Concentrations of 2399240Pu 9 137Cs,and 90Sr in the Waters of the Laurentian Great Lakes. Comparison of 1973 and 1976 Values James J. Alberts' Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 2980 1 Morris A. Wahlgren Radiological and Environmental Research Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439 Comparisons of the concentrations of 239,240Pu, 137Cs,and 90Srin the waters of four of the Great Lakes in 1973with those in 1976 show that the concentrations of 239,240Puand 137Cs have decreased somewhat in the upper pair, Lake Michigan and Lake Huron, but have remained unchanged in the lower pair, Lake Erie and Lake Ontario. There was no significant change in concentration of in any of the lakes. Studies of the submicrometer particle size and charge characteristics of 239,240Pu in these lakes demonstrate that it exists as a simple anionic complex or is associated with particles that are smaller than 30 A and negatively charged, as previously reported for Lake Michigan. For all three radionuclides, significant differences in concentration or degree of removal between the upper pair of lakes and the lower pair are attributable to the flushing and sedimentary characteristics of the four lakes.
Introduction Plutonium and the long-lived fission products 90Sr and 1,37Cs,which entered the biosphere as the result of nuclear weapons testing in the atmosphere, are particularly useful in studies of the processes that occur in natural water systems. There is no natural or other man-made source, their mode of entry in known, and their entry is well characterized with respect to time and amounts. About 90% entered in the period 1955-1966 and the remainder in the period 1966 to present. Wahlgren and Nelson ( I ) reported the total surface water concentrations of 239,240Pu,137Cs,and 9'4% in all five of the Laurentian Great Lakes during spring 1973. The authors observed relatively narrow concentration ranges for all of the isotopes and made calculations which indicated that 239,240Pu and 137Cshad been rapidly removed from the water column in all five lakes, with Lake Erie having the highest removal rate for both elements. Strontium-90 was apparently conservative in the water column, being controlled primarily by dilution and flushing processes. Additional studies of 239,240Pu in Lake Michigan have confirmed the reported concentration range for that lake and shown that the plutonium concentration follows a seasonal cycle in the surface waters while remaining relatively constant in deep waters (2, 3). In addition to the 2:39,240Pu values reported by Wahlgren and Nelson ( I ) , a concurrent study showed that the plankton concentration factor for 239,240Pu from water was the same for all five lakes ( 4 ) . Alberts et al. ( 5 ) have shown that 239,240Pu exists in Lake Michigan waters in a small, anionic form (apparently as a carbonate, bicarbonate, or hydroxide complex). If the chemical form of 2393240Pu in Lake Erie is similar to that in the other lakes, the apparent removal rate in Lake Erie must be due primarily to the differing physical parameters which control sedimentation and resuspension of that lake. The relatively high biological productivity of Lake Erie may also contribute to the accelerated removal of this element from the water column. Finally, the data provided by Wahlgren and Nelson ( I ) represent a base line to which newer data may be compared in an effort to detect deviations from expected trends and may be useful in understanding the mechanisms controlling the concentrations of these elements in surface waters. The study reported here allows each of the above points to 94
Environmental Science & Technology
be addressed for four of the five Great Lakes. The samples were all collected within a short time period so that temporal differences should be minor. In addition, the samples from both studies were collected and analyzed by the same laboratory using the same methodology so that the results should be directly comparable.
Met,hods Samples were collected aboard the U.S. Coast Guard Cutter Westwind during a single traverse of the Laurentian Great Lakes (Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario) over the period from June 30 to July 4,1976 (Table I). Two-hundred-liter surface water samples were obtained with a submersible pump placed at 3-m depth, while similar volumes of deep water were taken by multiple lowerings of a 30-L, PVC Niskin bottle. All water was passed through Nitex nets of 85- and 37-pm mesh and then filtered through 3.0- and 0.45-pm membrane filters (Millipore Corp). Two 50-L samples of filtered water were immediately spiked with 236Pu(1.2 dpm) as an internal standard, 100 mL of concentrated HC1 which lowered the pH to -2, and stable Cs and CsCl(5 mg of Cs/0.5 mL of 6 N HCl). An additional 50-L filtered water sample was passed (1 L/min) through successive ion-exchange resin beds containing 227 g of anion resin (Dowex 1A X 8,100-200 mesh, chloride form) and 227 g of cation resin (Dowex 50W X 8, 100-200 mesh, hydrogen form). The water which passed through the resin beds was discarded. For selected samples, a 50-L filtered water sample was passed through a Bio-Fiber 80 Miniplant (Bio-Rad Laboratories) ultrafiltration unit with an approximate size retention of particles greater than 30 000 molecular weight (M,) or -30-A diameter. The water which passed through the fiber bundle was retained and spiked as above. The fiber bundle was backflushed with deionized water which was, in turn, spiked and retained. The various water samples were analyzed for 239,240Pu by the method of Golchert and Sedlet (6),as modified by Nelson et al. ( 7 ) . One hundred milliliters of deionized water containing -4 nCi 85Srwas y counted in a 250-mL polyethylene bottle, and the solution was added to the initial water samples before separation of the plutonium. The resins were dried at 110 "C, ashed at 500 "C, dissolved in concentrated nitric acid, spiked with 236Pu(1.2 dpm), evaporated to dryness, dissolved in concentrated HBr to ensure isotopic exchange, evaporated to dryness again, taken up in nitric acid, and then analyzed for 239,240Puin the same manner as the water samples. The activity of 239,240Pu was determined by comparing the integrated count under that peak on an a spectrometer with that in the 236Pu internal standard peak. Cesium-137 and 90Sr were determined by using the 8 M HN03 feedstock and wash resulting from the plutonium analysis. One gram of ammonium molybdophosphate (AMP, Bio-Rad Laboratories) was added to the combined feedstock and rinse, stirred for 15min, and centrifuged. The precipitated AMP was reserved for 137Cs analysis and the supernatant for analysis. The 137Cs was analyzed by preparing a column consisting
0013-936X/81/0915-0094$01.OO/O @ 1981 American Chemical Society
Table I. Station Locations USCGS Westwind Laurentian Great Lakes Transect, June 30-July 4, 1976 station
latitude
iongltude
water depth, m
sample depth, m
Lake Michigan station 1
44'48"
86'22'W
260
3 220
Lake Huron station 2
45'20"
83'05'W
130
station 3
44'52"
82'28'W
61
3 125 3 55
Lake Erie station 4
42'02"
81'36'W
18
42'28.5"
79'55'W
38
43'25"
78'42'W
112
43'38.5"
77'44'W
162
station 5 Lake Ontario station 6 station 7
3 15 3 35 3 110
3 150
of 10 cm3 of cation-exchange resin (Bio-Rex 40) in a standard 50-mL Pyrex buret. The resin was preconditioned with 200 mL of 5%NaCl followed by 10 mL of 0.75 M NaOH. The AMP (containing the cesium) was dissolved in 30 mL of 0.75 M NaOH-2% EDTA and passed through the column at a rate of -2 mL/min. The column was rinsed with two 10-mL portions of deionized water and 250 mL of 0.3 M HC1. The cesium was eluted with 150 mL of 3 M HCl, and the eluant evaporated to dryness. (The column may be prepared for reuse with 100 mL of deionized water.) The cesium salt was dissolved in -10 mL of deionized water, and -0.5 mL of 10 M NaOH and 5 mL of 3%sodium tetraphenylboron solution was added. The solution was allowed to stand -15 min, and concentrated HC1 was added dropwise until the precipitate coagulated. After standing 30 min, the pH was adjusted to 1 2 or 13 with 10 M NaOH, and the suspension was filtered with suction through a weighed 25-mm membrane filter (0.45-pm Millipore). The precipitate was rinsed with deionized water and dried a t 60 'C. The weighed precipitate and filter were mounted on a 38-mm stainless steel planchet by using two-sided adhesive tape and covered with 8-pm Mylar film for 0counting. The counter used was an Argonne Laboratory built four-detector
gas-flow proportional counter. Backgrounds on the four detectors ranged from 0.38 to 0.61 cpm. After correcting for recovery of stable cesium, we computed the activity of 137Csby comparing the count rate with that obtained from a standard 137Cs stock solution counted under the same conditions. Four hundred milligrams of stable strontium was added to the 8 M HNO3 solution containing the 90Sr. Concentrated H N 0 3 was added until the volume was -900 mL and evaporated with stirring on a hot plate until Sr(NO& precipitated. decanted. The The solution was cooled and the "03 Sr(NO& was dissolved in -40 mL of deionized water and centrifuged if the solution was not completely clear. Four was added, and the hundred milliliters of concentrated "03 solution heated until the volume was less that 300 mL. The solution was cooled and the HNO3 decanted. The beaker was warmed gently to evaporate any remaining HNO:?. The salt was dissolved in 30 mL of deionized water and transferred to a 50-mL centrifuge tube. One-half milliliter of yttrium carrier was added, and the pH adjusted to -10 with NaOH. The supernate was centrifuged and decanted to the 250-mL polyethylene bottle in which the 85Srwas originally y counted. The pH was adjusted to 2 with HC1,0.5 mL of yttrium carrier was added, and the sample was counted to determine the recovery of s5Sr and stored at least 2 weeks. The sample was removed from the polyethylene bottle. The pH was adjusted to 10 with NaOH, and the sample was separated by centrifugation. The supernate was returned to the bottle, and the time of separation recorded. The precipitate was dissolved in a minimum of 6 M HC1 and diluted to 30 mL. A precipitate was formed and separated by adjusting the pH to 10 with NaOH and centrifuging. The two supernates were combined in the bottle, 0.5 mL of yttrium carrier was added, and the solution was reserved for future milkings. The precipitate was dissolved in 10 mL of 0.1 M HCl. The pH was adjusted to 3 with NaOH, and 0.5 mL of 10% oxalic acid was added. The solution was stirred for a few minutes, and the precipitate was filtered through a 25-mm membrane filter (0.45-pm Millipore). The precipitate was dried and mounted for 0counting as was described for 137Cs.The same detectors were used as for 137Cs, and counting was repeated often enough to establish the 64-h half-life of Following correction for the recovery of 8,5Sr, the activity of 90Srin the sample was computed by comparing the initial count rate of the with that obtained by using the same separations on a standard 90Sr-90Ystock solution. All water samples were filtered, and both the filters and the filtrate were analyzed for 239,240Pu. The amount on the filter
Table II. 2 3 9 ~ 2 4 0Concentrations P~ (fCi/L) in the Laurentian Great Lakes station
Lake Michigan station 1 Lake Huron station 2 station 3 Lake Erie station 4 station 5 Lake Ontario station 6 station 7
sample depth, m
fllterable
summer 1978 "dlssolved"
3 220
0.03 f 0.004 0.09 f 0.010
0.37 0.48
3 125 3 55
0.03 f 0.005 0.07 f 0.007 0.03 f 0.005 0.19 f 0.010
0.43 f 0.06 0.45 f 0.06 0.38 f 0.05 0.50 f 0.07
0.46 f 0.07 0.52 f 0.07 0.41 f 0.05 0.69 f 0.08
0.60 f 0.05
3 15 3 35
0.01 f 0.003 0.01 f 0.003 0.01 f 0.003 0.07 f 0.010
0.13 f 0.05 0.12 f 0.04 0.10 f 0.04 0.12 f 0.05
0.14 f 0.05 0.13 f 0.04 0.11 f 0.04 0.19 f 0.06
0.24 f 0.06
3 110 3 150
0.01 f 0.003 0.14 f 0.010
0.03 f 0.04 0.21 f 0.05 0.09 f 0.04 0.23 f 0.05
0.04 f 0.04 0.35 f 0.06
f 0.03 f 0.06
total
0.40 0.57
f 0.03 f 0.07
spring 1973
0.69
0.63
0.16
f 0.07
f 0.05
f 0.03
0.24 f 0.05 0.24
f 0.05
Volume 15, Number 1, January 1981 95
Table 111. Concentrations of O0Sr (pCi/L) in the Laurentian Great Lakes sample depth, m
Lake Michigan station 1
summer 1976
3
sprlng 1973
0.89
f 0.09
0.82
f 0.03
0.88
f 0.09
0.98
f 0.02
1.03 0.87
f 0.10 f 0.09
0.93
f 0.02
220 Lake Huron station 2
3 125
station 3
3 55
Lake Erie station 4 station 5
0.99 f 0.10 1.08 f 0.11 1.02 f 0.10 0.97 f 0.10
3 15
3 35
Lake Ontario station 6 station 7
0.98 f 0.10 1.22 f 0.12 1.26 f 0.13 1.20 f 0.12
3 110 3 150
1.10 f 0.02 1.06 f 0.03
1.28 f 0.03 1.28 f 0.03
was added to the amount in the water in calculating the total 239,240Puconcentration in the water.
Results and Discussion The total 239,240Pu concentrations obtained in this study are compared with those of Wahlgren and Nelson (1) in Table 11. The latter data represent averages of stations from the earlier work which best approximate the locations of the 1976 samples. Comparisons of these numbers show that the concentration of 239,240Puin the waters of the upper two lakes (Michigan and Huron) decreased ,between 1976 and 1973, while that in the lower two lakes (Erie and Ontario) remained unchanged. The data for Lake Ontario suggest that the 239,240Puconcentration may undergo seasonal cycling similar to that in Lake Michigan ( 2 , 5 ) .I t should be noted that the lake had begun to stratify at the time our samples were taken (to a depth of 29 m) as indicated by an XBT probe. Since the 1973 surface water samples from Lake Ontario were taken in the spring (prior to stratification), the values reported are probably those for the well-mixed water column value, i.e.,
similar to the deep-water values, as seen in Lake Michigan and the other lakes which have not undergone stratification. The data for 90Sr(Table 111)show that its concentration has remained unchanged with time in all of the lakes. Furthermore, the surface and deep-water values are constant with no indication of removal of this isotope from the surface waters with the onset of stratification. This is expected since the recent annual inputs of this radioisotope are nearly equal to the rate of radioactive decay (8) and strontium is considered to be conservative in the Great Lakes (9). The situation with 13jCs is not as clear as that for either 2::19,240Pu or 90Sr. The data for Lake Huron, Lake Erie, and Lake Ontario show that the l13jCsconcentrations either have slowly decreased or have remained unchanged in those waters (Table IV). The results for Lake Michigan suggest that the concentration of 137Csin that lake may be higher in 1976 than it was in 1973. However, the data from the station in the northern basin of the lake is inconsistent with a series of observations in the southern basin (IO)which indicates a slow decrease in 13jCs concentration with time. Hence, the indicated increase in 137Csconcentration may be an artifact as it is anomalous both to the observed trends in the other lakes and to a trend observed elsewhere in the same lake. A further means of gaining insight into the changes which have occurred in the isotope concentrations in the lakes is to compare the ratios of their concentrations for the two sampling periods (Table V). The 13TCs/90Srratio has remained unchanged in all instances. Similarly, the 239,240Pu/13iCs and 2:19,240P~/90Sr ratios have not changed significantly in the lower lakes. The only readily apparent changes in any of the ratios are those for 239~240Pu/90Sr in Lake Huron and both 239,240Pu/90Srand 239,240Pu/13jCsin Lake Michigan. The 2:19,240Pu/90Srratios appear to be reflecting the decreasing plutonium concentrations in the two lakes. However, the 239,240Pu/137Cs ratio in Lake Michigan still appears anomalously low and, again, may be the result of an artifact in the 137Csconcentration observed in northern Lake Michigan. Wahlgren and Nelson ( I ) calculated the residual fraction of fallout material remaining in the waters of the Great Lakes and found that the chemically dissimilar isotopes 239,240Pu and 137Cshad apparently been rapidly removed from the water column. Similar calculations using our data, updated input values ( 8 ) ,and the 20% contributions due to runoff used by
Table IV. Concentrations of 13’Cs (fCi/L) in the Laurentian Great Lakes statlon
Lake Michigan station 1
sample depth, m
station 3 Lake Erie station 4 station 5 Lake Ontario station 6 station 7
summer 1976 “dissolved”
total
sprlng 1973a
f1
1.1 2.7
59.0 49.1
60.1 51.8
40
33.7 31.8 30.2 27.9
35.4 34.7 31.3 37.6
44 f 5
55
1.7 2.9 1.1 9.7
3 15 3 35
1.3 0.9 0.5 6.4
14.7 17.1 9.9 13.9
16.0 18.0 10.4 20.3
22.5 f 3.5
3 110 3 150
0.2 16.4
24.9 20.0 21.6 17.9
25.1 36.4
26 f 2
3 220
Lake Huron station 2
fllterable
3 125
3
39 f 2
15 f 5.7
29
f2
a The stated errors are f l standard deviation of the mean values of 13’Cs concentrations for the stations which were averaged in the 1973 data to give values to approximate the locations oPthe 1976 stations. The 13’Cs concentrationsof the latter are from single grab samples and do not have a similar error term associated with those values.
96
Environmental Science & Technology
Table V. Ratios of Laurentian Great Lakes Isotope Concentrations 1976 total
Lake Michigan station 1 Lake Huron station 2 station 3 Lake Erie station 4 station 5 Lake Ontario station 6 station 7 mean
137~~/gO~r 1976 1973
1973 particulate
6.7 11.0
27 33
17.2
0.42
0.84
0.07
0.05
13.0 15.0 13.1 18.4
17 24 13 20
13.6
0.49
0.61
0.04
0.04
16.2
0.37 0.57
0.67
0.03 0.04
0.04
8.8 7.2 10.6 9.4
8.0 11 22 11
10.7
0.13 0.11 0.10 0.12
0.22
0.02 0.02 0.01 0.02
0.02
1.6 9.6 4.2 12.8 10.1
53 9
0.03 0.03 0.02 0.01 0.03
0.02
10.7
9.2
0.03 0.17 0.07 0.19 0.23
8.3 18 20.5
0.15
12.3
0.19 0.19 0.41
0.01
0.02 0.03
Table VI. Fraction of Isotope Remaining in Water Column 239,240~~ lake
1976
Michigan Huron
0.018 0.018
Erie
0.001 1 0.0090
Ontarioa
137cs 1973
1976
1973
0.028 0.022 0.0017 0.0100
0.051 0.023 0.0025 0.018
0.033 0.028 0.0028 0.019
m 1.06 0.84 0.23 1.22
90Sr
1973
0.92 0.87 0.24 1.32
a Because of the stratification of the surface waters of Lake Ontario and the subsequent anamolously low plutonium concentrations, calculations for plutonium remaining in this lake are based on dissolved plutonium concentrations of waters below the thermocline (Table 11).
Wahlgren and Nelson support the conclusion of those authors (Table VI). In addition, a comparison of the fractions remaining in the water column indicates that 239,240Pu is slowly being removed from the waters of the upper two lakes. The 37Csconcentrations have been unchanged or have been decreasing slowly, with the possible exception of northern Lake Michigan. ‘These data suggest that during the mid-1970s the continuing small inputs of 239,240P~ and 137Cswere nearly balanced by losses to the sediments. The residence times of 2.39,240Pu in all five Great Lakes have recently been estimated by using a coupled chain of lakes model, apd it was shown that the time dependence of observed concentrations of 239,240Pu in Lake Michigan from 1972-1977 could be approximated by assuming a residence time of 2.4 yr (11).It would appear that in the waters of the Great future concentrations of 239J40P~ Lakes may not decrease as rapidly as the residence time estimates would suggest. The reasons for this are (1)the apparent small but finite divergence between calculated steady-state and measured water concentrations, (2) a potential reversibility of sediment adsorption of 239,240Pu demonstrated recently in unpublished laboratory studies, and (3) the near bottom water concentration values of 239,240Pu for Lake Erie and Lake Ontario in this report. The anomalies observed here for all isotopes in Lake Erie were also observed in the calculations of Wahlgren and Nelson ( I ) . Those authors felt that this data plus other observations of plutonium and cesium behavior in Lake Michigan could be attributed to a strong biological influence on the controlling mechanism of these isotopes in the Great Lakes. Another possible explanation for such large differences, at least in the case of plutonium, could be differences in the chemistry of the za9,240Puin these waters. Alberts et al. ( 5 ) showed that 239,240Pu in Lake Michigan waters was present as a small (
