Enhancement of In Situ Bioremediation of BTEX-Contaminated Ground Water by Oxygen Diffusion from Silicone Tubing

A field study of oxygen-enhanced biodegradation was carried out in a sandy iron-rich ground water system contaminated with gasoline hydrocarbons. Prior to the oxygen study, intrinsic microbial biodegradation in the contaminant plume had depleted dissolved oxygen and created anaerobic conditions. An oxygen diffusion system made of silicone polymer tubing was installed in an injection well within an oxygen delivery zone containing coarse highly permeable sand. During the study, this system delivered high dissolved oxygen (DO) levels (39 mg/L) to the ground water within a part of the plume. The ground water was sampled at a series of monitors in the test zone downgradient of the delivery well to determine the effect of oxygen on dissolved BTEX, ground water geochemistry, and microbially mediated biodegradation processes. The DO levels and Eh increased markedly at distances up to 2.3 m (7.5 feet) downgradient. Potential biofouling and iron precipitation effects did not clog the well screens or porous medium. The increased dissolved oxygen enhanced the population of aerobes while the activity of anaerobic sulfate-reducing bacteria and methanogens decreased. Based on concentration changes, the estimated total rate of BTEX biodegradation rose from 872 mg/day before enhancement to 2530 mg/day after 60 days of oxygen delivery. Increased oxygen flux to the test area could account for aerobic biodegradation of 1835 mg/day of the BTEX. The estimated rates of anaerobic biodegradation processes decreased based on the flux of sulfate, iron (II), and methane. Two contaminants in the plume, benzene and ethylbenzene, are not biodegraded as readily as toluene or xylenes under anaerobic conditions. Following oxygen enhancement, however, the benzene and ethylbenzene concentrations decreased about 98%, as did toluene and total xylenes.     

A Proposed Method to Estimate In Situ Dissolved Gas Concentrations in Gas‐Saturated Groundwater

Gas‐saturated groundwater forms bubbles when brought to atmospheric pressure, preventing precise determination of its in situ dissolved gas concentrations. To overcome this problem, a modeling approach called the atmospheric sampling method is suggested here to recover the in situ dissolved gas concentrations of groundwater collected ex situ under atmospheric conditions at the Horonobe Underground Research Laboratory, Japan. The results from this method were compared with results measured at the same locations using two special techniques, the sealed sampler and pre‐evacuated vial methods, that have been developed to collect groundwater under its in situ conditions. In gas‐saturated groundwater cases, dissolved methane and inorganic carbon concentrations derived using the atmospheric sampling method were mostly within ±4 and ±10%, respectively, of values from the sealed sampler and pre‐evacuated vial methods. In gas‐unsaturated groundwater, however, the atmospheric sampling method overestimated the in situ dissolved methane concentrations, because the groundwater pressure at which bubbles appear (P_critical) was overestimated. The atmospheric sampling method is recommended for use where gas‐saturated groundwater can be collected only ex situ under atmospheric conditions.     

Diffusive Oxygen Emitters for Enhancement of Aerobic In Situ Treatment

Aerobic biodegradation can be enhanced within contaminant plumes by elevating typically low dissolved oxygen (D.O.) levels using materials or devices that passively release oxygen. We have developed passive devices that provide a uniform, steady, long-term source of oxygen by diffusion from pressurized polymeric tubing and report test results under lab and field conditions. Lab flow-through reactor tests were conducted to determine the diffusion coefficient (D) of oxygen through four readily available tubing materials. Oxygen diffusion was greatest through Tygon® 3350 platinum-cured silicone (D = 6.67 ± 10^-7 cm²/sec), followed by 2075 Ultra Chemical Resistant Tygon (1.59 ± 10^-7 cm²/sec), 2275 High Purity Tygon (5.11 ± 10^-8 cm²/sec), and low-density polyethylene (LDPE; 1.73 ± 10^-8 cm²/sec). Variable-pressure release tests with LDPE resulted in very close estimates of D, which confirmed that mass transfer is controlled by diffusion and that the concentration gradient is a valid approximation of the chemical potential driving diffusion. LDPE emitter devices were designed and installed in seven 8-inch-diameter well screens across a portion of a gasoline plume at a former service station. With the devices pressurized to 620.5 kPag (kilopascals gauge) late in the test, steady-state D.O. concentrations reached as high as 25 mg/L, comparing favorably to the value predicted using the mass-transfer coefficient estimated from the lab test (26.3 mg/L). The method can also be used to release other gases for other reasons: gaseous tracers (i.e., sulphur hexafluoride, helium, and argon), hydrogen (for reductive dechlorination), or light alkanes (for cometabolic biodegradation of methyl tertiary butyl ether [MTBE] or chlorinated solvents).     

In Situ Bioremediation of Toluene‐Polluted Vadose Zone: Integrated Column and Wetland Study

A vertical soil column setup integrated with wetlands is developed to study the biodegradation and transport of toluene, a light non‐aqueous phase liquid (LNAPL), in the subsurface environment. LNAPL‐contaminated water is applied to infiltrate from the top of the soil column. The observed and simulated breakthrough curves show high equilibrium concentration at top ports rather than at lower ports, indicating effective toluene biodegradation with soil depth. The observed equilibrium concentration of toluene is higher in the case of unplanted wetland, asserting an accelerated biodegradation rate in the planted case. A difference in the relative concentration of toluene between input and output fluxes at 100 h is found as 13.34% and 30.86% for planted and unplanted wetland setups, respectively. Estimated biodegradation rates show that toluene degradation is 2.5 times faster in the planted wetland setup. In addition, the difference in the observed bacterial count and dissolved oxygen prove that toluene degraded aerobically at a faster rate in the planted setup. Simulations show that as time reached 80–100 h, there is no significant change in concentration profile, thereby confirming the equilibrium condition. The results of this study will be useful to frame plant‐assisted bioremediation techniques for LNAPL‐contaminated soil–water resources in the field.     

In Situ Measurement and Numerical Simulation of Oxygen Limited Biotransformation

Enhanced subsurface biorestoration is rapidly becoming recognized as a valuable tool for the restoration of hydrocarbon-contaminated aquifers and sediments. Previous field and laboratory studies at a former wood creosoting facility near Conroe, Texas, have indicated that insufficient oxygen is the primary factor limiting the biotransformation of polynuclear aromatics (PNAs) in sediments and ground water at this site. A series of laboratory experiments and field push-pull injection tests were performed as part of this project to: (1) study the effect of low oxygen concentrations on the biotransformation of PNAs; (2) identify the minimum concentration of PNAs that could be achieved through the addition of oxygen alone; (3) confirm that enhanced subsurface biorestoration is feasible at this site; and (4) test an existing numerical model of the biotransformation process (BIOPLUME). The laboratory studies demonstrated that biotransformation of the PNAs was not inhibited at dissolved oxygen concentrations as low as 0.7 mg/L although this work did suggest that there may be a minimum PNA concentration of 30 to 70 μg/L total PNAs below which biotransformation was inhibited. The field push-pull tests confirmed that addition of oxygen was effective in enhancing the subsurface biodegradation of the PNAs. The minimum concentration achieved using oxygen alone was approximately 60 μg/L total PNAs. Minimal biotransformation of these compounds was observed without oxygen addition. The numerical model BIOPLUME was tested against monitoring data from the field experiments and appears to provide a good approximation of the biodegradation process.     

溶解氧对低洼盐碱地鱼塘物质循环的影响

对黄淮海平原低洼盐碱地鱼塘中溶解氧与物质循环之间的关系进行了初步研究. 结果表明池塘水体中的溶解氧含量取决于水体中藻类的光合作用及其它生物的耗氧作用扰动的方式时间可显著改变水体中溶解氧的含量. 池塘水体中异养细菌的数量变动除与水体中的有机物质含量等有关外与水体中溶解氧的含量密切相关. 尤其在底泥中这种变化更加明显. 池塘水体中的氨化作用强度磷的转化强度明显地呈现出受水体中的溶解氧含量影响. 当水体中的溶解氧含量增加时导致细菌的现存量及活性的增加使得氨化作用强度磷的转化强度也显著增加这种情况在底泥中表现的更加明显. 池塘水体中反硝化作用强度反硫化作用强度的变化则与水体中溶解氧含量的变化相反. 当水体中溶解氧的含量减少时水体中的反硝化作用强度反硫化作用强度反而大大增加. 这种作用在底泥中表现得也非常明显.     

Estimation of Biodegradation Rates Using Respiration Tests During In Situ Bioremediation of Weathered Diesel NAPL

Respiration tests were carried out during a seven month bioremediation field trial to monitor biodegradation rates of weathered diesel non-aqueous phase liquid (NAPL) contaminating a shallow sand aquifer. Multiple depth monitoring of oxygen concentrations and air-filled porosity were carried out in nutrient amended and nonamended locations to assess the variability of degradation rate estimates calculated from respiration tests.
The field trial consisted of periodic addition of nutrients (nitrogen and phosphorus) and aeration of a 100 m² trial plot. During the bioremediation trial, aeration was stopped periodically, and decreases in gaseous oxygen concentrations were logged semi-continuously using data loggers attached to recently developed in situ oxygen probes placed at multiple depths above and within a thin NAPL-contaminated zone. Oxygen usage rate coefficients were determined by fitting zero-and first-order rate equations to the oxygen concentration reduction curves, although only zero-order rates were used to calculate biodegradation rates. Air-filled porosity estimates were found to vary by up to a factor of two between sites and at different times.
NAPL degradation rates calculated from measured air-filled porosity and oxygen usage rate coefficients ranged up to 69 mg kg^-1 day^-1. These rates are comparable to and higher than rates quoted in other studies, despite the high concentrations and weathered state of the NAPL at this test site. For nutrient-amended sites within the trial plot, estimates of NAPL degradation rates were two to three times higher than estimates from nonamended sites. Rates also increased with depth.     

Solute Transport Simulation of Aquifer Restoration after In Situ Uranium Mining

In situ mining of uranium was conducted in two well fields at a research and development site in the Powder River Basin of Wyoming from March 1980 to July 1981. Subsequent aquifer restoration activities continued until December 1982. During the restoration phases of operation, complex pumping-injection schemes, necessitated by the absence of adequate disposal facilities, were employed by the mining company in an attempt to satisfy the conditions imposed by the mine permit.
An analytical "Random Walk" solute transport model was used to simulate mass transport mechanisms associated with the pumping-injection schemes used in the A-1 well field. Because the hydrogeology at the site is relatively simple; a more sophisticated model requiring extensive calibration was not required. The model was not used as a "predictive" tool, but as an analytical supplement to existing water quality and hydrogeologic data. As such, it provided a very useful and inexpensive means by which to evaluate the adequacy of the restoration techniques employed at the site and the effectiveness of the water quality monitoring program.
Model simulations suggest that the restoration methods used by the mining company were not adequate to remove residual lixiviant and actually tended to displace contaminants to a circular region between the well field and peripheral monitoring wells. An analysis of the water quality and operational history at the site suggests that the model results may be accurate. If this is true, the postrestoration water quality data collected at the site may not be an accurate measure of the effectiveness of aquifer restoration.