Groundwater Remediation Technologies Research Articles

Natural attenuation of chlorinated solvents at Area 6, Dover Air Force Base: groundwater biogeochemistry

Monitored natural attenuation (MNA) has recently emerged as a viable groundwater remediation technology in the United States. Area 6 at Dover Air Force Base (Dover, DE) was chosen as a test site to examine the potential for MNA of tetrachloroethene (PCE) and trichloroethene (TCE) in groundwater and aquifer sediments. A “lines of evidence” approach was used to document the occurrence of natural attenuation. Chlorinated hydrocarbon and biogeochemical data were used to develop a site-specific conceptual model where both anaerobic and aerobic biological processes are responsible for the destruction of PCE, TCE, and daughter metabolites. An examination of groundwater biogeochemical data showed a region of depleted dissolved oxygen with elevated dissolved methane and hydrogen concentrations. Reductive dechlorination likely dominated in the anaerobic portion of the aquifer where PCE and TCE levels were observed to decrease with a simultaneous increase in cis-1,2-dichloroethene ( cis-DCE), vinyl chloride (VC), ethene, and dissolved chloride. Near the anaerobic/aerobic interface, concentrations of cis-DCE and VC decreased to below detection limits, presumably due to aerobic biotransformation processes. Therefore, the contaminant and daughter product plumes present at the site appear to have been naturally attenuated by a combination of active anaerobic and aerobic biotransformation processes.

Transfer and commercialisation of contaminated groundwater remediation technologies

High costs and poor performance of conventional groundwater remediation technologies have brought a call for the deployment of innovative technologies capable of attaining regulatory standards while satisfying time and budget constraints. To develop an innovative technology in the laboratory and ultimately transition it to full-scale commercialisation, presents challenges at various levels. Scientific and engineering problems and regulatory and legal issues exist that must be dealt with when moving a technology from the laboratory to the field. Importantly, cost and performance data must be presented in a manner that convinces stakeholders that the technology can accomplish remediation more economically, safely and efficiently than conventional technologies. The challenges of transferring and commercialising innovative groundwater remediation technologies and strategies that may be used to help overcome these challenges are discussed. Case studies of groundwater remediation technology transfer are presented.

Kinetics and products of reactions of MTBE with ozone and ozone/hydrogen peroxide in water

Methyl- t-butyl-ether (MTBE) has become a prevalent groundwater pollutant due to its high volume use as a nationwide gasoline additive. Given its physicochemical properties, it requires new treatment approaches. Both aqueous O 3 and a combination of O 3/H 2O 2, which gives OH, can remove MTBE from water, making use of O 3 a viable technology for remediation of groundwater from fuel contaminated sites. Rate constants and temperature dependencies for reactions of MTBE with O 3 or with OH at pH 7.2, in a range of 21–45°C (294–318 K) were measured. The second-order rate constant for reaction of MTBE with O 3 is 1.4×10 18 exp(−95.4/ RT) (M −1 s −1), and for reaction of MTBE with OH produced by the combination of O 3/H 2O 2 is 8.0×10 9 exp(−4.6/ RT) (M −1 s −1), with the activation energy (kJ mol −1) in both cases. At 25°C, this corresponds to a rate constant of 27 M −1 s −1 for ozone alone, and 1.2×10 9 M −1 s −1 for O 3/H 2O 2. The concentration of OH was determined using benzene trapping. Products of reactions of O 3 and O 3/H 2O 2 with MTBE, including t-butyl-formate (TBF), t-butyl alcohol (TBA), methyl acetate, and acetone, were determined after oxidant depletion. A reaction pathway for mineralization of MTBE was also explored. Under continuously stirred flow reactor (CSTR) conditions, addition of H 2O 2 markedly increases the rate and degree of degradation of MTBE by O 3.

Humic acid enhanced remediation of an emplaced diesel source in groundwater.: 1. Laboratory-based pilot scale test

The enhanced solubility of petroleum-derived compounds in humic acid solutions is the basis for a new groundwater remediation technology. In this unique pilot-scale test, a stationary contaminant source consisting of diesel fuel was placed below the water table in a model sand aquifer (1.2×5.5×1.8-m deep) and flushed with water at a flow rate of 2 cm/h over 5 years. At 51 days, laboratory grade humic acid was added to the water and maintained at a level of approximately 0.8 g/l. The addition of humic acid had only a small impact on the aqueous transport of the BTEX components, which were rapidly dissolved from the diesel, but had a large effect on the flushing of PAHs, including methylated naphthalenes (MNs). Binding to aqueous humic acid enhanced the solubilization of MNs two- to tenfold. During aqueous transport, biodegradation of the BTEX and PAHs occurred, limiting the lateral and longitudinal extent of the diesel contaminant plume in the model aquifer. It appears that through enhanced solubilization, the overall biodegradation rate of the MNs was increased. As the various MNs were depleted from the diesel source, the MN plume shrank and then disappeared.

Funnel‐and‐Gate Performance in a Moderately Heterogeneous Flow Domain

AbstractThe funnel‐and‐gate ground water remediation technology (Starr and Cherry 1994) has received increased attention and application as an in situ alternative to the typical pump‐and‐treat system. Understanding the effects of heterogeneity on system performance can mean the difference between a successful remediation project and one that fails to meet its cleanup goals.In an attempt to characterize and quantify the effects of heterogeneity on funnel‐and‐gate system performance, a numerical modeling study of 15 simulated heterogeneous flow domains was conducted. Each realization was tested to determine if the predicted capture width met the capture width expected for a homogeneous flow domain with the same hulk properties. This study revealed that the capture width of the funnel‐and‐gate system varied significantly with the level of heterogeneity of the aquifer.Two possible remedies were investigated for bringing systems with less than acceptable capture widths to acceptable levels of performance. First, it was determined that enlarging the funnel and gate via a factor of safety applied to the design capture width could compensate for the capture width variation in the heterogeneous flow domains. In addition, it was shown that the use of a pumping well downstream of the funnel and gate could compensate for the effects of aquifer heterogeneity on the funnel‐and‐gate capture width. However, if a pumping well is placed downstream of the funnel and gate to control the hydraulic gradient through the gate, consideration should be given to the gate residence time in relation to the geochemistry of the contaminant removal or destruction process in the gate.

Remediation technologies for metal-contaminated soils and groundwater: an evaluation

Metals including lead, chromium, arsenic, zinc, cadmium, copper and mercury can cause significant damage to the environment and human health as a result of their mobilities and solubilities. The selection of the most appropriate soil and sediment remediation method depends on the site characteristics, concentration, types of pollutants to be removed, and the end use of the contaminated medium. The approaches include isolation, immobilization, toxicity reduction, physical separation and extraction. Many of these technologies have been used full-scale. This paper will review both the full-scale and developing technologies that are available. Contaminants can be isolated and contained to minimize further movement, to reduce the permeability of the waste to less than 1×10 −7 m/s (according to U.S. guidelines) and to increase the strength or bearing capacity of the waste. Physical barriers made of steel, cement, bentonite and grout walls can be used for isolation and minimization of metal mobility. Another method is solidification /stabilization, which contains the contaminants in an area by mixing or injecting agents. Solidification encapsulates contaminants in a solid matrix while stabilization involves formation of chemical bonds to reduce contaminant mobility. Another approach is size selection processes for removal of the larger, cleaner particles from the smaller more polluted ones. To accomplish this, several processes are used. They include: hydrocyclones, fluidized bed separation and flotation. Addition of special chemicals and aeration in the latter case causes these contaminated particles to float. Electrokinetic processes involve passing a low intensity electric current between a cathode and an anode imbedded in the contaminated soil. Ions and small charged particles, in addition to water, are transported between the electrodes. This technology have been demonstrated in the U.S. full-scale, in a limited manner but in Europe, it is used for copper, zinc, lead, arsenic, cadmium, chromium and nickel. The duration of time that the electrode remains in the soil, and spacing is site-specific. Techniques for the extraction of metals by biological means have been not extensively applied up to this point. The main methods include bioleaching and phytoremediation. Bioleaching involves Thiobacillus sp. bacteria which can reduce sulphur compounds under aerobic and acidic conditions (pH 4) at temperatures between 15 and 55°C. Plants such as Thlaspi, Urtica, Chenopodium, Polygonum sachalase and Alyssim have the capability to accumulate cadmium, copper, lead, nickel and zinc and can therefore be considered as an indirect method of treating contaminated soils. This method is limited to shallow depths of contamination. Soil washing and in situ flushing involve the addition of water with or without additives including organic and inorganic acids, sodium hydroxide which can dissolve organic soil matter, water soluble solvents such as methanol, nontoxic cations, complexing agents such as ethylenediaminetetraacetic acid (EDTA), acids in combination with complexation agents or oxidizing/reducing agents. Our research has indicated that biosurfactants, biologically produced surfactants, may also be promising agents for enhancing removal of metals from contaminated soils and sediments. In summary, the main techniques that have been used for metal removal are solidification/stabilization, electrokinetics, and in situ extraction. Site characteristics are of paramount importance in choosing the most appropriate remediation method. Phytoremediation and bioleaching can also be used but are not as well developed.

Commercial activities in remediation of soil and sediments and trends in the asia pacific region

Countries in the Asia Pacific region have not been widely recognised for leading in the development of technologies for soil, sediment, and groundwater remediation. Rather, countries in the region have relied on adopting remediation technologies and approaches developed in the Northern Hemisphere, and this approach has largely been successful. Trends in research and practise in the field of soil, sediment and groundwater remediation, indicate that there is relatively less reporting of international co-operation and development of remediation technologies in the Asia Pacific region compared with the Northern Hemisphere. The major barriers to adoption of such technologies have been the requirement for low implementation costs, and those regulatory agencies in the region have taken a non-prescriptive approach to enforcing contaminated site assessment and remediation. Unlike programs in the Northern Hemisphere (e.g. the Superfund Program in the US) where there has been special funding for the development of innovative technologies, there has been relatively little financing available for similar activities in Asia Pacific countries. Two case studies from Australia are presented on the commercial adaptation and application of (1) permeable barrier technology (for contaminated soil and groundwater) and (2) bioflushing of marine sediments contaminated with petroleum hydrocarbons.

Cyclodextrin‐Enhanced Vertical Flushing of a Trichloroethene Contaminated Aquifer

AbstractPilot‐scale testing of an innovative ground water remediation technology was conducted in a source zone of a trichloroethene‐contaminated Superfund site in Tucson, Arizona. The technology is designed to enhance the removal of low‐solubility organic contaminants from heterogeneous sedimentary aquifers by using a dual‐screened vertical circulation well to inject and extract solutions containing a complexing sugar (hydroxypropyl‐beta‐cyclodextrin (HPCD]). Prior to initiating the pilot test, tracer tests were conducted to determine hydraulic characteristics of the vertical flow field and to evaluate trichloroethene‐elution behavior during water flushing. The pilot test involved injecting approximately 4 m3 of a 20% HPCD solution into the upper screened interval of the well and extracting from the lower screened interval. The results of the pilot test indicate that the cyclodextrin solution increased the rate of trichloroethene removal from the aquifer. The concentrations of trichloroethene in the ground water extracted from the lower screened interval of the well increased by a factor of three (∼750 μg/L) in the presence of the cyclodextrin pulse, compared to concentrations obtained during previous water flushing (∼250 μg/L). Furthermore, the concentration of trichloroethene in water collected from the circulation well under static conditions was reduced to 6% of the levels measured prior to the test.

Instrumentation Design and Installation for Monitoring Air Injection Ground Water Remediation Technologies

An in situ instrumentation bundle was designed for inclusion in monitoring wells that were installed at the Wasatch Trailer Sales site in Layton, Utah, to evaluate in situ air sparging (IAS) and in-well aeration (IWA). Sensors for the bundle were selected based on laboratory evaluation of accuracy and precision, as well as consideration of size and cost. SenSym pressure transducers, Campbell Scientific Inc. (CSI) T-type thermocouples, and dissolved oxygen (DO) probes manufactured by Technalithics Inc. (Waco, Texas), were selected for each of the 27 saturated zone bundles. Each saturated zone bundle also included a stirring blade to mix water near the DO probe. A Figaro oxygen sensor was included in the vadose zone bundle. The monitoring wells were installed by direct push technique to minimize soil disruption and to ensure intimate contact between the 18 inch (46 cm) long screens and the soil. A data acquisition system, comprised of a CSI 21X data logger and four CSI AM416 multiplexers, was used to control the stirring blades and record signals from more than 70 in situ sensors. The instrumentation performed well during evaluation of IAS and IWA at the site. However, the SenSym pressure transducers were not adequately temperature compensated and will need to be replaced.

Removal of Perchlorate in Ground Water with a Flow‐Through Bioreactor

AbstractThe bacterium, perc1ace, has been shown to reduce perchlorate to less than the detection limit of 0.004 mg L−1 when grown on acetate under anaerobic conditions. In batch studies, the presence of nitrate does not significantly hinder the reduction of perchlorate. The ability of perclace to remove nitrate and perchlorate from ground water in a flow‐through system is described in this study. Celite‐packed columns of 300 ml were used to demonstrate the removal of perchlorate from ground water. At a flow rate of 1 ml min−1, perchlorate was removed from 0.738 mg L−1 to less than detectable levels and when the flow rate was 2 ml min−1 92 to 95% of the perchlorate was removed. Analysis of bacterial biomass at the completion of the study revealed that most of the bacterial growth was concentrated in the inlet area of the column. A circulating pump was added, with the idea that passing the ground water multiple times through the bacterially active zone might increase the efficiency of the column. In this experiment, perchlorate in ground water was reduced from 0.550 mg L−1 to nondetectable levels at a flow rate of 1 ml min−1. When the flow rate was increased to 2 ml min−1, 98% of perchlorate was removed and when the flow rate was 3 ml min −1 95% of perchlorate was removed. Rapid removal of perchlorate by perc1ace immobilized in a bioreactor may provide an efficient, cost‐effective technology for ground water remediation.

Evaluation of Polyalkastyrene Absorbent Beads for the Remediation of PCE in Groundwater

Abstract Passive groundwater remediation technologies are increasingly considered because they require very little long-term maintenance and are thus more cost effective than active systems such as pump-and-treat, which require the continuous upkeep of a groundwater pumping system and of an above-ground water treatment plant. Barriers are being installed in the ground to either prevent the flow of contaminated water from one property to another, or to remove the contaminants from the water as it passes through a porous reactive barrier. Absorbents which irreversibly incorporate spilled solvents into their structure have been developed to clean surface spills. This project was aimed at evaluating the performance of the absorbents with dissolved organic contaminants. It was found that while they did sorb aqueous phase compounds, it was through adsorption, not absorption and that it was therefore reversible. As adsorbents, the polymers marketed as Imbiber Beads® and Expandabeads® were not as effective as activated carbon.

Influence of chlorinated solvents on polarization and corrosion behaviour of iron in borate buffer

A new remediation technology for groundwater contaminated with chlorinated solvents is through contact with granular iron. To refine our understanding of the reaction mechanism, electrochemical and spectroscopic measurements were performed on iron electrodes in deaerated borate buffer containing an amount of a degradable (carbon tetrachloride) or non-degradable (dichloromethane) compound. The results of polarization measurements indicated that carbon tetrachloride acts as an oxidizer towards iron while dichloromethane is nonreactive. Magnetite and hydrated magnetite, identified by Raman spectroscopy, are the final products of the surface redox reactions. Based on electrochemical and spectral evidence, a new conceptual model for the reductive reactions is proposed.

Miscible fluid displacement stability in unconfined porous media:: Two-dimensional flow experiments and simulations

In situ flushing groundwater remediation technologies, such as cosolvent flushing, rely on the stability of the interface between the resident and displacing fluids for efficient removal of contaminants. Contrasts in density and viscosity between the resident and displacing fluids can adversely affect the stability of the displacement front. Petroleum engineers have developed techniques to describe these types of processes; however, their findings do not necessarily translate directly to aquifer remediation. The purpose of this laboratory study was to investigate how density and viscosity contrasts affected cosolvent displacements in unconfined porous media characterized by the presence of a capillary fringe. Two-dimensional flow laboratory experiments, which were partially scaled to a cosolvent flushing field experiment, were conducted to determine potential implications of flow instabilities in homogeneous sand packs. Numerical simulations were also conducted to investigate the differential impact of fluid property contrasts in unconfined and confined systems. The results from these experiments and simulations indicated that the presence of a capillary fringe was an important factor in the displacement efficiency. Buoyant forces can act to carry a lighter-than-water cosolvent preferentially into the capillary fringe during displacement of the resident groundwater. During subsequent water flooding, buoyancy forces can act to effectively trap the cosolvent in the capillary fringe, contributing to the inefficient removal of cosolvent from the aquifer.

Long‐Term Performance of an In Situ “Iron Wall” for Remediation of VOCs

AbstractThe use of granular iron for in situ degradation of dissolved chlorinated organic compounds is rapidly gaining acceptance as a cost‐effective technology for ground water remediation. This paper describes the first field demonstration of the technology, and is of particular importance since it provides the longest available record of performance (five years). A mixture of 22% granular iron and 78% sand was installed as a permeable “wall” across the path of a contaminant plume at Canadian Forces Base, Borden, Ontario. The major contaminants were trichloroethene (TCE, 268 mg/L) and tetrachloroethene (PCE, 58 mg/L). Approximately 90% of the TCE and 86% of the PCE were removed by reductive dechlorination within the wall, with no measurable decrease in performance over the five year duration of the test. Though about 1% of the influent TCE and PCE appeared as dichloroethene isomers as a consequence of the dechlorination of TCE and PCE, these also degraded within the iron‐sand mixture. Performance of the field installation was reasonably consistent with the results of laboratory column studies conducted to simulate the field behavior. However, if a more reactive iron material, or a higher percentage of iron had been used, complete removal of the chlorinated compounds might have been achieved. Changes in water chemistry indicated that calcium carbonate was precipitating within the reactive material; however, the trace amount of precipitate detected in core samples collected four years after installation of the wall suggest that the observed performance should persist for at least another five years. The study provides strong evidence that in situ use of granular iron could provide a long‐term, low‐maintenance cost solution for many ground water contamination problems.

Peer reviewed: hard times for innovative cleanup technology.

What can be done to remove market barriers to new groundwater and soil remediation technologies?

Enhanced Solubilization and Destruction of Tetrachloroethylene by Hydroxypropyl-β-cyclodextrin and Iron

Hydroxypropyl-β-cyclodextrin (HP-β-CD) enhances the solubility of tetrachloroethylene (PCE) in water both in static and in flowing systems. HP-β-CD does not decrease the interfacial tension between PCE and water and, therefore, should not mobilize immiscible-phase PCE in the subsurface. Rates for the reaction of PCE with metallic iron were measured in HP-β-CD solutions under static conditions. In flowing systems, metallic iron removed PCE, and no other chlorinated ethylene species were observed in the column effluent. In several such systems, recycling of the HP-β-CD solution took place following the reaction with iron. No downward mobilization of the PCE pool in the generator column was observed. The solubility enhancement and reaction with metallic iron are consistent with reversible formation of a stoichiometric HP-β-CD/PCE complex. A theoretical treatment of the reaction rates of complexed PCE on an iron surface was developed. This treatment suggests that any material that enhances the solubility of low-solubility organic substances may slow down the rate of reaction in aqueous solution. In many cases, the rate retardation should equal the degree of solubility enhancement. The combination of HP-β-CD and iron metal appears to be a promising groundwater remediation technology.

Hazardous waste storage, disposal, remediation, and closure

Hazardous waste storage, disposal, remediation, and closure

Applying Membrane Technology to Air Stripping Effluent for Remediation of Groundwater Contaminated with Volatile Organic Compounds

Remediation groundwater contaminated by volatile organic compounds (VOCs) requires cost- and technically-effective solutions. This paper reviews the options for VOC removal from remediation air streams, focusing on membrane separation. The basic separation science and technology, results of performance tests, and results of cost studies for membrane separation are presented. Competing technologies are discussed and compared with membrane separation. Membrane separation combined with air stripping will provide an economically and environmentally safe technology for remediation of VOC-contaminated groundwater and, as it matures, may become the preferred method.

The Application of In Situ Air Sparging as an Innovative Soils and Ground Water Remediation Technology

AbstractVapor extraction (soil venting) has been demonstrated to be a successful and cost‐effective remediation technology for removing VOCs from the vadose (unsaturated) zone. However, in many cases, seasonal water table fluctuations, drawdown associated with pump‐and‐treat remediation techniques, and spills involving dense, non‐aqueous phase liquids (DNAPLS) create contaminated soil below the water table. Vapor extraction alone is not considered to be an optimal remediation technology to address this type of contamination.An innovative approach to saturated zone remediation is the use of sparging (injection) wells to inject a hydrocarbon‐free gaseous medium (typically air) into the saturated zone below the areas of contamination. The contaminants dissolved in the ground water and sorbed onto soil particles partition into the advective air phase, effectively simulating an in situ air‐stripping system. The stripped contaminants are transported in the gas phase to the vadose zone, within the radius of influence of a vapor extraction and vapor treatment system.In situ air sparging is a complex multifluid phase process, which has been applied successfully in Europe since the mid‐1980s. To date, site‐specific pilot tests have been used to design air‐sparging systems. Research is currently underway to develop better engineering design methodologies for the process. Major design parameters to be considered include contaminant type, gas injection pressures and flow rates, site geology, bubble size, injection interval (areal and vertical) and the equipment specifications. Correct design and operation of this technology has been demonstrated to achieve ground water cleanup of VOC contamination to low part‐per‐billion levels.