Evaluation of Long‐Term Performance and Sustained Treatment at Enhanced Anaerobic Bioremediation Sites

This study evaluated the long‐term performance of enhanced anaerobic bioremediation (EAB) at chlorinated solvent sites to determine if sustained treatment processes were helping to prevent concentration rebound. A database of groundwater concentration versus time records was compiled for 34 sites, with at least 3 years of posttreatment monitoring data (median = 4.7 years, range = 3.0 to 11.7 years). Long‐term performance was evaluated based on order‐of‐magnitude (OoM) changes in parent compound concentrations during various monitoring periods. Results indicate that, relative to the pretreatment concentration, a median concentration reduction for all 34 sites of 1.0 OoM (90% reduction) was achieved by the end of the posttreatment monitoring period. No rebound was observed at 65% of the sites between the first year of posttreatment monitoring and the final year. During this posttreatment period, Mann‐Kendall trend analysis indicated that the concentration was stable or decreasing at 89% of the sites where a trend could be established (n = 27; 33% decreasing, 56% stable, 11% increasing). Statistical analysis indicates there is no evidence that the distribution of median concentration reductions after the first year of posttreatment monitoring was different than the distribution of median reductions 2 to 11 years later at the end of the monitoring period (p = 0.67). Similarly, statistical analysis indicates that there is no evidence that the distribution of median reductions for a larger set of sites (n = 84) with less than 3 years of posttreatment monitoring data (1.1 OoM; 92% reduction) was different than the distribution of median OoM reductions for the 34‐site dataset with longer monitoring periods (p = 0.80). This suggests that, at a typical site, a 3‐year monitoring period should be sufficient for evaluating performance. The results of this study indicate that, in the long term, after the end of active treatment, sustained treatment processes contribute to relatively modest concentration reductions but do mitigate rebound at the majority of EAB sites.     

Stardust-NExT,Deep Impact,and the accelerating spin of 9P/Tempel 1

The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13 year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 ± 0.025°/dy (41.335 ± 0.005 h) prior to the 2000 perihelion passage, 210.448 ± 0.016°/dy (41.055 ± 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 ± 0.030°/dy (40.783 ± 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 ± 0.012°/dy (40.827 ± 0.002 h) in the interval 2006–2010 following the 2005 perihelion passage. The period decreased by 16.8 ± 0.3 min during the 2000 passage and by 13.7 ± 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 ± 0.003°/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations.The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4–15) pole and longitude system. The possibility of a 180° error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42.We find that a net torque in the range of 0.3–2.5 × 10⁷ kg m² s^?2 acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than ～1° per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H₂O out-gassing from the surface is largely responsible for the observed spin-up.     

Impact of Recharge Through Residual Oil Upon Sampling of Underlying Ground Water

At an aviation gasoline spill site in Traverse City, Michigan, historical records indicate a positive correlation between significant rainfall events and increased concentrations of slightly soluble organic compounds in the monitoring wells of the site. To investigate the recharge effect on ground water quality due to infiltrating, water percolating past residual oil and into the saturated zone, an in situ infiltration experiment was performed at the site. Sampling cones were set at various depths below a circular test area, 13 feet (4 meters) in diameter. Rainfall was simulated by sprinkling the test area at a rate sufficiently low to prevent runoff. The sampling cones for soil-gas and ground water quality were installed in the unsaturated and saturated zones to observe the effects of the recharge process. At the time of the test, the water table was below the residual oil layer. The responses of the soil-gas and ground water quality were monitored during the recharge and drainage periods, which resulted from the sprinkling.
Infiltrated water was determined to have transported organic constituents of the residual oil, specifically benzene, toluene, ethylbenzene, and ortho-xylene (BTEX), into the ground water beneath the water table, elevating the aqueous concentrations of these constituents in the saturated zone. Soil-gas concentrations of the organic compounds in the unsaturated zone increased with depth and time after the commencement of infiltration. Reaeration of the unconfined aquifer via the infiltrated water was observed. It is concluded that water quality measurements are directly coupled to recharge events for the sandy type of aquifer with an overlying oil phase, which was studied in this work. Ground water sampling strategies and data analysis need to reflect the effect of recharge from precipitation on shallow, unconfined aquifers where an oil phase may be present.     

Uncertainty in floodplain delineation: expression of flood hazard and risk in a Gulf Coast watershed

This paper investigates the development of flood hazard and flood risk delineations that account for uncertainty as improvements to standard floodplain maps for coastal watersheds. Current regulatory floodplain maps for the Gulf Coastal United States present 1% flood hazards as polygon features developed using deterministic, steady‐state models that do not consider data uncertainty or natural variability of input parameters. Using the techniques presented here, a standard binary deterministic floodplain delineation is replaced with a flood inundation map showing the underlying flood hazard structure. Additionally, the hazard uncertainty is further transformed to show flood risk as a spatially distributed probable flood depth using concepts familiar to practicing engineers and software tools accepted and understood by regulators. A case study of the proposed hazard and risk assessment methodology is presented for a Gulf Coast watershed, which suggests that storm duration and stage boundary conditions are important variable parameters, whereas rainfall distribution, storm movement, and roughness coefficients contribute less variability. The floodplain with uncertainty for this coastal watershed showed the highest variability in the tidally influenced reaches and showed little variability in the inland riverine reaches. Additionally, comparison of flood hazard maps to flood risk maps shows that they are not directly correlated, as areas of high hazard do not always represent high risk. Copyright © 2012 John Wiley & Sons, Ltd.