Retention of Neptunium, Americium, and Curium by Diffusible Soil Particles John C. Sheppard”, Malcolm J. Campbell, and James A. Kittrick Department of Chemical Engineering and Department of Agronomy and Soils, Washington State University, Pullman, Wash. 99164
Todd L. Hardt Babcock and Wilcox, Lynchburg Research Center, P.O. Box 1260, Lynchburg, Va. 24505
Table I1 is a compilation of the physical and chemical properties of the soils. Radionuclides. The radiochemical purity of the radionuclides used in the distribution studies was established by a pulse analysis using a silicon surface barrier detector (25 keV fwhm) and a 1024-channel analyzer. The pulse height spectrum of each radionuclide (237Np,241Am,and 244Cm)indicated that less than 0.1% of other a-emitting radionuclides was present. As an additional precaution, a single-channelanalyzer was included as part of the counting system, making it possible to count only the radionuclide of interest. Chemical purity of the tracer solutions was established using a Si(Li) X-ray detector (280 keV fwhm a t 6.4 keV) and a 1024-channel analyzer. Examination of the characteristic L X-ray spectrum indicated no detectable contamination, and, in particular, neither Fe nor Ca was observed. Gel permeation chromatographic analysis of the aqueous phase before equilibration with the various soils indicated that 100%of the Am(II1) tracer was in the form of particles with radii less than 1nm, presumably Am3+,AmOH2+,and small hydrolytic polymers. Experimental Determination of Effective Distribution Ratios. Ten microliters, containing 0.25 pCi of the radionuclide of interest, was added to 10 mL of distilled water and a weighed amount, usually 1.00 g, of air-dry soil in a stoppered 50-mL polycarbonate centrifuge tube. The contents of the tube were equilibrated on a rotary shaker at room temperature (-25 “C) with samples removed periodically for analysis. Separate experiments with what is probably the least pHbuffered soil (FQ-1) indicated that during the course of the equilibration there was no appreciable change in pH from the
When some representative U S . soils were shaken with 241Am,244Cm,and 237Npin distilled water, it was found by centrifuging and other techniques that much of the 241Amand 244Cmand some of the 237Npwere bound to soil particles having colloidal dimensions. Effective distribution ratios (which depend upon physical details of the measurement technique) indicated that increasing retention of the actinides by most of the soils did not cease in 4-6 months. Furthermore, the proportion of actinide retained by potentially diffusible soil particles decreased with time. As the number of nuclear facilities increases, larger amounts of the actinide elements (Np, Pu, Am, and Cm) will be generated. While the probability of accidental releases of these elements is small in any individual case, the aggregate number of releases will inevitably increase. Because the actinide elements are radiotoxins, these releases may have public health implications. Rowe and Holcomb ( I ) have labeled these public health and societal impacts of the management of radioactive wastes as the “Hidden Commitment” of nuclear power. Thus, it is important to understand the chemical factors that influence the movement of the actinide elements through the environment. The physical processes controlling diffusional movement of the actinides (and, indeed, of some other elements) in the soil are poorly understood. It is often assumed that essentially all migration results from the actinide in the ionic form. This investigation examines the extent to which Np, Am, and Cm are retained by potentially diffusible soil particles under standardized soil-water conditions.
Experimental Soils. The soils were selected mainly for their relationship to sites of potential large-scale radionuclide releases. Some were taken from existing Department of Energy (DOE) facilities a t Hanford and Idaho Falls, and from the site of a potential fuel reprocessing facility being constructed by Allied Chemical Corporation at Barnwell, S.C. Other soils are related to concurrent DOE research into questions such as the uptake of P u and Am from soils by plants. Finally, soils were selected with widely varying chemical and physical properties that might be correlated with the distribution ratios. Table I contains a list of the soils used in this research, where they were obtained, and a code name which will be employed in the remainder of this report. Physical and chemical properties of each soil were determined by standard methods ( 3 ) .Soil pH was measured using a glass electrode and water-saturated soil paste. The mechanical analysis method of Bouyoucos ( 4 ) was used to determine percentages of sand, silt, and clay. Organic content of the soils was found by the dichromate oxidation method ( 5 ) and cation exchange capacity was determined using the sodium acetate extraction-saturation method (6).The physical properties of the BUR and RITZ soils were in agreement with those found by Routson (7). Physical and chemical properties of the MUS, FQ, and H F soils were in agreement with those determined by Relyea and Brown (8).Our results for the ID and HF soils agreed less well with those supplied by Miner (9). 680
Environmental Science & Technology
Table 1. Soils and Clays Used in the Effective Distribution Ratio Studies soil codea
sol1 name
locatlon
MUS BUR HF-A HF-B
Muscatine silt loam Burbank loamy sand
RITZ
Ritzville silt loam Fuquay sand (0-5 cm) Fuquay sand (5-15 cm) Fuquay sand (5-50 cm)
FQ-1 FQ-2
FQ-3 ID-A ID-B ID-C ID-D AHN
RKS
Aiken clay loam Aiken clay loam
Muscatine, Ill. Hanford, Wash. Hanford, Wash. Hanford, Wash. Hanford, Wash. Barnwell, S.C. Barnwell, S.C. Barnwell, S.C. Idaho Falls, Idaho Idaho Falls, Idaho Idaho Falls, Idaho Idaho Falls, Idaho Paradise, Calif. Placerville, Calif.
a MUS soil was obtained from R. C. Dahlman, Oak Ridge National Laboratory, Oak Ridge, Tenn. BUR and RlTZ soils were obtained from R. E. Wildung, BNWL. Richland. Wash. FQ-1. FQ-2, and FQ-3 soils were obtained from F. W. Boone, Allied Chemical, Barnwell, S.C. HF-A. HF-8. ID-A, ID-B, ID-C, and ID-D soils were obtained from F. J. Miner, Dow Chemical Co., Rocky Flats, Colo. HF-A and HF-B are essentially different samples of the BUR soil. AHN soil (2)was obtained from H. Nishita, UCLA, Los Angeles. Calif. RKS soil was obtained from R. K. Schultz, University of California, Berkeley, Calif.
0013-936X/79/0913-0680$01 .OO/O
@
1979 American Chemical Society
Table II. Physical and Chemical Properties of Soils Studied C.E.C., soii code
sand, %
Slit, %
clay,
MUS BUR HF-A HF-B
12.6
65.8
21.6
silt loam
76.0 65.2 83.6 32.0 91.2 91.6 94.6 42.6
2.8 5.8 3.8 12.0
60.4
21.2 29.0 12.6 56.0 7.8 5.4 1.6 39.4 19.4
1.o 3.0 3.8 18.0 20.2
83.4 49.2
8.8 28.4
7.8 22.4
loamy sand sandy loam loamy sand silt loam sand sand sand loam sandy clay loam loamy sand sandy clay loam
RlTZ FQ- 1 FQ-2 FQ-3 ID-A ID-B ID-C ID-D
=
cpm of actinide per unit supernatant vol cpm of actinide per unit mass of soil before equilibration where cpm is N or y counts per minute of r41Am,J%m, or "Np.. It should be noted that the denominator in Equation 1 differsfrom that usually employed in that the total actinide content of both phases, rather than the actinide content of the solid phase alone, is used. The Components of R*. From the definition of R * , it is clear that we can write the effective distribution ratio as a sum:
'
R*=R;+R,
org materlal, %
5.3
16.88
3.61
8.1 8.1 8.4 6.5
5.94 6.14 4.95 10.76
0.43 0.45 0.17 0.84
4.0 6.7 5.2 8.3 8.4
2.01 1.79 0.69 15.04 10.44
1.19 0.99 0.21 0.60 0.18
8.4 7.7
6.38 18.36
0.16 0.98
soil class
Oh
value given in Table 11. For our standard separations, sampling tubes were centrifuged a t 4500 rpm for 30 min, in a type 856 International centrifuge head, removing particles of density of 2.0 g ern-.' and >26 nm equivalent spherical radius. An aliquot of 100 pL was removed from the top 4 nm of supernatant to determine the element concentration. Aliquots were evaporated on stainless steel planchets, flamed, and covered with thin collodion film before being counted with the silicon surfacebarrier detector. Counting was continued until the error was less than 3%,except for samples having low counting rates. For these, the counting duration was extended only to 4000 or 8000 s, leading to somewhat larger errors. After sampling, the centrifuge tubes were returned to the shaker for further equilibration. This sequence was repeated a t intervals for 4-6 months, or until a constant distribution ratio was obtained. The data were converted to effective distribution ratios, R*, defined as:
R*
PH
mequivl 100 g
(2)
where R i is the particle component and R I the conventional, ionic component. True R I values require a demonstration that the radionuclide measured is truly ionic. In general, we find R I to be small compared to R i for our experimental conditions ( I O ) . (Additional experiments using Sephadex G-25M and Bio-Gel A-1.5M and centrifugation indicate the "ionic" contribution can be subdivided into two additional fractions. The Sephadex fraction includes actinide-bearing soil particles with radii less than 1 nm (equivalent molecular weight of 5000). This fraction includes actinide cations and their hydrolysis products and, presumably, small humic and fulvic acid complexes and represents less than 40% of R*. Bio-Gel A-1.5M experiments with Fuquay, Ritzville, and Burbank soils indi-
.
0
20
40 a, nm
60
80
Figure 1. S(a)for various rpm. R' is roughly proportional to the area under the S(a) curve, and therefore also varies with rpm. Soil particles are assumed to be spherical and have a density of 2.0 g cm-3 cated that most of the remaining actinide bearing particles were in the 1-10-nm range with peak molecular wights at 8000, 50 000, and 50 000. respectively. Contribution of soil particles with radii greater than 10 nm was almost negligible for the three soils studied.) R; is entirely dependent on the details of the technique used to separate soil particles of various sizes. For any specific separation method, we may define a function, S * ( a ) , which is t h e fraction of particles o f sizes a which is retained in t h e sample after separation. The method of calculating S * ( a ) is given in the Appendix. Generally, S * ( a ) = 1 for very small particles and falls to zero for particles above the cutoff size amax.Figure 1 shows the separation function for several centrifugation speeds, including our standard procedure used in measuring R* (4500 rpm and 4-mm sampling depth). The cutoff size in this case is 26-nm equivalent spherical radius; no particles larger than this are sampled, but a certain proportion of the particles just below the cutoff size is lost. Given this definition of S * ( a ) , we can write:
Ri =
La""'
S * ( a ) r ( a )da
(3)
where r ( a ) is the particle size-dependent distribution ratio, defined for each soil as r ( a ) da = (fraction of actinide on particles with radii in rangea to (a da))(M/V),where M is the total mass of the soil and V is the volume of the aqueous phase. The size-dependent distribution ratio will in fact depend upon the product of particle size distribution n ( a ){where n ( a ) da is the number per unit soil mass of particles with radii between a and ( a d a ) ] and another function, B ( a ) ,which
+
+
Volume 13, Number 6, June
1979
681
0 030
Np in AHN
-
1
ooo3'
0 1 a, nm
hypothetical size-dependent distribution ratios, A and B, as functions of particle size. The Stokes-Einstein diffusion coefficient
Figure 2. Two
curve, D, is also shown indicates how much actinide is attached to the average particle of size a : r ( a ) = n ( a ) B ( a ) Figure . 2 shows two hypothetical plots of r ( a ) . Also plotted is the diffusion coefficient, D , of potentially diffusible particles of each size, calculated from the Stokes-Einstein equation for particles in water. The observed diffusion coefficient would be an integral over the product of these factors. Soil A, because of the preponderance of small particles among those carrying the actinide, would evidence a greater potential mobility of the actinide than soil B, where the bulk of the actinide is attached to much larger, less mobile particles. If actinide molecules are retained by a continuous distribution of soil particle sizes, Figure 2 shows that a clear-cut distinction between mobile and immobile actinide does not exist. However, Figure 2 also shows that our arbitrary particle cutoff size of 26-nm equivalent spherical radius retains in R i the majority of particles with potential diffusional mobility. Retention of Np, Am, and Cm by Soil Particles. It was anticipated that when a 1-g soil sample was shaken continuously with 10 mL of distilled water, fine particles retaining actinides would detach from the soil matrix and remain dispersed. This was confirmed by centrifuging and gel filtration experiments which showed RI to be small compared to R i (10). Experiments using 0.2-pm filters (Nuclepore Corp., Pleasanton, Calif.) and a track films (using a modification of the procedure of Center and Ruddy (11))gave similar indications. We also studied the dependence of R* upon the time allowed for equilibration. Figure 3 shows a typical set of measurements. Over the times studied, R* generally showed three distinct segments. The first was a rapid quasiexponential decrease, which was followed by a slower exponential decrease with mean times of the order of weeks. Occasionally, a third segment, which suggested equilibrium conditions, was observed. A lower limit for measurable values of R* of 5 X g mL-l is imposed by the counting statistics. Because of the slow exponential decrease in R* values, experiments were continued 4-6 months. The possibility of surface adsorption on the centrifuge wall, causing the slow decrease in R *, was investigated by experiments with centrifuge tubes coated with silicone. No significant difference was observed between R* determined in the silicoated and uncoated centrifuge tubes. Since segment 2 predominates, these data were fitted to exponential curves of the form: R* = R i e - T / T b (4) where, for segment 2, Ri = the zero time value of R*, T = time, and 713 = the time required for R* to decrease to 0.37 of its original value. Table I11 summarizes values of -log Ri and 7 b obtained for the soils studied. Since R* X IO2 = % of total actinide in the 682
Environmental Science & Technology
Figure 3. Extrapolation of the 20- to 120-day segment of the curve to "zero time" yields R,. A typical plot of R' in g mL-' as a function of time, for Np in soil AHN
supernatant, a -log R i of 3, for example, implies 0.1% of the actinide in the supernatant and 99.9% removed by centrifuging. It is evident that for all actinides and all soils, the majority of the actinide has been removed by centrifuging. Further, much larger amounts of 237Npremain in the supernatant of each soil than of 241Amor 244Cm. Many of the R*s in Table I11 have now been resolved into two or more components by centrifugation and gel filtration experiments (10).As might be anticipated, for retention of Am and Cm by soils containing abundant small particles, R I is small compared to R i , and R* depends upon centrifuging conditions. For example, R* for MUS soil showed a reciprocal dependence on the square of the angular velocity, o,of the centrifuge rotor and had the form:
R*
= (6.1 f 2.5) X
+ (264 f 1
8 ) ~ ~
while for RITZ soils:
R* = (2.0 f 0.8) X
+ (0.599 f 0.005)t-1
where t is the centrifugation time. The slow decrease of R* with time in Table I11 parallels that observed by Cleveland and Rees (12) for their studies of the extractability of Pu(1V) and Am(II1) from Rocky Flat, Colo., soils. Rhodes (13)noted similar behavior for Pu(V1). In the case of the Np experiments, Np(V), the principal Np species at zero time, may be slowly reduced by organic matter to readily hydrolyzable Np(1V) and then adsorbed on soil colloids as suggested by Bondietti (14). However, regression analysis indicated no correlation between the organic content of the soils and the observed T b values for Am, Cm, or Np. Similarly, no correlation was found between 7 b and cation exchange capacity. The slow decrease in R* with time in Table I11 may represent growth of small particles, flocculation of particles, or redistribution of actinides from small to larger particles. Whatever the process, its rate is variable as indicated by columns 3,6, and 9 in Table 111, which disclose a wide variation in Tb between soils. Values of 7 b are generally larger for Np(V) than for Am(II1) or Cm(II1) for each soil, indicating that the increasing attachment of Np(V) to soil particles >26 nm is proceeding less rapidly than the other two actinides. Exceptions to this are BUR, HF-A, and HF-B, which are essentially the same soil, and can be used to estimate precision of the determinations. Table I11 also includes values of s y x , the standard deviation of the points about the second line segment. Generally the fit was good, but sources of error greatly exceeding the counting error clearly exist. The cause of this excess scatter is not known, but variations in sampling depth are suspected. Calculations based upon the solubility of Am(0H)S and Cm(OH), indicate that the actinide concentrations required for precipitation are orders of magnitude higher than the Am
Table 111. Zero ‘Time Effective Distribution Ratios of Am(lll), Cm(lll), and Np(V) for the Second of “Slow” Exponentially Decreasing Line Segment a 241Am soil
MUS
BUR HF-A HF-B
av F- 1 F-2 F-3 ID-A ID-B ID-C ID-D AHN
RKS RlTZ
av
--log R
3.50 f 0.44 2.61 f 0.49 2.32 f 0.13 2.79 0.32 2.53 2.38 f 0.34 0.64 2.48 2.31 f 0.71 4.04 & 1.00 3.22 4.03 f 1.05 0.18 3.75 4.01 4.23 3.14 A: 0.30 3.14 4: 0.66
* *
*
-log s y x
244cm T B , days
-log R
127
0.04 0.28 0.22 0.18 0.33
30 23
3.00 f 0.08 3.00 f 0.10 1.97 3.08 2.66 f 0.60 2.85 2.72
52 14
3.02 4.29 4.14
0.10 0.20 0.20 0.17
13 39
0.29 0.84 0.09 0.09
43 1
101 59
3.58 f 0.25 3.82 f 0.25 4.01 3.63 f 0.74 2.71 f 0.13 3.32 f 0.70
237Np -log R
T B , days
-log s y x
75
0.09 0.08 0.14 0.08
2.12 0.89 f 0.19 1.07 0.94 0.92 f 0.16 1.24 1.31 1.33 1.71 2.15 1.59 1.60 2.01 1.76 1.15 f 0.14 1.47 f 0.45
1395
0.05
35 27 91
0.11 0.07 0.14
0.01 0.05
43
0.17 0.19
7
7
0.06
-log Syx
T B , days
0.07
186 49 79 62
0.05 0.05 0.05 0.01
51 48 94
0.05 0.06 0.01 0.04 0.09 0.02 0.001 0.10
202 51 36 1 173 462 90
Distribution ratios with standard a Centrifuge conditions: International 856 centrifuge head, 4500 rpm, t = 30 min, sampling depth 4 f 1 mm. R‘ is in g mL-’. errors represent duplicate and triplicate determinations. BUR, HF-A, and HF-B are “Burbank” soils obtained from different investigators; therefore, R‘ values for these soils can also lbe compared, giving another indication of the precision of these experiments. The remaining R’s are the result of duplicate assays and counts. The distribution ratios are fitted to the form R’ = R, exp( T I T b ) .where RO is the “zero time” time value. Average for all 14 soils.
or Cm concentrations employed (3.2 X to 1.3 X M). Of course radiocolloids can somehow form a t orders of magnitude lower solution concentrations than can be explained by normal solubilities. I t has long been known that highly charged and readily hydrolyzable radionuclides such as Bi3+, Ac3+, and Th4+ bind to colloidal particles suspended in solution. In our experiments, the inert part of these radiocolloids as defined by Wahl and Bonner ( 1 5 ) is assumed to be soil minerals or other soil constituents. Conclusions The distribution ratio data in Table I11 suggest that correlations with chemical and physical properties of soils are difficult to obtain. Lack of precise distribution ratios and competition between cation exchange and complex formation by humic and fulvic acid are probably important factors. Since Am(II1) and Cm(II1) are adjacent actinide elements with almost equal hydrated radii, they would be expected to have almost ideintical distribution ratios. Therefore, differences of distribution ratios for these actinides must be explained on the b amax,where amaxis the particle cutoff size, amax = ( K d / X 1)1’2.
Acknowledgments
We are grateful for the assistance of Bernard Au who performed many of the N p distribution experiments. Todd Cheng performed the gel permeation experiments. Literature Cited (1) Rowe, W. D., Holcomb, W. F., Nucl. Tech., 24, 287 (1974). (2) Nishita, H., Hamilton, M., Steen, A. J., Soil Sei. SOC. Am. J., 42, 51 (1978). (3) Moodie, C. D., Koehler, F. E., “Laboratory Manual for Soil Fertility”, 3rd ed., Washington State University, Pullman, Wash., 1975. (4) Bouyoucos, J., Agron. J., 43,434 (1951). (5) Walkley, A., Soil Sei., 63,251 (1974). (6) Peech, M., Soil Sei., 59,25 (1945). (7) Routson, R. C., USAEC Document, BNWL-1464, March 1973. (8) Relyea, J., Brown, D. A., “Mineral Cycling Symposium”, Augusta,
Ga., 1976, in press. (9) Miner, F. J., Dow Chemical Co., Rocky Flats, Colo., 1975, private communication. (10) Unpublished research. (11) Center, B. M., Ruddy, F. H., Anal. Chem., 48,2135 (1976). (12) Cleveland, J. M., Rees, T. F., Enuiron. Sei. Technol., 10, 802 (1976). (13) Rhodes, D. W., Soil Sci. SOC.Am. Proc., 21,389 (1957). (14) Bondietti, E. A., “Agronomy Abstracts”, American Society of Agronomy, 1976, p 126. (15) Wahl, A. C., Bonner, N. A,, “Radioactivity Applied to Chemistry”, Wiley, New York, 1951, p 142. (16) Cunningham, E., Proc. R. Soc. London, Ser. A, 83,357 (1910). (17) Kononova, M. M., “Soil Organic Matter”, Pergamon Press, Headinton Hill, Oxford, 1966, p 103. (18) Myer, G. L., in “Transuranium Nuclides in the Environment”, Publication SM-199/105, International Atomic Energy Agency, Vienna, 1976, p 231. (19) Means, J. L., Crear, D. A,, Science, 200,1477 (1978).
Received for review October 17, 1977. Accepted January 11, 1979. This research was supported b y Department of Energy Grant No. EY-76-S-06-2221,Task 12, Modification 2.
Control of Fouling Organisms in Estuarine Cooling Water Systems by Chlorine and Bromine Chloride Dennis T. Burton” and Stuart L. Margrey Academy of Natural Sciences of Philadelphia, Benedict Estuarine Research Laboratory, Benedict, Md. 20612
The relative antifouling effectiveness of chlorine and bromine chloride under intermittent and continuous modes of application in low velocity flow areas was evaluated at an estuarine power plant located on the Chesapeake Bay. No significant difference in the control of fouling organisms was found on the average between similar concentrations of chlorine and bromine chloride. Significant differences in fouling were found between intermittent and continuous halogenation on both clean and prefouled surfaces. Continuous halogenation was more effective over an entire fouling season in controlling primary, secondary, and adventitious fouling communities than intermittent halogenation for periods up to 2 h per day. Continuous halogenation a t 0.3 mg/L total residual halogen was more effective than 0.1 mg/L total residual halogen during late spring and summer; no difference was found between the two concentrations during the early fall.
Several alternatives to chlorination for control of biological fouling in once-through cooling systems have recently been proposed (1, 2 ) t o satisfy compliance standards set forth in EPA’s Effluent Guidelines and Standards for Steam Electric Generating Source Categories ( 3 ) .Wackenhuth and Levine ( 4 ) and Bongers et al. ( 5 )demonstrated that bromine chloride could be used as an alternative to chlorine in controlling condenser biofouling a t two separate plants. The present study was initiated to evaluate the antifouling effectiveness of chlorine and bromine chloride in low velocity flow areas where estuarine waters are used for cooling purposes. The objectives of the study were: (a) t o test various intermittent and continuous chlorination and bromochlorination schemes for antifouling control in once-through cooling systems; (b) to determine optimal schemes for control of different fouling communities which occur over a fouling season; and (c) to test the 2-h discharge limitations of EPA’s Effluent Guidelines and Standards for Steam Electric Power Gener684
Environmental Science & Technology
ating Source Categories ( 3 ) .A preliminary report on a portion of the study has been given by Burton (6). Materials and Methods
Test Conditions. Three 20-day antifouling studies were conducted during the 1976 fouling season a t Baltimore Gas and Electric Company’s Calvert Cliffs Nuclear Power Plant (Calvert Cliffs) located on Chesapeake Bay. Calvert County, Md. The studies were conducted in late spring, summer, and early fall to determine optimal schemes for control of different fouling communities which occur over the fouling season. Chlorine and bromine chloride were tested simultaneously under the following conditions: (a) 0.2 mg/L total residual halogen (TRH) applied for 15 min every 3 h (average concentration and application duration allowed by EPA); (b) 0.5 mg/L T R H applied for 15 min every 3 h (maximum concentration and application duration allowed by EPA); (c) 0.1 mg/L T R H applied continuously; and (d) 0.3 mg/L applied continuously. The antifouling effectiveness of each halogenation scheme was determined by comparing weight change on fouling substrates (115 cm2; 18 in.2) exposed for identical time periods. Total dry weight, organic weight, and inorganic weight were used as measurements of fouling. The experimental apparatus used during the study has been described in detail by Margrey e t al. ( 7 ) . Briefly, the fouling substrates exposed to each test condition were housed in separate troughs which were supplied continuously with 19 (1k0.5) L/min ( 5 gal/min) of unfiltered Bay water a t a flow velocity of approximately 0.3 m/s (1 fth) past the fouling substrates. Stock solutions of halogenated water (approximately 40 mg/L TRH) were continuously made from filtered Bay water in stock head boxes, metered from the boxes a t specific volumes for each experimental concentration and mixed with Bay water before entering each experiniental trough. Timer activated solenoids were used to control the stock solutions entering the fouling troughs receiving intermittent doses.
0013-936X~79/0913-0684$01.00/0@ 1979 American Chemical Society
