CORRESPONDENCE
SIR: Your November ’74 Current Research section had a paper which showed that mussels were killed and oysters harmed by “diluted” outboard motor effluents. A quick calculation of this “low level” water pollution in relation to the number of boats needed to produce it shows: (a) the combustion products of 5 liters of fuel wound up in 2500 liters of seawater; this killed mussels; ( b ) it took 100 min for an 18-hp outboard to consume the 5 liters of fuel (these two steps are data from the paper); and (c) if all this happened on a water body 3 meters deep, it would require a straight 24-hr run by a group of boats each occupying a space 3 by 4 meters on the water surface. This is the supreme nightmare of all boaters-wall t o wall boats! Too bad the authors didn’t consider the effects on the boaters-fumes, noise, collisions, etc. I’d rather be an oyster. 2525 Blackwood Road Wilmington, Del. 19810
Donald W. Smith
Reply
SIR: Donald W. Smith very ably emphasizes the problem of relating preliminary data obtained in the laboratory to the real world. We did not state in the paper that wallto-wall outboard motors are a realistic probability-although there are sports fishermen who will argue the point, especially on opening day of the fishing season. We believe, however, that our initial statement, “Outboard motor effluent may be a significant source of petroleum pollution in the aquatic environment,” represents a realistic assessment of the danger to some productive estuarine environments. In point of fact, the 50-ppb n-hydrocarbon content of the outboard motor effluent used in our study seems reasonable a s an upper limit for a bioassay when compared with a value of 7 ppb of n-hydrocarbons we found in water near a boat-launching ramp in Puget Sound. Shellfish do not grow to maturity in the relatively unpolluted high seas as do tuna and salmon. Oysters and mussels are, like most aquacultured organisms, convenience crops grown close to home and close to active petroleum-powered boat traffic. Oysters and mussels are filter-feeders, concentrating particulate matter by pumping many liters of seawater daily and exposing themselves to food and to whatever else the water may hold. When petroleum leakage from boats occurs in an area of limited flushing, especially in shallow estuaries where oysters are grown, the potential for shellfish pollution becomes real as petroleum toxicants accumulate. Set in the context of sediment-incorporated petroleum components continuously replenished and released through time, the additional effluent from outboard motors could indeed represent an underestimated source of pollution to living marine resources. The data from our laboratory studies indicated that small amounts of petroleum from outboard motor wastes may adversely affect shellfish. We hope that someone will now have the opportunity to evaluate water quality, a t the ppb level, of productive marine environments shared above by man and below by valuable marine organisms sensitive to toxic components of petroleum. For John S. Finley, Northwest Fisheries Center, and Gary G. Gibson, Oregon Fish Commission, Robert C. Clark, Jr. Research Oceanographer Northwest Fisheries Center Seattle, Wash. 981 12
SIR: In July ’74 issue of Environmental Science and Technology, Sutton and Calder ( I ) state that “there is no mention in the literature of solubilities of the higher normal paraffins in high ionic strength media such as seawater.” Thus they studied the dissolution of several highermolecular-weight n-paraffins (n-Cl2 to n-Cz6) in distilled water and in one seawater sample. We wish to inform the readers of your journal that we have previously presented an in-depth study of hydrocarbon “solubility” in saline solutions and in seawater ( 2 ) using n-paraffin (n-& and n-Czo), aromatic, and isoprenoid hydrocarbons. We intentionally put the word “solubility” in quotes as our study indicated that the quantities of hydrocarbon passing a filter are very much dependent on many experimental factors. As we had cautioned readers of our work against interpreting values that we obtained as “absolute hydrocarbon solubilities,” we wish to stress this again to readers of Sutton and Calder ( 1 ) . These authors employed an identical criterion for “solubility” as we did; that material which passes a filter. However, they have not considered and therefore failed to indicate that the shaking and standing times of the hydrocarbon/water mixture, the type of filter used and its pore size, the amount of hydrocarbon added to a given water volume, all will affect the n-paraffin “solubility” values obtained (2-4). Sutton and Calder ( I ) also do not consider the important role that the dissolved organic matter in seawater plays in making n-paraffin hydrocarbons more “soluble” ( 2 ) . In considering the “solubility” behavior of hydrocarbons through an estuary and into the open ocean, we have found in studies on high ionic strength media that not only can the dissolved organic matter play an important role (especially in near shore waters), but that the combined effect of this material and the changing salinity in the media gives a nonlinear “solubility” trend ( 2 ) . We also found that n-paraffin “solubility” in seawater is dependent on the p H of the media (2). Therefore, it appears that Sutton and Calder ( I ) have presented an incomplete and hence misleading picture of the solubility behavior of hydrocarbons in estuarine waters between 0 and 35% salinity. Literature Cited (1) Sutton, C., Calder, J. A , , Enciron. Ski. Tech., 8,65447 (1974). (2) Boehm, P. D., Quinn, J . G., Geochim. Cosmochim. Acta, 37, 2459-77 (1973). (3) Peake, E., Hodgson, G. W., J . A m . Oil Chem. Soc., 43, 215-22 (1966). (4) Peake, E., Hodgson, G. W., ibid.,44, 696-702 (1967). Graduate School of Oceanography University of Rhode Island Kingston, R. I . 02881
Paul D. Boehm James G. Quinn”
Reply SIR: We would like to reply to the comments of Boehm and Quinn regarding our article in a recent issue of Enuironmental Science and Technology(1). We feel that many of these comments are either incorrect or irrelevant for reasons given below. I t was not our intention to present a “complete” picture of the behavior of hydrocarbons in water, but rather to determine, with as much rigor as the techniques available to us would permit, the true solubility of the hydrocarbons tested, to determine their empirical salting parameter, and to compare this parameter to one calculated from a rather simple theory. We did not attempt to mimic the real environment, but rather attempted to provide hard data which could serve as a partial basis for understanding it. We feel the critical reader will find adequate Volume 9, Number 4,April 1975 365
mention of the limitations of our experimental data with regard to real world situations and therefore should not be “mislead.” In their letter, Boehm and Quinn state that we failed to consider the effects of shaking, standing, and filtration methods on observed “solubility.” This is erroneous. In fact, we considered them very carefully and selected techniques which would give the best possible solubility data. Boehm and Quinn ( 2 ) equilibrated their solutions by shaking for 30 min on a metabolic shaker a t a rate of 120 strokes/min with an amplitude of 5 cm per stroke. They then allowed the solutions to stand for 30 min before filtering through a Whatman GF/C glass fiber filter. By agitating their solutions so violently, they imparted enough energy into the system to disperse the hydrocarbon solutions into very small droplets. In 30 min, probably only the larger droplets coagulate and rise to the surface, while a significant portion of the solute remains dispersed in droplets. The fraction of the total dispersed colloids which ultimately rise to the surface depends not only upon the buoyancy and size of the droplets, but upon the collision kinetics between droplets as well. Smaller droplets rise more slowly; however, collision and coagulation will result in larger droplets which rise faster. The collision rates between droplets depends upon the total number of droplets in each size range. This, in turn, depends upon the initial amount of hydrocarbon solute present as well as the energy imparted to the system (agitation rate and duration) ( 3 ) . Whether the droplets coagulate or not depends upon the height of the energy barrier arising from long-range electrical interactions between the droplets. The height of the energy barrier is dependent not only upon the nature of the substances composing the droplets but upon parameters affecting the nature of the solution such as ionic strength, pH, and dissolved organic compounds. The hydrocarbons appearing in their filtrate represent the sum of those truly dissolved, those dispersed as colloids small enough to pass through the filter, and those existing as larger colloids which may have been ruptured and pulled through the filter by the relatively harsh technique employed (200 mm Hg suction through filters of uncontrolled pore size). In summary, we feel that the paper of Boehm and Quinn ( 2 ) is kinetic in nature, and that their results reflect the effect of numerous parameters upon the rate of formation and the rate of disappearance of hydrocarbon droplets. In contrast to the experimental procedures of Boehm and Quinn, we equilibrated hydrocarbon solutions for 12 hr by gentle shaking. After shaking they were allowed to sit undisturbed for 24 hr. Thus we hoped not only to avoid the dispersal of the hydrocarbons as very fine drops, but also to allow sufficient time for any droplets formed to coagulate and rise to the surface. In addition, preliminary experiments showed that equilibration times in excess of 12 hr produced no further increase in concentration of n-paraffins in water which was passed through a 0.45 Fm filter. Thus we believe that our solubility values represent hydrocarbons in solution rather than submicron-sized droplets. Filtration was accomplished with very gentle suction (approaching gravity filtration) through 0.45 Frn Millipore filters. When greater suction was used, higher apparent solubilities were observed, presumabily through the rupture of droplets trapped on the filter. Both Boehm and Quinn ( 2 ) and Sutton and Calder ( I ) deal with the general subject of the behavior of hydrocarbons in aqueous solution. Each paper deals with a different aspect of the problem, with different methodologies. Neither paper answers all the questions on the subject, nor were they intended to. 366
Environmental Science & Technology
Literature Cited (1) Sutton, C., Calder, J. A., Enuiron. Sci. Tech., 8,654-9 (1974). J. G., Geochim. Cosmochim. Acta, 37, 2459-77 (1973). (3) Stokes, V. K., Harvey, A. C., 1973 Conf. on Prevention and Control of Oil Spills, Am. Petrol. Inst., 457-65 (1973).
(2) Boehm, P. D., Quinn,
Cities Service Oil Co.
Chris Sutton
P. 0. Box 50408 Tulsa, Okla. 741 50
Department of Oceanography Florida State University Tallahassee. Fla. 32306
John A. Calder*
Sir: In the paper by D. L. Johnson and R. S. Braman [ES&T, 8, 1003 (1974)], it is interesting to note that the mean of the total mercury concentrations recorded in air in the Tampa area (Table I) was about 19 ng/m3, whether determined for all stations or for rural and suburban stations alone, which is very close to the 20 ng/m3 worldwide average air mercury level estimated by Williston in 1968 (J. Geophys. Res., Vol 73, p 7051). My own calculations indicate that contributions of mercury from burning of fossil fuels to regional airborne mercury concentrations in the Carolinas are small (0.1 to 0.5 ng/m3), assuming that uniform atmospheric distribution occurs and that the average residence time of air masses over the Southeast is on the order of 3 days (an admitted oversimplification), The observations on mercury mobilization in soil are suggestive of needed research on ecosystem dynamics of mercury compounds. My research has indicated that native mammals in the Southeast exhibit mercury levels that are related to soil types in different physiographic regions ( R o c . , Southeastern Assoc. Game and Fish Commissioners, Vol. 28, in press). Hopefully, suitable techniques are now available for investigation of the cycling of mercury in natural systems. P. M. Curnbie D u k e Power Co. Charlotte, N.C. 28201 Reply
SIR: We agree with P. M. Cumbie that much more needs to be done on the ecosystem dynamics of mercury. Our techniques should be useful to that end. Statements about mercury concentrations need comment. We can find no estimate for a worldwide average mercury concentration in the referenced article by Williston (J. Geophys. Res., 73, 7051). Our data are in agreement with Williston in that a considerable variability in mercury content in air is observed, especially in the presence of ,‘ources of mercury. We found background levels of mercury were generally in the 3-6-ng/m3 range, approximately the same as reported by Williston. Open ocean concentrations, taken aboard ship and likely influenced by aerosolization of seawater, were an average of 6.5 ng/m3 (16 samples) for total mercury; of this, 1 ng/m3 (3 samples) was elemental mercury and the balance mercury (II) chloride type compounds. Our estimate of the effect of two coal-fired power plants on mercury in the Tampa Bay area is that they provide approximately 1 ng/m3 of the total mercury (15-33% of the 3-6 ng/m3 lower ambient levels). Consequently, coal-fired power plants may constitute a higher percentage source of general mercury-in-air than estimated by Cumbie. Robert S. Braman* University of South Florida David L. Johnson Tampa, Fla. 33620
