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Permeable reactive barriers: A sustainable technology for cleaning contaminated groundwater in developing countries
Desalination 248 (2009) 352–359
Permeable reactive barriers: A sustainable technology for cleaning contaminated groundwater in developing countries D.H. Phillips Environmental Engineering Research Centre, School of Planning, Architecture and Civil Engineering, Queen’s University of Belfast, Belfast BT9 5AG, United Kingdom Tel. þ44 (0)28 9097 55; Fax þ44(0)28 9097 42; email: [email protected] Received 31 January 2008; revised accepted 15 May 2008
Abstract Permeable reactive barriers (PRBs) are a proven technology for remediating contaminated groundwater, particularly on industrial and mining sites. PRBs are a sustainable technology that can operate over a long time scale with low maintenance. Over the past 10–15 years, there have been great strides in refining site characterisation techniques (i.e. geophysical techniques), developing/discovering reactive materials/sorbents (i.e. Fe0 filings), and the installation and design of PRBs (i.e. funnel-and-gate design) which have increased the cost-effeciveness of this technology. Prior to installation, careful consideration of the ease of removal of the PRB should be considered as part of the design. This is important as the PRB may eventually need to be decommissioned. PRBs are a sustainable site specific remediation technology that has the great potential to work well as a part of a larger scale integrated water resource management programme in developing countries. Keywords: Permeable reactive barriers (PRBs); Reactive materials; Sorbents; Remediation; Site characterisation; PRB designs
1. Introduction Permeable reactive barrier (PRB) technology has been sucessful in remediating a variety of groundwater contaminants including heavy metals [1], organics [2] and radionuclides [1,3]. Most PRBs have been installed on industrial, mining and agricultural sites around the world [1–3]. PRBs use the natural hydraulic gradient of the groundwater plume to move the contaminants
through the reactive zone giving it an advantage over traditional pump-and-treat technologies by being more cost effective and lower maintenance in the long-term [3]. Over the past decade, much work has been done on improving site characterisation techniques, developing reactive materials/sorbents, and the installation and design of PRBs. This work has increased the costeffeciveness of this technology making it a more viable remediation option for developing countries.
Presented at the Water and Sanitation in International Development and Disaster Relief (WSIDDR) International Workshop Edinburgh, Scotland, UK, 28–30 May 2008. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.05.075
D.H. Phillips / Desalination 248 (2009) 352–359 Geophysical techniques such as magnetic and ground probing radar are a form of non-invasive site characterisation that yields valuable information about site geology, in situ engineering properties, hidden cultural features, and contamination in the shallow subsurface. Non-intrusive investigations are a quick and cost effective means of obtaining data, especially when combined with old site plans, and are useful in planning the intrusive site investigations. These techniques are reducing the need for more expensive trial pitting or borehole drilling with lower risks and decreasing the chances of missing buried targets. Some of the instruments (EM units) are hand carried and generally do not contact the ground. All accessible areas of a site can be quickly surveyed (up to 2 ha/day) without disturbing the surface [4]. This is important because there may be considerable surface and near surface contamination on former industrial and mining sites. Comparing site historical plans with their geophysical surveys is a very beneficial ground truthing method and is generally part of the protocol of a study. A variety of reactive materials and sorbents, which can be used separately or in combination depending on the groundwater contamination, have been successful in remediating contaminated groundwater in PRBs. These materials, such as Fe0 filings [4], peat [3], limestone [4,8], granular activated carbon (GAC) [5,6] and zeolite [1], are easily available and some are fairly inexpensive. Benchscale treatability studies are carried-out in the initial screening of the reactive or sorbent material to plan the design of PRBs using site groundwater. Batch studes using a number of likely reactive and sorbent materials are conducted to determine the best performing materials. Then column tests are carriedout on the best performers. Column tests can give information towards the design of the PRB and indications on how an in situ PRB will perform [7]. Installation of PRBs is a crucial stage, especially in the excavation of geological material. Improved equipment and techniques used to excavate geological material without obstructing the flow of the contaminated groundwater plume in and out of the PRB has helped to increase the success rate of PRB performance. During installation, loose geological material and soil can be packed, smeared and fill void space that the contaminated groundwater flows through adjacent to the
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PRB [8,9]. Methods in depositing of the reactive material in to the PRB have also been refined to reduce particle size grading which can alter groundwater flow through the reactive zone [9]. Prior to installation, careful consideration of the ease of removal of the PRB should be considered as part of the design. This is important as the PRB may eventually need to be excavated due to the completion of remediation; therefore, decommissioning could become an issue [7]. Of the two basic designs, i.e. the continuous trench and the funnel-and-gate, the funnel-and-gate design with the reactive material placed in single or sequenced containers is probably the most cost-effective design. This is because the funnel-and-gate usually uses less reactive material than the continuous trench. The reactive material(s) is placed in the canister(s) (reactors) and can be removed if it needs to be replaced. This is an important consideration as there could eventually be built-up of contaminant concentrations in the reactive/sorbent material(s) from the remediation process. The containers can be designed to be reused at other sites once remediation is finished. PRBs are a sustainable site specific remediation technology that has the great potential to work well as a part of a larger scale integrated water resource management programe in developing countries. The objectives of the paper are to illustrate how PRBs are planned and installed, and highlight cost effectiveness which may allow them to be installed in developing countries. 2. Material and methods If PRB technology is considered one of the options for the remediation of contaminated groundwater at a site, there are a series of steps that should be taken to ensure that the PRB is viable, and that it is designed and installed properly. One of the first steps is to collect as much information about the site as possible, such as blueprints, geological maps, and records. This will help in ground truthing and determining contamination. What are the possible contaminants? Have there been any buildings on site? Buried debris may imped PRB installation and groundwater flow across the site. Knowing what contaminants may be present at the site will also allow for better health and safety plans.
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2.1. Site characterisation Non-invasive characterisation of the site using geophysical techniques, especially when used along with test pits and wells improves the detail of the site characterisation and is cost effective and time saving. Generally, a site is first cleared of surface objects (i.e. scrap metal, reinforced concrete rubble) that may interfere with subsurface features. A grid (i.e. 5 m) is surveyed out (i.e. using a Total Station Lecia 1010 Wild Heerbrugg surveyor). Further points are measured at the site boundaries and across the site to give a representation of height A.O.D. Electromagnetic (EM) and ground penetrating radar (GPR) instruments are portable units that can detect metal, debris, geological features, and contamination in the shallow subsurface. A Geonics EM61 time domain metal detector is used to survey for ferrous and non-ferrous metal objects down to 5 m. The instrument, operating at 75 Hz, is trawled across the site by the operator. The Geonics EM31 can map geological variations and any other feature associated with ground conductivity change down to 6 m. The EM31 is in the form of a boom that is carried about 1 m above ground. It can be used in a normal operating orientation (vertical dipole mode) or turned 90 to its long axis (horizontal dipole) and used to measure down boreholes. The Geonics EM38 also measures conductivity and is carried 15–20 cm above the ground. It is particularly sensitive to soil salinity which could be a characteristic of contaminated land/groundwater. Similar to the operation principle of the EM31, the EM38 surveys to the depth of 1.5 m in the vertical dipole, and 0.75 m in the horizontal dipole modes [10]. GPR is also used for high resolution shallow subsurface investigations. Ground truthing should be part of the protocol in a field investigation by comparing the geophysical results to historic plans and site visits.
several sampling intervals can be tested. Batch tests are carried-out in bottles or vials capped with inert septa. A known weight of reactive material and a known concentration and volume of contaminant solution (preferably the contaminated groundwater) are added to the vials. Samples are extracted at regular intervals and the contaminant concentration measured. Degradation curves can be made from this data, and degradation products can also be determined [7]. Column tests are used to collect detailed information on the degradation, precipitates, removal and/or sorption of contaminants by reactive/sorptive media. Removal rate data under a range of flow conditions, especially those that mimic groundwater flow velocity in the field can be determined with column tests. Contaminated or simulated groundwater is passed through the column at a known flow rate. The column design allows changes in contaminant composition and other parameters (e.g. major ions, pH) to be determined at the influent, effluent and along the column length (Fig. 1) [7]. Column dimensions are generally 10– 100 cm long, with a 2.5–3.8 cm inside diameter [8]. Glass columns are generally the least reactive or adsorptive with chlorinated organic compounds. However, no significant loss of organics have been found using Perspex columns [11]. Sampling ports along the length of the column should be constructed of stainless steel fittings or inert stoppers (i.e. Teflon coated or Viton). The ports should allow the sampling needle to be inserted into the centre axis of the column or a needle to be fixed in place in each port. A three-way port should also be positioned in the influent and effluent lines. All tubing and fitting for the influent and effluent lines should be composed of an inert material. Information from the column study can be used along with the site characterisation and modeling to help to design the field-scale PRB [7]. 2.3. Excavation
2.2. Treatability studies Batch tests are used to screen potential reactor materials. They can determine whether the contaminants are amenable to sorption, degradation, or precipitation by different types of media. They also can be used to compare treatment efficiency of reactive and sorptive materials. Contaminant degradation rates are measured quantitatively over time, and various parameters and
In conventional excavation of continuous trenches, the soil is removed and the trench is backfilled with reactive/sobent materials. When the emplacement of the reactive materials is completed, temporary retaining structures which support the walls will be removed from the ground. A backhoe comprised of a digging bucket on the end of an articuated arm is frequently used for rapid digging of shallow trenches [8] less that
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Fig. 1. Diagram and photograph of a column test.
10 m deep. A continuous trenching machine can also be used to create a narrow trench less than 7 m deep. It is comprised of a chain saw cutting belt and a trench box on the boom. The backfill of reactive/sorbent materials is carred-out with a hopper on the top of the trench box. The removal of soil and the backfilling of reactive materials are simutanously carried-out. This elminates the need for dewatering and temporary retaining structures [11,12]. An excavation of up to 15 m depth can be carried-out with a caisson for funnel-and-gate PRBs. Caissons with a circular cross-section are frequently used. After it has been installed, a large auger can be used to remove the soil within the caisson. The reactive/sorbent materials can be then backfilled into the caisson [11,12]. 2.4. PRB design The two common PRB designs are the continuous trench PRB and the funnel-and-gate system (Fig. 2a and b). The continuous trench PRB does not contain any structures, so the contaminant plume flows through the treatment zone using the natural hydraulic gradient. This PRB, which is perpendicular to groundwater flow direction, needs to be slightly larger than the crosssectional area of the contaminated groundwater inorder to capture the contaminants in both vertical and horizontal directions [9]. The top of the PRB should be at
least 0.60 m above the water table and the bottom of the PRB should be extended at least 0.30 m into a low permeability zone (i.e. clay), if it is present. The PRB thickness should be designed to provide sufficent residence time for the contaminants within the treatment zone to be completely treated. The funnel-and-gate system is composed of impermeable walls and at least one reactive zone. The funnel structure could be sheet piles or slurry walls. The function of the funnel is to intercept the contaminated groundwater and lead it to the treatment zone. The bottom of the funnel and reactive zone needs to be extended at least 0.30 m into the less permeable soil layer, while the top of the funneland-gate needs to be set at least 0.60 m above the water table [10]. The reactive material is directly implaced or filled into the reaction vessel(s) [9]. Multi-sequenced reactive barriers are also being installed, especially on sites with multiple groundwater contaminants such as gas works sites. Multi-sequenced PRBs use mulitiple reactive materials in more than one reactive zone to treat the contaminated groundwater [13] (Fig. 2c). 3. Results and discussion 3.1. Cost-effective groundwater clean-up with PRBs Over the past 10–15 years since the first PRBs were installed to remediate contaminated groundwater, there
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Fig. 2. Diagrams of PRBs. (A) Elevation view of a continuous trench or wall (after [14]), (B) plain view of a funnel-and-gate, and (C) elevation view of a multi-barrier.
have been great strides in reducing their costs. Compared to traditional pump-and-treat remediation, PRBs can be more costly in the initial stages, especially during installation. However, since PRBs are a passive system, which relies on the natural hydraulic gradient of the groundwater plume to move the contaminated groundwater through the treatment zone, the longterm costs are lower than traditional pump-and-treat operations and maintenance [14]. Many PRB reactive/sorbent materials can remediate a range of contaminates, while others are selective. PRBs can be used as part of a treatment train with other technologies to clean groundwater. PRBs are generally below ground and ‘out of sight’ so the site can be used for
other uses. However, there are some disadvantages of PRBs. Sometimes the PRB require more maintenance than originally planned. PRBs can also become clogged and the reactive material coated causing them to become less effective. Non-invasive site characterisation has also decreased the cost of PRB installation. Many of the magnetic and ground probing radar units are portable and can easily be transported into sites that may be remote. Data on the characteristics of a contaminated site can be gathered rapidly and can help in planning the invasive characterisation (trial pitting and boreholes). Data collected on buried features, geology and hydrogeology can be used to model or better plan for
D.H. Phillips / Desalination 248 (2009) 352–359 monitoring and remediation of contaminated land and groundwater. The suite of chemicals contributing to subsurface groundwater contamination may be electrically conductive, either because the contamination is acidic or contains salt. Therefore, the groundwater plume is also electrically conductive [4]. Importantly, the non-invasive site characterisation can also give indications if the site is not geologically and hydrogeologically fit for a PRB to be installed (i.e. absence of low permeability material to secure the bottom of the PRB), which can reduce money and time spent on a remediation option that may fail. Treatability studies may be carried-out at a number of stages of PRB selection including remediation options screening, PRB design and operation [7]. Treatability studies are used to determine the best reactive/ sorbent media, to help design the PRB, and to predict weaknesses in the PRB design. The cost of treatability studies can vary greatly, and should be proportionate to the total cost of remediation. Generally, treatability studies should cost <10% of the total remediation costs. To cut costs, reactive material from local sources can be screened in a treatability study. For example, activated carbon from coconut husk is used to treat arsenic contaminated groundwater [6]. Other media such as Fe0 [3], sawdust [1], plant material [1], and zeolite from volcanic rock [1] are also used in PRBs as well as microbes [1] in biobarriers. Using local sources may save money on purchasing the media and also renewing it. Additionally, the use of recycled materials makes the PRB a sustainable remediation option. The conventional excavation is a common PRB construction technique and is more suitable for shallow PRB systems such as a continuous trench <10 m deep. Operational cost can increase with the depth of excavation. The conventional excavation can be carried-out with common excavation equipment such as a backhoe, a clamshell and a caisson [12]. Because the trench is located in shallow strata, the whole installation of the PRB can be easily monitored. Some potential problems can occur with the conventional excavation. In addition to a decrease in the permeability of the PRB [11] from smearing, contaminated soil and water can potentially be exposed during the excavation raising health and safety issues. Nevertheless, the operational costs and time are low for continuous trenching [12]. Although, the funnel-and-gate PRB is physically easier to
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decommission, the continuous trench PRBs can still remain an option if the geology and hydrogeology is favorable for installation, and if there is a lack of funds and resources to install a funnel-and-gate PRB. In the funnel-and-gate PRB, reactive material is stored in canisters or reactors which can be removed after the PRB has completed the remediation or if the PRB has problems. This allows for removing of potentially further contamination from the site as a result of contaminant build-up in the reactive material. Additionally, funnel-and-gate PRBs generally require less reactive material. This is a great cost saver, if the reactive material is pricey. For these reasons, the funneland-gate design should be considered if funds are available. Therefore, it is important to take the design of the PRB in to consideration and how this will affect its long-term operation. 3.2. Case studies of PRBs There are over 100 PRBs in operation around the world at present; however, only a few are installed in developing countries. Examples are pilot-scale PRBs used to treat contaminated groundwater and leachate from uranium mines in Hungary and Bulgaria, respectively. The PRB in Pecs Hungary which was installed in 2002, is a continuous trench containing shredded Fe0 which removes uranium from the groundwater. Sand layers have been added to the up-gradient and down-gradient sides of the PRB to allow for better groundwater flow through the reactive zone. The PRB was emplaced in an underlying clay and geosynthetic clay liner. Groundwater U concentrations were reduced to <1% of influent concentrations after passing through the reactive zone in year 2003. Uranium concentrations were reduced from *1000 mg/L to *100 mg/L in monitoring wells near the PRB and <10 mg/L within the PRB. A negative performance issue is that a high amount of precipitates has been estimated to have formed in the PRB which may reduce groundwater flow through the PRB and reduce the reactivity of the Fe0. However, only a 1.6% loss in porosity was calculated and the PRB is predicted to have a 62 year lifespan [3]. The PRB in Western Bulgaria (installed 2004) treats acid drainage (pH 2.5–3.7) runoff from a uranium mine contaminated with radionuclides (mainly uranium and radium), heavy metals, arsenic
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Fig. 3. PRBs as part of a larger scale integrated water resource management programme in developing countries.
and sulphates. The PRB is part of a treatment train (connected series) in the form of an alkalizing limestone drain that removes Fe as hydroxides, the PRB for biosorption and microbial reduced sulphates, and a natural wetland. The barrier is a ditch/continuous trench with a reactive material consisting of a mixture of solid biodegradable organic material (plant and mushroom compost, wood chips, straw and cow manure), crushed limestone and zeolite saturated with ammonium phosphate. A mixed microbial community consisting of sulphate-reducing bacteria and other microorganisms is also present. Uranium, arsenic and non-ferrous metals were mainly removed in the PRB by the indigenous sulphate-reducing bacteria. Portions of the pollutants, and most of the radium, were sorbed onto the dead plant material in the PRB [1]. Another example of a PRB, but not in a developing country, is a funnel-and-gate PRB in Monkstown,
Northern Ireland, United Kingdom. This PRB, established in 1995, is a field-scale PRB developed to remediate a small but highly concentrated accumulation of a degreaser pollutant, trichlorethene (TCE), in gravely glacial till on an industrial site. TCE is a dense nonaqueous phase liquid (DNAPL) that is denser than water and has a tendency to settle in the subsurface as immiscible accumulations (slugs). The PRB consists of a long tube-like structure that holds Fe0 filing reactive material. This structure is emplaced in naturally occurring clay at the site. The TCE contaminated groundwater is funneled from the up-gradient position into the gate of the PRB where the vessel that holds the reactive material is present. The groundwater flows down through the Fe0 reactive zone in the PRB where it is treated and exits at the down-gradient. There is a decrease in up-gradient TCE concentrations in this Fe0 funnel-and-gate PRB over time which suggest that
D.H. Phillips / Desalination 248 (2009) 352–359 the PRB is remediating the TCE contaminated groundwater at the site. However, there is also a possibility that a great portion of the TCE slug was removed while installing the PRB which would cause the concentrations to be lower over time (Fig. 3). There is decrease in TCE as it moves through the PRB indicating that it is being remediated (dehalogenated) by the Fe0. A pre-existing TCE contamination occurs adjacent to the down-gradient portion of the PRB possibly due the clipping of the end of the TCE contamination slug when the PRB was being installed which hampers the down-gradient monitoring of this PRB; however, data shows that it is dramatically decreasing over time [2]. When PRBs were first being inplemented about 15 years ago, it was suggested that they become a remediation option for developing countries. This is because they are a sustainable remediation technology that requires little maintenance and operational cost over a long duration. Addtionally, reactive/sorbent materials such as peat, sawdust, activated carbon, zeolites, and limestone can be aquired easily and may be from a local source. Generally, PRBs are used for site specific remediation. However, in areas where adequate water supply is limited and there is contamination from industrial, agricultural or mining sites, if properly planned they could become part of a larger scale integrated water resources management programme. For example, an International Water and Sanitation Center (IRC) project DREAM (Drainage and Reuse of Effluents for Agricultural Management) has planned PRBs to be part of an integrated water resources management programme as a low cost approach for wastewater treatment for reuse in agricutural areas in developing countries where water supply is low [15].
References [1] S. Groudev, P. Geprgiev, I. Spasova, M. Nicolova, A. Angelov and L. Diels, Cleanup of acid mine drainage by means of a pilot-scale passive system, Annu. Univ. Min. Geol. ‘St. Ivan Rilski’ Part. 1. Geol. Geophys., 48 (2005) 217-220. [2] P. Beck, N. Harris and R. Sweeney, Design, Installation and Performance Assessment of a Zero Valent Iron Permeable Reactive Barrier in Monkstown, Northern Ireland, CL:AIRE Technology Demonstration Report, TDP3, 2001, 120pp.
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[3] K.E. Roehl, K. Czurda, T. Meggyes, F. Simon and D.I. Stewart, Long-term Performance of Permeable Reactive Barriers, Elsevier, New York, 2005. [4] US Army Corps of Engineers, Geophysical Exploration for Engineering and Environmental Investigations, EM 1110-1-1802, 1995. [5] V. Plagentz, M. Ebert and A. Dahmke, Remediation of ground water containing chlorinated and brominated hydrocarbons, benzene and chromate by sequential treatment using ZVI and GAC, Environ. Geol., 49 (2006) 684-695. [6] G.N. Manju, C. Raji and T.S. Anirudhan, Evaluation of coconut husk carbon for the removal of arsenic from water, Water Res., 32 (1998) 3062-3138. [7] Environment Agency, Guidance on the Use of Permeable Reactive Barriers for Remediating Contaminated Groundwater, National Groundwater and Contaminated Land Centre Report NOC/01/51, Environment Agency, Solihull, UK, 2002. [8] United States Environmental Protection Agency, Permeable Reactive Barrier Technologies for Contaminant Remediation, EPA 600-R-98-125, 1998. [9] A. Gavaskar, N. Gupta, B. Sass, R. Janosy and J. Hicks, Design Guidelines for Applications of Permeable Reactive Barriers for Groundwater Remediation, Battelle, Columbus, OH, Report No. AFRL-ML-WP-TR2000-4546, NTIS: ADA380005, 2000, 399pp. [10] US Department of Transportation, Determination of the Depth/Structure/Fracture of Bedrock, 2005, http:// www.cflhd.gov/agm/engApplications/Index.htm. [11] A. Gavaskar, N. Gupta, B. Sass, R. Janosy and D. O’Sullivan, Permeable Barriers for Groundwater Remediation: Design Construction and Monitoring, Battelle, Columbus, Ohio, 1998. [12] US Air Force, Design Guidance for Application of Permeable Barriers to Remediate Dissolved Chlorinated Solvents, DG 1110-345-117, 1997. [13] L. Dries Bastiaens, J. Vos, Q. Simons, M. De Smet and L. Diels, Comparison of different multibarrier concepts designed for treatment of groundwater containing mixed pollutants, in: A. Boshoff, B. Bone, eds., Permeable Reactive Barriers, International Symposium on Permeable Reactive Barriers, March 14, 2004, Queens University Belfast Northern Ireland, vol. 298, 2005, pp. 45-51. [14] Environmental Protection Agency, A Citizen’s Guide to Permeable Reactive Barriers, EPA 542-F-01-005, 2001, 2pp. [15] International Water and Sanitation Centre, Annual Report, The Netherlands, 2004, 24pp.
Permeable reactive barriers: A sustainable technology for cleaning contaminated groundwater in developing countries D.H. Phillips Environmental Engineering Research Centre, School of Planning, Architecture and Civil Engineering, Queen’s University of Belfast, Belfast BT9 5AG, United Kingdom Tel. þ44 (0)28 9097 55; Fax þ44(0)28 9097 42; email: [email protected] Received 31 January 2008; revised accepted 15 May 2008
Abstract Permeable reactive barriers (PRBs) are a proven technology for remediating contaminated groundwater, particularly on industrial and mining sites. PRBs are a sustainable technology that can operate over a long time scale with low maintenance. Over the past 10–15 years, there have been great strides in refining site characterisation techniques (i.e. geophysical techniques), developing/discovering reactive materials/sorbents (i.e. Fe0 filings), and the installation and design of PRBs (i.e. funnel-and-gate design) which have increased the cost-effeciveness of this technology. Prior to installation, careful consideration of the ease of removal of the PRB should be considered as part of the design. This is important as the PRB may eventually need to be decommissioned. PRBs are a sustainable site specific remediation technology that has the great potential to work well as a part of a larger scale integrated water resource management programme in developing countries. Keywords: Permeable reactive barriers (PRBs); Reactive materials; Sorbents; Remediation; Site characterisation; PRB designs
1. Introduction Permeable reactive barrier (PRB) technology has been sucessful in remediating a variety of groundwater contaminants including heavy metals [1], organics [2] and radionuclides [1,3]. Most PRBs have been installed on industrial, mining and agricultural sites around the world [1–3]. PRBs use the natural hydraulic gradient of the groundwater plume to move the contaminants
through the reactive zone giving it an advantage over traditional pump-and-treat technologies by being more cost effective and lower maintenance in the long-term [3]. Over the past decade, much work has been done on improving site characterisation techniques, developing reactive materials/sorbents, and the installation and design of PRBs. This work has increased the costeffeciveness of this technology making it a more viable remediation option for developing countries.
Presented at the Water and Sanitation in International Development and Disaster Relief (WSIDDR) International Workshop Edinburgh, Scotland, UK, 28–30 May 2008. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.05.075
D.H. Phillips / Desalination 248 (2009) 352–359 Geophysical techniques such as magnetic and ground probing radar are a form of non-invasive site characterisation that yields valuable information about site geology, in situ engineering properties, hidden cultural features, and contamination in the shallow subsurface. Non-intrusive investigations are a quick and cost effective means of obtaining data, especially when combined with old site plans, and are useful in planning the intrusive site investigations. These techniques are reducing the need for more expensive trial pitting or borehole drilling with lower risks and decreasing the chances of missing buried targets. Some of the instruments (EM units) are hand carried and generally do not contact the ground. All accessible areas of a site can be quickly surveyed (up to 2 ha/day) without disturbing the surface [4]. This is important because there may be considerable surface and near surface contamination on former industrial and mining sites. Comparing site historical plans with their geophysical surveys is a very beneficial ground truthing method and is generally part of the protocol of a study. A variety of reactive materials and sorbents, which can be used separately or in combination depending on the groundwater contamination, have been successful in remediating contaminated groundwater in PRBs. These materials, such as Fe0 filings [4], peat [3], limestone [4,8], granular activated carbon (GAC) [5,6] and zeolite [1], are easily available and some are fairly inexpensive. Benchscale treatability studies are carried-out in the initial screening of the reactive or sorbent material to plan the design of PRBs using site groundwater. Batch studes using a number of likely reactive and sorbent materials are conducted to determine the best performing materials. Then column tests are carriedout on the best performers. Column tests can give information towards the design of the PRB and indications on how an in situ PRB will perform [7]. Installation of PRBs is a crucial stage, especially in the excavation of geological material. Improved equipment and techniques used to excavate geological material without obstructing the flow of the contaminated groundwater plume in and out of the PRB has helped to increase the success rate of PRB performance. During installation, loose geological material and soil can be packed, smeared and fill void space that the contaminated groundwater flows through adjacent to the
353
PRB [8,9]. Methods in depositing of the reactive material in to the PRB have also been refined to reduce particle size grading which can alter groundwater flow through the reactive zone [9]. Prior to installation, careful consideration of the ease of removal of the PRB should be considered as part of the design. This is important as the PRB may eventually need to be excavated due to the completion of remediation; therefore, decommissioning could become an issue [7]. Of the two basic designs, i.e. the continuous trench and the funnel-and-gate, the funnel-and-gate design with the reactive material placed in single or sequenced containers is probably the most cost-effective design. This is because the funnel-and-gate usually uses less reactive material than the continuous trench. The reactive material(s) is placed in the canister(s) (reactors) and can be removed if it needs to be replaced. This is an important consideration as there could eventually be built-up of contaminant concentrations in the reactive/sorbent material(s) from the remediation process. The containers can be designed to be reused at other sites once remediation is finished. PRBs are a sustainable site specific remediation technology that has the great potential to work well as a part of a larger scale integrated water resource management programe in developing countries. The objectives of the paper are to illustrate how PRBs are planned and installed, and highlight cost effectiveness which may allow them to be installed in developing countries. 2. Material and methods If PRB technology is considered one of the options for the remediation of contaminated groundwater at a site, there are a series of steps that should be taken to ensure that the PRB is viable, and that it is designed and installed properly. One of the first steps is to collect as much information about the site as possible, such as blueprints, geological maps, and records. This will help in ground truthing and determining contamination. What are the possible contaminants? Have there been any buildings on site? Buried debris may imped PRB installation and groundwater flow across the site. Knowing what contaminants may be present at the site will also allow for better health and safety plans.
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2.1. Site characterisation Non-invasive characterisation of the site using geophysical techniques, especially when used along with test pits and wells improves the detail of the site characterisation and is cost effective and time saving. Generally, a site is first cleared of surface objects (i.e. scrap metal, reinforced concrete rubble) that may interfere with subsurface features. A grid (i.e. 5 m) is surveyed out (i.e. using a Total Station Lecia 1010 Wild Heerbrugg surveyor). Further points are measured at the site boundaries and across the site to give a representation of height A.O.D. Electromagnetic (EM) and ground penetrating radar (GPR) instruments are portable units that can detect metal, debris, geological features, and contamination in the shallow subsurface. A Geonics EM61 time domain metal detector is used to survey for ferrous and non-ferrous metal objects down to 5 m. The instrument, operating at 75 Hz, is trawled across the site by the operator. The Geonics EM31 can map geological variations and any other feature associated with ground conductivity change down to 6 m. The EM31 is in the form of a boom that is carried about 1 m above ground. It can be used in a normal operating orientation (vertical dipole mode) or turned 90 to its long axis (horizontal dipole) and used to measure down boreholes. The Geonics EM38 also measures conductivity and is carried 15–20 cm above the ground. It is particularly sensitive to soil salinity which could be a characteristic of contaminated land/groundwater. Similar to the operation principle of the EM31, the EM38 surveys to the depth of 1.5 m in the vertical dipole, and 0.75 m in the horizontal dipole modes [10]. GPR is also used for high resolution shallow subsurface investigations. Ground truthing should be part of the protocol in a field investigation by comparing the geophysical results to historic plans and site visits.
several sampling intervals can be tested. Batch tests are carried-out in bottles or vials capped with inert septa. A known weight of reactive material and a known concentration and volume of contaminant solution (preferably the contaminated groundwater) are added to the vials. Samples are extracted at regular intervals and the contaminant concentration measured. Degradation curves can be made from this data, and degradation products can also be determined [7]. Column tests are used to collect detailed information on the degradation, precipitates, removal and/or sorption of contaminants by reactive/sorptive media. Removal rate data under a range of flow conditions, especially those that mimic groundwater flow velocity in the field can be determined with column tests. Contaminated or simulated groundwater is passed through the column at a known flow rate. The column design allows changes in contaminant composition and other parameters (e.g. major ions, pH) to be determined at the influent, effluent and along the column length (Fig. 1) [7]. Column dimensions are generally 10– 100 cm long, with a 2.5–3.8 cm inside diameter [8]. Glass columns are generally the least reactive or adsorptive with chlorinated organic compounds. However, no significant loss of organics have been found using Perspex columns [11]. Sampling ports along the length of the column should be constructed of stainless steel fittings or inert stoppers (i.e. Teflon coated or Viton). The ports should allow the sampling needle to be inserted into the centre axis of the column or a needle to be fixed in place in each port. A three-way port should also be positioned in the influent and effluent lines. All tubing and fitting for the influent and effluent lines should be composed of an inert material. Information from the column study can be used along with the site characterisation and modeling to help to design the field-scale PRB [7]. 2.3. Excavation
2.2. Treatability studies Batch tests are used to screen potential reactor materials. They can determine whether the contaminants are amenable to sorption, degradation, or precipitation by different types of media. They also can be used to compare treatment efficiency of reactive and sorptive materials. Contaminant degradation rates are measured quantitatively over time, and various parameters and
In conventional excavation of continuous trenches, the soil is removed and the trench is backfilled with reactive/sobent materials. When the emplacement of the reactive materials is completed, temporary retaining structures which support the walls will be removed from the ground. A backhoe comprised of a digging bucket on the end of an articuated arm is frequently used for rapid digging of shallow trenches [8] less that
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Fig. 1. Diagram and photograph of a column test.
10 m deep. A continuous trenching machine can also be used to create a narrow trench less than 7 m deep. It is comprised of a chain saw cutting belt and a trench box on the boom. The backfill of reactive/sorbent materials is carred-out with a hopper on the top of the trench box. The removal of soil and the backfilling of reactive materials are simutanously carried-out. This elminates the need for dewatering and temporary retaining structures [11,12]. An excavation of up to 15 m depth can be carried-out with a caisson for funnel-and-gate PRBs. Caissons with a circular cross-section are frequently used. After it has been installed, a large auger can be used to remove the soil within the caisson. The reactive/sorbent materials can be then backfilled into the caisson [11,12]. 2.4. PRB design The two common PRB designs are the continuous trench PRB and the funnel-and-gate system (Fig. 2a and b). The continuous trench PRB does not contain any structures, so the contaminant plume flows through the treatment zone using the natural hydraulic gradient. This PRB, which is perpendicular to groundwater flow direction, needs to be slightly larger than the crosssectional area of the contaminated groundwater inorder to capture the contaminants in both vertical and horizontal directions [9]. The top of the PRB should be at
least 0.60 m above the water table and the bottom of the PRB should be extended at least 0.30 m into a low permeability zone (i.e. clay), if it is present. The PRB thickness should be designed to provide sufficent residence time for the contaminants within the treatment zone to be completely treated. The funnel-and-gate system is composed of impermeable walls and at least one reactive zone. The funnel structure could be sheet piles or slurry walls. The function of the funnel is to intercept the contaminated groundwater and lead it to the treatment zone. The bottom of the funnel and reactive zone needs to be extended at least 0.30 m into the less permeable soil layer, while the top of the funneland-gate needs to be set at least 0.60 m above the water table [10]. The reactive material is directly implaced or filled into the reaction vessel(s) [9]. Multi-sequenced reactive barriers are also being installed, especially on sites with multiple groundwater contaminants such as gas works sites. Multi-sequenced PRBs use mulitiple reactive materials in more than one reactive zone to treat the contaminated groundwater [13] (Fig. 2c). 3. Results and discussion 3.1. Cost-effective groundwater clean-up with PRBs Over the past 10–15 years since the first PRBs were installed to remediate contaminated groundwater, there
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Fig. 2. Diagrams of PRBs. (A) Elevation view of a continuous trench or wall (after [14]), (B) plain view of a funnel-and-gate, and (C) elevation view of a multi-barrier.
have been great strides in reducing their costs. Compared to traditional pump-and-treat remediation, PRBs can be more costly in the initial stages, especially during installation. However, since PRBs are a passive system, which relies on the natural hydraulic gradient of the groundwater plume to move the contaminated groundwater through the treatment zone, the longterm costs are lower than traditional pump-and-treat operations and maintenance [14]. Many PRB reactive/sorbent materials can remediate a range of contaminates, while others are selective. PRBs can be used as part of a treatment train with other technologies to clean groundwater. PRBs are generally below ground and ‘out of sight’ so the site can be used for
other uses. However, there are some disadvantages of PRBs. Sometimes the PRB require more maintenance than originally planned. PRBs can also become clogged and the reactive material coated causing them to become less effective. Non-invasive site characterisation has also decreased the cost of PRB installation. Many of the magnetic and ground probing radar units are portable and can easily be transported into sites that may be remote. Data on the characteristics of a contaminated site can be gathered rapidly and can help in planning the invasive characterisation (trial pitting and boreholes). Data collected on buried features, geology and hydrogeology can be used to model or better plan for
D.H. Phillips / Desalination 248 (2009) 352–359 monitoring and remediation of contaminated land and groundwater. The suite of chemicals contributing to subsurface groundwater contamination may be electrically conductive, either because the contamination is acidic or contains salt. Therefore, the groundwater plume is also electrically conductive [4]. Importantly, the non-invasive site characterisation can also give indications if the site is not geologically and hydrogeologically fit for a PRB to be installed (i.e. absence of low permeability material to secure the bottom of the PRB), which can reduce money and time spent on a remediation option that may fail. Treatability studies may be carried-out at a number of stages of PRB selection including remediation options screening, PRB design and operation [7]. Treatability studies are used to determine the best reactive/ sorbent media, to help design the PRB, and to predict weaknesses in the PRB design. The cost of treatability studies can vary greatly, and should be proportionate to the total cost of remediation. Generally, treatability studies should cost <10% of the total remediation costs. To cut costs, reactive material from local sources can be screened in a treatability study. For example, activated carbon from coconut husk is used to treat arsenic contaminated groundwater [6]. Other media such as Fe0 [3], sawdust [1], plant material [1], and zeolite from volcanic rock [1] are also used in PRBs as well as microbes [1] in biobarriers. Using local sources may save money on purchasing the media and also renewing it. Additionally, the use of recycled materials makes the PRB a sustainable remediation option. The conventional excavation is a common PRB construction technique and is more suitable for shallow PRB systems such as a continuous trench <10 m deep. Operational cost can increase with the depth of excavation. The conventional excavation can be carried-out with common excavation equipment such as a backhoe, a clamshell and a caisson [12]. Because the trench is located in shallow strata, the whole installation of the PRB can be easily monitored. Some potential problems can occur with the conventional excavation. In addition to a decrease in the permeability of the PRB [11] from smearing, contaminated soil and water can potentially be exposed during the excavation raising health and safety issues. Nevertheless, the operational costs and time are low for continuous trenching [12]. Although, the funnel-and-gate PRB is physically easier to
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decommission, the continuous trench PRBs can still remain an option if the geology and hydrogeology is favorable for installation, and if there is a lack of funds and resources to install a funnel-and-gate PRB. In the funnel-and-gate PRB, reactive material is stored in canisters or reactors which can be removed after the PRB has completed the remediation or if the PRB has problems. This allows for removing of potentially further contamination from the site as a result of contaminant build-up in the reactive material. Additionally, funnel-and-gate PRBs generally require less reactive material. This is a great cost saver, if the reactive material is pricey. For these reasons, the funneland-gate design should be considered if funds are available. Therefore, it is important to take the design of the PRB in to consideration and how this will affect its long-term operation. 3.2. Case studies of PRBs There are over 100 PRBs in operation around the world at present; however, only a few are installed in developing countries. Examples are pilot-scale PRBs used to treat contaminated groundwater and leachate from uranium mines in Hungary and Bulgaria, respectively. The PRB in Pecs Hungary which was installed in 2002, is a continuous trench containing shredded Fe0 which removes uranium from the groundwater. Sand layers have been added to the up-gradient and down-gradient sides of the PRB to allow for better groundwater flow through the reactive zone. The PRB was emplaced in an underlying clay and geosynthetic clay liner. Groundwater U concentrations were reduced to <1% of influent concentrations after passing through the reactive zone in year 2003. Uranium concentrations were reduced from *1000 mg/L to *100 mg/L in monitoring wells near the PRB and <10 mg/L within the PRB. A negative performance issue is that a high amount of precipitates has been estimated to have formed in the PRB which may reduce groundwater flow through the PRB and reduce the reactivity of the Fe0. However, only a 1.6% loss in porosity was calculated and the PRB is predicted to have a 62 year lifespan [3]. The PRB in Western Bulgaria (installed 2004) treats acid drainage (pH 2.5–3.7) runoff from a uranium mine contaminated with radionuclides (mainly uranium and radium), heavy metals, arsenic
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Fig. 3. PRBs as part of a larger scale integrated water resource management programme in developing countries.
and sulphates. The PRB is part of a treatment train (connected series) in the form of an alkalizing limestone drain that removes Fe as hydroxides, the PRB for biosorption and microbial reduced sulphates, and a natural wetland. The barrier is a ditch/continuous trench with a reactive material consisting of a mixture of solid biodegradable organic material (plant and mushroom compost, wood chips, straw and cow manure), crushed limestone and zeolite saturated with ammonium phosphate. A mixed microbial community consisting of sulphate-reducing bacteria and other microorganisms is also present. Uranium, arsenic and non-ferrous metals were mainly removed in the PRB by the indigenous sulphate-reducing bacteria. Portions of the pollutants, and most of the radium, were sorbed onto the dead plant material in the PRB [1]. Another example of a PRB, but not in a developing country, is a funnel-and-gate PRB in Monkstown,
Northern Ireland, United Kingdom. This PRB, established in 1995, is a field-scale PRB developed to remediate a small but highly concentrated accumulation of a degreaser pollutant, trichlorethene (TCE), in gravely glacial till on an industrial site. TCE is a dense nonaqueous phase liquid (DNAPL) that is denser than water and has a tendency to settle in the subsurface as immiscible accumulations (slugs). The PRB consists of a long tube-like structure that holds Fe0 filing reactive material. This structure is emplaced in naturally occurring clay at the site. The TCE contaminated groundwater is funneled from the up-gradient position into the gate of the PRB where the vessel that holds the reactive material is present. The groundwater flows down through the Fe0 reactive zone in the PRB where it is treated and exits at the down-gradient. There is a decrease in up-gradient TCE concentrations in this Fe0 funnel-and-gate PRB over time which suggest that
D.H. Phillips / Desalination 248 (2009) 352–359 the PRB is remediating the TCE contaminated groundwater at the site. However, there is also a possibility that a great portion of the TCE slug was removed while installing the PRB which would cause the concentrations to be lower over time (Fig. 3). There is decrease in TCE as it moves through the PRB indicating that it is being remediated (dehalogenated) by the Fe0. A pre-existing TCE contamination occurs adjacent to the down-gradient portion of the PRB possibly due the clipping of the end of the TCE contamination slug when the PRB was being installed which hampers the down-gradient monitoring of this PRB; however, data shows that it is dramatically decreasing over time [2]. When PRBs were first being inplemented about 15 years ago, it was suggested that they become a remediation option for developing countries. This is because they are a sustainable remediation technology that requires little maintenance and operational cost over a long duration. Addtionally, reactive/sorbent materials such as peat, sawdust, activated carbon, zeolites, and limestone can be aquired easily and may be from a local source. Generally, PRBs are used for site specific remediation. However, in areas where adequate water supply is limited and there is contamination from industrial, agricultural or mining sites, if properly planned they could become part of a larger scale integrated water resources management programme. For example, an International Water and Sanitation Center (IRC) project DREAM (Drainage and Reuse of Effluents for Agricultural Management) has planned PRBs to be part of an integrated water resources management programme as a low cost approach for wastewater treatment for reuse in agricutural areas in developing countries where water supply is low [15].
References [1] S. Groudev, P. Geprgiev, I. Spasova, M. Nicolova, A. Angelov and L. Diels, Cleanup of acid mine drainage by means of a pilot-scale passive system, Annu. Univ. Min. Geol. ‘St. Ivan Rilski’ Part. 1. Geol. Geophys., 48 (2005) 217-220. [2] P. Beck, N. Harris and R. Sweeney, Design, Installation and Performance Assessment of a Zero Valent Iron Permeable Reactive Barrier in Monkstown, Northern Ireland, CL:AIRE Technology Demonstration Report, TDP3, 2001, 120pp.
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[3] K.E. Roehl, K. Czurda, T. Meggyes, F. Simon and D.I. Stewart, Long-term Performance of Permeable Reactive Barriers, Elsevier, New York, 2005. [4] US Army Corps of Engineers, Geophysical Exploration for Engineering and Environmental Investigations, EM 1110-1-1802, 1995. [5] V. Plagentz, M. Ebert and A. Dahmke, Remediation of ground water containing chlorinated and brominated hydrocarbons, benzene and chromate by sequential treatment using ZVI and GAC, Environ. Geol., 49 (2006) 684-695. [6] G.N. Manju, C. Raji and T.S. Anirudhan, Evaluation of coconut husk carbon for the removal of arsenic from water, Water Res., 32 (1998) 3062-3138. [7] Environment Agency, Guidance on the Use of Permeable Reactive Barriers for Remediating Contaminated Groundwater, National Groundwater and Contaminated Land Centre Report NOC/01/51, Environment Agency, Solihull, UK, 2002. [8] United States Environmental Protection Agency, Permeable Reactive Barrier Technologies for Contaminant Remediation, EPA 600-R-98-125, 1998. [9] A. Gavaskar, N. Gupta, B. Sass, R. Janosy and J. Hicks, Design Guidelines for Applications of Permeable Reactive Barriers for Groundwater Remediation, Battelle, Columbus, OH, Report No. AFRL-ML-WP-TR2000-4546, NTIS: ADA380005, 2000, 399pp. [10] US Department of Transportation, Determination of the Depth/Structure/Fracture of Bedrock, 2005, http:// www.cflhd.gov/agm/engApplications/Index.htm. [11] A. Gavaskar, N. Gupta, B. Sass, R. Janosy and D. O’Sullivan, Permeable Barriers for Groundwater Remediation: Design Construction and Monitoring, Battelle, Columbus, Ohio, 1998. [12] US Air Force, Design Guidance for Application of Permeable Barriers to Remediate Dissolved Chlorinated Solvents, DG 1110-345-117, 1997. [13] L. Dries Bastiaens, J. Vos, Q. Simons, M. De Smet and L. Diels, Comparison of different multibarrier concepts designed for treatment of groundwater containing mixed pollutants, in: A. Boshoff, B. Bone, eds., Permeable Reactive Barriers, International Symposium on Permeable Reactive Barriers, March 14, 2004, Queens University Belfast Northern Ireland, vol. 298, 2005, pp. 45-51. [14] Environmental Protection Agency, A Citizen’s Guide to Permeable Reactive Barriers, EPA 542-F-01-005, 2001, 2pp. [15] International Water and Sanitation Centre, Annual Report, The Netherlands, 2004, 24pp.