographs
of two cores from Barrett Reservoir. Major storm derived layers and their time of deposition are indicated.
1567
Semi-arid lake sediments
IL
I
,
I
I
I
I
I
IO
20
39
‘lo
50
60
70
DEPTH
(cm)
Fig. 5. Pb-210 activity vs depth in Barrett Reservoir sediments.
layers in January 1974, January 1971, April 1967, April 1965 and February 1962. Referring to the X-ray, the light layers were assigned dates as shown. Using these dates the sedimentation rate is higher at the top of the core (4-5 cm/yr) and decreases to 2-3 cm/yr in the lower portions. The average sedimentation rate over the length of the core is 3.3cm/yr. Consideiable scatter is evident in the Pb-210 data (Fig. 5). Particularly anomalous is the very high Pb-210 activity at the 21-29cm depths which corresponds to the 1969 storm layer. During this major storm, considerable quantities of water and sediment were released form Morena Reservoir (immediately upstream from Barrett Reservoir). Thus, the sediment accumulated during this interval came from a different source than the rest of the core material. If this interval of high Pb-210 activity is ignored, there is a decrease in the Pb-210 activity with depth leading to an overall sedimentation rate of 3.4 cm/yr, in good agreement with the storm data. With such a rapid sedimentation rate, the Th-228/Th-232 method should produce finer resolution of the sedimentation rate due to the shorter halflife of Th-228 (T+ = 1.9 yr). Instead of the expected decrease in the Th-228/rh-232 ratio with depth, the ratio remains essentially constant to a depth of about 30 cm and then decreases (Fig. 6). If the two previous dating methods are valid, the 30 cm depth level corresponds to about 1960. With a 1.9yr half-life, the excess Th-228 should have been decayed. Polychaetes and burrowing molluscs are able to mix sediments down to depths of tens of centimeters. In sediments which have been so mixed, chemical properties such as Th-228/Th-232, Pb-210 activity, Pu-239 + 240 activities and elemental composition are constant to the mixed depths (GOLDBERG et al.,1978).In Barrett Reservoir sediments, however, not only are the above (with the exception of Th-228/I%-232) not constant in the top 3Ocm but distinct layering is also present in the X-ray. Thus, it is extremely unlikely that there
has been a physical homogenization of the top 30 cm of the sediment. The most probable explanation for the constant Th-228/l&232 ratio involves the very high water content of the sediment (Fig. 4) and the irrigational activities of the worms. The Th-232 contents of the sediments of the lake are very high (8.3-13.5 ppm). On on a biogenous sediment-free basis, the content would be about 24 ppm. Therefore, considerable Ra-228 is generated by the decay of Th-232 and in part would enter the pore waters. The irrigational activities of burrowing molluscs and polychaetes have been shown to exchange interstitial water with the overlying water on the time scale of weeks (GOLDBERG et al. 1978). This, in conjunction with the high water content of the sediments (the divers were unable to distinguish the sediment-water interface) would enable a constant flux of Ra-228 through the top 3Ocm of sediment. Any Th-228 generated by the decay of Ra-228 during this passage would precipitate out since Th is known to have a
lo-
01
0
I
I
I
I
I
,\
IO
20
30
40
50
60
1 70
cm
Fig. 6. Corrected Th-228/l%-232 ratios vs depth in Barrett Reservoir sediments.
1568
K. K. BERTINE. S. J. WALAWENDER and
Table 4. Cs-I 37 activities and Pu-239 + 24O/Cs-I37 ratios in Barrett Reservoir sediments and Wilson Creek (dry) bed sediments
Depth (cm) ______1-5 21-24 24-27 32-34 43.5-45.2 53.0-55.3 58.7-60.0 61.5-64.5 76.0-80.2 Wilson creek bottom sediment
CS'7' dpm/kg 6,670 12,600 16,300 2,537 4,695 798 187 159 66 3,770
pu.“'+:'4",Cs'17
0.01s 0.032 0.027 0.019 0.063 0.023 0.043 0.022 __ 0.012
short residence time in waters (GOLDBERGand ARRHENIUS 1958). Thus, the Th-228/I&232 ratio would be constant to the 30cm irrigational depth. Below this depth, the Th-228 in the sediments would not be supported by Ra-228 and would decay with a 1.9 yr half-life. The sedimentation rate in the interval below 30cm determined by the Th-228/Ih-232 decay is 3.7cm/yr (Fig 6), similar to the rates derived in the two previous methods. The Pu-239 + 240 activity and the Pu-238/ Pu-239 + 240 ratio can be used to check the sedimentation rates obtained previously. One factor which must be considered before using plutonium isotopes as a dating tool is whether the plutonium has remained fixed in the sediment. A check on this was made by determining the Cs-137 contents (also produced from atmospheric weapon testing) in 8 samples (Table 4). In general the G-137 values follow the Pu-239 + 240 values as reflected in near constant Pu/Cs ratios. The expected increase in this ratio due to Cs-137 decay with depth in the sedimentary column was not discernible, most probably due to the low counting statistics at the lower depths. However, there is no evidence that either plutonium or cesium has migrated extensively within the sedimentary column. Plutonium isotopes became measurably evident in coastal marine sedimentary strata deposited in the early 1950s and increased rapidly during the next five years (KOIDE et al. 1975; GOLDBERGet al. 1977). The Pu-239 + 240 activities in Barrett Reservoir sediments are given in Fig 7. Pu-239 + 240 was just measurable at the 62-64cm depth. Its activity increased rapidly up to 55 cm depth level. This period of rapid increase corresponds to the late 1950s by the other dating techniques as expected. The Pud238/Pu-239 + 240 activity ratio in environmental samples increased consequent to the burn up of the SNAP device in 1964, and this increase was detected in envrionmental samples by 1964-1966. The increased ratio in Barrett Reservoir sediments occurred in the interval between 36 and 41 cm depths. Using the previous dating techniques, this interval represents the time period April 1%5-summer 1966,
M. KOIDE
thus the plutonium isotopes are in reasonable agreement with the previous time frame. A last check on the sediment accumulation rate can be made using the Cu contents of the sediments. CuSO, was added to the reservoir as an algicide in the summer of 1956. Increased Cu contents are found in the 61-65 cm depth strata which was previously assigned the time of 19561957 by the other dating techniques. In conclusion, all five dating techniques converge at a sediment accumulation rate of about 3.4cm/yr. Thus, even though each individual dating technique is not definitive on its own, the fact that all five give approximately the same rates, supports the 3.4 cm/yr sedimentation rate. Using this sedimentation rate, a density of solids of 2.6g/cm3, and the average water content of the sediments (69.5%) the average sediment flux is computed to be 1.28 g/cm’/yr. The sediment flux for 1976 from the suspended sediment trap was 0.89 g/cm’/yr. Considering that that year was not a major storm year, the sediment fluxes are in reasonable agreement. Atmospheric fluxes
It was shown in a previous section that direct atmospheric fallout is a minor source of sediment at Barrett Reservoir. However, GOLDBERGet al. (1976) and KOIDEet al. (1975) have demonstrated that there can be a magnification of a direct atmospheric fallout flux for bomb produced radionuclides due to the accumulation of soil debris by water runoff into the reservoir from the adjoining land where the surface soil debris contains a significant atmospheric component of these radionuclides. A magnification factor
r-
Fig 7. Pu-239 + 240 activity and per cent PLI-238/Pu-239 + Pu-240 vs depth in Barrett Reservoir sediments.
1569
Semi-arid lake sediments Table 5. Elemental contents of the soluble and insoluble fraction of atmospheric
particulates collected
in a bucket during May 1976 at Barrett Reservoir compared to average values for 1976 at La Jolla, California and Ensenada. Baja California. (Values in ppm) (ND = not detected) !!a
Al -
Cd -
go%
ND ND
86,900 5,800
ND ND
ND ND
Insoluble ND 64,700 Soluble 2 ND Ensenada, Baja, California(i) Insoluble ND 82,140 Soluble ND h>
1 3 2 0.5
Barrett Insoluble La U;j;;leCAcl)
pb
&l
32 17
460 26
518 332
344 132
41 17
1,190 314
750 339
1,064 214
24 1
193 29
264 29
cu
Ff
k&l
31 ND
62 29
58,300 3,700
899 1,303
m ND
109 3
281 174
41,875 165
ND ND
42 1
86 14
43,570 Lxrl
g
v ND ND
121 ND
(I)HODGEet al. (1978).
of 4-5 compared to the integrated atmospheric fallout was found in Tokyo moat sediments (GOLDBERGet al. 1976) and a factor of 2 in California coastal sediments KOIDE et al. 1975). These magnification factors were arrived at by comparing the total fluxes of plutonium isotopes to the deposit with those of direct fallout. By utilizing the Pu-239 + 240 data for Barrett Reservoir (and linearly interpolating between points), an integrated value of 15 mCi/km’ is found. The average stratospheric fallout based on soil profiles for 3&4O”N latitude obtained by HARDY et al. (1973) is 1.8 mCi/km’. Thus, there is a magnification factor of about 8 at Barrett Reservoir. This factor is larger than the factors found in the other areas due to the rugged relief, semiarid environment with small vegetative cover, large drainage area to basin ratio and heavy rainfall over a small portion of the year. All these contribute to an efficient removal of surface soil debris with the infrequent rains. Therefore, if other atmospherically transported pollutants behave similarly to plutonium, it is possible that the sediments at Barrett Reservoir may contain Table 6. Characteristics Sample Depth (cm) l-2 4-5 7-8. 9-10 12-13 20-21 23-24 24-25 26-27 29-30 35.8-37.7 41.5-43.5 48.8-50.2 58.7-60.0 61.5-63.0 63.0-64.5 64.5-66.0 66.0-69.0 69.0-73.3 73.3-73.8 73.8-76.0 76.0-80.2 80.2-81.7 81.7-82.7 G.C.A. 42/l-
% Dry Weight 24.4 26.6 29.1 35.7 31.4 29.2 33.8 37.2 25.3 18.1 21.0 20.8 23.9 41.1 37.8 40.7 33.5 45.0 43.6 33.2 23.3 35.3 28.1
measurable levels of atmospheric pollutants due to the entry of remobilized soil debris containing these atmospheric pollutants even though direct atmospheric fluxes of solids are small compared with the other sediment sources. The contents of Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn were determined on the bucket atmospheric samples. The results on the soluble and insoluble fractions are given in Table 5. With the exception of Mn, they are in reasonable agreement with values found in San Diego particulates by HODCE et al. (1978). The elevated Mn content most probably is a reflection of the railing removed at the dam as discussed earlier. Compared with their contents normally found in sediments, Pb and Zn contents in the sediments could be increased by a large influx of atmospheric particulates. The metal contents of Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn were determined at selected depths in the sediments. In general, storm layers were not sampled. A few were included toward the bottom of the core in addition to the 1969 storm layer.
of Barrett Reservoir sediments
X HCl Residue
x Quartz
x Plagioclase
84.2 84.3 84.7 86.5 85.7 84.0 82.5 83.4
66.5 64.5 62.7 83.7 88.2 76.0 77.6 84.7
3.1 2.9 3.1 3.1 2.8 4.0 3.9
11.5 11.3 13.0 8.5 13.4 14.4 11.5
84.7 83.6 84.0 85.5 85.6 85.6
60.8 67.0 al.1 62.5 82.8 81.7 83.4
3.0 3.6 3.4 4.0 4.7
% Remaining (5OOOC)
17.1 14.8 11.9 22.0 13.3
% Total Organic
% CaCO3
x Opal
z CH?O
5.5 10 10 4 4 15 6 5
32 30 33 35 32 17
11.5 11.5 9.8 9.5 9.8 14.3 16.3 17.3
49 51.5 53 48.5 46 46
6 3 19 4 3 4
29
13.3 11.0 15.0 12.0 11.3 11.8
48
27
30.5 25 27 27 28
49
64.5 41 41 43
CO ppm 21.9 22.8 23.5 28.9 29.1 19.8 27.9 28.7 22.6 22.7 28.4 16.0 22.0 24.2 22.1 19.9 19.6 21.8 22.1 10.1 15.3 22.1 26.3 25.6
Semi-arid lake sediments REFERENCES
CONCLUSIONS
An investigation of direct atmospheric fallout, suspended particulates, and core material was made to decipher the historical record of anthropogenic metal fluxes to the sediments of Barrett Reservoir located in a remote area 30 km east of the metropolitan center of San Diego. Direct atmospheric fallout was found to contribute only a small percentage to the sediment flux to the reservoir even when compared to those months when no stream influx occurred. Fortunately though, in this setting of semiarid climate, rugged relief and incomplete vegetative cover, there is a magnification factor of about 8 (as indicated by the Pu-239 + 240 data) for atmospheric fallout when surface soil debris (containing a significant atmospherically derived component) from the drainage basin is washed into the reservoir during infrequent rains. Thus, in spite of a high sedimentation rate, an anthropogenic Pb input to the sediments was found. This magnification effect explains the high anthropogenic Pb flux, 18.3pg Pb/cm’/yr vs background 15.5 pg/cm2/yr, for this inland region compared to the fluxes determined for West Coast offshore sediments. Even though Barrett Reservoir was not the ideal case of constant particle-by-particle deposition, due to seasonal&y of riverine input, it is possible to glean some information about metal fluxes from its record. In such a non-ideal case, it is necessary to integrate the results of several dating methods and to keep the basic assumptions
of each method
1571
firmly in mind.
However, for those elements with small anthrooogenie inputs, the historical record of pollutant influx may not be decipherable above natural variations due to seasonal effects. Acknowledgements-This work was supported by NSF grant, DES74-19872 and AEC grant AT(04-3)34, project 84. The manuscript benefited greatly from the review of E. D. GOLDBERG. We would especially like to thank MEL AUBERYand BILL NYE of the Water Utilities Department, City of San Diego, California, for their help in securing the samples and interpreting the results.
BERTINEK. K. and GOLDBERGE. D. (1977) History of
heavy metal pollution in southern California coastal zone-reprise. Environ. Sci. Technol. 11, 297-299. BERTINE K. K. and MENDECK M. F. (1978) Industrial&+ tion of New Haven, CT, as recorddd in’reservoir sediments. Environ. Sci. Technol. 12, 201-207. BRULANDK. W., BERTINEK., KOIDE M. and GOLDBERG E. D. (1974) History of metal pollution in southern California coastal zone. Environ. Sci. Techno/. 8, 425-432. GOLDBERGE. D. (1958) Determination of opal in marine sediments. Mar. Res. 17, 178-182. GOLDBERGE. D. and ARRHENIUS G. D. S. (I 958) Chemistry of Pacific pelagic sediments. Geochim. Cosmochim. Acta 13, 153-212. GOLDBERGE. D., GAMBLEE., GRIFFIN J. J. and KOID M. (1977) Pollution history of Narragansett Bay as recorded in its sediments. Esturaine Coastal Mar. Sci. 5, 549-561. GOLDBERGE. D., HODGE V., KOIDE M. and GRIFFIN J. J. (1976) Metal pollution in Tokyo as recorded in sediments of the Palace Moat. Geochim. J. 10, 16>174. GOLDBERGE. D.. HODGEV.. KOIDEM.. GRIFFINJ.. GAMBLE E., BRICKER0. P., MAT~SOFFG., HOLDRENd. R. and BRAUNR. (1978) A pollution history of Chesapeake Bay. Giochim. Acta 42, 1413-1425. HARDYE. P., KREY P. W. and VOLCHOKH. L. (1973) Global inventory and distribution of fallout plutonium. Nature 241, 444-445. HODGE V., JOHNSONS. R. and GOLDBERGE. D. (1978) Influence of atmospherically transported aerosols on ~surface ocean water composition. Geochem. J. 12, 7-20. HUTCHINSON G. E. (1975) A Treatise on Limnology. Volume III. Limnological Botany, pp. 148-l 5 I, 30&302, 401402. Wiley. KOIDEM. and BRULANDK. W. (1975) The electrode position and determination of radium by isotope dilution in sea water and in sediments simultaneouslv with other natural radionuclides Anal. Chim. Acta 75: l-19. KOIDE M., BRULANDK. W. and GOLDBERGE. D. (1972) Th-228/Th-232 and Ph-210 geochronologies in marine and lake sediments. Geochim. Cosmochim. Acta 37, 1171-1187. KOIDE M., GRIFFIN J. J. and GOLDBERGE. D. (1975) Records of plutonium fallout in marine and terrestrial samples. J. Geophys. Res. 80, 4153-4162. KUZNETSOVS. I. (1970) The Microjlora of Lakes and its Geochemical Activity, pp. 429436 (trans. from Russian). University of Texas Press. Z~JCHTBAUER H. (1974) Sedimentary Petrology Part II. Sediments and Sedimentary Rocks 1. pp. 218, 221-222. (trans. from German). Wiley.
0016-7037/78/1001-1559302.00/0
Chronological strategies and metal fluxes in semi-arid lake sediments K. K. Department
BERTWE*
and S. J. WALAWENDER
of Geological Sciences, San Diego State University, San Diego, CA 92182, U.S.A.
and M. KOIDE Scripps Institution of Oceanography,
La Jolla, CA 92093, U.S.A.
(Received 4 April 1978; accepted in revised form 16 June 1978)
Ahatraet-The largest flux of sediments to a semi-arid lake (Barrett Reservoir) was stream influx after the winter rains. During the times of no stream flow, resuspension of previously deposited material dominated. Direct atmospheric transport was minor but atmospheric particulates were evident in the sediments due to stream influx of particles containing a significant atmospherically derived component. Only an anthropogenic lead influx was measurable in the sedimentary record due to the high sedimentation rate. In semi-arid areas where intermittent stream flow occurs, more than one technique must be used to obtain an overall sedimentation rate.
INTRODUCTION RECENT years the historical records maintained in the sediments of lakes (BERTINE and MENDECK 1978;
IN
et al. 1976), estuaries (GOLDBERG et al. 1977, 1978) and marine sediments (BRULAND et al. 1974; BERTINEand GOLDBERG1977) have been util-
GOLDBERG
ized to arrive at anthropogenic metal fluxes through time to these areas. The intent of this study was to look at the historical record of atmospheric metal transport to a reservoir in an isolated area east of San Diego, CA, where local sources of metal contamination were minimal. The use of the recently deposited sedimentary material to reveal anthropogenic metal fluxes depends upon obtaining the sediment accumulation rate. This has normally been accomplished by the Pb-210 method (see KOIDE et al, 1973). However, Pb-210 results are open to alternate interpretations in areas such as the one studied where particle-by-particle deposition may not occur over the whole year due to seasonality of rainfall and intermittent stream flow. Therefore, a variety of additional techniques must be employed in order to obtain a valid sedimentation rate. These geochronologies involve the use of storm layers, the times of addition of algicides, Th-228F-232 radionuclides, artificial radionuclide geochronologies and measuring the fallout of suspended sediment over the period of a year. Since the mineralogy and thus the chemistry of the suspended particulates collected over the time period of a year varied with seasonal river flow, the range in metal contents of these particulates was also measured to * Present address: Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A. Contribution No. 38 from the San Diego State University Center for Marine Studies.
see if such variations would complicate the interpretation of the historical record. Whether the atmospheric fluxes of the metals could significantly affect the sediment metal contents was addressed by collecting atmospheric particulates with a bucket sampler and comparing the metal fluxes therein with the sediment metal fluxes. Finally, the historical record was sought in two cores. RESERVOIR
DESCRIPTION
Barrett Reservoir is located 33 km east of San Diego, California in the Cleveland National Forest. It was built by damming a steep sided river valley in 1921-1922 to provide flood control and storage for the municipal water supply of the City of San Diego. It is 2.3 km long, with an average width of 0.25 km and a usual depth of 20 m at the dam. The intermittent streams draining into Barrett flow after the winter rains and the occasional spring snow melt, but usually not during the summer-fall seasons. Due to the lack of extensive vegetative cover, dust derived from the surrounding hills has often been observed in the valley. Since the reservoir is free from direct entry of pollutants, the entry of any pollution is anticipated to be via the atmosphere from distant sources. METHODS Sample collection Suspended particuhtes. A funnel type suspended sediment trap (designed by A. Soutar) was suspended 5 m below the water surface and 5 m above the bottom in the water column about 100 m from the dam at Barrett Reservoir for a year beginning 13 November 1975. The top of the funnel consists of a 22.7 x 22.7 cm gridded (1 I_&) plastic. SOUTAR (personal communication) extimates that the trap efficiency is between 85 and 100%; 90% is used in the calculations. The particulates were collected at approximate monthly intervals. After extraction, they were dried at 1 10°C weighed and ground. The flux of the solids was then computed from the weight, the surface area of the trap, the time exposed and the trap efficiency.
1559
K. K. BERTINE,S. .I. WALAWENDER and M. KOIDE
1560
Atrnosphrric particulatrs. In order to measure atmospheric input to the reservoir. a bucket sampler with grid (see HODGEet al. 1978) was placed on the water surface immediately above the suspended sediment trap for the period 9 April 1976 to 18 May 1976. At the end of that period, the feathers, bees and other insects were removed. Then the solids were removed from the bucket with successive washes (totalling 150ml) of distilled water. This mixture of solids and water was rotated at 1 rps for a day at room temperature, centrifuged, and the soluble and insoluble fractions dried at 110°C and weighed. The fluxes of the soluble and insoluble materials were computed. Seditnents. Two cores were taken from Barrett Reservoir. A short core (66cm) was taken in February 1975 and a longer core (80 cm in length) in April 1976 by divers gently placing polyvinyl chloride tubes into the sediment. The two cores were immediately frozen. A 1cm thick slab was sawed from each core and X-rayed to ascertain the presence of sediment structure. The remainder of the core was then scraped to remove any contamination from the sawing. The top 32cm of the shorter core was sampled at one cm interval and the remainder according to the layering evident in the X-radiograph. The longer core was sampled only in its deeper portions according to the layering overlapping the same bottom layers of the shorter core.
Opal contents were ascertained by the method of GOLDBERG(1958). Quartz and paglioclase contents were determined by X-ray diffraction.
RESULTS Sources of
sediment
The monthly flux of particulate material to the suspended sediment trap varied over the year from 0.36 to 1.68 g/cm’/yr (Table I). The integrated flux was 0.89 g/cm’/yr. Although the two highest fluxes in the suspended sediments occurred during the months with the highest rainfall (Table I) there is no correlation between rainfall and particulate flux. Five sampling intervals of about equal duration had similar fluxes (0.3W.46g/cm2/yr) yet the rainfall over those intervals was highly variable, ranging from 0.02 to 7.19 cm. Changes in biological productivity over the time period of the year do not account for the lack of correlation. The sum of the opal, CaCO, and organic (as CH,O) contents account for between 46.0 and 62.8% of the solids (Table 2). Correcting for these Ana/.vfica/ rwthods percentages, the non-biologic fluxes in the above Elemental concentrations. The solids were totally dissamples are still very uniform, ranging between 0.16 solved using successive applications of HCI. HF, HNO, and 0.21 g/cm2/yr (Table 1). and HC104. The concentrations of Ag, Al. Cd, Co, Cr. Cu. Fe, Mn. Ni, Pb. V and Zn were determined by atomic During many of the months there was no stream absorption spectrophotometry using a Varian AA6. The flow into Barrett Reservoir, yet significant fluxes of hydrogen background corrector was used when the wavenon-biologic particulates were collected in the suslengths were under 3000A. pended sediment traps. Geochronology. Pb-210 and Th-228/Th-232 were deterThe flux of the atmospheric fallout, determined mined following the method of KOIDE and BRULAND (1975). Pu-239 and 240 and Pu-238 were determined by from one bucket sample, was 0.0035g/cm2/yr. Comthe method of KOIDE et al. (1975). 13’Cs was analyzed paring these values with the mineral flux in the susby the addition of Cs carrier, followed by a separation pended sediment trap, direct atmospheric fallout can of Cs with ammonium phosphomolybdate and further puraccount for only about 1-2’4 of the sediment flux. ification from 4”K by use of Bio-Rex-40 cation exchanger. The Cs separate was precipitated then quantified for yield The possibility that the one atmospheric particulate as Cesium chloroplatinate (Cs,PtCI,). The “‘Cs activity flux in atypical must be considered. HODGE et al. (1978) was determined with an anticoincidence low background found only slightly higher atmospheric fluxes in beta counter. nearby La Jolla, California and Ensenada, Baja CaliParriculate composition. Organic carbon and CaCO, fornia, Mexico. Considering that the sea saIt comcontents were measured using a Leco carbon analyzer.
Table I. Fluxes to Barrett Reservoir suspended sediment traps, rainfall and resultant water level rise during the period 13 November 1975 to 4 November 1976 Flux (g/cm~/year)
(8) Period Collected
No. Days
llO°C Dry Weight
Total
Opal
cacoj
Organic (CH?O)
0.10 0.15 0.62 0.11 0.03 0.08 0.27 0.50
0.04 0.05 0.00 0.02 0.11 0.12 0.13 0.19
0.05 0.05 0.15 0.10 0.05 0.06 0.13 0.17
So"Rainfall biogenic(') (cm)
GaWXl? rise (m)
y 11/13/75-12/18/?5 12118/75-l/21176 l/21/76-2127476 '/27/76-4/Y/76 4/Y/76-5/18/76 5/18/76-6129/76 6/29/76-a/5/76 a/5/76-1114176
35 34 37 41 39 42 37 91
15.76 19.50 79.03 21.54 19.66 22.80 47.15 180.00
0.36 0.46 1.68 O.i,l('f 0.40 0.42 1.00 1.56
Total 356 Bucket 4/Y/76-5118/76 (insol.) 39 4/g/76-5110176 (sol.) 39
405.44
0.89
0.1323 0.0708
0.0023 0.0012
“‘Probably low since trap was moved to dam by currents. ‘z)Approximate since water was being pumped from reservoir. c3’Computed from total flux minus organic fluxes.
0.17 0.21 0.91 0.18 0.16 0.16 0.47 0.70
7.19 0.08 17.12 7.19 4.72 0.02 1.45 7.67
0.2 0.0 7.0 2. 4 0.6(-l 0.0 0.0 3.0
1561
Semi-arid lake sediments
Table 2. ~ineralo~
%
of sus~nd~
% CH20(1)
%
11/13/77-12/U/75 12118175~01/21176
28 33
~~~f2~fJ~O2f2Jf?6 OZf2J~~6-04~0~~~6 04/09/76-05/18/76 05/18/J+06/29/76 06/29/76-08/05/76 08/05/76-L1/04/76
37 28 20 19 27 32
sediments at Barrett Reservoir
11.5 10.5 0 5
28 29.5 13 12
91
Quartz
Ka0lilIiCe(2) Biotite
90Feldspar
13.5 11.8
6 7
28 34
2y$3f 12.0 14.3 13.3 11.0
lb 3.5 7.5 8 9 6
17 23 25 30 30 20
1.29 1.69 3.58 2.74
1.10 1.43 1.51 3.16
(“Calculated from organic carbon contents. (“Ratio based on peak areas from X-ray diffraction. @)Higb leaf content.
ponent is higher there, the agreement is reasonable. The fluxes at La Jolla were constant over the year studied whereas those at Ensenada were about a fattor of 3 higher in the winter months. Even applying a factor of 1.5 to the atmospheric flux at Barrett Reservoir, it is still less than 5% of the sediment flux. During the summer months when there is no stream flow into Barrett Reservoir, the atmospheric and biogenic fluxes account for only about half the fluxes. Other processes that can put materials into the suspended sediment trap include wave erosion of the banks, resuspension of previously deposited sediment and authigenic mineral formation. X-ray diffraction of the suspended particulates indicates the presence of calcite, quartz, hornblende, biotite, feldspar and kaolinite (Table 2). No significant authigenic mineral formation other than calcite is evident. Since oxygen is frequently depleted in the bottom waters, large amounts of amorphous iron oxides are unlikely to be present. During strong wind conditions it was observed that waves with white caps were formed and subsequently the water became quite turbid and cliffs of from 30 to 6Ocm were found eroded in the banks. Turbid waters were especially noticeable around the delta areas of the streams. Two different seasonal sources for the sediment are also indicated by their mineralogies. In the months where stream transport dominates, the particulates
are characterized by higher kaolinite and lower biotite, quartz and feldspar contents (Table 2) than in the months where erosion and resuspension dominate. The question of whether the resuspended material is derived from old material or recently deposited material can be answered by looking at the radioactive nuclides Th-232, its daughter Th-228, Pu-239 + 240 and Pb-210. Because of the very short half-life of Th-228 (T, = 1.9yr), an excess of Th-228 relative to Th-232 indicates the presence of recent materials (KOIDE et al. 1972). Th-228/I&232 ratios measured on three of the suspended sediment samples range from 1.12 to 1.55 (Table 3) indicating that there is a significant recent component. Environmental Pu-239 + 240 has been generated primarily from atmospheric weapon testing and is measurable in sediments deposited in the early 1950s (KOIDE et al. 1975; GOLDBERG et al. 1976). Its environmental levels reached a maximum in about 1963 and have decreased since. The Pu-239 + 240 activities on an organic-free basis in the suspended particulates range between 79 and 190dpm/kg (Table 3). The spring-summer months have slightly lower activities than the winter months. However, there is no marked difference between the resuspension dominated months of summer and the stream transport
Table 3. Radionuclides in Barrett Reservoir suspended sediments and atmospheric fallout
Suspended Sediments 1113/?5-12tl8175 12/X3/75-1/21/76 l/21/76-2/27/7u 2/27/76-4/Y/76 4/9/76-5/18/76 5/10/76-6/29/76 6f29/76-a/3/76 815/76-U/4/76
Th22e/Th232 1.55+0.06 1.12t0.02 1.35t0.04
Organic Free pu239+240 Pb210 dpm/kS dpm!g
,aa239+240
Pb210 dpmlcm /year
14.7 13.6 9.6 14.5 10.5
0.032 0.039 0.153 0.027
2.5 2.8 a.7 2.5
0.016
147
12.0
0.024 0.069 0.103
1.7 1.1
148
10.8 11.1
357270
38.1 5.0
189 190 169 152 103 151
pb21o/p"z39+2+0 78 71 57 95
102 71
5.2 8.4
75 a2
Bucket 4/9/76-5/B/76 (Insoluble) 4/g/76-5118176 (soluble)
0.0008
0.09 0.006
107 .
K. K. BERTINE. S. J.
1562
WALAWENDER
and M.
KOIDE
19551956 1957 1959 1959 I%0 1961 1962 I%3 1964 1965 1966 1967 1966 1969 1970 I971
1972
1973
1974 1975
1976
Fig. 1. Rainfall and water height measured at the dam at Barrett Reservoir from 1955 to 1976. dominated months of winter. This supports the conclusion that the sediment trap material in summer
is probably composed of resuspended sediment that was recently transported to the delta areas or to the banks of the reservoir by streams or by slope wash during the preceding rainy season. The primary source of Pb-210 (T+ = 22.3 yr) in sediments, which is unsupported, is from atmospheric Rn-222 (T+ = 3.84 days) which enters the air from rocks and soils after formation from its parent Ra-226 (T+ = 1620 yr). Atmospheric Rn-222 decays to Pb-210 which is then removed in precipitation. The other source of Pb-210 is sedimentary Ra-226 and can enter with older sediments in which radioactive equilibrium conditions probably exist. The Pb-210 activities (on an organic-free basis) in the suspended sediment range from 8.1 to 14.5 dpm/g (Table 3). Again there is no marked decrease in the resuspension dominated months indicating that the reworked sediment had recently been brought to the reservoir. A comparison of the Pb-2lO/Pu-239 + 240 ratio in the April-May suspended sediment sample (102) with that in the insoluble atmospheric particulates (107) collected at the same time indicates that the suspended sediment is mostly composed of unsupported Pb-210. Biogenic input
The sum of the opal, calcite and organic carbon (as CHrO) contents range between 46.0 and 62.8% of the solids (Table 2). Opal accounts for between 19 and 37x, calcite between 0 and 30% and CHrO between 9.0 and 24.3%. The calcite contents and fluxes are higher in the spring-summer months (Tables 1 and 2) coinciding with the algal bloom in Barrett Reservoir. Several varieties of freshwater algae and higher plants produce encrustations of calcite (rarely aragonite) during periods of active photosynthesis when
dissolved CO1 becomes depleted and HCO; is used instead as a carbon source (see discussions by ZUCHTBAUER 1974; HUTCHIN~~N 1975; and KUZNET~OV 1970). The reaction may be written as: Ca’ + + 2HCO;
- p,antS CaCOJ + CO, + HzO.
Such a mechanism would explain the higher amounts of CaCO, in the suspended particulates during those months of the year when photosynthetic activity was greatest. The large fluctuations in the CaCOJ contents account for most of the variability in the opal patterns. On a CaCOs contents account for most of the variability in the opal patterns. On a CaCO,-free basis the percent opal is more uniform ranging between 27 and 37%. The higher fluxes of organic matter (as CH,O) resulted from two different processes. The two major storms brought a large quantity of leaves and twigs to the reservoir resulting in higher organic contents. Secondly, algal decay contributed significant organic matter during the last two sampling periods when the reservoir became eutrophic with a moderate fish kill. Affects of the difirent elemental content;
particulate sources on the
The question remains: to what extent has the elemental composition of the sediments been affected by the different sources of the particulates? Al has been used for the normalization of other elemental concentrations and fluxes for both sediments (see e.g. BRULAND et al. 1974) and atmospheric particulates (e.g. HODGE et al., 1978). The argument is made that Al represents detrital crustal material of constant composition and any deviation from constant Al content with time or depth in a sedimentary column rep
1563
Semi-arid lake sediments
wm Zn
304
I . . . I , NDJFMAMJJASON
I
,
,
1 I
c
Fig. 2. Metal contents (on an organic-free basis) in suspended sediments collected from November
1975 to November 1976. resents *dilution by biological and/or anthropogenic activities. An analysis of the Al contents (on an organic free basis) in the suspended particulates indicates that the above assumption is not valid for Barrett Reservoir sediments due to changes in mineralogy over the year (Fig 2). The months with significant stream input have higher Al contents and high kaolinite/biotite ratios (Table 2). The resuspension dominated months have much lower Al contents and lower kaolinite/biotite ratios. Since kaolinite contains about 21% Al compared with about 7.5% Al for biotite, the changes in the relative kaolinite and biotite contents will affect the Al content of the particulates. Therefore, the normalization method of metal/Al cannot be used for these sediments. Instead, a preferable way
to assess variations in elemental contents over the year is by expressing them on a CaCO,, opal and organic matter (as CHIO) free basis (Fig. 2). These results are somewhat imprecise since they reflect the cumulative errors of all analyses. Unfortunately for the interpretation of the elemental variations, the City of San Diego Water Utilities Department replaced the badly weathered steel railing atop the dam during the first few months of our field program. It was cut off by acetylene torch in late October-early November 1975. Mn and Cd contents in the suspended particulates (on an organic-free basis) co-vary over the year (Fig. 2). Both are exceptionally high during the first sampling interval when the railing was being cut down. With succeeding sampling periods their concen-
K. K. BERTINE, S. J. WALAWENDER and M. KOIDE
1564
ments may not be decipherable within the sedimennations decrease to minimum values in the summer tary column. months. The gradual decrease is interrupted by the storm of 6 February which considerably diluted the Historical record Mn and Cd contents. The X-ray pictures revealed distinct layering (Fig. 3) In late December a new railing of steel was welded which could be correlated from one core to the other. into place. During the interval when the new railings The strata possessed different colors. The light layers were being welded in place, the Fe, Cu, Zn and Ag on the X-ray were brown and characterized by lower contents are higher in the suspended sediments. These water contents (Fig. 4) whereas the dark layers were increases are most probably a result of the welding almost black with higher water contents. The light process. brown layers seen to represent periods of storm depoCuSO, was added as an algicide in late August sition 1976. The Cu content in that sample increases to The top 13 cm of the shorter core was black with 609 ppm (on an organic free basis). Mn, Fe, Co and worms followed by a light brown layer (13-19cm). Ni contents are also higher in this sediment compared Other light layers occurred at 2427, 3 l-32, 37.7-39.5, to the other months (Fig. 2). The higher Mn and Fe 5658.7 and 6&66cm depths. The remainder of the concentrations may result from a greater precipitation core was black. Small 2-3 cm worms were found to of their carbonates. The Co and Ni may either be a depth of about 30cm. They show up on the X-ray sorbed onto these carbonates or have resulted from as black dots. Their frequency decreased with depth. impurities in the CuSO,. Soon after its deployment, the 27 February 1976 to Sediment ,fiuxes 9 April 1976 suspended sediment trap was carried by In cores which were deposited under non-uniform currents to the dam. Contamination associated with its proximity to the dam is suspected to account for conditions, it is difficult to put a time frame into the sediment. Thus, five different methods (storm periodithe high Zn, Mn. Ni, Pb and Cd contents in this city, Pb-210, Th-228/Th-232, Pu isotopes and the sample. Aside from the above elemental changes ascribed .: single addition of CuSO,) were used to arrive at an rate for Barrett Reservoir to man’s activities at the dam, the contents of Fe, I overall sedimentation (keeping in mind that most of the sedimentation Cu. Zn, Cr, Co and Pb show no obvious variations occurs rapidly during storm periods as shown by the due to seasonal factors with their different sediment suspended sediment traps). sources. Thus, atmospheric anthropogenic inputs of The pattern of light and dark layers in the sedithese elements may be decipherable within the sediments was compared with rainfall data and water rise mentary column of Barrett Reservoir. On the other measured at the dam (Fig. 1). The latter is more imhand V is generally slightly higher in the winter portant since the rains vary in duration and intensity months. It does not correlate with stream input. Ni and thus in effectiveness at bringing sediment to the is slightly higher in the months with high stream reservoir. Major storm layers should have been detransport, probably reflecting the mineralogy changes. posited during January and March 1973, January Ag seems to be lower in stream transported particu1969, December 1966. March 1957 and minor storm lates. Thus, any atmospheric transport of these ele-
H
H
H
HHHH
I4
IH
so t
-
_ 40
_
--
-
_
-
-
-
I-
3
-
-30-
3
__-
3
_--
$ 0 s
-
--
-
--
-
-
--
20 -
-
-
--- -
__
-
-
.-
10 StlORT
~:: I
0
10
20
: 30
40
DEPTH
Fig. 4. Per cent dry weight of the sediments vs depth measured from the X-radiographs,
CORE
.L~___ SO
QO
iota3
CORE
~,~~~: 70
80
km)
in the core. Extent of storm are indicated by arrows.
derived
sediment,
1.565
._ 20
‘1. 1 30 :f ‘:- 40 -
CM
L 50 _:.:Y - 60 :-
;- 80 Fig. 3. X-radiographs
of two cores from Barrett Reservoir. Major storm derived layers and their time of deposition are indicated.
1567
Semi-arid lake sediments
IL
I
,
I
I
I
I
I
IO
20
39
‘lo
50
60
70
DEPTH
(cm)
Fig. 5. Pb-210 activity vs depth in Barrett Reservoir sediments.
layers in January 1974, January 1971, April 1967, April 1965 and February 1962. Referring to the X-ray, the light layers were assigned dates as shown. Using these dates the sedimentation rate is higher at the top of the core (4-5 cm/yr) and decreases to 2-3 cm/yr in the lower portions. The average sedimentation rate over the length of the core is 3.3cm/yr. Consideiable scatter is evident in the Pb-210 data (Fig. 5). Particularly anomalous is the very high Pb-210 activity at the 21-29cm depths which corresponds to the 1969 storm layer. During this major storm, considerable quantities of water and sediment were released form Morena Reservoir (immediately upstream from Barrett Reservoir). Thus, the sediment accumulated during this interval came from a different source than the rest of the core material. If this interval of high Pb-210 activity is ignored, there is a decrease in the Pb-210 activity with depth leading to an overall sedimentation rate of 3.4 cm/yr, in good agreement with the storm data. With such a rapid sedimentation rate, the Th-228/Th-232 method should produce finer resolution of the sedimentation rate due to the shorter halflife of Th-228 (T+ = 1.9 yr). Instead of the expected decrease in the Th-228/rh-232 ratio with depth, the ratio remains essentially constant to a depth of about 30 cm and then decreases (Fig. 6). If the two previous dating methods are valid, the 30 cm depth level corresponds to about 1960. With a 1.9yr half-life, the excess Th-228 should have been decayed. Polychaetes and burrowing molluscs are able to mix sediments down to depths of tens of centimeters. In sediments which have been so mixed, chemical properties such as Th-228/Th-232, Pb-210 activity, Pu-239 + 240 activities and elemental composition are constant to the mixed depths (GOLDBERG et al.,1978).In Barrett Reservoir sediments, however, not only are the above (with the exception of Th-228/I%-232) not constant in the top 3Ocm but distinct layering is also present in the X-ray. Thus, it is extremely unlikely that there
has been a physical homogenization of the top 30 cm of the sediment. The most probable explanation for the constant Th-228/l&232 ratio involves the very high water content of the sediment (Fig. 4) and the irrigational activities of the worms. The Th-232 contents of the sediments of the lake are very high (8.3-13.5 ppm). On on a biogenous sediment-free basis, the content would be about 24 ppm. Therefore, considerable Ra-228 is generated by the decay of Th-232 and in part would enter the pore waters. The irrigational activities of burrowing molluscs and polychaetes have been shown to exchange interstitial water with the overlying water on the time scale of weeks (GOLDBERG et al. 1978). This, in conjunction with the high water content of the sediments (the divers were unable to distinguish the sediment-water interface) would enable a constant flux of Ra-228 through the top 3Ocm of sediment. Any Th-228 generated by the decay of Ra-228 during this passage would precipitate out since Th is known to have a
lo-
01
0
I
I
I
I
I
,\
IO
20
30
40
50
60
1 70
cm
Fig. 6. Corrected Th-228/l%-232 ratios vs depth in Barrett Reservoir sediments.
1568
K. K. BERTINE. S. J. WALAWENDER and
Table 4. Cs-I 37 activities and Pu-239 + 24O/Cs-I37 ratios in Barrett Reservoir sediments and Wilson Creek (dry) bed sediments
Depth (cm) ______1-5 21-24 24-27 32-34 43.5-45.2 53.0-55.3 58.7-60.0 61.5-64.5 76.0-80.2 Wilson creek bottom sediment
CS'7' dpm/kg 6,670 12,600 16,300 2,537 4,695 798 187 159 66 3,770
pu.“'+:'4",Cs'17
0.01s 0.032 0.027 0.019 0.063 0.023 0.043 0.022 __ 0.012
short residence time in waters (GOLDBERGand ARRHENIUS 1958). Thus, the Th-228/I&232 ratio would be constant to the 30cm irrigational depth. Below this depth, the Th-228 in the sediments would not be supported by Ra-228 and would decay with a 1.9 yr half-life. The sedimentation rate in the interval below 30cm determined by the Th-228/Ih-232 decay is 3.7cm/yr (Fig 6), similar to the rates derived in the two previous methods. The Pu-239 + 240 activity and the Pu-238/ Pu-239 + 240 ratio can be used to check the sedimentation rates obtained previously. One factor which must be considered before using plutonium isotopes as a dating tool is whether the plutonium has remained fixed in the sediment. A check on this was made by determining the Cs-137 contents (also produced from atmospheric weapon testing) in 8 samples (Table 4). In general the G-137 values follow the Pu-239 + 240 values as reflected in near constant Pu/Cs ratios. The expected increase in this ratio due to Cs-137 decay with depth in the sedimentary column was not discernible, most probably due to the low counting statistics at the lower depths. However, there is no evidence that either plutonium or cesium has migrated extensively within the sedimentary column. Plutonium isotopes became measurably evident in coastal marine sedimentary strata deposited in the early 1950s and increased rapidly during the next five years (KOIDE et al. 1975; GOLDBERGet al. 1977). The Pu-239 + 240 activities in Barrett Reservoir sediments are given in Fig 7. Pu-239 + 240 was just measurable at the 62-64cm depth. Its activity increased rapidly up to 55 cm depth level. This period of rapid increase corresponds to the late 1950s by the other dating techniques as expected. The Pud238/Pu-239 + 240 activity ratio in environmental samples increased consequent to the burn up of the SNAP device in 1964, and this increase was detected in envrionmental samples by 1964-1966. The increased ratio in Barrett Reservoir sediments occurred in the interval between 36 and 41 cm depths. Using the previous dating techniques, this interval represents the time period April 1%5-summer 1966,
M. KOIDE
thus the plutonium isotopes are in reasonable agreement with the previous time frame. A last check on the sediment accumulation rate can be made using the Cu contents of the sediments. CuSO, was added to the reservoir as an algicide in the summer of 1956. Increased Cu contents are found in the 61-65 cm depth strata which was previously assigned the time of 19561957 by the other dating techniques. In conclusion, all five dating techniques converge at a sediment accumulation rate of about 3.4cm/yr. Thus, even though each individual dating technique is not definitive on its own, the fact that all five give approximately the same rates, supports the 3.4 cm/yr sedimentation rate. Using this sedimentation rate, a density of solids of 2.6g/cm3, and the average water content of the sediments (69.5%) the average sediment flux is computed to be 1.28 g/cm’/yr. The sediment flux for 1976 from the suspended sediment trap was 0.89 g/cm’/yr. Considering that that year was not a major storm year, the sediment fluxes are in reasonable agreement. Atmospheric fluxes
It was shown in a previous section that direct atmospheric fallout is a minor source of sediment at Barrett Reservoir. However, GOLDBERGet al. (1976) and KOIDEet al. (1975) have demonstrated that there can be a magnification of a direct atmospheric fallout flux for bomb produced radionuclides due to the accumulation of soil debris by water runoff into the reservoir from the adjoining land where the surface soil debris contains a significant atmospheric component of these radionuclides. A magnification factor
r-
Fig 7. Pu-239 + 240 activity and per cent PLI-238/Pu-239 + Pu-240 vs depth in Barrett Reservoir sediments.
1569
Semi-arid lake sediments Table 5. Elemental contents of the soluble and insoluble fraction of atmospheric
particulates collected
in a bucket during May 1976 at Barrett Reservoir compared to average values for 1976 at La Jolla, California and Ensenada. Baja California. (Values in ppm) (ND = not detected) !!a
Al -
Cd -
go%
ND ND
86,900 5,800
ND ND
ND ND
Insoluble ND 64,700 Soluble 2 ND Ensenada, Baja, California(i) Insoluble ND 82,140 Soluble ND h>
1 3 2 0.5
Barrett Insoluble La U;j;;leCAcl)
pb
&l
32 17
460 26
518 332
344 132
41 17
1,190 314
750 339
1,064 214
24 1
193 29
264 29
cu
Ff
k&l
31 ND
62 29
58,300 3,700
899 1,303
m ND
109 3
281 174
41,875 165
ND ND
42 1
86 14
43,570 Lxrl
g
v ND ND
121 ND
(I)HODGEet al. (1978).
of 4-5 compared to the integrated atmospheric fallout was found in Tokyo moat sediments (GOLDBERGet al. 1976) and a factor of 2 in California coastal sediments KOIDE et al. 1975). These magnification factors were arrived at by comparing the total fluxes of plutonium isotopes to the deposit with those of direct fallout. By utilizing the Pu-239 + 240 data for Barrett Reservoir (and linearly interpolating between points), an integrated value of 15 mCi/km’ is found. The average stratospheric fallout based on soil profiles for 3&4O”N latitude obtained by HARDY et al. (1973) is 1.8 mCi/km’. Thus, there is a magnification factor of about 8 at Barrett Reservoir. This factor is larger than the factors found in the other areas due to the rugged relief, semiarid environment with small vegetative cover, large drainage area to basin ratio and heavy rainfall over a small portion of the year. All these contribute to an efficient removal of surface soil debris with the infrequent rains. Therefore, if other atmospherically transported pollutants behave similarly to plutonium, it is possible that the sediments at Barrett Reservoir may contain Table 6. Characteristics Sample Depth (cm) l-2 4-5 7-8. 9-10 12-13 20-21 23-24 24-25 26-27 29-30 35.8-37.7 41.5-43.5 48.8-50.2 58.7-60.0 61.5-63.0 63.0-64.5 64.5-66.0 66.0-69.0 69.0-73.3 73.3-73.8 73.8-76.0 76.0-80.2 80.2-81.7 81.7-82.7 G.C.A. 42/l-
% Dry Weight 24.4 26.6 29.1 35.7 31.4 29.2 33.8 37.2 25.3 18.1 21.0 20.8 23.9 41.1 37.8 40.7 33.5 45.0 43.6 33.2 23.3 35.3 28.1
measurable levels of atmospheric pollutants due to the entry of remobilized soil debris containing these atmospheric pollutants even though direct atmospheric fluxes of solids are small compared with the other sediment sources. The contents of Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn were determined on the bucket atmospheric samples. The results on the soluble and insoluble fractions are given in Table 5. With the exception of Mn, they are in reasonable agreement with values found in San Diego particulates by HODCE et al. (1978). The elevated Mn content most probably is a reflection of the railing removed at the dam as discussed earlier. Compared with their contents normally found in sediments, Pb and Zn contents in the sediments could be increased by a large influx of atmospheric particulates. The metal contents of Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn were determined at selected depths in the sediments. In general, storm layers were not sampled. A few were included toward the bottom of the core in addition to the 1969 storm layer.
of Barrett Reservoir sediments
X HCl Residue
x Quartz
x Plagioclase
84.2 84.3 84.7 86.5 85.7 84.0 82.5 83.4
66.5 64.5 62.7 83.7 88.2 76.0 77.6 84.7
3.1 2.9 3.1 3.1 2.8 4.0 3.9
11.5 11.3 13.0 8.5 13.4 14.4 11.5
84.7 83.6 84.0 85.5 85.6 85.6
60.8 67.0 al.1 62.5 82.8 81.7 83.4
3.0 3.6 3.4 4.0 4.7
% Remaining (5OOOC)
17.1 14.8 11.9 22.0 13.3
% Total Organic
% CaCO3
x Opal
z CH?O
5.5 10 10 4 4 15 6 5
32 30 33 35 32 17
11.5 11.5 9.8 9.5 9.8 14.3 16.3 17.3
49 51.5 53 48.5 46 46
6 3 19 4 3 4
29
13.3 11.0 15.0 12.0 11.3 11.8
48
27
30.5 25 27 27 28
49
64.5 41 41 43
CO ppm 21.9 22.8 23.5 28.9 29.1 19.8 27.9 28.7 22.6 22.7 28.4 16.0 22.0 24.2 22.1 19.9 19.6 21.8 22.1 10.1 15.3 22.1 26.3 25.6
Semi-arid lake sediments REFERENCES
CONCLUSIONS
An investigation of direct atmospheric fallout, suspended particulates, and core material was made to decipher the historical record of anthropogenic metal fluxes to the sediments of Barrett Reservoir located in a remote area 30 km east of the metropolitan center of San Diego. Direct atmospheric fallout was found to contribute only a small percentage to the sediment flux to the reservoir even when compared to those months when no stream influx occurred. Fortunately though, in this setting of semiarid climate, rugged relief and incomplete vegetative cover, there is a magnification factor of about 8 (as indicated by the Pu-239 + 240 data) for atmospheric fallout when surface soil debris (containing a significant atmospherically derived component) from the drainage basin is washed into the reservoir during infrequent rains. Thus, in spite of a high sedimentation rate, an anthropogenic Pb input to the sediments was found. This magnification effect explains the high anthropogenic Pb flux, 18.3pg Pb/cm’/yr vs background 15.5 pg/cm2/yr, for this inland region compared to the fluxes determined for West Coast offshore sediments. Even though Barrett Reservoir was not the ideal case of constant particle-by-particle deposition, due to seasonal&y of riverine input, it is possible to glean some information about metal fluxes from its record. In such a non-ideal case, it is necessary to integrate the results of several dating methods and to keep the basic assumptions
of each method
1571
firmly in mind.
However, for those elements with small anthrooogenie inputs, the historical record of pollutant influx may not be decipherable above natural variations due to seasonal effects. Acknowledgements-This work was supported by NSF grant, DES74-19872 and AEC grant AT(04-3)34, project 84. The manuscript benefited greatly from the review of E. D. GOLDBERG. We would especially like to thank MEL AUBERYand BILL NYE of the Water Utilities Department, City of San Diego, California, for their help in securing the samples and interpreting the results.
BERTINEK. K. and GOLDBERGE. D. (1977) History of
heavy metal pollution in southern California coastal zone-reprise. Environ. Sci. Technol. 11, 297-299. BERTINE K. K. and MENDECK M. F. (1978) Industrial&+ tion of New Haven, CT, as recorddd in’reservoir sediments. Environ. Sci. Technol. 12, 201-207. BRULANDK. W., BERTINEK., KOIDE M. and GOLDBERG E. D. (1974) History of metal pollution in southern California coastal zone. Environ. Sci. Techno/. 8, 425-432. GOLDBERGE. D. (1958) Determination of opal in marine sediments. Mar. Res. 17, 178-182. GOLDBERGE. D. and ARRHENIUS G. D. S. (I 958) Chemistry of Pacific pelagic sediments. Geochim. Cosmochim. Acta 13, 153-212. GOLDBERGE. D., GAMBLEE., GRIFFIN J. J. and KOID M. (1977) Pollution history of Narragansett Bay as recorded in its sediments. Esturaine Coastal Mar. Sci. 5, 549-561. GOLDBERGE. D., HODGE V., KOIDE M. and GRIFFIN J. J. (1976) Metal pollution in Tokyo as recorded in sediments of the Palace Moat. Geochim. J. 10, 16>174. GOLDBERGE. D.. HODGEV.. KOIDEM.. GRIFFINJ.. GAMBLE E., BRICKER0. P., MAT~SOFFG., HOLDRENd. R. and BRAUNR. (1978) A pollution history of Chesapeake Bay. Giochim. Acta 42, 1413-1425. HARDYE. P., KREY P. W. and VOLCHOKH. L. (1973) Global inventory and distribution of fallout plutonium. Nature 241, 444-445. HODGE V., JOHNSONS. R. and GOLDBERGE. D. (1978) Influence of atmospherically transported aerosols on ~surface ocean water composition. Geochem. J. 12, 7-20. HUTCHINSON G. E. (1975) A Treatise on Limnology. Volume III. Limnological Botany, pp. 148-l 5 I, 30&302, 401402. Wiley. KOIDEM. and BRULANDK. W. (1975) The electrode position and determination of radium by isotope dilution in sea water and in sediments simultaneouslv with other natural radionuclides Anal. Chim. Acta 75: l-19. KOIDE M., BRULANDK. W. and GOLDBERGE. D. (1972) Th-228/Th-232 and Ph-210 geochronologies in marine and lake sediments. Geochim. Cosmochim. Acta 37, 1171-1187. KOIDE M., GRIFFIN J. J. and GOLDBERGE. D. (1975) Records of plutonium fallout in marine and terrestrial samples. J. Geophys. Res. 80, 4153-4162. KUZNETSOVS. I. (1970) The Microjlora of Lakes and its Geochemical Activity, pp. 429436 (trans. from Russian). University of Texas Press. Z~JCHTBAUER H. (1974) Sedimentary Petrology Part II. Sediments and Sedimentary Rocks 1. pp. 218, 221-222. (trans. from German). Wiley.