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Innovative Soil and Groundwater Remediation Alternative

Accelerated Remediation Technologies, LLC (ART)
Integrated Remediation System

Introduction                                                                                      
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Numerous technologies have been implemented at sites worldwide to remediate contaminated groundwater.   Some of the most commonly used technologies such as air sparging, vapor extraction and ex-situ (above-ground) air-stripping are based on physical removal of contaminants; however, significant shortcomings are inherent with each method. This document will briefly present a review of ex-situ air stripping and air sparging/vapor extraction technologies along with an evaluation of their strengths and weaknesses.  A summary of currently available, in-well air stripping technologies will also be presented.  A description of an innovative technology that combines the elements of air stripping and air sparging/soil vapor extraction will be discussed.  Results of laboratory simulation and actual field implementation of this innovative remediation system will also be presented. 

1.0  Ex-situ Air Stripping

Ex-situ air stripping has been used in conjunction with pump-and-treat systems to remedy contaminated groundwater since the 1950s to treat water impacted with contaminants including petroleum hydrocarbons, chlorinated hydrocarbons and pesticides.  Pump-and-treat technology is based on extracting contaminated groundwater from the subsurface for above-ground treatment.  The characteristics of the contaminants will dictate the approach selected for the above-ground treatment for the water.  Air stripping is a technology in which volatile organics are partitioned from groundwater by greatly increasing the surface area of the contaminated water that is exposed to air. This technology is well proven and has been implemented worldwide.  

Although air stripping towers can be designed to remove up to 99.9% of a contaminant, extracting all of the contaminated groundwater to the surface has been a challenge.  Contaminants in the subsurface are either adsorbed to the soil in the vadose zone or may exist in form of free-product on the groundwater table or at the bottom of the aquifer, depending on the relative density of the compound.  Another portion will be dissolved in the groundwater.  Within the dissolved zone, small droplets of free phase contamination may be suspended in groundwater.  As a result of groundwater movement and fluctuation, those droplets become locked in what is referred to as the “dead zone” or pore spaces that are not connected. The effects of water flushing as a result of pump-and-treat remediation are minimal in treating and cleansing the saturated zone/dead zone.  Accordingly, this may explain why pump-and-treat technology may require in excess of 30 years to achieve acceptable clean up levels. Large volumes of water must be pumped to the surface, treated and properly disposed.  The disposal process requires a long list of analysis and permits for regulatory compliance. Considering the extended project life, quarterly chemical analysis/monitoring, remediation design, equipment, operation and maintenance – costs and project management can become unwieldy.

2.0  Air Sparging

Air sparging is a technology that has been used extensively in recent years as a result of increased desire to implement in-situ rather than ex-situ remedial measures.  Air sparging is based on the same physical removal concept as air stripping. Pressurized air is bubbled through a contaminated aquifer.  The air travels horizontally and vertically through the carrying the contaminants to a vapor extraction system.  Air sparging has several advantages when compared to pump-and-treat technology, including the following: 

        All groundwater treatment is performed in-situ

        Contaminants are treated at the point of location

        Shorter project life results in significant cost savings

        Operation and maintenance costs are reduced

        May be implemented for a wider range of subsurface conditions

        Includes the injection of fresh air in the subsurface resulting in enhanced biodegradation processes.  (Thus another treatment is occurring along with air sparging to expedite the remediation of the subsurface). 

However, numerous deficiencies are associated with in-situ air sparging.  The number and magnitude of the shortcomings may vary depending on site conditions, location and contaminant concentrations including: 

        With air sparging technology, treatment may not be as effective since all of the design parameters are estimated and may be significantly different from actual values.  Conversely, with an air stripping tower the water flow rate, contaminant concentrations and tower dimensions are known.  Therefore, air flow rate to achieve acceptable removal rates to remedy the water stream can be accurately calculated

        The radius of influence (cleaning zone) depends on the hydraulic conductivity of the subsurface material.  High conductivity soils (e.g. sandy soils) can allow air bubbles to float vertically shortly after leaving the sparging point, resulting in a very small radius of influence.  Conversely, if the hydraulic conductivity is low, the radius of influence will be larger, but air injection will be difficult based on the soil type

        The radius of influence is usually assumed based on a pilot test.  It is common to assume that subsurface conditions at the site are homogeneous.  In reality, air flow rate sparged at one point may not be adequate for another location at the same site  

        Air sparging technology design and implementation requires specialized expertise to minimize the potential for spreading contaminants.

 A remedial technology that combines the advantages of air-stripping and air sparging methods that includes in-situ active treatment (e.g. bioremediation), would be more effective and ideal.  This technology would compensate for shortcomings associated with pump-and-treat such as long project life and costs of water disposal and the deficiencies related to air sparging such as limited radius of influence and removal rate.  This technology is discussed below.

3.0  In-well Air Stripping Technology

Attempts to create this improved technology were made by Stanford University (NoVOCTM), and by IEG Technologies Corporation (Underduck-Verdampfer-Brunnen (UVB)).  The in-well air stripping, an extension of air sparging technology, involves the creation of a circulation cell around a well in which groundwater is cycled.  

With the NoVOCTM technology, a blower introduces air to produce bubbles in a sparging well. The well is equipped with a deflector plate that separates two screens.  When the sparged air encounters the deflector plate, the bubbles break, re-combine and then re-infiltrate the vadose zone to be extracted through the upper screen.  With the UVB technology, air-lift pumping occurs in response to negative pressure induced at the well head by a blower.  Vacuum draws water into the well through the lower screen. As a result, air is introduced through a diffuser plate located within the upper, screened section.  The air bubbles provide air-lift pump effect that moves water up in the well. A submersible pump is installed to insure that water flows from bottom to top.  A stripping reactor consisting of fluted and channeled columns is installed to facilitate transfer of volatile compounds to the gaseous phase. 

Although the UVB approach may be considered as significant improvement over the NoVOCTM technology, major shortcomings remain, including: 

        The radius of influence of a well is only based on the water drawn to the well as a result of head reduction created by vacuum pressure.  Accordingly, the radius of influence is unlikely to be significant

        The stripping reactor will require a vault or a large diameter well.  These are difficult to construct, costly, and may not be feasible in most cases - especially if the contamination is underneath structures or at an operating facility.  

Other technologies such as the Density Driven Convection (DDC) system have been developed.  These technologies utilized the addition of nutrients to enhance in-situ biodegradation; however, they are based on the UVB or the NoVOCTM technologies and maintain their shortcomings. 

In the next section, an innovative remediation system is presented.  This technology is based on proven, accepted techniques and will be referred to as the “ART Integrated Remediation System”. 

4.0  ART Integrated Remediation System

Accelerated Remediation Technologies, LLC (ART) has recently developed and patented an effective, innovative remedial technology that relies on well-proven and established concepts.  The ART technology combines in-situ air stripping, air sparging/soil vapor extraction and enhanced bioremediation in an innovative wellhead system.  The system is designed to accommodate a four inch well and be very cost effective when compared with other, stand-alone remediation technologies.  Figure 1 (below) provides a schematic of the well components. 

 

                                 

          Figure 1

 4.1  Technical Description

The air sparging component results in lifting the water table.  This lifting of the water in the well causes a net reduction in head at the well location, which results in water flowing toward the well.  Vacuum pressure (the vapor extraction component) is be applied atop of the well point to extract vapor from the subsurface.  The negative pressure from vacuum extraction results in water suction that creates additional water lifting (mounding) and a net lower gradient. This further enlarges the radius of influence.  

A submerged pump is placed at the bottom of the well to recirculate water to the top for downward discharge through a spray head.  The water cascades down the interior of the well similar to what occurs in an air stripping tower. Enhanced stripping via air sparging near the bottom of the well will occur simultaneously. In essence, the well will act as a subsurface air stripping tower. In addition to the air stripping effected by the pumping/cascading, the pumped, stripped, highly oxygenated water will flow down well annulus and over the “mounded” water back in to the aquifer. This will set up a circulation zone surrounding the well that will further enhance cleanup.  Effects of the different forces on the groundwater table in relation to the wellhead technology are shown in Figure 2 (below). 

Multi-surface packing may be placed in a well to increase the effectiveness of air stripping as shown in Figure 3 (below).  In most cases, however, in-well packing will probably not be necessary.

               

                                  Figure 2 and 3

 In summary, contaminants are stripped from water as a result of the combined effects in-well air stripping and in-well air sparging.  The radius of influence/cleaning zone will be created by a combination of three forces: 

1.   Negative gradient as a result of the lifting of the groundwater caused by air sparging at the bottom of the well

2.   Additional, negative gradient and water lifting resulting from the application of vacuum extraction at the top of the well

3.   Subsurface water circulation surrounding the well induced by a submersible pump placed at the bottom of the well. 

All of these different components can be integrated and installed in a 4-inch groundwater well.  Cost of this technology is in the range of air sparging technology alone, since the costs of added pump and piping will be compensated for by the elimination of a separate vapor extraction point and associated trenching and construction.

4.2  Laboratory Simulation

Laboratory experimentation was necessary to simulate the effectiveness and utility of the of the ART Integrated Remediation System. The main objective of the laboratory simulation was to determine the behavior of the water when extracted from the bottom of a well and sprayed back in the same well above the groundwater surface.   Due to the different forces acting in the same well, it was not clear whether the pumped water is replaced by water in the well void, or replaced by water extracted from the aquifer.  Additionally, it was not clear if the water sprayed atop the water table remains in the well void or flows away from the well to be replaced by contaminated water from the subsurface. 

4.2.1  Laboratory Set-up

Laboratory experimentation included simulation of different forces.  To achieve the objectives, an 85-gallon, glass, fish tank was filled with a medium-grained, well-graded sand.  The ART well was constructed of inch, PVC pipe and placed near the center of the tank.  The well was manually slotted to simulate a screen. A small, fountain pump with a pumping rate of approximately 0.2 gallons per minute (gpm) was placed at the bottom of the well to lift the water approximately 18 inches for spraying near the top of the well.  The well also contained a tube placed at its bottom and connected to an air pump to sparge air at the bottom of the well.  A fish tank aeration pump with a capacity of approximately 0.2 standard cubic feet per minute (SCFM) was used to effect the air sparging.

The tank was slightly sloped approximately 0.25% to simulate natural groundwater flow conditions. Four monitoring wells (MWs) were simulated using inch, PVC pipe and slotted similar to the in-situ air stripping well.  The first well (piezometer) was placed next to the air stripping well to measure well conditions since. This was done because ART well measurements could not be taken due to different tubes and pump obstructions.  The second well, MW-1, was located 12 inches upgradient of the ART well.  MW-2 and MW-3 were placed 12 and 24 inches, respectively, downgradient of the well.  

4.2.2 Procedure

The procedures consisted of measuring water levels in the tank prior to and at different intervals after sparging and pumping.  Water levels were measured by inserting a small-diameter wooden dowel in the well and measuring the wetted length of the sticks.  Dedicated sticks and similar procedures were used for each well to minimize potential for measurement errors.  Water levels were measured in each monitoring point prior to starting sparging.  Water levels were also measured following sparging, at different increments until stabilization, and following air sparging pumping.  Water levels are shown in the following table in inches from the bottom of the tank.  

 

 

Well Piezometer

 

MW-1

up-gradient

 

MW-2

down-gradient

 

MW-3

down-gradient

 

Distance From Well

 

0

 

12

 

12

 

24

 

Starting Water level

 

16

 

16.25

 

16.1

 

16.25

 

Water Level after sparging only (5 minutes)

 

16.15

 

 

16.65

 

16.15

 

16.25

 

Water Level after Sparging only (10 minutes)

 

16.25

 

16.6

 

16.65

 

16.25

 

Water Level after Sparging only (25 minutes)

 

16.25

 

16.6

 

16.5

 

16.5

 

Water Level after Sparging & pumping (35 minutes)

 

16.25

 

16.65

 

16.6

 

16.5

 4.2.3  Laboratory Simulation Findings

As expected, water levels were elevated when sparging occurred.  This occurrence has been exhibited at thousands of sites.  However, the main objective of this experiment was to determine the behavior of the water sprayed through the well atop of the groundwater surface.  After starting the submersible pump, water was visually and clearly observed migrating away from the ART well at a significant rate.  It was apparent that pumped water does not sink back into the ART well, but rather it flows over the vacuum/sparged-induced, mounded water table and away from the ART well. The pumped water was observed traveling a distance of approximately 18 inches away from the well until water movement was no longer visually detected.  

It was also observed that water in the vicinity of the well contained a large number of small bubbles. This appears to be a result of air sparging and stripping which significantly increased the air content in the water – thus elevating dissolved and suspended air that reduced the water density.  As a result of the water density reduction, sparged and stripped water was pushed to the surface of the water table.  Further, the stripped/sparged water was not allowed to sink in the well but migrated outward, over and down the higher-density water interface away from the well. Simultaneous replacement by higher density water from the well surroundings was also observed in the well.  

4.2.4  Laboratory Experiment Conclusions

The laboratory simulation was successful in deriving the following conclusions: 

        Air stripped water will flow away from the well and it does not appear to sink back in the same well.

        Air stripped water has significantly lower density than surrounding water and will float on the surface and flow down the mounding water away from well.

        The radius of influence of the air striping well is larger than initially thought and is in the range of three times the water column in the well. 

It can be confidently concluded that the ART Integrated Remediation System will result in a more effective remedial process, a shorter project life and thus lower expenses and a larger radius of influence than sparging alone.  The air stripping technology appears to be the most practical, promising and effective to remedy chemically impacted groundwater, especially hydrocarbon, chlorinated and recalcitrant compounds. 

5.0    Case Study

ART Integrated Remediation System was implemented at an industrial site in Minnesota where tetrachloroethene (PCE) was detected in soil and groundwater.  A national environmental consulting firm installed in-situ soil vapor extraction and groundwater air sparging systems in 1995 to remedy soil and groundwater at the site.  As of early 2001, PCE concentrations remained elevated and it was apparent that remediation had become static.  Authorization was granted to install an ART Integrated Remediation System on May 17, 2001. 

5.1  General Site Description

The facility occupies an area approximately six acres with a 16,000 square feet structure.  An industrial laundry operation was established at the site in the mid-1970s.  A dry cleaning process was used at the site for several years.  The topography of the site is relatively flat with a river located approximately 1,500 feet to the east.  Subsurface soils at the site consist of fine sand mixed with silt, loam and organic sediments. 

5.2  Subsurface Conditions

Several soil borings and groundwater monitoring wells were installed at the site in 1994.  PCE was detected at varying depths at several locations.  The highest level of PCE in soils was 47,000 ppb. Highest levels of PCE in groundwater were encountered in MW-2 at 20,000 ppb and have fluctuated throughout the last seven years.  The last sampling round prior to the installation of the ART Integrated Remediation System detected PCE at 2,700 ppb.  TCE and cis-1,2-DCE were also detected at concentrations of up to 250 and 110 ppb, respectively.  

5.3  ART Integrated Remediation System Implementation

As a result of the impending sale of the site, the owner needed a remedial technology that would provide more effective results and reduce contamination at the site to acceptable levels in a shorter period of time, while controlling additional costs.  The owner selected the ART Integrated Remediation System that was approved by the consultant and the state regulatory agency. 

One ART extraction well was installed approximately 18 feet upgradient of monitoring well MW-2. The well was extended to a depth of approximately 20 feet below the groundwater table where a submersible pump was placed.  The general construction details are as shown in Figure-1. 

5.4  Results

MW-2 was used as the main monitoring point to gage the effectiveness of the technology.  Immediately prior to the implementation of ART Integrated Remediation System alternative, PCE and dissolved oxygen (DO) concentrations were approximately 2,700 ppb and 1.23 ppm, respectively.  MW-2 was sampled on May 29, 2001, thirteen days after the implementation of ART system.  Chemical analysis indicated that PCE concentrations were reduced by approximately 90% to 240 ppb.  DO concentration increased from approximately 1.23 to 9.57 ppm (near saturation).  TCE and DCE concentrations were reduced to below detection.  PCE concentration reduction was also exhibited in wells approximately 70 feet down gradient from the ART well. 

These results clearly demonstrate that the ART Integrated Remediation System is a very effective technology that was able to jump-start site remediation and significantly reduce contaminant concentrations over a period over a relatively short period of time.  Further, the ART Integrated Remediation System demonstrated that it could achieve more in a few weeks than air sparging alone had achieved over a period of several years.  It is anticipated that the ART system will continue to demonstrate contaminant reduction over a shorter period of time than that of alternate technologies. 

6.0  Technology Conclusion

It can be concluded from the case study/field test and laboratory simulation that the ART Integrated Remediation System is a very effective remediation alternative.  It was proved to be significantly more effective than air sparging.  Significant shortcomings associated with air sparging and air ex-situ air stripping have been eliminated.  The ART technology is based on proven concepts that have been used with varying levels of effectiveness at thousands of sites over the years.  When integrated as a technology system, these concepts were found to be synergistic and far more effective than past remediation approaches.


Interested in the ART System....contact:  mshiang@advtech.net


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