Abstract:Rainfall simulations allow for controlled comparisons of runoff and erosion among ecosystems and land cover conditions. Runoff and erosion can increase greatly following fire, yet there are few rainfall simulation studies for post-fire plots, particularly after severe fire in semiarid forest. We conducted rainfall simulations shortly after a severe fire (Cerro Grande) in ponderosa pine forest near Los Alamos, New Mexico, USA, which completely burned organic ground cover and exposed unprotected soil. Measurements on burned plots showed 74% of mineral soil was exposed compared with an estimated 3% exposed prior to the fire. Most of the remaining 26% surface area was covered by easily moveable ash. Rainfall was applied at 60 mm h 1 in three repeated tests over 2 days. Runoff from burned plots was about 45% of the total 120 mm of applied precipitation, but only 23% on the unburned plots. The most striking difference between the response of burned and unburned plots was the amount of sediment production; burned plots generated 25 times more sediment than unburned plots (76 kg ha 1 and 3 kg ha 1 respectively per millimetre of rain). Sediment yields were well correlated with percentage bare soil (r D 0Ð84). These sediment yields were more than an order of magnitude greater than nearly all comparable rainfall simulation studies conducted on burned plots in the USA, most of which have been in grasslands or shrublands. A synthesis of comparable studies suggests that an erosion threshold is reached as the amount of soil exposed by fire increases to 60-70%. Our results provide sediment yield and runoff data from severely burned surfaces, a condition for which little rainfall simulation data exist. Further, our results contrast post-fire hydrologic responses in forests with those in grasslands and shrublands. These results can be applied to problems concerning post-fire erosion, flooding, contaminant transport, and development of associated remediation strategies.

The results from several field experiments on methods to control soil erosion, biointrusion, and water infiltration were used to design and test an enhanced landfill cover that improves the ability of the disposal site to isolate buried wastes. The performance of the improved cover design in managing water and biota at the disposal site was compared for 3 yr with that obtained from a more conventional design that has been widely used in the industry. The conventional cover design consisted of 20 cm of sandy loam topsoil over 108 cm of a sandy silt backfill, whereas the improved design consists of 71 cm of topsoil over a minimum of 46 cm of gravel, 91 cm of river cobble, and 38 cm of sandy silt backfill. Each plot was lined with an impermeable liner to allow for mass balance calculation of water dynamics. Results over a 3-yr period, including 2 wet yr, demonstrated that the improved design reduced percolation of water through the landfill cover by a factor of >4 over the conventional design. This decrease in percolation was attributed to a combination of increased evapotranspiration from the plant cover and the effect of a capillary barrier embedded in the enhanced cover profile in diverting water laterally in the cover. The field data are finally discussed in terms of its usefulness for waste management decisions to be made in the future for both new and existing landfills at Los Alumos, NM, and at other semiarid waste disposal sites.

No abstract

This study was initiated to determine the kinetics of cesium transport in East Twin Lake, a 5-ha, natural semi-drainage lake which lies at an elevation of 2880 m in the north-central Colorado Rockies. One kilogram of 133Cs (as 133CsC1) was introduced into the water of East Twin Lake on 15 September 1970. Samples of water, seston, sediment, amphipods, zooplankton, trout and two species of vegetation were obtained over a 393-day period following the dosing event. Stable cesium was measured by neutron activation analysis.The loss of 1%k from water occurred in two phases. The rapid phase accounted for the loss of 60% of the cesium from water and had a loss half-time of 0.5 days while the slow phase had a loss half-time of 130 days. The seston fraction of each water sample contained from 25 to 80% of the 133Cs present in each liter of unfiltered water. Bottom sediments were identified as the major site of deposition of the '=Cs dose. The sediment, at 393 days post-dosing, accounted for 82% of the l%Cs inventory in the lake whereas water and seston accounted for only 3 and 14%, respectively. Amphipods and zooplankton reached equilibrium with the water within about 3 weeks after the lSCs administration and achieved concentration factors (wet weight) of about 700 and 150, respectively. During mid-winter, however, these organisms showed increased concentrations of lSCs with concentration factors ranging up to 3700 and 350 for amphipods and zooplankton, respectively. Trout accumulated the 1BCs more slowly than the invertebrates and reached a maximum concentration factor of about 5600 some 260 days following the dosing event.Absolute quantities as small as 10-8 g of lSCs were detectable. Concentrations of133Cs and fallout lS7Cs in trout muscle were significantly correlated which indicated that the kinetic behavior of the isotopes in this component of the lake was similar. Furthermore, the stable tracer obviates the radiation protection problems that would arise from contaminating a public water with a radiosotope of cesium.

A review of actinide behavior in the environment is presented with emphasis on chemical, physical, and biological factors that influence actinide mobility in ecosystems. Available data from terrestrial and fresh water ecosystems suggest that physical processes which result in the transport of soils and sediments dominate in the translational movement of plutonium and, as well, dominate in the transport of this element through lower trophic levels. Exceptions to that statement occur in arctic ecosystems and in deep oceans. Regardless of mode of transport, plutonium levels in higher trophic levels including man are very low indicating the low solubility of this element in the environment. Very few data on the behavior of the other actinides in the environment are currently available although theoretical considerations and limited laboratory experiments suggest that many of the actinides are more mobile than plutonium.

A field study was conducted in 1977 on 238, 239Pu and 137Cs availability to zucchini squash (Curcurbita melopepo, hybrid seneca) and green bush beans (Phaseolus vulgaris, Landreths stringless) grown under home‐garden conditions in an area at Los Alamos National Laboratory used for treated radioactive liquid waste disposal. Radionuclide concentrations were measured as a function of tissue type, height above the soil, fertilization regime, and for the squash, food‐cleansing procedures. Analysis of variance procedures were used to analyze the data.Ratios of the concentration of a radionuclide in oven‐dried vegetation to dry soil ranged from 0.0004 to 0.116 for the Pu isotopes, and from 0.051 to 0.255 for 137Cs. Fertilization with cattle manure reduced the Pu concentration ratios by 30% and 137Cs by 50%. Vegetative parts sampled within 20 cm of the ground surface were contaminated about four times as much as those parts growing further from the ground surface. About 65% of the contamination was removed by washing, indicating the presence of surficial contamination. The 50‐year radiation dose commitment to humans consuming vegetables from the garden plot would be less than 0.05 mrem and would be due almost entirely to 137Cs.

Data are presented on the availability of tritium, cesium‐137, and plutonium to honey bee colonies foraging in the environment surrounding the Los Alamos Scientific Laboratory. Sources of these radionuclides in the laboratory environs include liquid and atmospheric effluents and buried solid waste.Honey bee colonies were placed in three canyon liquid waste disposal areas and were sampled frequently, along with honey, surface water, and surrounding vegetation, to qualitatively determine the availability of these radionuclides to bees (Apis mellifera) and to identify potential food chain sources of the elements.Tritium concentrations in bee and honey samples from the canyons increased rapidly from initial values of < 1 pCi/ml moisture to as much as 9.2 nCi/ml in 75 days after placement of the hives in the canyons. Seasonal patterns in foraging activities as influenced by weather and food availability were apparent in the data. It appears that several sources of tritium were utilized by the colonies, including surface water in the canyons and vegetation receiving tritium from atmospheric effluents and buried solid waste.Concentrations of cesium‐137 and plutonium were generally low or undetectable in bees throughout the study. However, levels of both nuclides increased by factors of 10–20 in bees from two of the canyon study areas during a 3‐month period in 1973. It was speculated that the liquid effluents in the two canyons were the source of the increased concentrations in bee samples, since this water was the only significant source of 137Cs in the environs.The existence of at least three radionuclide sources in the Los Alamos Scientific Laboratory (LASL) environs complicates the interpretation of the data. However, it is apparent that honey bees can acquire 3H, 137Cs, and Pu from multiple sources in the environs.

Determining the appropriate criteria and designs for hazardous waste landfill covers has spawned much discussion within the environmental remediation arena. Ve y little reliable comparison of various technologies exists. Researchers at Los Alamos National Laboratory studied the relative hydrologic performance of four landfill cover designstwo capillary barrier designs, one modified EPA RCRA design, and one control cover. Monitoring the fate of natural precipitation for nearly four years showed that the covers with barrier layers more effectively reduced deeppercolation than the control cover. Although none entirely eliminated deeppercolation, the RCRA cover, incorporating a clay hydraulic barrier, most effectively controlled it. The two capilla y barriers reduced deep percolation, but significant amounts were stillproduced. Over POpercent of allpercolation through the covers, and lateralflow within the covers, occurred during Februa y through May each year, primarily as a result of snowmelt, early spring rains, and low evapotranspiration. The study also showed that gravel mulch su$ace treatments (70-to 80-percentground cover) reduced runoff and erosion. Despite additional shrubsplanted on one, the two plots receiving thegravel mulch treatmentsexhibited equally enhanced amounts of evapotranspiration.Landfilling, the oldest method of waste disposal practiced by humans (Hagerty, 19731, enjoys widespread use for hazardous waste disposal. Today, satisfactory landfill technology must prevent leaching of contaminants to surrounding soil or underlying groundwater. Remediating previously contaminated sites often relies on containment; it is cost-effective, generates little by-product waste, and can be designed to meet regulatory requirements. Successfully containing wastes in landfills or contaminated sites depends almost entirely on the landfill cover.Most containment failures result from interactions of water with the landfill covers, as documented by operating experience at major low-level radioactive waste disposal sites since the early 1940s (Duguid, 1977;Jacobs et al., 1980;Hakonson et al., 1982;Herzog et al., 1982). Landfill covers seem particularly susceptible to failure during snowmelt periods when large inputs of water occur and when evapotranspiration is low (Nyhan et al., ). Choosing the best cover design for a particular site becomes a critical decision in the overall landfill design or remediation strategy.Unfortunately, few field studies have evaluated the relative performance of cover design alternatives and whether they meet EPA's performance requirements. Previous work generally examined individual processes affecting cover performance, for example: erosion (Hakonson et al.). While each of these processes are important, data are needed to evaluate them acting together under field conditions.Although directly measuring water balance could best evaluate the performance of cover designs, few studies have attempted to do so (Healy et al., 1989;Nyhan et al., 1990;Campbell et al., 1991;Limbach et al., 1994...