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| U N I V E R S E M A G A Z I N E - F A L L 1 9 9 7 | THE SMELL OF FRESHLY PLOWED SOIL: A Natural History of the Underground DUST Soil ecology doesn’t mean much if you can’t keep the soil down on the farm. Every year, about five tons per acre of Washington State soil take to the air. Anyone who’s spent any time in eastern Washington is familiar with the blustery, hazy, gritty days of late summer and early autumn, when many people with respiratory problems simply leave the area. Those who can’t, stay indoors or wear masks, and the rest of us wipe our eyes and hawk the rich loess out of our nasal passages. This might all have remained just a fact of life in semi-arid farm country, had it not been for the passage of the Clean Air Act of 1990. Among other things, the act called for the monitoring and control of particulate matter smaller than 10 microns. (A micron is one-millionth of a meter, and 10 microns is about one-seventh the thickness of a human hair.) Ten microns is small enough for a dust particle to remain suspended in air, to blow freely in the wind—and to dodge the body’s barricades and enter the lungs along with the air upon which it was suspended. As part of a project called the Columbia Plateau PM10 Project (or CP3), more than 40 soil scientists, engineers, economists, microbiologists, archaeologists, agronomists, and others at WSU and across the state have been searching for ways to alleviate the problem. They are gathering massive amounts of information on the nature of the windblown dust problem and how to curb it. Spokane and the Tri-Cities are two of several urban areas throughout the West that must clean up their air, somehow, to comply with clean air standards. Sitting in the bowl that it does, Spokane has an air quality problem to start with, what with inversion-trapped automobile and industry emissions, wood smoke in the winter, and grass field smoke in early autumn. Add a few tons of “fugitive dust,” and the city, says agricultural engineer Keith Saxton, who coordinates the CP3, becomes “tremendously interesting to study.” According to a recent report issued by the Natural Resources Defense Council, it also makes Spokane’s the eighth dirtiest air in the country, following Phoenix’s and that of six California cities. Because no one had any idea what to do about it, the EPA allowed Spokane to focus in its original air quality implementation plan on everything but agricultural dust while the CP3 gathered information. Spokane must present its compliance plan to the EPA by the end of 1997, and Saxton and the 20-odd project scientists are currently writing up summary results of the project. As scientists, he says, most of the investigators would like “more data and about 10 more years,” but the EPA and USDA have deadlines. Assessing the results of the extraordinarily interdisciplinary study, Saxton says, “We’re much smarter now than four years ago.” But in terms of standards and policy, the group has in a sense found itself “shooting at a moving target.” The EPA recently included in the clean air standards a provision for natural events, including large forest fires, volcanoes, and “significant events” of windblown dust. “What is a significant event?” asks Saxton. “If windstorms occur on a frequency probability basis, what level of control do you want to talk about?” The definition of “significant” essentially establishes a yet-to-be-determined cut-off point for whether an area needs to do anything about its windblown dust or not. Establishing that cut-off point is tricky, to say the least. Also, the EPA is considering moving the size standard of particulate matter from 10 microns to 2.5 microns. When the agency suggested this, it implied at first that agricultural issues would just go away, because agriculture doesn’t produce dust particles as small as 2.5 microns. “Well, that’s probably true east of the Mississippi River,” says Saxton. But Western dust, like everything else out here, is different. Atmospheric scientist Candis Claiborn is looking at dust from two perspectives. She and Brian Lamb, both of civil engineering, and others are developing computer models that track wind patterns and the effect of different wind conditions on windblown dust. She is also working with Jane Koenig of the University of Washington Medical School on an epidemiological study of the health effects of particulate matter. Koenig collects Spokane hospitalization records, outpatient records, and emergency room data, correlating them with amounts of particulate matter in the air. Claiborn is monitoring air quality in Spokane with the Spokane County Air Pollution Control Authority. The current standard of 10 microns and smaller, says Claiborn, is intended to rule out particles that would not make it into the respiratory tract. But if you graph the size distribution of particles in the air, she says, you find two peaks. The larger size particles, like windblown dust, tend to occur naturally. The smaller tend to be manmade, such as combustion products and aerosols. The chemical contents of the two factions are quite different. What makes appropriate regulation of air quality so complex is that the cutoff between the two is not clean. There’s a dip in the graph between the two, somewhere between 1 and 3 microns. The 2.5 -micron figure has traditionally been used to represent the far right hand side. But unfortunately, Western dust is a little different. “There’s also some naturally occurring material around 2.5 microns and a little smaller,” says Claiborn. In Spokane, at certain times of the year, a significant amount of the PM 2.5 is naturally derived. Why our dust is different has to do with where it came from. Most of the Palouse soils and many of the soils over the entire Columbia Plateau are formed from loess, windblown silt, and fine sand. Loess has been blowing onto, and around, the area for as long as 1 to 2 million years. It has always been a dusty place, says soil scientist Alan Busacca. Because of the area’s aridity, the soils are low in organic content. Organic matter, or humus, is important in aggregating the soil, or holding it together. The soil also has considerable volcanic ash, though not as much as is popularly believed. And it is not altogether clear what makes some soils susceptible to blowing and others not. “There’s a popular mythology that soils of the Columbia Plateau are volcanic,” says Busacca, puzzling over a general refusal by reporters and Congressional staffers to get their facts straight. During the recent uproar over low Conservation Reserve Program approval rates in Washington State, it was argued repeatedly that Washington farmers were mistreated, because their soils are highly erodible —which is because the soil is volcanic. The first two points might be true. The last is not, says Busacca. Of the mineral material that makes up the Columbia Plateau’s soils, about 10 percent is volcanic glass. Curiously, he has found that some of the most highly erodible soils, such as that around the Horse Heaven Hills, contain almost no volcanic glass. On the other hand, some of the least erodible soils, in Whitman and Spokane counties, have the highest glass content. Because of the origin of the soils, Busacca was interested also in establishing some historical context. If the soil is particularly dusty, then hasn’t it always been dusty? And if it has been, is there anything we can do about it? To work this out, he chose a lake in central Washington that has no inlet or outlet. In other words, virtually all of the sediment that settles into Fourth of July Lake falls out of the air above it. Busacca and archeologist Peter Mehringer took core samples of sediment from the lake bottom and are analyzing the record the sediments reveal. Their core goes back about six or seven thousand years, though their main interest was the last few hundred. Busacca concentrated on the mineral content of the sediment. Mehringer, whose specialty is the analysis of fossil pollen, focused on the pollen contained in the core. Identifying the pollen gives them a picture not only of the local ecology, but also a record of the area’s agricultural history. Identifying pollen extracted from lake silt admittedly is a bit tricky. For example, says Busacca, you can’t tell the difference between Idaho fescue and wheatgrass, two native prairie grasses. But you can tell the difference between native grasses and wheat. The pollen grains of domesticated grasses are huge, much larger than wild grasses. The first wheat pollen shows up at about 65-70 centimeters into the core. Pollen from the introduced Russian thistle, or tumbleweed, occurs at about the same place. Busacca also measured the PM10 content of the sediment. From the deepest measured so far, about 120 centimeters up to 80 centimeters, the mean diameter of the dust falling into the lake measures about 60 microns up to very fine sand size. From about 80 centimeters to the top of the core, the mean diameter suddenly changes to around 30 microns. So the size of the average dust particle falling into the lake changed dramatically from the time before the 80-centimeter depth. Before 80 centimeters, there is notable natural variation in particle size. But from 80 centimeters to the surface, the size is relatively consistent. Initially, Busacca was puzzled, because the first appearance of foreign pollen comes in a little higher, about 65 centimeters. But Mehringer told him it was “exactly what I’d expect.” When there’s a disturbance in the plant community, there’s a lag before the impact shows up in pollen. In other words, when the native prairie of the Columbia Plateau was disturbed as grazing, and then farming, began, it took a while for the wheat pollen to show up in the sediment record. Wheat pollen is heavy, for one thing, and does not travel far. Mehringer thinks that the real start of farming and grazing disturbance on the Columbia Plateau occurs at about 85 centimeters. Busacca explains further that natural dust contains larger particles eroded from sandy soils that tend to blow out and have little vegetative cover. Sand dunes, for example. What’s indicated in the sediment record is that once the natural vegetation had been stripped away, suddenly the finer textured soils could blow away as well. From about 120 centimeters to 70-80 centimeters, they measured about 0.05 gram of dust per cubic centimeter of core sample. That translates to a dustfall of about 0.05 gram per square centimeter of land area every three years. That is fairly consistent over the years, says Busacca, with some natural variation in the number of dust storms. But from 70-75 centimeters on up to where the ash from the 1980 eruption of Mount St. Helens starts to obscure the record, they found a steady increase in the amount of sediment falling in the lake. The amount goes from about 0.05 gram per square centimeter before farming and rises steadily to 0.15 gram. “So there’s maybe a doubling ... of the dust falling out of the atmosphere from agriculture,” says Busacca. There is also an important aspect that is easily overlooked, he says—there were significant dust storms and a dusty atmosphere before farming and grazing began. At about 40-45 centimeters, the core shows the largest amount of dust—probably indicating the Dust Bowl years. Since virtually nothing was known about the relationship between PM10 and windblown agricul- tural dust before the project started, much of the CP3 project’s effort is simply understanding where it comes from, how it is generated, and how to predict the severity and frequency of “wind events.” This is accomplished in a variety of ways. Saxton, for example, packs a portable wind tunnel to different sites throughout the area and looks at how various soil types under various management practices respond to various wind conditions. Ann Kennedy has developed a means of “fingerprinting” soils by analyzing the fatty acids of microorganisms that claim those particular soils as home. Consulting a library of soil fingerprints she and Busacca compiled, she is able to look at PM10 samples gathered in Spokane and determine where the soil came from. With the results of this and much other work, says Saxton, researchers can figure out where the worst of the windblown dust is coming from. The most susceptible region appears to be the Pasco-Ritzville dryland region. Along with developing an understanding of the problem, others, under the coordination of soil scientist Robert Papendick, are approaching it more directly by developing or recommending management practices to control it. Such practices might include annual cropping, cover crops, alternative tillage practices, and leaving more crop residue on top of the soil. According to Saxton, farmers have been quite willing to participate. They are keenly aware of the various problems resulting as the borders between rural and urban become less distinct, and they understand the problem of windblown dust as one of natural resource protection. Not only does blowing dust damage crops and clog their sinuses the same as city dwellers, it also strips their fields of top soil. | U N I V E R S E M A G A Z I N E H O M E | |
