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.