The effectiveness of the electrokinetic (EK) process in treating soil enriched in chromite ore processing residue (Cr–soil) was investigated. An electric gradient of 1.2 V/cm was applied to induce the movement of electrolyte solution in the soil cell for 17–43 days. Simulated rainwater was used as EK processing fluid. Results show low removal efficiency of total Cr (2–7%) because the insoluble form of chromite dominates in the Cr–soil. However, more than 95% of leachable Cr(VI) can be removed by controlling the pH of the processing fluid at 9.0 during experiments. The removal of Cr from Cr–soil was characterized by the high Cr(VI) concentration collected at the anode, and the presence of Cr precipitates collected at cathode. The electric energy requirement under all experimental conditions varied from 1.3 to 5.8 kW h/m³. Results indicated that the EK is an effective process in mitigating the leaching of Cr(VI) from Cr–soil.

To test the inherent hexachlorobenzene (HCB) dechlorination ability of indigenous consortia in Taiwan, microbes from four non-HCB-contaminated river sediments were collected and directly introduced to dechlorination experiments without any acclimation and enrichment. In natural sediment-extracted waters, when amended with 5 g/L of yeast extract, all four sets of sediment microorganisms initiated HCB dechlorination after 60 days. Without yeast extract, there were only two sets of microbes-dechlorinated HCB during a 150-day incubation period. On the other hand, in the artificial synthetic media amended with 5 g/L of yeast extract, all microbes from four sediments showed HCB-dechlorinating activity after 45–60 days of lag phases and completed dechlorination in latter 30 days. In all HCB-dechlorinated sets, dechlorination was accompanied by significant methane generation and the drop of oxidation-reduction potential (ORP). The results indicated that there were HCB-dechlorinating consortia grouped by the indigenous microorganisms in the river sediments of Taiwan, even though these sediments had not been contaminated by HCB before. In addition, the dechlorination could occur when the environmental conditions were suitable for the dechlorinating consortia to be enriched, and then the ability for HCB decomposition was built up.

An integrated slurry-phase remedial system was developed for cleanup of polycyclic aromatic hydrocarbon (PAH) contaminated soil in Singapore. The bench-scale remedial system consisted of a chemical treatment to initiate the degradation of PAHs followed by a biological treatment. The 20% (weight/volume) slurry of PAH contaminated soil was mixed continuously at 180 rpm in the 1 L bioreactor. PAH contaminated soil was synthesized by spiking naphthalene, phenanthrene, and pyrene onto uncontaminated model soil. Fenton’s reagent was added during chemical treatment to initiate degradation of PAHs. The removal efficiency of chemical treatment was 76%. The active PAH-degrading strain of bacteria was isolated from a slurry-phase reactor, where garden soil was used as a source of indigenous microorganisms. The sequencing of 16S rDNA showed that this bacterial strain was close to Pseudomonas beteli (identities=99%, 553 from 556 position). The application of isolated bacteria for bioremediation of PAH contaminated soil resulted in a 96–97% removal of PAHs. It was possible to use the cultivated PAH-degrading microorganisms as an effective inoculum to start up the remediation process. The proposed integrated chemical-biological remedial system was capable of treating PAH contaminated soil efficiently in terms of high-ring PAH degradation, less expensive process, and short treatment time required.

The objective of this research is to investigate the intrinsic biodegradation potential of the municipal solid waste (MSW) landfill ecosystem for chlorinated aliphatic hydrocarbons (CAHs). The research was conducted in two complementary systems: simulated landfill bioreactors and batch degradation experiment in serum bottles. Refuse samples excavated from a MSW landfill were tested in laboratory bioreactors operated with leachate recirculation and gas collection. Tetrachloroethene (PCE) and trichloroethene (TCE) were added to reactors and maintained at 20 μM each in leachate to simulate the long-term exposure of refuse microorganisms to CAHs in landfills. Anaerobic microbial cultures were derived from reactors at two different stages of refuse decomposition: active refuse decomposition stage representing young landfills and maturation phase representing aged landfills. These cultures were tested in serum bottles for their abilities to biodegrade target CAHs. Results of this study show that MSW landfills have an intrinsic reductive dechlorination capacity for PCE and TCE. The decomposition of refuse also enhanced reductive dechlorination in this study. This research suggests the potential to develop engineering strategies to promote chlorinated solvent degradation concurrently with refuse decomposition in MSW landfills.

The availability of Fe(III) for microbial reduction was measured in macrocosms containing contaminated aquifer sediment with benzaldehyde as the organic substrate. Repeated benzaldehyde additions initially resulted in an increase in Fe(II) and decrease in Fe(III). However, after ∼400 h total Fe(II) and Fe(III) stabilized and both ethanol and benzylalcohol began to accumulate, indicating a shift in microbial processes from iron reduction to fermentation. Addition of Fe(III) as amorphous–oxyhydroxide, goethite, and hematite to the Fe(III) depleted sediment stimulated ethanol and benzylalcohol degradation. Washing the sediment with ${\mathrm{CaCl}}_2$ and NaAc to remove sorbed and/or precipitated Fe(II) also stimulated ethanol and benzylalcohol degradation indicating that Fe(II) present on the sediment surface was limiting Fe(III) bioavailability. Selective chemical extraction of the sediment with either 0.5 M HCl or oxalate did not provide a useful indicator of the total bioavailable Fe(III).

Risk-based corrective action (RBCA) has been becoming an accepted approach for the remediation of contaminated sites. This paper described the application of four different risk assessment approaches, including the North Carolina risk analysis framework (Framework), Illinois tiered approach to correction objectives, RBCA tool kit for chemical releases (RBCA tool kit), and exposure and risk assessment decision support system, to conduct a risk evaluation task and demonstrate the application strategy at a fuel-oil spill site. After the risk evaluation processes, the soil and groundwater remediation goals for target compounds (total petroleum hydrocarbon, benzene, toluene, ethylbenzene, and xylenes) were determined. The calculated remediation levels met with the requirements for minimum target risk levels (i.e., cancer risk of $1\times {10}^{-6}$ and hazard quotient of one). After completion of the RBCA process, the remediation time and cost could be minimized in some cases due to the adoption of more achievable and flexible remediation goals. Among these applied risk assessment methods, the RBCA tool kit was an appropriate and feasible tool for the future risk assessment application due to its thorough considerations of the exposure pathway and transport model selection. Results from this study provided a streamlined process and guidelines for future risk assessment work at petroleum–hydrocarbon contaminated sites in Taiwan. Based on the findings from this study, RBCA was believed to be a more sound and defensible basis for site closure.

Spatial heterogeneity of microbial populations (total heterotrophs or total bacteria (AODC), iron reducers, sulfate reducers, methanogens, aerobic, and anaerobic protozoa) and geochemical parameters (hydrocarbon concentrations, inorganics, grain size, sediment Fe) were studied at gasoline contaminated aquifers located at Rocky Point, N.C. and Pope Air Force Base (AFB), N.C. At Rocky Point, heterogeneity was studied on samples collected on vertical scale (2–3 cm and 0.3 m intervals) and horizontal scale (>40 m). At Pope (AFB) heterogeneity was studied in samples collected on vertical scale (7.5 cm intervals) and horizontal scale (>20 m). Distribution of microbial populations at Rocky Point showed that there is little variability in 2–3 cm spaced samples, while there is substantial variability over 0.3 m intervals. Elevated numbers of iron reducers and anaerobic protozoa were measured at specific depth intervals that corresponded with reduced contaminant concentrations. On the horizontal scale little variability was observed in microbial and geochemical parameters. At Pope AFB, distribution of microbial populations varied on horizontal and vertical scales. Shifts in the dominance of trophic groups were observed on vertical scale and dominant microbial processes varied on the horizontal scale. In addition aqueous inorganic analysis and sediment Fe analysis showed that geochemistry at the two aquifer varied. At Rocky Point extractable Fe(III) was greater relative to Pope AFB and significantly greater fraction of total Fe(II) was on the solid phase (>98%) relative to Pope AFB (60–72%). The measured heterogeneity suggests that techniques utilizing blended sediment for the measurement of in situ biodegradation rates and vertically averaged measurement of hydrocarbons in the field will provide misleading results.

An optimal model is developed for determining the well sites and their pumping rates from the upper aquifer and lower aquifer in a two-aquifer system, which is designed to prevent the water quality in lower aquifer from contamination of the polluted upper aquifer by observing a set of constraints determined by the drawdowns between the upper and lower aquifers. The physical model of the complex simulation area may be simplified as a two-aquifer system, which consists of an upper phreatic aquifer and a lower confined aquifer with a leaky stratum between them. First, a numerical quasithree-dimensional mathematical equation is introduced to govern the flow of the two-aquifer system. Second, the generation of the response matrix in two-aquifer system is presented by separating the numerical quasithree-dimensional mathematical equation into three equations. Then the objective management strategy for the upper and lower aquifers is designed to construct a linear programming model, the optimal groundwater management model, with its constraints through the response matrix approach in a two-aquifer system. The developed model with data from Qiqihar City, Heilongjiang Province, China is used for allocating the withdrawal amount optimally among wells in the upper and lower aquifers without reducing the water quality in the lower aquifer and satisfactory results are obtained.

Large oil fields are distributed within the loess plateau of northwest China, where severe soil contamination by petroleum hydrocarbons has been found in several locations. A prevention measure is to modify the topsoils with surfactants to enhance the attenuation capacity in the vadose zone. In this paper, both cationic surfactant hexadecyltrim–ethylammonium bromide (HDTMA-Br) and anionic surfactant—sodium dodethylbenzene sulfonate were used to modify the natural loess soils. The sorption of organic contaminants in the modified soils can be characterized by linear isotherms. By using toluene as an indicator of petroleum hydrocarbons, partition and adsorption contributions to the sorption amount were determined quantitatively. The approach displayed enhanced soil sorption capabilities by 2–10 times. Column tests showed that the retardation capacity of the HDTMA-modified soil for p-nitroaniline solution increased the breakthrough time at least three times as compared to the natural loess soil. Using the suggested surfactants would significantly enhance the topsoil’s ability to delay transport of petroleum leachates.

Chemical reduction of hexachlorobenzene (HCB) by metallic iron was investigated. Since the percentage of HCB transformation is almost the same as that of chloride ion production, no chlorinated intermediates in the solution was formed during the reduction reaction. The solution pH went to around 8.0 even though the initial pH was 10 or lowered to 3. The reaction is pseudofirst order with respect to HCB concentration. The optimum dosage of iron particles for the dechlorination of HCB varies with the HCB concentration. For example, when the initial concentration of HCB was $1.0\times {10}^{-5} M\text{,}$ the highest transformation rate of HCB occurred with using at 350 g/L of iron particles; the optimum dosage shifted to around 500 g/L when the HBC concentration was $2.72\times {10}^{-5} M\text{.}$