Introduction and Related Research

Plutonium occurs throughout the earth’s environmental systems, though usually in quantities so small that they are barely detectable. Because this artificial element is so toxic, it is necessary to identify those few locations where the concentrations are likely to be the highest. Because almost all plutonium released into the environment is ultimately attached to soil and sediment particles, the behavior of constantly changing natural transport systems such as water and sediment flows provide the key to understanding the ultimate geographic disposition of the element. The general purpose of the work discussed in this book is to explain the distribution of plutonium in the Northern Rio Grande system of northern New Mexico and southwestern Colorado by forging a link among the available data and general principles of environmental sciences such as hydrology, geomorphology, and radioecology. Between 1945 and 1952, Los Alamos National Laboratory handled large amounts of plutonium as part of the Manhattan Project (the effort to construct the first atomic weapons) and as part of the weapons programs related to the early years of the cold war. During this time, the laboratory emptied untreated plutonium waste into the alluvium of Los Alamos Canyon. After 1952, the laboratory released relatively small amounts of treated plutonium waste. Although the vertical movement of plutonium through the alluvial materials has been largely limited to the upper 10 m,4 the horizontal movement of the contaminants has had much larger dimensions. The plutonium was adsorbed onto sedimentary particles, and so the fate of those sediments is also the fate of the plutonium. Natural processes of erosion have resulted in substantial movement of contaminated sediments through the canyons. Research during the 1960s and early 1970s showed that since the war years, surface flows within the laboratory’s boundaries had redistributed at least some of plutonium. Laboratory researchers later estimated that fluvial (river-related) processes in Los Alamos Canyon had probably removed significant quantities from the laboratory area by carrying the plutonium into the Rio Grande. They predicted that early in the twenty-first century almost all of the plutonium would have been emptied from Los Alamos Canyon into the Rio Grande.

Sediment and Plutonium Storage Downstream from Cochiti

Downstream from White Rock Canyon and the reaches discussed in Chapter 9, the Rio Grande takes on a different character because of the presence of Cochiti Dam at the lower end of the canyon. From that point downstream, the river’s present appearance and behavior reflect the influence of the dam, which was closed in 1973. Although the channel has become narrower throughout the length of the Rio Grande since the 1930s, this change is most pronounced south of Cochiti Dam. Downstream from the Los Lunas representative reach (which ends near Bernardo), the character of the Rio Grande changes radically. Immediately below Bernardo, the Rio Puerco joins the main river, bringing with it a huge load of sediment. The Rio Grande Valley becomes much wider below Bernardo, and the twentieth-century narrowing of the channel, aided by engineering works, is even more pronounced than in upstream areas, and the vegetation community is dominated by tamarisk. The final three representative reaches discussed in this chapter share the features of great valley width, extensive channel changes, and widespread impacts of engineering works. The Peña Blanca reach, a 5-km channel section, represents conditions common along 40 km of the Northern Rio Grande between Cochiti Pueblo (site of Cochiti Dam) and the confluence with the Jemez River. The river passes Peña Blanca, a settlement based on irrigated agriculture dating from the early nineteenth century. The reach is typical of the conditions in a portion of the river where the flood plain is several times the width of the channel and where the channel has been exceedingly unstable. The reach is also instructive concerning the results of levee construction (in 1953) and dam closure (in 1973). The behavior of the channel in the Pena Blanca reach between the early 1940s and about 1990 has consistently included locational instability and progressive adjustment from a broad-braided configuration to a narrow, straighter alignment. In the 1940s, the channel was wide and unstable, with numerous major and minor threads, but the gradual reduction in water yield and radical reduction in the annual flood peaks resulted in the progressive isolation and closure of secondary channels.

Plutonium and Los Alamos

The plutonium in the Northern Rio Grande is entirely artificial. Small amounts of plutonium may have formed in exceptionally rich uranium deposits in south-central Africa, but for practical purposes, until its manufacture in 1939, the element did not occur in the earth’s environment. Although the detailed story of the origins of plutonium are beyond the scope of this book, a summary of that history does clarify the issues regarding plutonium in the Northern Rio Grande in the late twentieth century. The purposes of this chapter are to review the origins of plutonium and to examine briefly the nature of that element. Modern nuclear physics, which ultimately led to the production of plutonium, began with the publication of the discovery of X-rays by Wilhelm Conrad Röntgen in 1896. His work showed that the physical world was much more complicated than previously thought and that energy could be emitted from substances. In the same year, Henri Becquerel of Paris showed that uranium emitted radiation, and soon thereafter Marie and Pierre Curie coined the term radioactivity to describe the emissions they recorded from two newly discovered elements, radium (named after its radiative properties) and polonium (named after Marie Curie’s home country of Poland). Between 1898 and 1902, Ernest Rutherford of Cambridge University and, later, McGill University explored processes of radioactive decay that generated free electrons (beta radiation) and bursts of energy (gamma radiation) and discovered that some elements changed their basic properties during the emission. Rutherford termed these changes transmutation and laid the philosophical foundations for understanding atomic structure. The transmutation of elements was a significant addition to the rapidly expanding knowledge about the number and types of elements in the natural world. Between 1894 and 1900, William Ramsey enlarged the periodic table with an entire family of inert gases, and by 1903 more than a dozen radioactive elements were known. By 1903, it was obvious that the decay process explained many observed elemental changes: Americans Bertram B. Boltwood and Herbert N. McCoy showed that radium descended from uranium, and Otto Hahn connected several types of thorium.

General Lessons and Conclusions

The initial investigations reported here offer some broad lessons from the analysis of a specific example. Application of the techniques and results in the Los Alamos case to other areas or cases would require modifications, and even the conclusions about plutonium in the Northern Rio Grande are more first approximations than final answers. After reviewing the lessons of the Los Alamos work, this chapter summarizes some of the natural scientific lessons, with particular reference to the changing Northern Rio Grande and some observations about the interactions between natural science for plutonium and the associated public policy and politics surrounding the issue. Because this work is a beginning rather than an ending, this chapter concludes with some speculations on the future of plutonium in the Northern Rio Grande. The lessons from the particular case of Los Alamos and the Rio Grande extend far beyond northern New Mexico. As a “test bed,” the laboratory, its plutonium releases, and the data-rich Rio Grande provide generalizations useful to researchers, monitors, regulators, decision makers, and managers of other systems and locations. This chapter reviews these lessons as two distinct groups: general guidelines and specific sediment-sampling procedures. First are several general guidelines that should direct any effort at assessing the plutonium system of a river affected by industrial-waste disposal, an accidental release during transportation, or distribution from a nuclear detonation involving surface materials or from atmospheric fallout. Second, because of the overriding importance of sediment in the transport and storage of plutonium, several specific procedures should be followed when sampling soils and sediment, to ensure accurate interpretations of the results. A refined sampling and monitoring program for plutonium in sediment should be driven by a philosophy that has the following general principles. 1. Obtain an accurate inventory of sediment-bound plutonium at the source location. If the source of contaminated sediments in a river system is a known mass, such as a well-defined waste-disposal site, a tailings accumulation, or another readily measured mass of material, a reasonable estimate of the total inventory of plutonium in the source is possible.

Annual Plutonium Budget for the Rio Grande

A mean annual plutonium budget for the Northern Rio Grande provides an accounting of the amounts of plutonium moving into and out of various reaches of the river during a typical year. Such a budget is a basis for assessing the rates of plutonium transport and the location of storage along the river. The budget presented in the following pages is for bedload and suspended sediments. It does not include plutonium in water because water-borne plutonium is such a small portion of the total in the system (as discussed in Chapter 7). The budget as calculated here requires data concerning sediment and plutonium concentrations in the sediment. The sediment discharge data that are available from U. S. Geological Survey gaging sites (Chapter 4) define the overall framework for budget construction. A reasonably detailed picture is possible for the river system from the Rio Grande at Embudo and the Rio Chama at Chamita southward to the Rio Grande at San Marcial (for locations, see Figure 3.9) where the river empties into Elephant Butte Reservoir. Data collected by Los Alamos National Laboratory and published in the annual surveillance reports by the laboratory’s Environmental Studies Group and later by the Environmental Surveillance Group provide plutonium concentrations for bedload and suspended sediments. The calculations for each site in this study used mean values of plutonium concentrations from all measurements at or near the site. Table 8.1 reviews the sources of plutonium concentration data for each of the sediment-gaging sites in the regional budget calculations. Unfortunately, the sites for collecting the plutonium data were not always colocated with the gaging sites that produced the sediment discharge data. In addition, most of the plutonium concentration data are for bedload sediments because of the manner in which the workers collected samples. In some cases, the best estimates of plutonium concentrations in suspended load for gaging sites are from concentrations found in sediments of the nearest reservoir downstream because those sediments are likely to have been in suspension before their emplacement on reservoir floors. The assumption that the mean concentration is a useful representative value seems reasonable given that in those reaches with relatively large amounts of data, concentration values do not show temporal or geographic trends.

Reparian Vegetation

The interaction among water, sediment, landforms, and human environmental manipulation on the Northern Rio Grande has produced a distinctive assemblage of plants in the riparian (or near-channel) community. The fluvial landforms and the sediment of which they are composed are often not immediately visible in field investigations because of the dense cover of riparian vegetation. In aerial photography—the primary source of data for historical river-channel change and sedimentation- riparian vegetation is often the only aspect of the near-channel environment that is amenable to interpretation and mapping. Vegetation also provides information about the date of emplacement of the sediments on which it grows, information useful in tracking contaminants introduced into the system during known time periods. Vegetation communities therefore provide useful keys to identifying the distribution of near-channel sediments and the contaminants they contain. This chapter briefly reviews the origin and changes in riparian vegetation in the study area, including its connections with geomorphic systems. Almost all major rivers in the American Southwest have undergone considerable geomorphic and vegetation change since the early nineteenth century when channel margins were the sites of bogs, lakes, abandoned meanders (sloughs), and marshes. Most major rivers had broad, sandy channels with braided configurations and meandering low-flow channels. Even small tributaries had marshy areas created by beavers. The riparian vegetation originally evolved in association with frequent extensive flooding. Removal of the beavers, the development of gullies and arroyos, land-management schemes, changes in climate, and the construction of dams changed the streams into single-thread or compound channels that flooded less often. The Rio Grande’s recent history is typical of the larger region except for the extensive recent engineering works that restrict the active channel and flood plains. There are few detailed descriptions of the channel and riparian vegetation before major human intervention, but generally, most firsthand observers indicate that the Northern Rio Grande was broad and shallow, with meandering subchannels frequently altered by flooding. After channel migration, cottonwood, willow, and cattail colonized the newly exposed alluvial surfaces. Early in the twentieth century, the cottonwood groves near the river rarely developed trees more than about 10 m high before more changes in the channel destroyed them.

Engineering Works

The hydrologic, sedimentologic, and geomorphic processes of the Northern Rio Grande as outlined in the previous chapters do not operate under natural, undisturbed conditions. Numerous engineering structures and activities have modified the processes and forms, and so an explanation of the movement and storage of contaminants in the system requires knowledge of the channelization and dam construction in the region. Channelization works are usually directed toward controlling the horizontal position of the channel, keeping it aligned in an economically advantageous arrangement, and maintaining a clear path for floodwaters to prevent them from spilling over the banks. The imposition of an artificial, stable channel on a naturally unstable system is rarely completely successful, but even with partial success, the newly defined system is a radical departure from the natural one. Floodwaters usually flow through modified channels at higher velocities than they do through natural channels, and so they may transmit more sediment in the channel. Low flows, however, may deposit sediment in the engineered channel, thereby reducing its efficiency and raising its bed. The abandonment of previously active minor channels or braided sections provides new areas of colonization for riparian vegetation, which may enhance sedimentation when flows exceed the capacity of the designed channel. The construction of dams obviously disrupts river processes in the reservoir area but has indirect effects throughout the river system because of newly instituted controls on flood flows, normal low flows, and sediment discharges. The first engineering structures on the Rio Grande probably appeared about A.D. 1200. With the collapse of irrigation societies in the Salt and Gila River valleys in Arizona and in tributaries of the San Juan River in Colorado and New Mexico, migrants moved into the Rio Grande Valley. By the time of the Spanish incursions in the middle and late sixteenth century, the native population had developed extensive irrigation systems along the entire Northern Rio Grande to support numerous pueblos.4 Diversion works on the main stream probably consisted of brush and boulder structures that directed the water into canal entrances through the low banks. These structures probably washed away with each spring flood.

The Northern Rio Grande Basin

In northern New Mexico, the environmental plutonium bound to sedimentary particles is the most mobile in river systems, particularly the Rio Grande. This chapter describes the physical characteristics of the drainage basin into which Los Alamos National Laboratory has released plutonium. I review those characteristics of the basin that most strongly influence the movement of sediment and its associated plutonium: landforms, geology and soils, climate, vegetation, and precipitation. Precipitation and elevation provide the energy that is the primary driving force behind river processes in the Northern Rio Grande Basin. The geographic variation in stream flow and the temporal characteristics of its magnitude and frequency explain how water, sediment, and contaminants such as plutonium move through the system. An accurate accounting of stream flow is therefore essential to the development of a basinwide budget for water, sediment, and contaminants. Calculations for the mechanics of sediment transport (and the transport of associated contaminants) thus depend on measurements of stream flow from a variety of places within the system. In this chapter I examine the basic data for stream flow in the basin and then define and explain the temporal and geographical variation in the system’s river flows. The result is a regional stream-flow budget. The portion of the Northern Rio Grande emphasized in this book consists of the watershed upstream from the U.S. Geological Survey stream gage on the Rio Grande at San Marcial, at the headwaters of Elephant Butte Reservoir. The drainage network in this 71,700-sq-km area is the principal mechanism for the surface transport and storage of plutonium. The Rio Grande begins as a trickle of meltwater from a semipermenant snowbank at Stoney Pass in the San Juan Mountains in southwestern Colorado. Steep mountain tributaries are the primary sources of water, joining the main stem as it trends southeastward to the San Luis Valley and the Alamosa, Colorado, area. Additional mountain waters from the Rio Conejos, which drains the southern San Juan Mountains in southern Colorado, join the main stream as it flows southward into New Mexico. The northern Sangre de Cristo Mountains in Colorado generate surface runoff, but relatively little reaches the main river.

Simulation of Sediment and Plutonium Dynamics

The empirical data reviewed in the previous chapters indicate that low levels of plutonium can be found in sediments of the Rio Grande system. It is not readily apparent why the concentrations are low, given that the concentrations in sediments of the upper Los Alamos Canyon are one to three times greater than those in the main river. The explanation of observed concentrations probably lies in the complexities of the water and sediment system. Flash floods on the tributary occasionally evacuate some of the relatively plutonium-rich sediments into the Rio Grande, but when they enter the main river they are subject to two river processes that produce low plutonium concentrations in sedimentary deposits: mixing and dispersal. The concentrations are diluted when the sediments from Los Alamos Canyon combine with the sediments from the Upper Rio Grande and from tributaries, which contain fewer contaminants. The dispersal of plutonium on a scale of tens of kilometers along the river also generally lowers the concentrations, although the river processes deposit the contaminated materials in specific places rather than diffusing them completely throughout the river system. The consequences of the river’s complex contaminated sediment processes can be illustrated by means of direct measurement, laboratory experiments, or numerical simulation, but only the last alternative is feasible. Because the mixing, diffusion, and deposition in the Rio Grande cannot be directly observed, no detailed empirical data about them are available. And in order to be feasible, laboratory experiments must duplicate the significant components of the real system using flumes, and physical models of the system require changes in scale that may result in inaccurate representations of the actual system. The sediment in physical models must be smaller than that in their real counterparts, for example, but because the water cannot be “scaled down” in the model, the fine sediment in the laboratory behaves differently from the coarser sediment it represents in the real system. Therefore, the only possible detailed analysis of the system of contaminant transport and storage in the Rio Grande is through a numerical simulation model.

Sediment and Plutonium Storage Upstream from Cochiti

The foregoing chapters demonstrated that large amounts of sediment and much of the plutonium entering the Northern Rio Grande have been stored along the river channel. A composite budget analysis gives the quantities of materials involved annually, but except in very broad terms it does not describe where the materials are stored. It is a matter of scale: The budget indicates the overall quantities of sediment and plutonium stored in the system but does not reveal on a local scale where one might search for the materials. The next chapters show that the storage process has particular geographic characteristics and that in representative reaches it is possible to map those sediments that were deposited during the years of maximum input of plutonium into the system. These critical deposits are likely to contain more plutonium than are similar deposits of other years. In this way, the evidence of environmental change along the river provides a guide for determining the fate of plutonium in the system. A sampling program for assessing the storage of plutonium along the Northern Rio Grande depends on the development of the connections among vegetation communities, fluvial landforms, sedimentary deposits, and plutonium contents. Although it is not possible here to map and interpret completely the entire 313 km of river from Espanola to San Marcial, limited reaches can serve as representatives of larger portions of the whole. Eleven representative reaches, each about 3 to 6 km long, provide information on the entire study area because each representative reach exemplifies the conditions that obtain over a much larger portion of the total length of the river. My selection of the representative reaches began by reviewing the entire river by aerial photography and then directly in the field. The river divides itself into sections based on the geomorphologic conditions as modified by engineering works. Each representative reach illustrates the conditions within one larger section. For example, the Frijoles representative reach is similar to other relatively short reaches throughout White Rock Canyon.