Title:
Introduction
Text: The passive flux meter (PFM) was developed at the University of Florida and has been demonstrated at several sites across the country as part of an Environmental Security Technology Certification Program (ESTCP) project.
The PFM was designed to measure contaminant and groundwater flux in saturated zones simultaneously when deployed in the subsurface via a borehole or a monitoring well.
Typically, several PFMs are deployed along a transect to measure the contaminant and groundwater flux. PFMs have been deployed to monitor chlorinated solvents, hydrocarbons, pesticides, and dissolved inorganics such as phosphate, nitrate, and metals.
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Title:
Theory (1 of 2)
Text: The PFM is a permeable unit that can fit tightly in a screened well or boring.
Organic/inorganic contaminants present in the groundwater, which flow through the meter, are retained by the hydrophobic and/or hydrophilic permeable sorbents in the meter. The contaminant mass intercepted and retained on the sorbent is used to quantify cumulative contaminant mass flux.
The sorptive media also releases water soluble tracers called ‘resident tracers’ into the groundwater that are displaced from the sorbent at rates proportional to the groundwater flux. Therefore, the soluble tracers are used to quantify the cumulative groundwater flux.
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Title:
Theory (2 of 2)
Text: The mass flux can be readily determined under the following conditions:
(1) the permeable sorbent captures and retains the contaminant from groundwater flowing through the meter;
(2) the contaminant can be extracted from the sorbent or analyzed in the sorbed state for purposes of quantifying the mass captured; and
(3) the contaminant does not undergo degradation inside the PFM.
The PFM works on the assumption that the groundwater flow and the contaminant transport are parallel and also that the contaminant transport is advective, uniform, and horizontal. The desorption of resident tracers released in the groundwater by the PFM is assumed to be linear, reversible, and instantaneous.
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Title:
Passive Flux Meter Application
Text: The PFM is deployed for a period ranging from days to months into a boring or a monitoring well. The current cost of PFM deployment and data evaluation ranges from approximately $150 to $200 per linear foot of saturated thickness investigated.
During this entire time, the meter is exposed to the groundwater flow and contaminants. At most sites, as shown here, a series of PFMs are deployed in a row perpendicular to the groundwater flow.
After the meter is removed, the sorbent is carefully extracted to quantify the mass of all contaminants intercepted and the residual masses of all resident tracers are also quantified. Contaminant mass is used to calculate time-averaged or cumulative contaminant flux, while residual resident tracer mass is used to calculate time-averaged or cumulative groundwater flux.
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Title:
Data Collection and Interpretation
Text: The figure shown here is a cross-section of the sorbent matrix.
The first picture depicts the tracer displaced to the right in the direction of the groundwater flow. The residual mass of the tracer is used to quantify cumulative water flux. The mass of contaminant sorbed on the matrix in the same way is used to quantify the contaminant flux.
The mass flux using PFM is calculated using a function that is dependent upon (1) specific discharge in the PFM, (2) mass of contaminant sorbed, (3) length of the sorbent matrix, (4) retardation of contaminant on the sorbent, and (5) relative mass of a resident tracer retained in the PFM.
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Title:
Performance Assessment
Text: PFMs are ideally suited for evaluating pre- and post-remediation mass flux values. Most conventional techniques for assessing technology performance rely upon mass-balance criterion.
A typical remedial action objective will be to achieve maximum contaminant level (MCL) values for the contaminant in groundwater. This approach neglects the fractional mass removal of the remedial option in place. The combination of fractional mass removal estimates with an accurate quantification of the pre- and post-treatment mass flux of contaminants is required to evaluate the effectiveness of aggressive source removal.
The figure shown here is an example of pre- and post-remediation mass flux determinations made using PFMs. The figure represents the mass flux distribution after the source zone treatment application at Hill Air Force Base, Utah. TCE was the major contaminant at this site. The TCE mass flux was significantly reduced post-remediation, which demonstrates effective application of the remediation technology within the source zone.
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Title:
Advantages/Limitations
Text: Advantages:
PFMs generate spatial information on cumulative water and contaminant mass flux.
Uncertainty associated with the deployment of the technology in the field is determinable.
The instrument generates local estimates of horizontal aquifer conductivity.
Small waste volumes are produced by the deployment of PFMs on-site.
Inexpensive
Limitations:
Relationship between water flow through the sorbent and release of tracers governs the data procurement.
The system generally works with very low volumes of aquifer.
The effectiveness in which contaminant is extracted from the sorbent matrix can alter the entire mass flux data.
Degree of flow convergence and divergence around the well with a PFM in place is very essential to understand for mass flux determination.
Does not function in all wells.
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