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Groundwater Remediation Techniques

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Christina's Assignment Previous Groundwater Policy and Management Next Case Study: Aberdeen Proving Ground

Groundwater Remediation Techniques

Polluted groundwater can potentially spread over a large area and interact with groundwater wells or make its way back to surface water, possibly infecting human water sources and sensitive environments. Because of the threat this poses for humans and sensitive environments, prevention and remediation is important and can be accomplished by several remedy approaches. For example, the contamination can be contained to prevent further migration from the source, the contamination can be removed from the groundwater and treated, or the contamination can be treated or destroyed in situ [8]. The different remedy types are listed in Figure 6.

Figure 6: Remediation techniques taken from EPA [20]

The remedy types can be accomplished by remediation techniques that rely on physical, chemical, and biological methods to treat groundwater. For a particular site, several approaches and techniques are compared based on the process, applicability, advantages, limitations and concerns, site-specific parameters, and costs[26]. The technique or techniques that are selected are based on the contaminant and site characteristics, regulatory requirements, costs, and time constraints [26, 27]. Below are brief descriptions of containment, pump and treat, and several in situ techniques and a more detailed explanation of natural attenuation.

A. Containment

Polluted groundwater can be contained by physical barriers, which prevent flow of water and/or contaminants into or out of an enclosed region[13]. There are two categories of barriers: vertical and horizontal. Vertical barriers block horizontal flow and are usually in the form of a wall [13]. These include compacted clay walls, slurry trench cutoff walls, grout curtains, mixed-in-place seepage barriers, vibrating beam walls, sheet piling, ground freezing, and hydraulic barriers. Horizontal barriers prevent vertical flow and consist of covers, liners, and bottom seals [13]. The type of barrier that is used depends on the nature of the contamination and the site conditions. Barriers can be used to control the source of groundwater pollution as well as the groundwater plume [13].

B. Pump and Treat

Pump and treat has been one of the most commonly used methods for groundwater remediation. For this method, extraction wells are placed at various locations in a contaminated aquifer and the contamination is removed with pumped water [20, 26]. The pumped water is then treated and either placed back into the aquifer or placed into a surface body of water. This method can used for all types of dissolved contaminants and can prevent the contaminants from spreading and remove the contaminant mass but it can require a significant amount of time (5-50 years) and money and lacks effectiveness for contaminants that adsorb to soil and fractured rock or clay soils [20, 26]. Adding surfactants to the groundwater can facilitate the groundwater pumping process by increasing the mobility and solubility of the contaminants sorbed to the soil [26].

C. In situ Techniques

Air sparging is an in situ technique that can be used for groundwater remediation of volatile and semi-volatile groundwater contaminants. Air is injected into the saturated zone to volatize the contaminants dissolved in the groundwater and sorbed to the soil and promote biodegradation by increasing oxygen concentrations. The volatilized contaminants can flow into the vadose zone where they are either biodegraded or removed by a soil vapor extraction [26]. This method allows for contaminant removal by several different in situ mechanisms but is only applicable to contaminants that can be volatilized and biodegraded and requires homogenous and relatively permeable soil conditions [26, 28].

Biosparging is another in situ remediation technique similar to air sparging but involves injecting both air and nutrients into the saturated zone. This method relies on degradation of the contaminants by organisms indigenous to the site [26, 29]. It is mostly used for petroleum contamination and is often used in conjunction with soil vapor extraction systems. The treatment times are relatively short (6 months to 2 years) but is only applicable for areas that have homogenous permeable soil, unconfined aquifers, and no free product present [26].

Bioslurping is a newer treatment technology that combines the methods of bioventing, providing oxygen to enhance biodegradation, and vacuum-enhanced pumping to recover free product [26]. This method allows for the removal of LNAPLs as well as vapors from the vadose zone. This method can be cost-effective but may not be applicable in areas that have low permeability [26].

In situ treatment walls, ultraviolet (UV)-oxidation technologies, and groundwater circulation are other classes of emerging treatment methods for groundwater pollution. UV-oxidation methods rely on the use of an oxygen-based oxidation, ozone or hydrogen peroxide, along with UV light to oxidize the contaminated groundwater [26]. These methods are applicable to several different contaminants including ones for which other methods fail but may require additional and ongoing treatment. Also, problems might occur with using the oxidizers [26].

In situ treatment walls rely on the natural movement of water to move the contaminant through a treatment wall, where the contaminants are either trapped or transformed into harmless products that can flow out of the wall [26]. Treatment walls work best for volatile organic compounds, semi-volatile organic compounds, and inorganic compounds. The type of contaminant determines the material of the wall, which influences the chemical process for treatment. Sorption, precipitation, and degradation are three of the most common chemical processes [26]. Different types of walls can be built depending on the plume characteristics. Permeable reactive trenches and funnel and gate systems are two of the main type of walls that are used [26]. Treatment walls can only be used in aquifers that have depths within reach of the trenching equipment and often require the reactive medium to be replaced as it can lose its reactive capacity over time [26].

Groundwater circulation wells continuously remove contaminants from groundwater by circulation that causes vapor stripping. A circulation pattern is created by drawing water into and pumping it through a well and then reintroducing it without reaching the surface [26]. This method is not applicable for NAPLs but can be used for volatile organic compounds, semi volatile organic compounds, fuels, as well as pesticides and inorganic compounds [26]. The circulation wells can be used in most soil types but are ineffective in areas with strong natural flow, low conductivity, and shallow aquifers [26].

Horizontal wells were originally developed for the petroleum industry and to install underground utilities but have recently been applied to groundwater remediation [26, 30]. Horizontal wells are designed for hydraulic contaminant and/or mass removal [30]. The wells are often used in conjunction with in situ techniques such as bioremediation, air sparging, and vacuum extraction and can be installed underneath surface structures, such as buildings that are inaccessible to vertical wells [26]. This technology is only applicable to certain depths but is cost-effective as it requires fewer wells and allows for contaminant removal then vertical wells [26, 30].

D. Natural Attenuation

Natural attenuation, also called monitored natural attentuaion, is a remediation technique that relies on natural processes to contain the spread of contamination and reduce the concentration and amount of pollutants in groundwater [31]. This approach does not use engineered methods but focuses on monitoring and verification. Natural attenuation can be used for a variety of contaminants including petroleum hydrocarbons, nonchlorinated solvents, chlorinated aliphatic and aromatic compounds, and wood treating wastes [26, 31]. Although it can be used for a large number of pollutants and it is a cost-effective remediation technique that is environmentally friendly, its application requires extensive analysis of the site and potential natural attenuation processes [31].

The idea of natural attenuation became popular in the 1990s when the high costs of engineered systems were matched by unsuccessful attempts to restore contaminated aquifers to background or drinking water standards [32]. This prompted research into the natural capabilities contaminated subsurface environments to degrade or transform the contaminants without human intervention. In 2000, National Research Council (NRC) [10] reported that natural processes was used alone, without engineered steps to enhance them, at more than 15,000 sites where fuels from underground storage tanks leaked into groundwater [10]. More recently it has been used and proposed for other contaminants, such as chlorinated solvents, nitroaromatics, heavy metals, and radionuclides [10].

Natural attenuation can be a cost-effective, environmentally-friendly alternative to the engineered remediation methods. However, there is some controversy and skepticism with its use, especially at large sites where the public is actively involved [10]. Community members who are directly affected by contaminated sites view natural attenuation as a “do-nothing” approach and feel that the responsible parties are free from the financial burden of site remediation without adequately protecting public health and the environment [10, 33]. This controversy is compounded by discrepancies in the definitions coming from the different governing agencies and scientific institutions.

Natural attenuation has been given a range of definitions by different groups (see Figure 7). The details of the individual definitions are based on the objectives they can achieve [33]. The objectives of earlier definitions focused on the fate of the contamination, mostly reductions in time and space. Later definitions evolved to have objectives that distinguished this from protecting humans and sensitive environments [33]. In a broad sense all of the definitions highlight an endpoint to the objectives and the processes by which the objectives are achieved.

Figure 7: Definitions of Natural Attenuation by different agencies taken from Rittmann [33].

The processes that make up natural attenuation can be separated into destructive and nondestructive [34]. Destructive processes reduce the contaminant mass by transformation to less harmful forms [26, 34]. Biodegradation, radioactive decay, and abiotic reactions are examples of destructive processes. Nondestructive processes reduce contaminant concentrations or mobility [26]. These processes include dilution, dispersion, adsorption, plant uptake, and volatilization [26, 34].

Biodegradation is an important mechanism for natural attenuation. In the process naturally occurring microorganisms break down or degrade contaminants into less toxic or non-toxic substances [26, 35], supported by the available electron donors, electron acceptors, and nutrients [31]. There are several processes that the microorganisms use to break down the contaminants including: fermentation, aerobic respiration, and anaerobic respiration [36]. During fermentation, organic contaminants act as both the electron donor and acceptor and are broken down by a series of enzyme-mediated reactions [26]. During aerobic respiration, the contaminant is broken down by a series of enzyme-mediated reactions, in which oxygen serves as an external electron acceptor. This can happen metabolically or cometabolically. Metabolically the contaminant serves as the growth substrate for the microorganisms. Cometabolically, the contaminant is degraded by a nonspecific enzyme without benefiting the microorganisms provided that a separate growth substrate is present to create the enzyme. In anaerobic respiration, the contaminant is broken down by a series of enzyme-mediated reactions in which nitrate, ferrous iron, sulfate, carbon dioxide, and other oxidized compounds serve as electron acceptors in the absence of oxygen [26]. For some contaminants, such as chlorinated aliphatic hydrocarbons, the contaminants can serve as the electron acceptor and allow for degradation to occur supported by the availability of electron donors, such as hydrogen.

There are several factors that affect biodegradation of contaminated groundwater. Farhadian et al. [35]discussed several of the factors that have been widely described in the literature, which deal with the pollutant, groundwater, and the site. For the pollutant, the source, concentration, and toxicity need to be considered as well as the chemistry (e.g. solubility, transport, adsorption, dispersion and volatility) and the biodegradability. For the groundwater, the chemistry, physics and microbiology (e.g. presence of a competent biodegrading population of microorganisms) need to be considered. The chemistry of the site (e.g. nutrient sources and presence of electron donors and acceptors) also needs to be considered along with the soil mechanics.

The NRC [10]states that for natural attenuation to be adequate the natural processes must protect humans and the environment from harmful exposures. Destruction (mainly biodegradation and radioactive decay) and very strong immobilization (mainly precipitation and strong surface complexation) have been accepted and proven as doing this [10, 33]. Destruction transforms the contaminant to a harmless product that offers no risk even if comes into contact with humans or the environment [33]. Very strong immobilization keeps the contaminant from moving away for the source zone and also prevents contact with humans or the environment [33].

Utilizing natural attenuation as a remediation technique requires evidence that natural processes at the site are immobilizing or destroying the contamination to an extent that is sufficient to protect public health and the environment [10]. Obtaining this evidence requires monitoring and evaluation; linking measurements from the site to a site model and “footprints” of the underlying mechanisms [10]. Footprints are changes in concentrations of reactants or products of the biogeochemical processes that transform or immobilize the contaminants [10]. The footprints occur because the biochemical process consumes or produces other materials (such as oxygen, inorganic carbon, and chloride) in established stoichiometric ratios to the loss of the contaminant [33]. The processes responsible for natural attenuation can be established and documented by observing contaminant loss coupled to one or more of these footprints. Examples are shown in Figure 8.

Figure 8: Examples of footprints for groundwater contamination taken from Rittmann [33].

NRC [10] identified the three basic steps to document natural attenuation: develop a site conceptual model, analyze site measurements, and monitor the site [10, 33]. The site conceptual model should identify important features of the flow, transport, and reaction processes occurring at the site. Important features include where the contaminants are located and at what concentrations, and which types of natural processes could theoretically affect the contaminants[10, 33].

The second step is sampling and chemically analyzing the groundwater. The groundwater should be analyzed for footprints of the natural attenuation processes. Quantitatively coupling the formation of footprints to the loss of contaminants provides evidence that a natural attenuation process is responsible for reductions in contaminant concentration [33]. Loss of contaminant without the corresponding footprints suggests that the lower contaminant concentration may be an artifact. Artifacts can include missing the plume, transfer of the contaminant to another location or phase, or losses during sampling [33].

The third step for natural attenuation documentation is site monitoring. Long-term monitoring should occur to ensure that the regulatory requirements are achieved and documented attenuation processes continue to occur [10] Long-term monitoring is important because many source areas are poorly defined and release the contamination at rates and over a time period that cannot be predicted [33]. Also, it is important to ensure that the conditions which support the biogeochemical process allowing transformation or immobilization of the contaminants are maintained.

The success of natural attenuation can be affected by several factors [10, 32]. One is the contaminant and the site. Natural attenuation processes are contaminant and site specific such that the types of settings that provide the most favorable conditions for natural attenuation depend on the type of contaminant. This causes issues for poorly understood chemical classes and subsurfaces that are heterogeneous. Another factor is changing environmental conditions with time, which can influence the processes that originally controlled the remediation. This causes problems when trying to determine long-term sustainability. A third factor is with mixtures of contaminants, which can act differently than when present as individuals. This can cause problems when trying to determine the different responses to the natural processes. Finally, some natural processes can cause contaminant transformations into products that are more toxic or more mobile than the parent compound, which makes the contamination worse than better.

There are numerous examples of natural attenuation being used as a remediation practice. Natural attenuation has been particularly useful for sites contaminated with fuels. An example is in Bemidji, Minnesota where a buried oil pipeline ruptured in 1979 and spilled an estimated 3,200 barrels of crude oil [32]. The oil infiltrated the subsurface and served as a long-term, continuous source of hydrocarbon contaminants that dissolved in and was transported with the groundwater.

Aerobic and anaerobic biodegradation were found as the most important natural attenuation processes affecting the hydrocarbon plume. The biodegradation reactions caused a number of footprints near the aqueous plume, including decreased oxygen and hydrocarbon concentrations and increased dissolved iron, manganese, and methane concentrations [32]. This indicated and was simulated to show that aerobic, manganese- and iron-reducing, and methanogenic microorganisms degraded the dissolved hydrocarbons and the degradation followed the temporal evolution of the redox conditions in the aquifer. Aerobic degradation took place first. Once oxygen was consumed, an anoxic zone developed and manganese reduction and iron reduction and methane production took place, releasing dissolved phase manganese and iron, and methane. Simulation predicted that 40% of the hydrocarbon degradation occurred aerobically and 60% occurred anaerobically (Mn reduction: 5%, Fe reduction: 19%, methanogenesis: 36%) [32].

Usage and Trends of Groundwater Remediation Techniques

Pump and treat has been the most commonly used remediation technique in the past as seen in Figure 9. The EPA [20] reported that nearly 90% of record of decisions (RODs) used Pump and treat from 1987 to 1992. This decreased to 30% in 1998 and in 1998 and averaged at approximately 35% in 2005. Natural attenuation was used in less than 10 % of RODs from 1986 through 1991, but then increased every year until it peaked at 48% in 1998. Its use then declined to 10% in 2002 after which it has increased steadily to 49% in 2005. RODs that used in situ groundwater treatment generally increased from none in 1986 to 31% in 2005. Vertical engineered barrier usage remained less than 10% for all years and other remediation techniques were less than 25% from 1986 through 1997, but then increased rapidly to 2005.

Figure 9: The percentages of RODs that selected groundwater techniques taken from EPA [20]. The combined percentages for all remedies in a given year total more than 100 percent because a ROD may select multiple remedies and may be counted in more than one category.

Figure 10 shows the percentages of pump and treat, in situ treatment, and natural attenuation used as remediation techniques at NPL sites [20]. This figure also shows that pump and treat is the most commonly used remediation technique. However, often remediation techniques are used in conjunction with each other to treat different parts of the plume. For example pump and treat can be used to control plume migration and areas of the plume where contaminant concentrations are lower. In situ techniques may be used for areas that are difficult to treat using pump and treat, such as hot spots, NAPL source zones, tight clays, fractured rock, and areas with heterogeneous hydrogeology and natural attenuation may be used to treat areas of the plume where contaminant concentrations are relatively low but that still remain above remediation goals [20].