In Situ Biorestoration as a Ground Water Remediation Technique

In situ biorestoration, where applicable, is indicated as a potentially very cost-effective and environmentally acceptable remediation technology. Many contaminants in solution in ground water as well as vapors in the unsaturated zone can be completely degraded or transformed into new compounds by naturally occurring indigenous microbial populations. Undoubtedly, thousands of contamination events are remediated naturally before the contamination reaches a point of detection. The need is for methodology to determine when natural biorestoration is occurring, the stage the restoration process is in, whether enhancement of the process is possible or desirable, and what will happen if natural processes are allowed to run their course.
In addition to the nature of the contaminant, several environmental factors are known to influence the capacity of indigenous microbial populations to degrade contaminants. These factors include dissolved oxygen, pH, temperature, oxidation-reduction potential, availability of mineral nutrients, salinity, soil moisture, the concentration of specific pollutants, and the nutritional quality of dissolved organic carbon in the ground water.
Most enhanced in situ bioreclamation techniques available today are variations of hydrocarbon degradation procedures pioneered and patented by Raymond and coworkers at Suntech during the period 1974 to 1978. Nutrients and oxygen are introduced through injection wells and circulated through the contaminated zone by pumping one or more producing wells.
The limiting factor in remediation technology is getting the contaminated subsurface material to the treatment unit or units, or in the case of in situ processes, getting the treatment process to the contaminated material. The key to successful remediation is a thorough understanding of the hydrogeologic and geochemical characteristics of the contaminated area.     

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

Vapor 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.     

Ground Water Remediation Using an Extraction, Treatment, and Recharge System

Ground water remediation of volatile organic compound (VOC) contamination at a site in Michigan was initiated as a result of a consent agreement between the Michigan Department of Natural Resources (MDNR) and the responsible party. Under the direction of the MDNR, the responsible party conducted a remedial investigation/feasibility study using federal guidelines to define the extent of contamination at the site and to select a response action for site remediation. The selected alternative included a combination of ground water extraction, treatment, and recharge, and soil flushing. The extraction system withdraws ground water from various depths in heavily contaminated areas. The ground water is treated using an air stripper. A spray distribution system spreads effluent from the stripper over a recharge basin constructed over the most contaminated areas. Additional contaminant removal is achieved by volatilization from the spray and percolation through the gravel bed. Recharge water moves downward through the contaminated soils, thus flushing residual soil contaminants. The initial operating data demonstrated that the system can effectively remove trichloroethylene (TCE) from ground water (approximately 95 percent overall removal efficiency). The annualized capital and operation and maintenance (O & M) costs of the remedial action were estimated for several operating periods (15, 20, and 30 years).     

Spodic Material for In Situ Treatment of Arsenic in Ground Water

The leaching of chromium-copper-arsenic salts from old wood preservation sites is a threat to ground water at many places in Sweden. The installation of in situ reactive barriers is an attractive "passive' technique to prevent the further spreading of contaminants. The use of peat as a reactive barrier material has been suggested for heavy metals, but this material was expected to be unsatisfactory for arsenic (As). Therefore, the feasibility of using spodic B horizon material for the retention of arsenic was tested in laboratory column experiments. Contaminated soil was taken from an old preservation site and leached under conditions designed to imitate the field conditions. The arsenic load during the three-month duration of the test corresponded to a load at the field site during three years. The B horizon material proved to be efficient for retention of arsenic, despite the observation that As(III) dominated the As speciation. The As(III) concentration was reduced from 1 to 3 mg dm⁻³ to < 0.02 mg dm⁻³. Pure peat was, as expected, not suited as a reactive barrier for As, and a mixed B horizon/peat reactive barrier also proved unsatisfactory for the removal of As. It is therefore important to separate the B horizon material from any peat that is used to sorb heavy metals. Before applying the B horizon reactive barrier technique in the field, the effect of the naturally occurring variability of the reactive compounds should be tested. The inclusion of oxidizing agents in the barrier could possibly improve the lifetime considerably. Furthermore, the influence of the flow rate should be evaluated since the kinetics of the arsenic adsorption is relatively slow.     

Field Test of the In Situ Permeable Ground Water Flow Sensor

Two in situ permeable flow sensors, recently developed at Sandia National Laboratories, were field tested at the Brazos River Hydrologic Field Site near College Station, Texas. The flow sensors use a thermal perturbation technique to quantify the magnitude and direction of ground water flow in three dimensions. Two aquifer pumping tests lasting eight and 13 days were used to field test the flow sensors. Components of ground water flow as determined from piezometer gradient measurements were compared with ground water flow components derived from the 3-D flow sensors. The changes in velocity magnitude and direction of ground water flow induced by the pump were evaluated using flow sensor data and piezometric analyses. Flow sensor performance closely matched piezometric analysis results. Ground water flow direction (azimuth), as measured by the flow sensors and derived in the piezometric analysis, predicted the position of the pumping well accurately. Ground water flow velocities measured by the flow sensors compared well to velocities derived in the piezometric analysis. A significant delay in flow sensor response to relatively rapid changes in ground water flow was observed. Preliminary tests indicate that the in situ permeable flow sensor provides accurate and timely information on the velocity magnitude and direction of ground water flow.     

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.     

Internet Listservices as a Valuable Resource for Environmental Professionals in Soil and Ground Water Remediation

In 1957, four computers across the United States were linked to form the first version of what we now know as the Internet. The Internet has grown beyond what anyone working in the U.S. Department of Defense Advanced Research Projects Agency (DARPA) more than 40 years ago could have imagined. Yet despite the phenomenal growth of Internet users, and access to information in all fields of endeavor, finding genuinely useful and helpful resources is still a challenge. This article distils information on the most useful Internet listservers for environmental professionals working the site remediation.     

Case History of a Large-Scale Air Sparging/Soil Vapor Extraction System for Remediation of Chlorinated Volatile Organic Compounds in Ground Water

A large-scale air sparging/soil vapor extraction (AS/SVE) project constructed within coastal plain sediments in New Jersey has demonstrated substantial progress toward remediating ground water through removal of volatile organic compounds (VOCs). Potential concerns identified prior to project implementation regarding hydraulic mounding, reduction in hydraulic conductivity, development of air channels, and the absence of hydraulic containment were assessed and addressed through testing and operational features incorporated into the project. At the project site, AS/SVE has successfully reduced the presence of many VOCs to undetectable levels, while reducing the concentrations of the remaining VOCs by factors of two to 500. The physical agitation caused by air sparging, and incomplete transformation from sorbed and nonaqueous phases to the vapor phase, appears to temporarily increase VOC concentrations and/or mobility of dense nonaqueous phase liquids (DN APLs) within source areas at the project site, but this is addressed in terms of subsequent removal of VOCs by properly placed downgradient treatment lines and VOCs by properly placed downgradient treatment lines and DNAPL recovery wells. This case study identifies and evaluates project-specific features and provides empirical data for potential comparison to other candidates AS/SVE sites.     

In Situ Electromigration as a Method for Removing Sulfate, Metals, and Other Contaminants from Ground Water

Electromigration is proposed as an in situ method for preconcentrating contaminants in ground water prior to pumping and treating. In earlier investigations by the senior author and co-workers, it was found that Cu in synthetic ground water migrated strongly to a Pt cathode and plated out as metallic copper. In the present study, carbon electrodes were inserted into a laboratory column of fine quartz sand that was saturated with a lower concentration of CuSO₄ solution. A fixed potential of 2.5 V was applied, causing dissolved Cu and SO₄ to accumulate strongly at the cathode and anode, respectively. Only minor plating-out of Cu took place on the carbon electrodes. In addition to the use of carbon electrodes, the present research also investigated the effects of a lower concentration of metal, accumulation of SO₄ adjacent to the anodes, adsorption of Cu on the sand, and competition by moving ground water.
At an imposed voltage of 2.5 V and in the presence of 65 mg/L of dissolved Cu and 96 mg/L of SO₄ (0.001 M CuSO₄ solution), electrolysis of water caused large changes in the pH and speciation of the aqueous components, as well as precipitation of solid Cu-hydroxides. Significant retardation of Cu occurred in the presence of ground water flowing at an average intergranular velocity of 0.2 m/day, but only minor retardation at water velocities of 1.9 and 2.9 m/day.
Sulfate tends to migrate strongly to the anodes, suggesting that in situ electromigration may offer a useful new method for preconcentrating such highly soluble ions as SO₄, NO₃, and CI that are difficult to remove by conventional pump-and-treat methods. A number of potential problems exist that should be addressed in a field test.     

Arrays of Unpumped Wells for Plume Migration Control by Semi-Passive In Situ Remediation

Arrays of unpumped wells can be used as discontinuous permeable walls in which each well serves both as a means to focus ground water flow into the well for treatment and as a container either for permeable reactive media which directly destroy dissolved ground water contaminants or for devices or materials which release amendments that support in situ degradation of contaminants within the aquifer downgradient of the wells. This paper addresses the use of wells for amendment delivery, recognizing the potential utility of amendments such as electron acceptors (e.g., oxygen nitrate), electron donors (primary substrates), and microbial nutrients for stimulating bioremediation, and the potential utility of oxidizers, reducers, etc., for controlled abiotic degradation. Depending on its rate and constraints, the remedial reaction may occur within the well and/or downgradient. For complete remediation of ground water passing through the well array, the total flux of amendment released must meet or exceed the total flux demand imposed by the plume. When there are constraints on the released concentration of amendment (relative to the demand), close spacing of the wells may be required. If the flux balance allows wider spacing, it is likely that limited downgradient spreading of the released amendment will then be the primary constraint on interwell spacing. Divergent flow from the wells, roughly two times the well diameter, provides the bulk of downgradient spreading and constrains maximum well spacing in the absence of significant lateral dispersion. Stronger lateral dispersion enhances the spreading of amendment, thereby increasing the lateral impact of each well, which allows for wider well spacing.