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 |