Fate and transport of the Exxon Valdez oil spill Part 4 of a$ve-part series
I
.
.
.
Jerry A. Galt William J. Lehr Debra L. Payton iVational Oceanic and Atmospheric Administration Seartle, WA 98115 In the early hours of March 24, 1989, the tanker Exxon Valdez ran aground on Bligh Reef in Prince William Sound, Alaska (Figure 1). Eight of the ship's 11 cargo tanks and three of its seven ballast tanks were perforated, releasing approximately a quarter million barrels of North Slope crude oil into the surrounding waters. Thus began the largest and most expensive spill in U S . waters. 202 Environ. Sci. Technol., Voi. 25. No. 2, 1991
Prudhoe Bay crude oil is relatively heavy, with an API (American Petroleum Institute) density of 27 and an intermediate sulfur-content range (0.82%). It has a pour point below 0 "C and a low total aqueous solubility ( 2 M mg/L), as do most crude oils (I). Studies of Prudhoe Bay oil spilled in open water show even lower total dissolved hydrocarbons, ranging from 1 to 3 p a (2). Measurements of its mass fraction of aromatics, generally considered to be the most toxic hydrocarbons in oil (3),show an aromatic fraction of less than 20% (4). ?he asphaltene and wax content, important in water-in-oil emulsion formation, are ahout 2% (4) and 4% (S), respectively. Experiments with Prudhoe Bay crude oil suggest that it tends to
.
form a fairly stable emulsion (6). Because crude oil is a mixture of thousands of different hydrocarbons, its properties change as it weathers. The volatile components evaporate quickly. Some of the medium-sized polycyclic aromatic hydrocarbons are slightly soluble, and some of the pmducts of solar and microbial degradation are highly soluble. Figure 2 shows the physical processes at work in the early weathering of a typical spill. The summary provides an estimate of the fate of the spilled oil from the Enon Valdez. The environment Prince William Sound is a complex fjord-type estuarine system. Tide heights range from 3 to 4 m. The major net CUI-
This article not subjea lo U.S. copyright. Published 1991 American Chemical Society
\ '
Barren Islands
Afognak Island
Environ. Sci. Technol.. Vol. 25, No. 2, 1991 203
-
1
I '
II
. rent is a counterclockwise flow that enters the Sound through Hinchinhrook Entrance and exits through Montague Strait. This current is concentrated along the eastem shore of Knight Island with most of the flow going between Knight and Seal islands. Based on observations of the oil movement, the maximum speed along the axis of this current has a net displacement of between 15 and 25 km a day. A rough computer simulation of this current is shown in Figure 3. Such a circulation pattern could be caused by several factors. The most likely explanation is that it is the inertial tendency for water flowing inward through Hinchinbrook Entrance on a flood tide to continue in a northerly direction along the eastern edge of the Sound. The ebb tide would have no specific direction but would tend to drain uniformly. The net effect is a cyclonic circulation pattern (7). Outside of Prince William Sound, the major current systems that affect the flow over the Alaskan Continental Shelf can be thought of as comprising two components. The first is caused by the large-scale Gulf of Alaska Gyre, which leads to a westerly flow over the shelf. This current is generally much less than 0.5 mls, but reaches a maximum near the shelf break and is typically about 0.5 mls in that region. The second component is caused by a relatively strong near-shore current, the Alaska Coastal Current (8).This current is caused by a pressure gradient set up by fresh water runoff from the coast and is typically 10-25 km wide between Montague Island and the western end of the Kenai Peninsula. This current varies in speed, depending on the amount of fresh water 204
Environ. SCL Technol., VoI. 25,No. 2,1991
that enters the system; speeds of up to 1.7 mls have been observed (9). During the first week of April 1989, the Alaska Coastal Current was considerably slower than its maximum; typical speeds were 0.2 mls. Even at this reduced level, the Alaska Coastal Current was the dominant process transporting oil from Prince William Sound. As the Alaska Coastal Current moves beyond the Barren Islands, it is deflected north around the end of the Kenai Peninsula where it then flows west, turns south of Cape Douglas, and enters Shelikof Strait (Figure 4). A small portion of the current closest to the Kenai Peninsula shoreline actually moves north along the coast and enters lower Cook Inlet. The regional wind pattern is affected by a semipermanent low-pressure system over the Aleutian Island chain. The circulation around this low-pressure area provides Prince William Sound with a predominantly easterly wind, except in the summer when the winds are mostly from the west. Topography controls local wind patterns, particularly drainage winds out of the major fjord arms, and it was found necessary during the spill to model these local patterns in several parts of the Sound. For example, Figure 5 shows a wind pattern used for the area at the entrance to Prince William Sound. Storms are not uncommon and visibility in the area is often restricted by fog. The steep shorelines along much of the coasts of Smith, Eleanor, Ingot, and Naked islands cause reflected or standing wave patterns. These wave patterns have a convergence node just offshore that can trap oil close to shore, without letting the oil actually beach. Then, given a sudden change in the wind (and
wave patterns), the oil will float away from shore and appear as a secondary source or new patch of oil. Fresh-water runoff from major glaciers and streams leads to another process that may affect the transport of surf a c e pollutants. In this process, relatively fresh water spreads as a lens, pushing out from boundary fjords. As the lens spreads, it forms aconvergence line along its leading edge that tends to trap flotsam (or oil) and inhibit its movement into the fjord or near-shore region. This process was particularly important in the early pat? of the spill, both inside Prince William Sound and along the Kenai Peninsula.
The data It is pretty clear that no oil spill in recent U.S. history has been studied as much as the Exxon Valder spill. Moreover, these investigations and studies will certainlv continue. There is no doubt that response personnel and environmental scientists will be able to broaden their knowledge base about oil spills from these efforts. It is also true that no oil spill in recent history captured the attention of the press and public as much as that of the E u o n Vddez. As a result, hundreds of reporters looking for stories and many naive observers were seeing a major oil spill for the first time. Unfortunately, during the height of a spill response there is little quality control of information. Thus, many misconceptions were passed on, which left millions of readers and viewers overwhelmed with information that tended to be more sensational than true. This happened in all areas of spill response, but the greatest problem for trajectory analysis and for understanding the movement and spreading of the oil was false positive sightings. Reports of floating oil came in daily from dozens of sources. Hundreds of overflight maps were prepared. During the course of the spill, ice, internal waves, kelp beds, natural organics coming from kelp beds, plankton blooms, cloud shadows, and guano washing off rocks were all reported at one time or another as oil. These, of course, were in addition to the hundreds of true reports of oil, of which there was a good deal. Newspaper, TV, and news magazine accounts typically treated all reports as factual; the most common representation of the spill was a black blob extending from Prince William Sound to somewhere in the Aleutians. It is easy to see why the several hundred million people who were interested in the spill and had no other sources of information thought that the spill looked like a six-hundredmile-long parking lot.
Faced with this kind of confusion, it was admittedly difficult to get an accurate picture of where the oil spill moved and what it was like. There were several techniques, however, that helped. The first of these was to concentrate on trained observers (as time went on there were a lot more of these) and on technical data from remote sensing equipment or drifting buoys. The second technique was to use computers and trajectory analysis routines that accounted for oil movement caused by winds and currents. During a spill, such models are used for forecasts, but after the fact they become very useful in analyzing what happened. Each day, or several times each day, the model was checked against observations. If sighted oil patches “swam” upstream (or moved against strong winds), then they were treated as false positives. If, on the other hand, the leading edge of the slick or individual patches of oil were seen to outrun or lag behind the computer projections, then the hydrodynamic current estimates were suspect and the model was adjusted accordingly and rerun. Using these methods, the history and coverage of the spill can be reconstructed in a relatively reliable way.
The spill From an observational point of view, the spill can actually be looked upon as two separate spills: one inside Prince William Sound caused by a rapid release of oil from the tanker, and a second in the Gulf of Alaska caused by a slow leak of oil out of Montague Strait. Inside Prince William Sound. When the Exwon Vddez ran aground, the release of the oil was almost instantaneous. There was little wind or wave activity to affect the movement of the spill or cause any significant amount of water-in-oil emulsion formation. The major physical processes occurring, therefore, were spreading of the slick by gravity and surface tension, and evaporation of the lighter weight hydrocarbns by surface transfer processes. We and others (Hanna, S. R.; Drivas, P., personal communication) are studying the distribution of the pollutant cloud above the oil in the early stage of the spill. As is typical for crude oil spills, the spreading was not uniform. Rather, there were patches of thicker oil surrounded by a larger area of sheen. The center of the spill slowly drifted to the southwest (Figure 6). These conditions persisted throughout March 24 and March 25. With the cyclonic residual circulation pattern and the drainage winds, it was apparent even at this early stage in the spill that the possibility of oil traveling into the eastern or southeastern segments of Prince William Sound was minimal. Environ. Sci. Technol., VOI. 25,NO. 2, 1991 205
During the third day of the spill, March 26, the region experienced a major windstorm. This had a profound effect on the spilled oil, dramatically changing its appearance, character, and distribution. The dominant wind direction during the storm was from the east to northeast. However, with drainage winds coming out of the northern fjord arms and Port Wells, this translated into a northeast to north-northeast wind over the central area of the spill. As a result, oil moved rapidly between Naked and Smith islands towards Montague Strait. In addition to simply moving the oil, the storm supplied a tremendous amount of mixing energy that affected the spilled oil in three important ways. The first effect was that the more or less contiguous slick was ruptured into bands and streaks and spread over a significantly larger area. Typically under such conditions, slicks will cover large areas, of which 90% or more is on the order of microns thick, leaving most of the oil in relatively small, n m w hands that are associated with vertical movement in the water column (convergence zones). The second effect of the storm was that mixing processes were dramatically increased. Evaporation was enhanced, with an estimated 15-20% of the total 206 Environ. Sci. Technol., VoI. 25, No. 2,1991
oil being lost by the end of the storm. In addition, breaking waves caused by the wind led to the dispersion of oil droplets into the water column. Natural surfactants enhance this process, acting somewhat like a dispersant, so that small droplets appear to be in solution and rapidly mix to very low concentrations. This action significantly weathered the oil. Actual dissolution of the slick would be expected to account for only around 1% of the mass balance (2). The third major effect of the storm was that a significant fraction of the remaining oil formed a water-in-oil emulsion (mousse). The water content of the mousse was tested and found to be about 70%. Therefore, the oil remaining at this point (i.e., what hadn’t evaporated or been lost through weathering processes) was nearly tripled in volume. The mousse also had different physical properties than the original oil, most notably increased viscosity and adhesion capabilities, making it more likely to stick to beach material hut less likely to penetrate the sediment whenever the oil came in contact with land. It also greatly impeded the natural weathering processes. One process that, although often reported, almost certainly did not occur to any significant extent, was the sinking
of the oil to the bottom of the Sound. Crude oils rarely achieve densities that exceed seawater, even when extensively weathered. Sediment particles may attach to the oil in the near-shore region, causing the combined material to sink. However, studies carried out in the Sound soon after the spill indicated that there was little or no sediment-laden water near the oil slick. In the open water, what was reported as sinking oil was probably oil washed over by waves and temporarily invisible. As the storm progressed, the oil slick arched southwest and then southsouthwest, first impacting beaches along the southeast coast of Naked Island and then grounding in large quantities on Smith, Little Smith, and Eleanor islands (Figure 6). By the end of the storm, the oil had weathered, mixed, emulsified, and moved so that a clearly new phase of the spill was at hand. Scattered but heavy concentrations of floating oil were centered in the apex between Naked, Smith, and Eleanor islands. From this junction, channels lead in all directions, but, because of persistent current and wind patterns the oil was expected to move south and/or west. The areas of special interest then became Montague Strait and Knight Island Passage. Oil quickly moved through both of these passages. This major bifurcation continued throughout the spill, giving two branches to the trajectory problem each of which acted differently. As oil entered the northem end of Montague Strait between Smith and Eleanor islands during the fourth and fifth days of the spill (March 27 and 28), the oil was quickly spread out, thereby impacting the coast of Eleanor, Ingot, and Knight islands, with lesser concentrations beaching on Seal Island and Applegate Rock. By the end of March 28, the leading edge of the oil was between Latouche Island and the southern end of Montague Island by March 29, it had moved beyond Montague Strait into the Gulf of Alaska. This relatively quick trip through Montague Strait, with shoreline oiling primarily along the eastern shore of the Knight Island group, is typical of the movement of most of the oil that entered the northern end of Montague Strait. During the first few weeks of the spill, there were several exceptions to this typical flow, two of which were particularly significant. The first exception occurred on March 29, when there were northwest winds in the Smith, Naked, and Knight Island apex region and throughout the northem end of Montague Strait. As a result, some of the oil moving between Smith and Eleanor islands moved away from the strong currents along the western side of Mon-
FIGURE 6
Dlstribution of floating oil, Prince William Sound
tague Strait to the weaker currents by Green Island. This resulted in severe oiling of the northern part of Green Island and was the eastern limit of significant oiling in Montague Strait. The southern end of Knight Island, where Knight Island Passage meets Montague Strait, was the site of another exceptional flow that had a major effect on the movement of the oil. The flow down the western side of Knight Island is predominantly southerly, but it is much weaker than that down the eastern side of Knight Island. In Knight Island Passage, this flow is sufficiently weak that during some phases of the spring neap tidal cycle the flood tide currents are strong enough to reverse the direction of the flow in the southern part of the Passage. This means that as oil drifts south past Point Helen, it enters an area where the current may be flooding west into Knight Island Passage. This deflects the oil in such a way that when the tides ebb, the trajectory leads down the westem side of Latouche Island, threatening Elrington Island, Latouche Passage, Evans Island, and the Sawmill Bay hatchery. This was first noticed on March 30, which was about halfway between neap tides (weakest period) and spring tides (strongest period). Over the next week these tides increased each day
and subsequently sent larger and larger pulses of oil into the passage between Latouche and Evans islands. The oil that moved west between Naked Island and Eleanor Island entered an area that has virtually no steady current patterns, where winds dominate the trajectories. By the end of the first major storm, oil had entered this area in relatively high concentrations. During the next couple of days (Days 4 and 5 of the spill) the oil moved south under the influence of northerly winds, and heavy concentrations went ashore on Eleanor Island, particularly in Northwest Bay. Heavy oil also moved past Ingot Island and onto the northwest parts of Knight Island. Some patches moved west nearly to Port Nellie Juan, and light shoreline impacts were observed between Main Bay and Eshamy Bay. As patches and bands of oil reached the southem part of this area they entered Knight Island Passage and came under the influence of a weak current system that canied them south between 5 and 8 km a day. As this slow drift continued, day-to-day variations in the wind pushed the oil hack and forth, but the dominant direction was such that most shoreline impacts were from northwest Bay to Herring Bay with some oil actually passing through the channels
north and south of Ingot Island and back into the current system of Montague Strait. Over the next two weeks most of the oil that moved west between Naked Island and Eleanor Island followed this general pattern. Eventually some of the oil following this path was seen to pass through Bainbridge Passage and Prince of Wales Passage into Port Bainbridge and then into the Gulf of Alaska. On April I C 1 1, a patch of oil that was relatively large (compared to what was left floating in the area) and that had been held offshore by a standing wave pattern, moved away from Smith Island. Under the influence of a very strong easterly wind, this oil moved between Naked and Eleanor islands to form a large patch between Northwest Bay and Lone Island. Over the next week, this patch moved back and forth. When more easterly winds developed, moderate amounts of the oil washed ashore on Lone Island, southeastern Perry Island, Culross Island, and Applegate Island. Lighter concentrations of oil also moved up Perry Passage and down into Port Nellie Juan.
Gulf of Alaska Floating oil first exited Prince William Sound through Montague Strait about March 30 (Figure 6). After that, Environ. Sci. Technol., VoI. 25,No. 2, 1991 207
oil more or less continuously spilled into the Gulf of Alaska, reaching its maximum sometime within the next week. By the second week of April (a little over two weeks into the spill), between 20% and 25% of the oil had moved into the Gulf of Alaska, primarily through Montague and Latouche straits, with lesser amounts passing through Port Bainbridge. Although the Prince William Sound system continued to feed oil into the Gulf of Alaska, the amount of oil coming from the Sound was greatly reduced. By mid- to late-April these small isolated patches of light to moderate oil were diminished even more. By April 5, the leading edge of the oil was south of the Chiswell Island group, extending a little more than 15 km from just off of the headlands across the width of the coastal current. This movement in the coastal current progressed around 10-13 km a day. Just west of the Chiswell Island group, the bathymetric contours become more complex, with a fairly large bank extending south of the islands. In this area, the coastal current is deflected offshore, and large eddies tend to spin off between the current axis and the shore (typically somewhere south of Granite Cape). Floating oil followed these patterns, with the majority deflected south and away from shore, while a smaller portion tended to get caught up in the eddies west of the Chiswell Islands. By April 9, the oil’s leading edge, which had continued to move southwest, was about 60 km south of Nuka Sound. From here, it could be traced in a more or less continuous series of streaks and patches to Montague Strait, a distance of about 160 km. This was the greatest distance that the slick could be traced as a continuous entity once it left Prince William Sound. Beyond that point, the leading edge feathered out into broken patches of sheen. On windy days, this process was exaggerated and the slick would appear to shrink; on calm days, it would extend, but never beyond this point as a single, connected series of oil patches. Despite the more or less continuous appearance of the oil up to this time, it was noted that over the segment of the Alaska Coastal Current that contained floating oil, the fraction of the surface actually covered was very small (on the order of a few percent). Thus there were many thin lines of floating oil separated by large areas of clear water. Beyond Nuka Sound, the oil was in individual patches or streaks. There was no longer a simple line that could be followed out from Prince William Sound, and reconnaissance became more difficult. Nonetheless, the oil remaining in these scattered patches, al208 Environ. Sci. Technol., Vol. 25,No. 2,1991
though often difficult to see from the air, still represented significant hazards to offshore birds, floating sea otters, and shoreline when it was blown ashore. The resulting hits of oil were patchy and generally widely scattered, with many relatively clear areas in between. Considering the nature of the Alaska Coastal Current and the drainage, or offshore, winds and currents within fjords, it was likely that oil beaching would be concentrated along offshore islands, coastal headlands, and eastward facing spits or promontories. By the same token, relatively few coastal impacts would be expected within fjords. This was generally what occurred-all of the major fjords showed relatively little or no oil moving into them, and offshore islands all were moderately to heavily oiled at some time. By the time oil reached the vicinity of the Chugach Islands, it was in the form of widely scattered patches and lines of sheen. Typically these patches were comprised of “tar balls,” which are small pieces of mousse that vary from less than one inch to several feet in diameter. In some instances, strong surface convergence patterns in the currents (caused by fresh water mixing or wind shear) collected tar balls into streaks where they coalesced and mixed with other debris and flotsam to form a continuous line of mousse. Widely scattered tar balls can also obviously coalesce along a beach and yield a continuous band of mousse in the intertidal zone. By the third week in April, scattered patches of oil were moving between Afognak Island and the western end of the Kenai Peninsula. This path includes the Barren Islands with the Kennedy Entrance to the north and Stevenson Entrance to the south. The vast majority of the oil passing the Barren Islands moved along the Alaska Coastal Current across the mouth of lower Cook Inlet and into Shelikof Strait. Only a small fraction of the oil moved north along the eastern side of lower Cook Inlet. As scattered oil patches moved across lower Cook Inlet and into Shelikof Strait, they were met with relatively large amounts of fresh water flowing out of Cook Inlet. This fresh water causes a strong convergence band that wraps around Cape Douglas and extends down the northern side of Shelikof Strait. This convergence zone coalesced a good deal of scattered oil, thereby appearing to reconstitute the spill. More serious than the visual appearance of a large band of oil was the fact that many birds that sleep on the water during the night were drawn into convergence lines and mixed with the oil. This explains the sudden appearance of large numbers of
oiled birds along the Katmai coast. During the oil movement across lower Cook Inlet and down Shelikof Strait, wind events grounded a number of patches, resulting in widely scattered light-to-moderate coastal impacts. Once again, the heaviest shoreline impacts were seen on beach segments that faced the predominant currents and winds. This led to scattered oil along the Katmai coast and a moderate concentration at Cape Douglas. During April, however, spotty shoreline oiling also took place on Afognak, Raspberry, and Kodiak islands because of some northerly wind events. By the time the remnants of the oil reached the end of Shelikof Strait, they were so widely scattered that isolated tar balls were about all that could be found. Currents generally turn south around the western end of Kodiak Island, and some tar ball spatter was seen on the Trinity Islands and eventually on Chirikof Island. Along the coast of the Alaskan Peninsula a few tar balls were discovered in the Chignik area.
Summary The floating oil distribution can be summarized as follows. Heavy amounts of floating oil were present in southwest Prince William Sound for about two weeks. Some shorelines were heavily oiled during this time. Significantly reduced amounts (by an order of magnitude) were present for another two weeks. After this time, light sheensoften caused by leaching from oiled beaches-were observed, but represented minor amounts of pollutant. In the Gulf of Alaska during the first two weeks of April, scattered patches of heavy oil were present slightly offshore between Montague Island and the Chiswell Island group. Between the Chiswell and the Barren islands the same period saw even more widely scattered patches. Beyond the Barren Islands, only widely scattered patches of mousse were observed around midApril. The exception in the area was the strong convergence zone south of Cape Douglas and in the eastern end of Shelikof Strait where bands of mousse coalesced. Shoreline impacts were widely scattered and generally light. An estimated 35% of the oil evaporated or dispersed into the water column in the Sound, mostly in the first two weeks. An additional 40% affected the shoreline inside Prince William Sound, mostly on Smith, Eleanor, Ingot, and Knight islands, with secondary amounts on Green and Latouche islands. Perhaps 25% of the original spill left the Sound as floating oil. Only about 10% of the oil made it beyond Gore Point, and 2% actually got as far as Shelikof Strait.
References Bobra, M. A. “A Catalogue of Crude Oil and Oil Raduct Ropenies”: Environmem Canoda 1989, Repon EE-I 14. Mackay. D.: McAuliffe. C. D.Oil Chem. r d u r . 1989.5. I. Fingas. M. F.: Duvall. W. S.: Stevenson, G. B. “The Basics of Oil Spilt Cleanup”: Eniironnanr Canada, 1979. Experimental data from the Environmental Emergencies Technology Division. Technical Service Branch. Environmental Protection Service, Environmmr Conada 1984-89. Mackay. D.: Zagorski. W. Studies of Water-In-Oil Emulsions, Environmmr Conoda 1982, Repon EE-34. MacKay. D. et al. “Development and Calibration of an Oil Spill Behavior Model”: United Stater Coast Guard Repon CG-D27-X3. 1982. Muench. R. D. Schmidt. G. M. “Variations in the Hydrographic Structure of Prince William Sound”: Institute of Marine Science. Sea Grant Repon R75-t.
HERE IT IS! THE BRAND NEW EDITION OF THIS “MUST-HAVE” REFERENCE!
l”7C ,711.
Just published! The new edition of CCF, containing the most current information on:
Johnson, W.R.: Royer. T. C.: Luick. J. L. J . Cmphyr. Res. 1988, 93. 12423. Johnson, W. R.: R o y . T. C. Deep SPO Rt.wo,r.h 1986.33, 1127.
2,160 deportments of chemistry, chemicol engineering, biochemistry, and phormoceuticol/medicinol chemistry in the US ond Conada, including: complete mailing addresses and phone numbers separate listings for each deportment department chair and head listings degrees granted b y each department nototion for two- and three-year colleges alphabetical listings by state
All three authors work for the Modeling and Simulation Studies Branch of the Hazardous Materials Response Division of the National Oceanic and Atmospheric Administration, Seattle. WA.
18,357foculfy members in these deportments,
including:
ma’or teaching fields higkest degree earned academic rank
Two alphabetical indexes-one by institution and one by focuhy member name-make look-up quick and easy. Academic departments, libraries and personnel offices, recruiters, students, soles and marketing personnel-you’ll find yourself turning again and ogoin to this directory.
Call toll free (800) 227-5558 and c h a r g e to your c r e d i t Jerry A. Galt leads the Modeling Branch.
His Ph.D. in physical oceanography is from the University of Washingron.He has worked in the field of oil spill research and response for the past I5 years.
card-or mail the coupon below today. I I I I I I I I I I I I I I I I Pleose send -copy(ier) of College Chemistry Forulties, 8th Edition, at $74.95 each (US & Conoda), $89.95 wch (export).
Poyment enclosed (moke checks payable to American Chemical k i e t y ) . Purchase order enclosed. P.O.ll Chorge my MaslerCordfVISA American Express Diners ClubICorte Blanche AICOUII~ If
Expires
Phone Y
Noms of cardholder Signolure Ship books 10:
Name Address City, State, ZIP
William J. Lehr holds a Ph.D. in physics
from Washington State University. He has been actiee in oil spill research since 1979. Debra L. Payton holds B.S. and M.S. degrees from the University of Washingron. She has worked with the modeling group on oil spill research and response since 1978.
ORDERS FROM INDIVIDUALSMUST BE PREPAID. Prepaid ond credit cord orders receive h e porloge and handling. Prices ore quoted in US ddlarr and (IO wbiect lo change without notice. Plwre allow 4 4 weekt lor deliwry. Foreign payment must be mode in US currency by inbrnolional money order, UNESCO coupons, or US bank drak. Order hrough your locol bookreller or direcly hom ACS. To chorge your books by phone, CAU TOU FREE (800)227-5558.h i l his mder lorm with your payment o purcbre order to Am&on C M i l Wkly, DistributionOIPwe apt.209, P.O. Box 57136, Wesl E d Itlltion,
Worhi@on, DC Zoo37
Environ. Sci. Technol.. Vol. 25, No. 2. 1991 209
