11.3 Well Drawdown, Specific Capacity, and Interference
Key Takeaways
- Drawdown is the difference between the nonpumping water level and the pumping water level at a well or observation point.
- Specific capacity is Q / s, commonly reported as gpm per ft of drawdown, and it is a practical field indicator of well performance.
- Confined steady radial-flow equations use transmissivity and a logarithmic radius term because the flow area expands with distance from the well.
- Interference occurs when drawdown cones from nearby pumping wells overlap; in many confined-aquifer exam problems, drawdowns can be superimposed.
- A falling specific capacity during a constant-rate test can indicate well loss, clogging, boundary effects, aquifer dewatering, or delayed drainage.
Drawdown and Well Performance
The April 2024 PE Civil WRE specification explicitly lists well and drawdown analysis. The exam can ask for a direct calculation, but it can also ask what a pump test result means for water supply, dewatering, or site impacts. Start every well problem by separating water levels, pumping rate, aquifer response, and well efficiency.
Core Terms
Static water level is the water level before pumping or after recovery. Pumping water level is the level during pumping. Drawdown, s, is static level minus pumping level at the same location. In a pumping well, measured drawdown includes aquifer drawdown plus additional head loss near and inside the well. In an observation well, drawdown better represents aquifer response because it does not include screen entrance and wellbore losses from the pumped well.
Specific capacity is:
Specific capacity = Q / s
It is often reported in gpm/ft. A larger value means more yield for each foot of drawdown. It is not a fixed aquifer constant because it can change with pumping duration, pumping rate, well condition, turbulent well losses, boundaries, and seasonal recharge.
| Item | Formula or meaning | Exam use |
|---|---|---|
| Drawdown | s = static level - pumping level | Basic water-level calculation |
| Specific capacity | Q / s | Quick well performance comparison |
| Transmissivity | T = K b | Confined radial-flow analysis |
| Radius of influence | R | Approximate outer limit of drawdown cone |
| Interference | Sum of overlapping drawdowns | Multiwell and field operation questions |
Radial Flow Equations
For steady confined radial flow to a fully penetrating well, a common form is:
Q = 2 pi T (h2 - h1) / ln(r2 / r1)
A drawdown form is:
s = Q ln(R / r) / (2 pi T)
These equations reflect that the cylindrical flow area increases as radius increases. Do not use a rectangular Darcy area for a well unless the prompt intentionally simplifies the geometry.
For an unconfined aquifer under Dupuit assumptions, a common steady equation is:
Q = pi K (h2^2 - h1^2) / ln(r2 / r1)
The heads h1 and h2 are saturated thicknesses above the aquifer base. If drawdown is large compared with saturated thickness, using a confined approximation can overstate or understate the result depending on how the problem is framed.
Pump-Test Workflow
- Record static water level, pumping rate, pumping water level, time, and observation-well distances.
- Compute drawdown at each measured point.
- Compute specific capacity if well performance is requested.
- Use observation-well data when estimating aquifer transmissivity because pumped-well data include extra well losses.
- Check whether the assumptions match the formula: confined or unconfined, steady or transient, fully penetrating or partial penetration, constant rate, and appropriate boundary conditions.
- Compare late-time trends for recharge boundaries, impermeable boundaries, delayed yield, or nearby pumping.
Interference
When two wells pump near each other, their cones of depression overlap. In many PE-level confined-aquifer problems, the aquifer response is treated as linear, so drawdowns from separate wells can be added at a point. This superposition idea is useful for wellfield spacing, dewatering systems, and evaluating whether a nearby production well will be affected.
Interference is not always bad. A dewatering system uses intentional interference among wellpoints or deep wells to lower groundwater across an excavation. For water supply wells, excessive interference can reduce available drawdown, increase pumping cost, and trigger impacts at property boundaries or wetlands.
Practical PE WRE Traps
Specific capacity is Q divided by drawdown, not drawdown divided by Q. A lower pumping water level means larger drawdown. Pumping-level depths below ground surface must be converted carefully if static and pumping levels are given as elevations. Also watch logarithms: radial-flow equations usually use natural log unless the formula supplied by the problem uses log base 10 with a conversion factor.
A well has a static water level 42 ft below ground surface. During a constant-rate test at 450 gpm, the pumping water level stabilizes at 60 ft below ground surface. What is the specific capacity?
Two production wells pump from the same confined aquifer. At a nearby observation point, Well A alone would cause 4.2 ft of drawdown and Well B alone would cause 2.8 ft. If superposition is appropriate, what combined drawdown should be expected?