The use of the

Membrane Interface ProbeIn PHASE II Site Investigations:

Capabilities, Data Interpretation, Correlation with Analytical Data and Limitations of the Technology

ASTM accepts the use of field screening and field analytical methods in the course of Phase II ESA and highlights the need for a set of standard operating procedures and protocols:

BACKGROUND

P ERMEABL E M EM BRANE

VOLATILE ORGANIC CONTAMINANTS IN SOIL

E LECTRICAL CONDUCTIVITYPRO BE

CARRIER GAS SUP PLY(F ROM MIP CONTROL LER )

GAS RETURN TUBE (TO CO LL ECTOR)

Electric Conductivity (EC)

dipole array

VOC molecules

In the soil

Semi-permeable

membrane

Carrier gas supply

Carrier gas return

to detectors

The Membrane Interface Probe (MIP) is rapid, high-resolution field screening technology that provides information about relative concentrations of VOCs in the subsurface, and Electrical Conductivity of the soil.

•The MIP uses a thin film fluorocarbon polymer membrane approx. 6.35mm in diameter which stays in direct contact with the soil during MIP logging. •The thin film membrane is impregnated into a stainless steel screen which serves as a rigid support for the fluorocarbon polymer. •The down-hole, permeable membrane serves as an interface to a detector at the surface. •Volatiles in the subsurface are getting transferred across the membrane and partition into a stream of carrier gas where they are swept to the detector. The membrane is heated in order to facilitate VOC transfer and self-cleaning.

BACKGROUND

What is Membrane Interface Probe?

A set of Standard Operating Procedures (SOP) and a preliminary “MIP Output Interpretation” guide were developed in 2000 by ZEBRA Environmental as a result of documentation requirements for performing work under Federal Contracts, using the input provided by EPA Region I and the US Navy.

DOCUMENTATION

CAPABILITIES

The list of successfully detected to date compounds includes:

• Full range of BTEX compounds

• Methane

• TBA (more research required)

• Chlorinated solvents (PCE through VC)

• Chlorofluorocarbons (freon)

Detected compounds:Detected compounds:

CAPABILITIES

Detection Limits:Detection Limits:

Consistent BTEX detection by the MIP system is observed at concentrations as low as 400 ppb. The detection limits for individual compounds are affected by numerous factors that vary from one site to another. A site-specific detection limit should be established for each site.

CAPABILITIES

Depth Limitations:Depth Limitations:

MIP logging has been performed successfully to the depth of 90’ BG.

The depth limitation for the MIP logging on a particular site depends on three factors:

• Subsurface material compaction and bedrock presenceThe MIP generally has the same limitations as other types of Direct Push equipment in that

regard, including Geoprobe sampling systems and the majority of CPT equipment.

• Pore pressureHigh pore pressure conditions sometimes encountered in saturated clay formations below

50’ BGS can cause water breakthrough across the membrane, rendering the equipment temporarily inoperable.

• Boiling point of a target compoundZEBRA’s SOP requires an operator to allow the membrane to reach a temperature in excess of the boiling point (BP) of a target compound. The BP increases with depth due to an

increase in the barometric pressure. For some compounds, starting at a certain depth, that requirement can not be met due to limitations of the probe’s heater output, thus leading to a

a reduction in sensitivity and, consequently, increased detection limits.

DATAINTERPRETATION

Vapor phase:

This log was recorded in weathered shale above the water table, adjacent to a pump island at a gas station. The probe hit refusal at 7’ BGS. The detector response was produced by gasoline vapors present in the subsurface, above the free-phase gasoline at a lower depth.

Flat elevated sections of the graphs indicate detector saturation at a given Range and Attenuation setting

LNAPL

.      

 LNAPL (diesel fuel):

DATAINTERPRETATION

NOTE: LNAPL signatures exhibit certain variations for different hydrocarbons and different site conditions. A correlation must be established for proper data interpretation

DATAINTERPRETATION

Dissolved contamination zone

Dissolved phase:

Dissolved petroleum hydrocarbons can be reliably detected by the MIP system at concentrations of 400 ppb and above; dissolved phase halogenated solvents have been consistently detected at concentrations as low as 190 ppb (using ECD)

Soil Conductivity Data Interpretation:

HAND-AUGERED

HOLE

WET FINE SILTY SAND CLAY SILTY SAND INCREASING

SILT CONTENT

FRESH WATER ZONE SALT WATER INTRUSION

NOTE: actual soil conductivity values for each type of soil vary from one region to another. A correlation with conventional and/or rotosonic soil sampling must always be established for each site.

DATAINTERPRETATION

CORRELATINGDATA

1

10

100

1000

10000

100000

1000000

10000000

Analytical Data

MIP Data

Series3

Correlation Data:

The correlation between analytical data and the MIP data is typically in the range

of 70 to 80%, when raw data is used. Further data processing, such as baseline adjustments and data normalization, significantly improve the correlation.

Correlation coefficient = 0.722

This graph shows correlation by plotting both data sets in a logarithmic scale

CORRELATINGDATA

Most common data processing errors:

The MIP response is normally measured as the magnitude of a detector’s response above its baseline. A detector’s baseline signal varies depending on many factors.

BASELINE

DETECTOR RESPONSE

CORRELATINGDATA

Most common data processing errors, cont’d:

1. By far the most common mistake is to use a detector signal’s absolute value for data interpretation. This can lead to erroneous interpretation, especially when MIP data is being used for preparing cross-sections, or in any kind of automated data processing.

On the examples above, both graphs show almost identical response magnitude, though with different baselines. A person using this data, without accounting for the difference in baselines, could come to a conclusion that the detector response in the first case is higher than in the second.

CORRELATINGDATA

Most common data processing errors, cont’d:

2. Another typical mistake, is not accounting for fluctuations of the detector baseline that occur due to subsurface conditions. On the graph below, the initial (“dry”) baseline changed dramatically as soon as the probe entered the water table. The detector’s response must be measured in relation to this new baseline.

QUALITYASSURANCE

QA/QC:

• Monitoring of physical parameters

of the MIP systemParameters such as carrier gas pressure and

flow rate, heater temperature cycling, trip time

for individual compounds and detector baselines are being constantly monitored by the crew to ensure consistency of physical parameters of the system, which is the key for obtaining reproducible results.

• Detector response testing

In the beginning of each day, and subsequent to any adjustment or repair done to the MIP equipment, a series of tests is performed specifically ensuring a consistent sensitivity levels of each detector to a group of contaminants. An effort is made to perform testing with specific compounds of concern for each site.

• Detector cleaning and trunkline purging

A new, stricter maintenance schedule is currently being implemented, which includes shorter time intervals between detector cleaning and jet replacement, and stricter purging requirements for all the gas tubing.

RELIABILITY

Improving reliability:The reliability of MIP equipment has been an issue since it’s introduction. In the beginning, it was not uncommon to have up to 30% downtime on a MIP project.

In the course of two years of development by ZEBRA’s MIP team, a large number of equipment modifications and procedural changes has been implemented that resulted in the decrease of on-site down time to below 5%. The improvements include:• adding a high level of redundancy to most of the critical components of the system

• incorporation of new connectors and fittings• adding additional system monitoring instruments

allowing to detect impending malfunctions earlier,

before damage occurs• dedicated data acquisition computer• implementation of new Standard Operating Procedures• improved personnel training

PRODUCTIVITTY

Production rates:

The production rate of the MIP logging is affected by many factors, such as:

• the number of logging locations• the depth of logging• subsurface conditions• access restrictions• probe hole abandonment requirements• weather

The average production rate has recently increased as a result of improvements to the MIP system design and is currently in the range of 120’ to 300’ of logging per day.

1-800-PROBE-IT1-800-PROBE-IT

www.zebraenv.comwww.zebraenv.com

ZEBRA Environmental Corporat ion 2001ZEBRA Environmental Corporat ion 2001Parts of this presentation reproduced under permission of EA EngineeringParts of this presentation reproduced under permission of EA Engineering

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