Assessment of the Performance of Engineered Waste Containment Barriers (2007)

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

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