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Article Evaluation

Evaluating Content

Paragraph 1: Introduction

• Describes NAPLs as "organic liquid contaminants" and links to wikipedia pages for each of these words.
  • Mentions hydrophobic nature (doesn't mix with water) and links to wikipedia page.
  • Lists examples, including oil, gasoline, petroleum with a functional citation.

Paragraph 2: Things of Note

• Notes that NAPLs are POPs and links to wikipedia page. Also included are two functional citations.
• Mentions that NAPLs often enter waterways via non aqueous-phase solutions, includes citation but does not define what that is.
• Makes the distinction between dense NAPLs and light NAPLs (links to DNAPL Wikipedia page, which surprised me!)

Major questions left unanswered

• Why do we care about NAPLs??
• Where does their name come from?
• Why is it significant that they are immiscible with water?
  • Ecological effects? Financial? Effects on human health?
• How do DNAPLs and LNAPLs affect the environment differently? Is one more pervasive than the other?
• How many different spheres of the environment in which they exist?
• What are the major sources?
• How do we deal with NAPLs? Are there both in-situ and ex-situ considerations?

Evaluating tone

Tone is neutral. No personal pronouns or subjective statements. The article reads very simplistically and lacks detail.

Evaluating Sources

Three sources are included. All are books published by the National Academy Press. They are all lengthy, but appear relevant and trustworthy. They seem to cover bioremediation strategies in detail.

Evaluating Talk Page

The only information on the talk page is 1) this article is part of the WikiProject Geology 2) it's the subject of a course assignment (and has me listed as the assigned editor!)

Source Evaluation

The following information will be personal notes related to the sources I plan to cite.

The Subsurface Environment (Textbook)

Includes seven sections, which each include subsections, followed by a conclusion.

Major takeaways:

Soil can be divided into two main regions: the unsaturated or vadose zone and the saturated zone or water table. Aquifers occur in the saturated zone. The unsaturated zone involves a combination of soil, water, and air, while the saturated zone only involves soil and water.

In the subsurface, water can move in practically any direction due a variety of acting forces. There is gravity, concentration gradients or "hydraulic head," uptake by plants, etc.

NAPLs are not great because it doesn't take a lot to create a lot of contamination. They have the capacity to dissolve slowly over time into groundwater as well as vaporize into the air in the unsaturated zone or adhere to soil particles.

NAPLs can exist both in the unsaturated zone or the saturated zone. Typically, they have to reach a certain concentration in the vadose zone before there is enough mass for gravity to pull it down into the saturated zone. Once it reaches the saturation zone, its fate is determined by its density: LNAPLs float on top of the water table while DNAPLs sink.

In terms of remediation, LNAPLs are easier to deal with than DNAPLs. While LNAPLs can be sucked out using a well, DNAPLs have to be dealt with using more advanced engineering. These projects have the capacity to make the problem even worse by allowing DNAPLs to seep into cracks in the parent material, at which point they're nearly impossible to identify or remove.

Time Domain Reflectometry technique for measuring NAPLs in soils

Includes and abstract and five sections, followed by a conclusion.

Major takeaways:

Measuring NAPL content is challenging because many of the traditional procedures are invasive and costly. It's important to measure NAPL content for agricultural and environmental studies. So, scientists need alternatives.

Geophysical methods such as TDR provide an alternative to the traditional drilling/welling techniques. TDR takes advantage of NAPL's electric permittivity to detect it in soils of variable saturation. TDR can also be used to measure NAPL content during decontamination procedures.

NAPLs are relatively insoluble in water. Some of the ways they enter subsurface systems are: diffusion from abandoned industrial sites, poor disposal practices, and accidental spills from places like petrol stations and refineries. NAPLs are dangerous because they are relatively mobile through soil systems and are toxic even in very small amounts.

Ground Water Issue: Dense Nonaqueous Phase Liquids

Background and introduction, seven sections

Major takeaways:

This particular paper aims to provide a conceptual framework for how DNAPLs move and are distributed in terms of their phases. There are four possible phases of DNAPL which include gaseous, solid, water, and immiscible hydrocarbon. It also provides summaries of monitoring, remediation, and modeling technologies in addition to providing the primary parameters for site characterization.

DNAPLs encompass a variety of chemical compounds, which are generally categorized based on whether they are halogenated and their volatility. Volatility largely influences the extent of contamination in the groundwater and soil. Chlorinated solvents are the most prevalent type of DNAPL. Other types include "solvents, wood preserving wastes, coal tars, and pesticides."

The extent of residual saturation determines whether the DNAPL will enter the saturated zone. Residual saturation is defined as "the volume of hydrocarbon trapped in the pores relative to the total volume of pores." The volume of the DNAPL will either be exhausted by residual saturation OR it will pool above the impermeable layer in a "bowl shape." The phase distribution of DNAPLs is largely variable, meaning it can change even within the same site during different phases of remediation, but understanding phase distribution helps to determine which tools are viable for site characterization and remediation. Different phases have different mobilities; the soluble and volatile components are the most mobile. Also, anthropogenic activities, such as "unsealed geotechnical boreholes and improperly sealed hydrogeological investigation sampling holes and monitoring wells," can influence the movement of DNAPLs.

Some important characteristics of DNAPLs include density, viscosity, solubility, vapor pressure, volatility, interfacial tension, and wettability. Some important porous media parameters include soil permeability, porosity, organic carbon, moisture, structure, and particle size distribution. Some important subsurface media characteristics include capillary force/pressure, pore size distribution/initial moisture content, stratigraphic gradeitn, and groundwater flow velocity. Gathering these parameters is complex and expensive, especially because the subsurface is heterogeneous and irregular in nature.

Scientists can measure the direction/velocity of groundwater flow, but this method isn't always successful to determine the flow of the DNAPL. Soil/aquifer material is tested via drill cuttings/cores, gas chromatography, and high pressure liquid chromatography. Some methods of phase separation include squeezing and immiscible displacement, techniques, vacuum centrifugation, and paint filter tests. Some methods to determine the 3D extent of DNAPL presence include electrical resistivity and transparent bailer. The paper notes that caution must be taken when drilling because it is easy to unknowingly drill through a DNAPL pool, causing it to drain further down into the aquifer.

Some remediation technologies include vacuum extraction, biodegradation, groundwater pumping, and soil flushing. Also, physical barriers can be used to limit flow. It should be noted that some DNAPLs are highly corrosive, creating further engineering challenges. Surfactants are highly effective for NAPL recovery, but they are also expansive and potentially ecologically damaging. Often, the only available means of recovery is the soluble components. One way to remediate the volatile components is through soil vacuum extraction, which causes the volatile contaminants to evaporate where they can be recovered and treated. This method is applicable for chlorinated solvents. Another way to enhance mobility is through the injection of steam. Immiscible hydrocarbons don't biodegrade well due to insufficient oxygen levels.

Chlorinated Solvent and DNAPL Remediation: An Overview of Physical, Chemical, and Biological Processes

Includes introduction and four sections followed by a conclusion

Major takeaways:

Remediating chlorinated solvents is a complicated issue, and restoration to pristine conditions may not be possible. Initially, the remediation technologies were limited to "pump and treat" technologies, but it became clear that the volume of groundwater for extraction was too high. As a result, between the early 1980s and mid 2000s, newer technologies were developed that both built on previous technologies and innovated new ones. Includes a list of common chlorinated solvent contaminants: tetrachloroethylene, trichloroethylene, cis-1,2-dichloroethylene, trans-1,2-dichloroethylene, vinyl chloride, 1,1,1-trichloroethane, 1,1-dichloroethylene, 1,2-dichloroethane, carbon tetrachloride, chloroform, and methylene chloride. These solvents can become distributed as small globules and ganglia within the geological matrix in addition to pooling on top of the low permeability layer, and they can remain there for decades/centuries supplying groundwater with contamination. Cites some examples of organizations that stood out in the effort to develop new technologies. In 1980, the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) was passed, which brought focus to detoxification/removal of hazardous wastes.

Physical Processes

When a contaminant is highly volatile, it can be stripped using air movement. When a contaminant is less volatile, heat can be used to bring the contaminants to near-boiling levels and induce more volatilization. Soil vapor extraction or SVE is perhaps the most popular removal method for chlorinated solvents and involves applying air to remove the solvents through the open interstitial pores in soil. Multiphase extraction or MPE can be used to simultaneously extract soil vapor, dissolved contaminants, and immiscible liquid. This method is used specifically in the vadose zone, so air-sparging can be used to transport contaminants there where they can then be vacuum extracted. However, this method can also cause further lateral or vertical migration.

Some methods of thermal treatment include steam stripping, three-phase and six-phase electrical resistive heating, and microwave heating. There are high costs associated with these treatments. Another high-cost treatment is the addition of surfactants, which can be either solubilizing (increase the solubility of contaminants) or mobilizing (allows the DNAPL to be displaced by continuous flooding).

Containment strategies involve physical structures of hydraulic barriers, including slurry barriers, vibrating beam barriers, deep soil mixing, jet grout walls, sheet piling, and geomembrane liners, which prevent groundwater from flowing through the treatment zone. They can be modified in various ways.

Chemical Processes

This involves using redox processes to create a harmless product. It is the most direct and fast method to remediate chlorinated solvents, but they are also challenging because of both competitive reactions that limit their effectiveness and the physical byproducts that might lead to even more spreading. The appropriate treatment depends on the specific contaminant and the specific reagent, and comprehensive site characterization data is necessary to determine the appropriate treatment. The greatest limiting factor in effectiveness is whether the subsurface conditions will allow for effective application.

Biological Processes

This can be either aerobic or anaerobic, also including sequential anaerobic/aerobic bioremediation applications. Most of these strategies rely on the addition of organic carbon to stimulate biodegradation. Some of the listed aerobic microorganisms include methanotrophs, propane-oxidizers, ammonia-oxidizers, and toluene oxidizers. Dissolved oxygen is limiting in these processes and can be supplied artificially through various strategies include air sparge/SVE. Also mentions bioaugmentation and the fact that its effectiveness is limited. The most popular biological remediation strategy is anaerobic and involves reductive dechlorination. Describes how organic carbon can be applied into the subsurface. One of the more recent discoveries was of certain bacteria, dehalorespirers, that are capable of direct dehalogenation of specific chlorinated solvents.

The effectiveness of these biodegradation strategies has been questioned because NAPLs are not bioavailable if they are not dissolved. That said, these strategies are applicable for dissolved-phase plumes or in tandem with other strategies. Natural attenuation as an ongoing measured parameter has become increasingly important as part of the site management strategy.

Conclusion

• Most important factors to consider when choosing a strategy include the complexity/heterogeneity of subsurface systems, the phase distribution of the contaminant, the behavior of those respective phases, and the ability for us to actually detect the contaminant

• The complexities of site conditions often restrict the extent to which it can actually be remediated

Groundwater Contamination by Chlorinated Solvents: History, Remediation Technologies and Strategies

Four sections

Major takeaways:

Includes many different studies in the 1970s/1980s surrounding the discovery and subsequent research of chlorinated solvents. Stanley and Eliassen, 1960 provides an overview of the understanding of groundwater contamination based on those perceptible with human senses. The development of gas chromatographic methods in the 1970s allowed for the identification of chlorinated solvents in groundwater. Some of the first records of chlorinated solvents in the US were in New Orleans (Marc, 1974) and California (Reinhard et al., 1979; Roberts et al., 1978b).

For cleanup technologies, the ones that stood out the most initially were air stripping and activated carbon adsorption. Over time, air stripping transitioned from an ex-situ to in-situ remediation strategy. It was discovered that GAC is most effective when remediating air as opposed to water.

The movement of chlorinated solvents began to be characterized in the late 1970s/1980s (Roberts et al., 1978a and Roberts et al., 1980). It was found that sorption was an important driver of movement because it caused retardation, transfer to aquifer solid material, and complicated/prolonged remediation.

Bouwer et al., 1981; Vogel and McCarty, 1985 were early contributors to the discovery that chlorinated solvents can be broken down biologically in a process called natural attenuation. They discovered that this led to the production of harmful intermediate chemicals.

Superfund was created after a 1978 report where contaminants that had been dumped into Love Canal were contaminated schools and homes with organic chemicals including chlorinated solvents, leading to further research.