Natural attenuation is currently being applied as a remedial technology at many petroleum hydrocarbon and chlorinated compound contaminated sites. Although information on the use of natural attenuation at these sites is abundant, information on sites contaminated with polycyclic aromatic hydrocarbons (PAH) is limited. An assessment report by the National Research Council on natural attenuation indicates that the current understanding of the fate and transport of PAH compounds at contaminated sites is “moderate” and the likelihood of success in the application of natural attenuation at these sites is expected to be “low,” given the current level of understanding. The purpose of this paper is to review documented work on natural attenuation of PAH-contaminated sites and summarize information to improve our level of understanding and address important issues for the implementation of natural attenuation at these sites. The main processes affecting the attenuation of PAH compounds are sorption and biodegradation. The relative contribution of each of the two attenuation processes is unclear. The few studies available tend to focus on the degradation of low molecular weight PAHs such as naphthalene, acenaphthylene, and phenanthrene. The estimated first-order decay rates of naphthalene, acenaphthylene, and phenanthrene from the various studies were 0.00057–0.0063 day⁻¹, 0.00027 day⁻¹, and 0.000027 to 0.063 day⁻¹, respectively. Some of the issues that need further investigation include: (1) an understanding of the solubility and dissolution of PAH NAPLs; (2) the interactions and effects of the more soluble low molecular weight PAHs on the sparingly soluble high molecular weight PAHs; and (3) the utilization of electron acceptors other than oxygen during microbial degradation of PAHs under complex mixture conditions. Overall, the natural attenuation of low molecular weight PAHs appears to be promising for the sites investigated.

Monitored natural attenuation has become accepted as an appropriate “technology” for treating petroleum hydrocarbon-contaminated groundwater. Commonly, the monoaromatic compounds benzene, toluene, ethylbenzene, and xylenes (BTEX) and oxygenated additives, e.g., methyl tert-butyl ether (MTBE), are the major constituents of regulatory importance. Although a variety of naturally occurring attenuation mechanisms may reduce BTEX and MTBE concentrations, intrinsic biodegradation is the primary destructive mechanism. Attenuation by anaerobic biodegradation is important for BTEX compounds because of the relative abundance of anaerobic electron acceptors as compared with dissolved oxygen. Field studies indicate that MTBE is also biodegradable in shallow aquifers, but at a slower rate relative to BTEX compounds; thus, dispersion and dilution may also be important MTBE attenuation mechanisms. To demonstrate that natural attenuation is occurring, it is critical to document the proposed natural attenuation processes in the field. These processes often cause measurable “footprints.” At a large number of petroleum-hydrocarbon release sites, natural attenuation processes have been observed and documented and found to control the extent of migration of BTEX contaminants. Although the presence of MTBE may compromise the use of natural attenuation at some sites due to the greater mobility and persistence of MTBE compared to BTEX, some data indicate that MTBE plumes do eventually stabilize.

Long-term monitoring is an integral part of the natural attenuation remediation process. It is needed to monitor the migration of a contaminated groundwater plume over time and to evaluate the extent of natural remediation that is occurring. It involves the placement of four types of monitoring wells: up-gradient, side-gradient, impacted source, and down-gradient wells. The monitoring of groundwater samples from these wells should provide information to evaluate if the objectives of the natural attenuation process are being met. The sampling analytical plan depends upon the type of contaminants involved. The frequency of sampling should be sufficient to detect in a timely manner any changes in plume contaminant degradation behavior. It should also be flexible enough to make changes, should there be a need. The monitoring program should be continued until the objectives of the natural attenuation process have been met, and possibly longer, if necessary, to ensure that health and environment threats are protected. If during the monitoring process the plume behavior is found to change for the worse (i.e., plume migration is faster or further than before), a contingency plan should be in place to protect the down-gradient receptors. The contingency plan usually requires an engineered remediation plan to reduce the negative impact of the plume on the down-gradient receptors.

Poplar trees represent one vegetation option for phytostabilization of heavy metal-contaminated sites. The objective of this study was to investigate the suitability of deep-planted hybrid poplars for phytostabilization of a heavy metal-contaminated site. Tree survival was greatly improved when trees were planted using cattle manure as a soil amendment as compared to no amendment. Survival after 3 years with manure ranged from 18 to 56% for four poplar hybrids, while without manure, the range was 0–18%. Increases in trunk diameter and tree height were also greater with manure than without after the first growing season. Zinc, Cd, and Pb concentrations were as high as 3,606, 44, and 33 mg kg⁻¹, respectively, in leaves, and there was little influence of manure amendment or hybrid. Zinc phytotoxicity may have been a factor in tree survival. Average metal concentrations in plant tissues followed the order: $\mathrm{leaves}>\mathrm{bark}>\mathrm{twigs}>\mathrm{wood}$ for Zn and Cd, and $\mathrm{bark}>\mathrm{wood}>\mathrm{twigs}\cong \mathrm{leaves}$ for Pb. Manured treatments supported higher rates of both photosynthesis and transpiration. Comparisons among varieties indicated that DO1 and Imperial Carolina hybrids had more than twice the rates of photosynthesis of the other two hybrids and that each variety was unique in terms of water-use efficiency (WUE), with Imperial Carolina being higher than any other hybrid. This combination of high rates of photosynthesis and high WUE suggests a unique suitability of this hybrid for remediation of this site. More extensive characterization of the hybrids under a variety of conditions, including drought, is likely to reveal more about inherent suitability of each hybrid for site-specific remediation.

Four soil samples, collected from a National Priorities Listed site contaminated with uranium, thorium, and radium, were leached for two, seven, and 30 days. The nonleached soils, leached soils, and leachates were analyzed using gamma spectroscopy, alpha spectroscopy, and delayed neutron activation. Unleached and leached soils had low radionuclide activities, and four different leaching solutions had no significant observable effect upon the resulting leachate activity. Three of the soils produced leachates with uranium activities below the expected primary drinking water standard of 30 μg/L for uranium, but the leachate from the fourth soil had an unacceptably high concentration of uranium (653 μg/L) despite the low activity of the original, unleached soil. Distribution coefficients calculated for uranium ranged from 100 to 12,000 mL/g, while distribution coefficients for ${}^{226}\mathrm{Ra}\text{,}$ ${}^{228}\mathrm{Ra}\text{,}$ ${}^{228}\mathrm{Th}\text{,}$ ${}^{137}\mathrm{Cs}\text{,}$ and ${}^{40}K$ ranged from 1 to 20 mL/g. The uranium distribution coefficient was strongly correlated to the leachate alkalinity, suggesting that conditions resulting in high alkalinity yield a high potential to leach uranium.

Simulation models can be efficient tools, provided they are sufficiently validated. In the United States, the HELP model was developed to simulate the water balance of landfills, especially of liner systems. HELP version 3 was examined under the humid climate of Germany in an extensive validation study. The aim of this paper is to put the validation results into practice by showing the potential and limitations of applying the HELP model for surface cover systems using examples of practical planning. The examples cover the components of surface covers: topsoil and vegetation, a compacted layer at the bottom of the topsoil, drainage layers and drainage nets, geomembranes, and several liners. Potential errors are shown and classified, and recommendations for critical application cases are given. In summary, the HELP model is a suitable tool for instructed users, but good knowledge of the model and its behavior and critical review of the simulation results are essential. A German enhancement of the HELP model, HELP 3.50 D, was developed. It fixes errors shown in this article and enhances some processes.

Nearly 2/3 of the total population in the affluent State of Kuwait include working expatriates. A significant percentage of these individuals are replaced annually by new incoming workers, often with differences in socio-economic and life-cycle traits. Naturally, this changing population composition affects—among other factors—the quantity and the mix of the daily generated solid waste by households. This paper reports the findings of a research study aimed at the determination of the quantity and mix of the current household solid waste in Metropolitan Kuwait. The paper also examines the compatibility between household-reported and landfill site-measured percent compositions of the daily solid waste. Findings point to a slight increase in quantity of the daily generated solid waste by households over the last four years. The study has also confirmed that, while households are reasonable estimators of the quantity of the solid waste they generate daily, their estimation of the composition of their solid waste suffers from gross inaccuracies.