Appendix B
Methods for Monitoring Engineered Barrier Performance

Parameter

How Measured

Use

Comments

1. Phreatic surface (water table)

Observation (monitoring) wells

Establish hydraulic gradient in uppermost aquifer or perched groundwater surface

Monitoring zone depends on screened interval

2. Hydraulic head in groundwater

Vibrating wire, pneumatic, and standpipe (Casagrande) piezometers

Establish hydraulic gradients and groundwater flow velocities

Flow velocities are based on permeability values; requires knowledge of point of measurement to establish elevation head

3. Constituent chemical concentrations in groundwater

Chemical analysis of groundwater samples for organic and key inorganic constituents

Establish background concentrations and concentration gradients and detect releases

Representative background values may be difficult to establish in complex geologies

4. Subsurface distribution of chemical concentrations

Electrical and acoustic surveys

Identify breaches in barriers and preferred groundwater flow paths

Rarely used in practice

5. Surface projection of extent of chemical concentrations

Geophysical surveys (e.g., electrical resistivity, EM, GPR)

Identify and map groundwater plumes of certain contaminants

Rarely used in practice

6. Volumetric moisture content in soil (θ)

Time domain reflectometry

Determine wetting front and determine indirectly unsaturated hydraulic conductivity (k) and soil suction (ψ) via established k versus θ and ψ versus relationships

Provides a direct measurement of moisture content, which also can be determined indirectly through measurement of soil suction (see 7) and use of an established soil-water characteristic curve (ψ vs. θ)

7. Soil suction (ψ)

Gypsum blocks, psychrometers, suction lysimeters, tensiometers

Establish soil suction gradients and infer seepage under unsaturated flow conditions

Range of suctions measured varies depending on instrument

8. Percolation through barriers

Pan lysimeters (underdrains)

Establish leakage rates for bottom barriers before and after waste emplacement and for covers

Accuracy of measurement is a function of boundary conditions

9. Gas-phase constituent concentrations and flow rates through cover systems

Gas/air samples analyzed using handheld instruments and/or flux chambers

Determine quantity and quality of gas emissions and air quality

Complex geospatial modeling may be required to analyze downwind measurements obtained from tracer tests; point measurements from flux chambers may not capture emission patterns; results of questionable quality

10. Gas-phase constituent concentrations in gas collection systems

Subsurface probes (see above) placed at the mouth of boreholes

Establish constituents of concern, identify releases, and establish concentration gradients

Provides a direct indication of the performance of the gas collection system and an indirect indication of cover performance



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Assessment of the Performance of Engineered Waste Containment Barriers Appendix B Methods for Monitoring Engineered Barrier Performance Parameter How Measured Use Comments 1. Phreatic surface (water table) Observation (monitoring) wells Establish hydraulic gradient in uppermost aquifer or perched groundwater surface Monitoring zone depends on screened interval 2. Hydraulic head in groundwater Vibrating wire, pneumatic, and standpipe (Casagrande) piezometers Establish hydraulic gradients and groundwater flow velocities Flow velocities are based on permeability values; requires knowledge of point of measurement to establish elevation head 3. Constituent chemical concentrations in groundwater Chemical analysis of groundwater samples for organic and key inorganic constituents Establish background concentrations and concentration gradients and detect releases Representative background values may be difficult to establish in complex geologies 4. Subsurface distribution of chemical concentrations Electrical and acoustic surveys Identify breaches in barriers and preferred groundwater flow paths Rarely used in practice 5. Surface projection of extent of chemical concentrations Geophysical surveys (e.g., electrical resistivity, EM, GPR) Identify and map groundwater plumes of certain contaminants Rarely used in practice 6. Volumetric moisture content in soil (θ) Time domain reflectometry Determine wetting front and determine indirectly unsaturated hydraulic conductivity (k) and soil suction (ψ) via established k versus θ and ψ versus relationships Provides a direct measurement of moisture content, which also can be determined indirectly through measurement of soil suction (see 7) and use of an established soil-water characteristic curve (ψ vs. θ) 7. Soil suction (ψ) Gypsum blocks, psychrometers, suction lysimeters, tensiometers Establish soil suction gradients and infer seepage under unsaturated flow conditions Range of suctions measured varies depending on instrument 8. Percolation through barriers Pan lysimeters (underdrains) Establish leakage rates for bottom barriers before and after waste emplacement and for covers Accuracy of measurement is a function of boundary conditions 9. Gas-phase constituent concentrations and flow rates through cover systems Gas/air samples analyzed using handheld instruments and/or flux chambers Determine quantity and quality of gas emissions and air quality Complex geospatial modeling may be required to analyze downwind measurements obtained from tracer tests; point measurements from flux chambers may not capture emission patterns; results of questionable quality 10. Gas-phase constituent concentrations in gas collection systems Subsurface probes (see above) placed at the mouth of boreholes Establish constituents of concern, identify releases, and establish concentration gradients Provides a direct indication of the performance of the gas collection system and an indirect indication of cover performance

OCR for page 113
Assessment of the Performance of Engineered Waste Containment Barriers Parameter How Measured Use Comments 11. Leachate hydraulic head on the primary liner Vibrating wire piezometers and liquid-level measurements in sumps using drop-down resistivity probes Assess the performance of the leachate collection and removal system Measurements beyond sumps are rare, although vibrating wire piezometers on the liner have performed well in some cases 12. Volumetric seepage in the LCRS and LDS Pumped volume or flow meter, depending on the system Evaluate the effectiveness of LCRS and the primary liner system Can provide an indirect assessment of cover performance, LCRS efficiency, liner integrity, and development of clogging 13. LCRS continuity Dye testing and pumping tests Indicates any clogging in the LCRS Rarely used in practice 14. Leachate constituent concentrations Chemical analysis of leachate samples for organic and inorganic constituents Identify constituents of concern and evaluate the potential for mass flux of contaminants and degradation of the barrier system (e.g., hydraulic conductivity) May be misleading (with respect to constituents of concern) due to chemical transformation within the liner system and subgrade 15. Geomembrane continuity Electrical leak detection using conductive geomembranes or wire grids placed below membranes Establish the location and frequency of defects in geomembranes Typically used only in CQA, as the measuring techniques are ineffective when soil or waste cover on the geomembrane exceeds a meter or more 16. Settlement (surface and at depth) Survey markers, settlement forks, extensometers Determine settlement of cover systems Total and differential settlements are required to assess cover performance 17. Temperature of soil and geosynthetic barrier components Thermocouples Estimate the service life of geosynthetics, determine thermal gradients, and conduct heat and moisture transfer analysis Historically, rarely used in practice, but some recently reported field studies indicate measurement is important 18. Vertical barrier continuity Geophysical methods, field measurements of hydraulic conductivity of slurry walls and of heads and constituent concentrations inboard and outboard of the wall Identify defects in vertical barriers Geophysical methods have potential but are rarely used in practice; hydraulic conductivity measurements are employed primarily for CQA via tests on field-recovered samples 19. Vertical barrier leak detection Wells, drainage layers installed along the midsection of vertical barriers Determine the amount of leakage and thus the performance of vertical walls Results of questionable reliability; rarely used in practice; requires installation of the collection and removal system in the barrier; integrity of half of the thickness of the barrier is assessed 20. Radioisotope concentrations Total radiation dose Identify releases and establish concentration gradients Primarily of concern for low-level radioactive waste NOTES: CQA = construction quality assurance; EM = electromagnetic; GPR = ground-penetrating radar; LCRS = leachate collection and removal system; LDS = leak detection system; TDR = time domain reflectometry.