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SOIL VAPOUR EXTRACTION AS AN INVESTIGACION, MONITORING AND REMEDIATIONDECIDING TOOL

Maite Garcia1, Juan Fernndez

1and Laia Juncosa

1

1TUBKAL INGENIERIA, S.L., Joan Gamper 25, E-08014 Barcelona (www.tubkal.com)

Key words: VOC, XCOV, SVE, UST, gas stations

Summary

After more than 300 vacuum assays performed by TUBKAL INGENIERIA, this paper describes SVEtests applied for site quality investigation and control purposes and it includes some practicalsimplified examples that show the capabilities of the technique.

1 Introduct ion

Conventional sampling techniques often do not access to mostpolluted areas or contamination sources when these are locatedbelow tanks, pipelines or other underground facilities; a typical

case in gas stations. XVOC pollution is also difficult to hit whensampling, as they percolate with a narrow, fingering profile. Theresult is that, even when soil samples from a site investigationcomply with soil quality criteria, there may be an undetectedcontamination problem to cope with.

These limitations in site investigation are most worrying when soilquality evaluation is critical, such as in a property transfer, whenthere is no shallow groundwater to detect potential soilcontamination or in sensible residential use of land.

To overcome these limitations, TUBKAL INGENIERIA started touse the Soil Vapour Extraction (SVE) technique, traditionally

applied for soil remediation, to detect VOC contamination leaks orsources in unsaturated soils. The grounds are that if SVEmobilises VOC present in soil through the application of vacuum,this same technique could also be used to evaluate VOC soilquality trough the quality of soil gas extracted, giving informationfor a bigger volume of soil (10-20 m radius from the extractionpoint) than punctual and discrete soil sampling.

2 Method

An SVE test consists of extracting soil-air from a piezometer (or vapour detector) and periodicallymeasuring air flowrate, vacuum in extraction and monitoring points, and quality of the air extracted(VOC, O2, CO2, T, etc). Before the test ends, an air sample should be taken for VOC laboratory

analysis. That is, a SVE test is performed in a similar way than SVE remediation technique but it onlylasts a few hours and therefore more frequent data needs to be compiled.

Before executing SVE tests on the field, some aspects must be taken into consideration:

- Piezometers (or vapour detectors) should be adequately constructed to apply vacuum (mainly,that there is a good surface seal and enough open filter to air, that is unsaturated soil) andthey should be located near all potential VOC sources (UST, pipelines, etc.).

- Nearby underground infrastructures may cause air preferential pathways, altering results andfinally disabling the detection of potential contamination sources. This can also be the casedepending on the soil layers geometry with different permeability and the depths of sources.

- A previous theoretical performance estimation is recommended. SVE flowrate may beevaluated using Johnson equation (1991) which takes into account the site-specificpermeability. Therefore, time for the SVE test can be predicted so that it reaches the requireddistance or radius of control to cover nearby potential contamination sources. On the otherhand, a vacuum pump should be selected according to the site-specific permeability.

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Interpretation of results considers that soil-air is extracted from a homogenous cylinder in the subsoil,with increasing radius as times elapses. On this base, the relation between time and radius of controldepends on air flowrate, height of soil layer where air moves and its porosity.

rcontrol = [ tcontrol x Q / ( x bsoilx nsoil)] ; tcontrol = x r

2control x bsoilx nsoil / Q

Where rcontrol radius of control (m)tcontrol time of test (h)Q air flowrate (m

3/h)

bsoil thickness of soil layer where air moves (m)nsoil effective porosity (-)

For each extraction point VOC data (usually from PID, ppmv) should be plotted versus time andversus distance. Interpretation of VOC evolution versus distance over the actual site plan will confirmthe control of all potential contamination sources and it may indicate leaking points or polluted areas.See examples included at the end of this paper. Focus should also be placed on VOC data evolutionin terms of time and distance, as it may give more information about contamination profile.

VOC mobilization rate (kg/day) is also evaluated and plotted as described for VOC data. Thisparameter would help overcome misinterpretation due to dilution effects in high permeability cases.The mobilization rate is also a remediation deciding criteria. Mobilisation rate may be estimated on thebasis of in-situ VOC measures (ppmv) but it should be recalculated with laboratory analysis (mg/m

3),

as in some cases there may be some relevant differences between both data.

T = Q x CVOCx 24 x 10-6

Where T mobilisation rate (kg/day)

Q air flowrate (m3

/h)CVOC VOC concentration (mg/m

3)

CVOC= PID x FC x PMcomp / VM or CVOC= PID x FC x 10-6x VOC

On CVOC VOC concentration (mg/m3)

PID PID measurement, usually based on isobutylene (ppmv)FC PID correction factor for specific VOC (-)PMVOC VOC molecular weight (g/mol)VM gas molar volume (22,4 l/mol at 1 atm and 25C)VOC VOC gas density at specific pressure and temperature (mg/m

3)

High VOC measures in extracted air and/or high VOC mobilisation rates indicate a VOC polluted area

nearby the extraction point or, if tests are repeated periodically, a potential recent leak.

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Other SVE test data to evaluate are:

- Radius of influence for each SVE test, which is the maximum distance where the vacuum isdetected. Radius of influence should be higher than radius of control to guarantee theobjectives of the SVE test. Radius of influence should be measured on site trough monitoringpoints and, ideally, for different SVE operating conditions (flowrates and vacuums). Althoughnot recommended, radius of influence may also be estimated by another Johnson equation

(1991).

- Air permeability in the basis of Johnson equations (1991) to check the soil layer that allows airmobilisation with vacuum.

- Changes in SVE operation parameters according to flowrate and vacuum evolution in terms oftime (and distance). This sort of changes may be caused by sudden air intrusion through anearby slope or another relevant underground infrastructure, which may distort the idealcircular situation. Oxygen and carbon dioxide data may give some extra hints about the causeof a change in operating parameters.

The detailed methodology for SVE tests and their interpretation for gas stations are detailed in

chapters 2 and 3 of the document Methodological guide to perform vacuum tests in vapour detectorsin gas stations developed in 2012 by TUBKAL INGENIERIA under contract of the public Agncia deResidus de Catalunya

1.

3 Application Example: periodic monitoring o f soil quality in a gas station

Yearly application of SVE tests in an urban gas station with 4 piezometers installed as vapourdetectors demonstrated a gasoline leak around one UST, later confirming that it was a result ofdefective tightness in the supply pipe.

SVE tests performed in years2009 and 2010 indicatedsimilar VOC background

levels in the subsoil of thegas station (VOC around 100ppmv and maximum of 400ppmv and mobilisation ratesbelow or around 1 kg/day forall tested points).

Nevertheless, SVE tests in 2011 showed an increase in values, mainly in vapour detector CV1 whereVOC measures in extracted air were above 3.000 ppmv and mobilisation rate was calculated ofaround 12 kg/day through air samples analysis. Compiled data in 4 vapour detectors indicated that themost polluted area was nearby CV1 and between CV1 and CV4; approximately at a distance of 2-3 mfrom CV1 and 6-7 m from CV4. See graphics and site map interpretation.

1http://www20.gencat.cat/docs/arc/Home/LAgencia/Publicacions/Sols%20Contaminats/sols_guia_carburants.pdf

Date PointDuration(hh:mm)

Flowrate(m3/h)

Vacuum(mbar)

VOC(ppmv)

Rate(kg/day)

CV1 3:00 35 185 400 1,3

CV2 6:00 22 185 100 0,3CV3 3:00 32 187 < 100 0,1

2010

CV4 3:30 45 185

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After leak confirmation and repair, remediation was easily performed by SVE in the piezometer CV1until VOC lecture in extracted air was reduced to 10 ppmv in less than 3 months; around 400 kg of

gasoline were removed. The subsequent SVE tests concluded effective remediation of soil andindicated once again low background VOC levels, which allow continuing with periodic vapourmonitoring.

GAS STATION - CV1

0

1.000

2.000

3.000

4.000

0 1 2 3 4 5 6 7

Distance (m)

VOC(p

pmv)

2011

2009

2010

GAS STATION - CV2

0

200

400

600

800

1.000

0 1 2 3 4 5 6 7 8 9

Distance (m)

VOC

(ppmv)

2011

2009

2010

GAS STATION - CV3

0

100

200

300

400

500

0 1 2 3 4 5 6 7 8

Distance (m)

VOC

(ppmv)

2011

2009

2010

GAS STATION - CV4

0

200

400

600

800

1.000

0 1 2 3 4 5 6 7 8 9

Distance (m)

V

OC

(ppmv)

2011

2009

2010

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4 Application Example: site investigation at a chemical facility

PCE and TCE groundwater contamination in a deep aquifer was detected and a chemical facilitynearby was required to perform a first site investigation.

Soil and groundwater investigation included drilling and installation of 4 deep piezometers (45 m)

located considering groundwater flow direction and 9 shallow vapour detectors (5 to 8 m) all nearbypotential contamination sources. Conventional soil gas survey was not implemented as subsoil is veryheterogeneous with interbedded clay layers with fine sand layers.

Only very low levels of XVOC were detected in the analysed soil samples; concentrations ranged frombelow detection limit to a maximum of 0,33 mg/kg of PCE. These low levels respond to vapourmigration but they show that XVOC source areas, if so, have not been identified nor prospected. Onthe other hand, groundwater contamination was confirmed but these specific results did not provideeither sound information about potential sources.

Afterwards, SVE tests wereperformed and XVOC contaminationwas detected in soil air quite in a

small area of the property; PIDmeasures were above 1.000 ppmv in4 of the 13 points tested, reaching amaximum of 5.800 ppmv inpiezometer Pz4. Air samplesanalysed showed high concentrationsof PCE and, in a lesser proportion,TCE and 1,2-cis-dicloroethylene.Mobilisation rates were as high as 54kg/day in piezometer Pz4, indicatinga high polluted source area nearby.

SVE tests results showed that contamination source was not related to different ASTs containingXVOC solvents (either now or in the past); the most polluted area happened to coincide with an old pitlocated in a corner of the site although not clearly defined in geometry (now all the facility is paved),where several chemical products were probably dumped some years ago. But even the 2 soil samplesanalysed from the borehole in this area (Pz4) did not show high XVOC concentrations.

PointDuration(hh:mm)

Flowrate(m3/h)

Distance(m)

VOC(ppmv)

Rate(kg/day)

Pz1 5:45 145 8,7 50 0,6

Pz2 4:25 160 8,0 110 1,1Pz3 4:05 110 6,4 100 0,8

Pz4 5:50 118 7,9 5.800 54,0

Cv1 3:30 65 10,2 150 0,8

Cv2 4:00 103 13,7 25 0,2

Cv3 2:00 78 8,4 402 2,7

Cv4 3:00 50 8,3 270 1,2

Cv5 3:00 85 10,8 1.200 8,5

Cv6 1:30 80 7,4 20 0,2

Cv7 1:55 72 7,9 2.500 15,5

Cv8 3:00 92 11,2 110 0,8

Cv9 1:55 77 8,2 1.800 10,5

CHEMICAL FACILITY - CV5

0

500

1.000

1.500

2.000

2.500

3.000

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Distance (m)

VOC

(ppmv)

0

3

5

8

10

13

15

R

ate(kg/dia)

CHEMICAL FACILITY - CV7

0

500

1.000

1.500

2.000

2.500

3.000

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Distance (m)

VOC

(ppmv)

0

3

5

8

10

13

15

R

ate(kg/dia)

CHEMICAL FACILITY - CV9

0

500

1.000

1.500

2.000

2.500

3.000

0 1 2 3 4 5 6 7 8 9

Distance (m)

VOC

(ppmv)

0

3

5

8

10

13

15

Rate(kg/dia)

CHEMICAL FACILITY - Pz4

0

1.000

2.000

3.000

4.000

5.000

6.000

7.000

0 1 2 3 4 5 6 7 8 9

Distance (m)

VOC

(ppmv)

0

10

20

30

40

50

60

70

Rate(kg/dia)

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Tests also showed that SVE was very effective as a remediation technique and installation wasoptimised with only 4 SVE points. In 4 months more than 1.500 kg of XVOC (PCE and TCE mainly)have been effectively removed from the unsaturated soil and remediation proceeds.

5 Summary and Conclusions

SVE tests can be used as a powerful investigation tool to detect VOC sources or hot spots inunsaturated soil, either to further optimize soil investigation efforts or to decide whether it is advisable

to proceed with remediation.

Moreover, if applied periodically in an existing network of vapour piezometers, this technique can alsobe used to detect future soil contamination from new VOC leaks from the facility; making SVE tests avery useful and competitive environmental tool to survey soil quality in gas stations and in otherfacilities with underground VOC tanks and pipelines.

References

Agncia de Residus de Catalunya (ARC), Guia metodolgica per a la realitzaci dassaigs de buit(proves deficincia) en captadors de vapors en estacions de servei , July 2012

ARC, Guia de prevenci de la contaminaci del sl per a les activitats potencialment contaminants delsl sota lepgraf CCAE 50500: venda al detall de carburants de lautomoci , March 2009.

US Army Corps of Engineers. Engineering and Design Soil Vapour Extraction and Bioventing, EM1110-1-4001, June 2002.

US EPA, Detecting Leaks, Successful Methods Step-by-Step November 1989

US EPA, Tank Issues, Design and Placement of Vapour Monitoring Wells, October 1990

US EPA, Soil Vapour Extraction Technology - Reference Handbook, February 1991

US EPA. Standard Test Procedures For Evaluating Leak Detection Methods: Vapour-Phase Out-Of-Tank Product Detectors, March 1990.

US EPA, Background hydrocarbon vapour concentration for underground fuel storage tanks, 1992

US EPA, Decision-Support Software for Soil Vapour Extraction Technology Application:HyperVentilate, February 1993.

US EPA. Guidelines for Vapour Monitoring, April 1998.