Estimates of contaminant mass flux can improve decision making, reduce remediation costs by targeting areas of high flux, and in some instances aid in predicting remediation performance—especially with regard to determining the feasibility of monitored natural attenuation. However, there are limitations in developing mass flux estimates. Making estimates from integral pumping tests or pump and treat systems can provide good estimates of mass discharge but may not be effective—even with multiple pumping wells in identifying horizontal or vertical high versus low flux zones. If a sampling point approach is taken point, the heterogeneity of the flow system may still result in an under or over estimation of flux even with relatively closely spaced sampling points and detailed characterization of hydraulic conductivity and groundwater flow fields. Collection of data for either of these will add to the costs of the site characterization or remedy performance efforts. In short, mass flux estimates can have high degrees of uncertainty, which can be significant and difficult, but not impossible, to quantify. The Table below gives advantages and limitations of flux measurements in general and those related to specific techniques.
Advantages and Limitations of Mass Flux and Mass Discharge Estimates
1
| Method |
Advantages |
Limitations |
| All Mass Flux Methods |
- Improves source strength characterization.
- Improves potential to understand where high-contaminant-strength areas are and to focus remediation accordingly.
- Improves assessment of natural and enhanced attenuation.
- Direct measurement of contaminant loading to receptors.
- Potential basis for relevant and measurable performance requirements.
|
- Potential increase in characterization and/or monitoring costs.
- Uncertainties related to subsurface heterogeneities.
- May require long times for fluxes to reach equilibrium after treatment.
|
| Point and Transect Sampling |
- Greater spatial information on flux and variations.
- Less purge water disposal needed.
- No change in natural flow regime.
|
- Increased cost for sample points and analyses.
- Higher risk for error in mass discharge estimates due to missed high-flux zones.
- Beed for high-resolution characterization, especially hydraulic conductivity.
- Greater risk of interpolation errors.
|
| Passive Flux Meter |
- Provides an estimate of both groundwater and contaminant fluxes.
- Probably good for analytes.
- Does not require bringing gas (e.g., nitrogen, air) or electricity into the field.
- No purge water to dispose of.
- Flux measurements can be used for process control, remedial action performance assessments, and compliance monitoring.
|
- Releases resident tracers into the groundwater, which may require regulatory approval.
- For measurements taken in excess of 60 m, the current PFM design can be difficult to implement (ESTCP 2007).
- In very heterogeneous aquifers, the number of wells needed to provide adequate flux assessments will increase (ESTCP 2007).
- Only measures contaminants passing through well screen (as opposed to integral pumping tests which draw water from around the wells).
- Must be retrieved before resident tracers are exhausted.
|
| Well Capture or Integral Pump Test |
- Reduced interpolation error.
- Greater certainty of capturing all of the mass at a given location.
- Low potential for missing high flux zones.
|
- Increased costs for water treatment and disposal.
- Potential for error due to under- or overcapture of plume.
- Loss of spatial information.
- Potential capture of water that may not migrate under natural flow regimes.
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