Design, Construction and Operation of a Funnel and Gate In-Situ Permeable Reactive Barrier for Remediation of Petroleum Hydrocarbons in Groundwater
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DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL ANDGATE IN-SITU PERMEABLE REACTIVE BARRIER FORREMEDIATION OF PETROLEUM HYDROCARBONS IN
TERRY McGOVERN1, TURLOUGH F. GUERIN2, STUART HORNER1and BRENT DAVEY3
1 SRS Australia Pty. Ltd., Environmental Consultants, Werribee, Victoria, Australia; 2 ShellEngineering, NSW State Office, P.O. Box 26, Rosehilly, New South Wales 2142, Australia; 3 Prpic
Davey Consulting, Murrumbeena, Victoria, Australia( author for correspondence, e-mail: firstname.lastname@example.org, phone: 61-417-124453)
(Received 27 November 2000; accepted 4 April 2001)
Abstract. A white spirit spill at a factory site located in a residential area of south eastern Australialed to contamination of shallow groundwater that fed into a nearby river. The contaminated ground-water contained toluene, ethyl benzene, and xylene and n-alkanes in the C6C36 fraction range. Afunnel and gate permeable reactive barrier was designed and built, based on preliminary pilot scaletests using peat as the medium for the gate and the work conducted is presented as a case study.The technical effectiveness of the funnel and gate, over the 10 month operating period in which datawas collected, indicates that peat represents an effective material for use in the gate component offunnel and gate remedial systems. The application of the funnel and gate technology represented asubstantial saving to the client and was effective in preventing ongoing pollution of the nearby river.The construction of the funnel and gate system also incurred the minimum disturbance to the publicaccess areas between the facility and the river.
Keywords: case study, dissolved phase, efficiency, groundwater, passive remediation, peat, per-meable barriers, petroleum hydrocarbons, remediation, sparging
Treatment walls, or permeable reactive barriers, first reported by McMurty andElton (1985), involve construction of permanent, semi-permanent, or replaceableunits across the flow path of a dissolved phase contaminant plume (Starr andCherry, 1994; Vidic and Pohland, 1996). As the contaminated groundwater movespassively through the treatment wall, contaminants are removed by physical, chem-ical and/or biological processes, including precipitation, sorption, oxidation/reduc-tion, fixation, or degradation. These barriers may contain agents that are placedeither in the path of contaminant plumes to prevent further migration or immedi-ately downgradient of the contaminant source to prevent plume formation (Vidicand Pohland, 1996).
Water, Air, and Soil Pollution 136: 1131, 2002. 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Several methods have been developed for the installation of permeable treat-ment walls (Eykholt and Sivavec, 1995; Steimle, 1995). The majority of experiencewith installation of these walls is with relatively shallow emplacements (
DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL AND GATE 13
previously been observed with peat used to remove dissolved and free phase andpetroleum hydrocarbons from water (Stehmeier, 1989). In addition, contaminantrelease or resolubilisation may occur.
The funnel and gate system is an application of a permeable reactive barrierfor in situ treatment of dissolved phase contamination (Starr and Cherry, 1994).Such systems consist of low hydraulic conductivity cut-off walls (e.g. 1 106 cms1) with one or more gaps that contain permeable reaction zones. Cutoff walls(the funnel) modify flow patterns so that groundwater primarily flows through highconductivity gaps (the gates). The type of cutoff walls most likely to be used in cur-rent practice are slurry walls, sheet piles, or soil admixtures applied by soil mixingor jet grouting. At the current time there are relatively few published reports on thefull-scale design and operation of funnel and gate systems and related systems fordissolved phase petroleum hydrocarbons (Bowles et al., 2000; Schad and Schulze,2000) with even fewer reporting the use of peat (Kao and Wang, 2000). The currentarticle presents the findings of a case study on the use of a funnel and gate systemin Australia.
A white spirit petroleum hydrocarbon spill occurred at a factory facility in SouthEastern Australia in December 1997. This volatile petroleum hydrocarbon wasused routinely as a solvent at the facility (Table I). A leaking underground storagetank (UST) caused a quantity of the white spirit (20003000 L) to leak throughthe scoria fill material in which the UST was located, into the soil of the upperembankment at the rear of the facility. A quantity of the white spirit emerged atthe base of the slope, discharging into a spoon drain, and then via a culvert, flowedinto the nearby river. A proportion of the leaked white spirit was absorbed by thesoil in the embankment and an amount entered the groundwater underlying the site(Figures 1 and 2).
3. Study Objective
The aim of the remedial works conducted in this study was to intercept the dis-solved phase plume of petroleum hydrocarbons present in the shallow aquifermoving toward the river, using a funnel and gate permeable barrier, and reduce con-centrations to less than regulatory limits. A funnel and gate system was consideredto be the most cost-effective way for treating petroleum hydrocarbon contaminatedgroundwater and preventing further contamination of the river. In the proposed re-mediation strategy, contaminated groundwater flow would be directed towards thegate which would then be subject to two treatment processes, applied in sequence,within the funnel and gate system. The processes applied were (1) air sparging, as
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DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL AND GATE 15
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TABLE IComposition of White Spirita
Component Name % (v/v)
C6C9 Benzene 0.07%Toluene 0.04%Ethyl benzene 1.10%Total xylene 20%
Total C6C9 23%C10C14 n-Alkanes 77%C15C28 n-Alkanes
DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL AND GATE 17
TABLE IIDevelopment of the remediation strategy after the White Spirit spill
Key activity Date Tasks
Control and containment December 1997 Boom and sorbents placed in river to captureof spill spilled solvent leaving the site
Removal of leaking tanks
Initial characterization December 1997 Sampling of soil and groundwaterof soil and groundwater February 1998 Developed model of contaminationcontamination and its at the siteextent
Selection of remediation March 1998 Selection of funnel and gate optionstrategy
April 1998 Feasibility study to determine optimumconfiguration of peat
Identified options for remediatingpetroleum hydrocarbon contaminated soil
Implementation of MarchMay 1998 Installation of funnel and gateremediation strategy
Remediation of contaminated soil excavatedfrom the site
Monitoring July 1998 Initial 10 month groundwaterMay 1999 sampling and analysis program was
conducted to assess effectives of funneland gate system
Review of effectiveness March 1998 Soil remediation was validated by samplingof remediation strategy August 1998 and analysis of the recirculating air during
bioventing, and sampling and analysis of thesoil in the biovented piles for BTEX andn-alkanes
September 2000 Three monthly groundwater sampling andOnwards analysis program is planned
of the site, as identified in monitoring wells (MW) 9 and 10. Varying thicknesses offill comprise the upper and lower embankments which is of a highly heterogeneousnature.
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4.2. SITE HYDROGEOLOGY
A highly localised, shallow perched water table is present in the fill material whichcomprises the embankment between the factory and the nearby river. The regionalwatertable is expected to be located at approximately the same depth as the levelof the river. Residual clays from the weathered Silurian siltstones tend to be imper-meable, and as such perched water tends to be located at the fill-natural sandy clayinterface.
4.3. AQUIFER TESTING
Aquifer recovery tests were carried out on monitoring wells MW1, MW2, MW3,MW4, MW5, MW6, MW7, and MW8 on 20 November 1998. The procedureinvolved the removal of a known volume of groundwater from the well and mon-itoring the subsequent recovery with time. The hydraulic properties of the fillmaterial were investigated (by slug test) to determine the hydraulic conductivityof this material. The Hvorslev method (Fetter, 1988) was used to calculate thehydraulic conductivity in the immediate proximity to each well tested.
4.4. CONSTRUCTION OF FUNNEL AND GATE SYSTEM
A volume of1600 m3 of petroleum hydrocarbon contaminated soil was excavatedas part of establishing the funnel and gate system. This was placed in 3 bioventedpiles at the facility (refer to biobanks in Figure 1). Details of the remediation of thissoil are not given in this report. The funnel and gate system was comprised of animpervious barrier membrane (i.e. the funnel), directing groundwater into the treat-ment area (i.e. the gate). The gate consisted of a sparging unit upgradient of a blendof peat materials. The funnel component, designed to intercept the contaminantplume before it entered the nearby river, consisted of a 0.75 mm thick High-Density Polyethylene (HDPE) impervious barrier membrane positioned verticallyin the cut-off trench to capture and redirect incident groundwater over the lengthof the spill area, parallel to the nearby river. The gate was composed of sequentialtreatment systems comprising a sparging unit emplaced in basaltic scoria, followedby blended peat materials. The funnel was designed to intercept groundwater flowfrom areas directly downgradient of the spill site, as well as adjacent areas in whichlateral migration of the plume may be occurring. The dimensions of the funneltrench are 27 m long 5 m deep 0.6 m wide, excavated to a level of 0.5 mbelow the fill-natural siltstone interface.
4.4.1. Gate ConstructionA stepwise procedure was undertaken so as to ensure the final integrity of thegate and prevent the collapse of the riverbank. This was achieved in the followingmanner. Chain mesh fencing (2 m 15 m long section) was laid flat along theriver-embankment interface and fastened to the ground using 1.5 m long steel star
DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL AND GATE 19
Figure 3. Transect through the funnel and gate system (as built) (refer to Section AA, Figure 2).
pickets to aid long term stability. Sheet piling was inserted into the embankmentparallel to the river bank, at a distance of approximately 4.0 m from the river,dividing the areas to be excavated for both components of the gate system. The areadesignated for the peat (between the river bank and the sheet piling) was excavatedto a depth of 0.5 m below the siltstone-fill interface, and infilled with peat to a depthof 0.5 m below surface. The remaining volume was filled with natural soil overlyinga layer of A15 Type Bidim Geofabric to prevent fine soil particles mixing with andclogging the peat and scoria filtration media. The area was then sown with grassseed in order to promote bank stability. Excavated soil was removed for bioventing.Subsequent to the emplacement of the peat, soil adjacent to the opposite side of thesheet piling was excavated to a depth of 0.5 m below the fill-natural siltstone stratainterface, and a horizontal sparging tube and scoria was emplaced. Thirty cubicmeters of peat (on a wet volume basis of estimated at 40% moisture) was used inthe gate.
4.4.2. Sparging Component of GateThe section of the gate that initially encounters influent groundwater consists of avolume (5 m3) of porous basaltic scoria. The scoria was emplaced over a sub-merged, perforated air sparging pipe, creating fine air bubbles, which permeate thewater column within the gate, delivering oxygen to enhance biodegradation. Airwas supplied to the system by a 12-cfm compressor, supplying air at 0.9 m3 hr1.Soil gas and air concentrations of petroleum hydrocarbons were monitored at thesite using a Foxboro TVA 1000A Flame Ionization Detector (FID) and a Photo-Ionization Detector (PID) unit. A transect diagram through the gate is given inFigure 3.
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4.4.3. Peat Composition and Implications for Contaminant RemediationAfter the groundwater was sparged (as it passed through the scoria as describedabove), the groundwater then passed through a peat mixture, immediately down-gradient of the sparging system. Previous studies indicate the high sorptive ca-pacities of humic materials (such as peat) for petroleum hydrocarbons (Kao andBorden, 1997). Peats with low fibre contents and high lignin pyrolysis materialand ash content have been shown to be the most effective peats to remove free anddissolved phase petroleum hydrocarbons from groundwater (Cohen et al., 1991).The ability to both adsorb dissolved and free phase petroleum hydrocarbons fromgroundwater, and provide a catalytic surface on which microbial activity can occur,indicated the suitability of peat for use in groundwater treatment (Cohen et al.,1991). Cohen et al., (1991) have indicated that a wide range of peats is effectivein removal of petroleum hydrocarbon contamination from water, with rates of re-moval efficiency between 63 and 97%. Using this information, a locally availablepeat, known commercially as Biogreen Humic Reed Sedge, with low fibre and highhumic content was selected for use onsite.
4.4.4. Peat Blending and Implications for Hydraulic ConductivityAn important consideration in the design and construction of funnel and gate sys-tems is that the treatment media be of an equal or greater hydraulic conductiv-ity than that of the surrounding substrate, to allow incident groundwater to bechanneled through the treatment aperture. This is particularly important as pre-vious studies have shown that the variability of the saturated hydraulic conduct-ivity of peat can be as high as 10 orders of magnitude, depending on the typeof peat. Accordingly, hydraulic conductivity tests were performed on a varietyof peat/sphagnum peat/cocoa fibre blends, in order that an optimal mixture, withrespect to sorptive capacity, air circulation and hydraulic conductivity, be achieved.Tests were conducted using the falling head test method (Schwartz and Domen-ico, 1997). Equipment consisted of a column of packed, saturated peat. The fall inhydraulic head over time (K) was measured from a manometer tube, and substi-tuted in the following equation:
K = 2.3(aL/A(t1 t0)) log10 h0/h1 ,where
a = cross sectional area of the stand pipe,L = length of sample,A = cross sectional area of sample tube,(t1 t0) = elapsed time required for a head fall from h1 to h0,h0 = starting height of water in the manometer tube,h1 = end height of water in the manometer tube.
DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL AND GATE 21
A blend of 70% peat (Biogr...