π Lesson 12
D5
Measuring and Interpreting Piezometric Levels in Multi-Aquifer Systems
Piezometric level is the height to which water would rise in a sealed tube drilled into an aquifer, showing how much pressure the groundwater is under.
π― Learning Objectives
- β Explain the physical meaning of piezometric head and distinguish it from water table elevation
- β Analyze piezometric data from nested piezometers to identify hydraulic connectivity or barriers between aquifers
- β Calculate vertical hydraulic gradient between two aquifers using measured piezometric levels and stratigraphic separation
- β Apply Hvorslev and ASTM D5084 methods to interpret aquifer response during slug tests in multi-layered systems
π Why This Matters
In mining, inaccurate interpretation of piezometric levels can lead to catastrophic slope failures, uncontrolled inflows into excavations, or ineffective dewatering designs. For example, a higher piezometric level in a deep confined aquifer may drive upward seepage through weak clay layers β causing heave or piping beneath a pit floor. Understanding these levels across multiple aquifers isnβt academic: it directly controls whether your dewatering wells will succeed, your slope stability models are valid, and your environmental compliance holds.
π Core Principles
Groundwater in confined aquifers is under pressure due to overlying low-permeability layers (aquitards), resulting in piezometric levels that may lie above the top of the aquifer β even above ground surface (artesian conditions). In multi-aquifer systems, each aquifer has its own piezometric surface; differences reflect vertical hydraulic conductivity of intervening aquitards and historical or ongoing pumping stresses. Critical concepts include: (1) piezometric head = elevation head + pressure head (in meters water equivalent); (2) hydraulic head continuity across aquitards is not assumed β leakage occurs per Darcyβs law; (3) nested piezometer installations (multiple screened intervals in one borehole) allow direct comparison of heads across strata without spatial aliasing.
π Vertical Hydraulic Gradient Between Aquifers
This gradient determines direction and magnitude of inter-aquifer flow. A positive gradient (Ξh > 0) from Aquifer 1 to Aquifer 2 indicates downward flow if Aquifer 1 is upper; sign convention depends on coordinate system. Used in analytical leakage models (e.g., Hantush, 1967) and numerical calibration.
Vertical Hydraulic Gradient (i_z)
i_z = (h_1 β h_2) / (z_1 β z_2)Dimensionless ratio quantifying driving force for vertical inter-aquifer flow.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| h_1 | Piezometric head in Aquifer 1 | m ASL | Head in upper or reference aquifer |
| h_2 | Piezometric head in Aquifer 2 | m ASL | Head in lower or target aquifer |
| z_1 | Midpoint elevation of Aquifer 1 screen | m ASL | Geometric center of monitoring interval |
| z_2 | Midpoint elevation of Aquifer 2 screen | m ASL | Geometric center of monitoring interval |
Typical Ranges:
Tightly layered glacial till sequence: 0.001 β 0.05
Fractured basalt over sedimentary bedrock: 0.1 β 0.5
π‘ Worked Example
Problem: A nested piezometer in a coal mine site records: Piezometric level in Upper Sand Aquifer = 182.4 m ASL; Piezometric level in Lower Limestone Aquifer = 179.1 m ASL. The mid-depths of the aquifer screens are at 175.3 m ASL and 162.8 m ASL, respectively. Calculate i_z.