Well hydraulics explores how groundwater flows into wells during pumping. It's crucial for understanding aquifer behavior and well performance. Key concepts include drawdown, cone of depression, and aquifer properties like transmissivity and storativity.

Pumping tests are essential for determining aquifer characteristics and well efficiency. These tests involve monitoring water levels in pumping and observation wells, then analyzing the data using methods like Theis and Cooper-Jacob to estimate aquifer properties and well performance parameters.

Well Hydraulics

Principles of well hydraulics

• Well hydraulics describes groundwater flow into a well during pumping
• Drawdown is the water level lowering in a well and surrounding aquifer due to pumping
  • Measured as the difference between static water level (before pumping) and pumping water level
  • Increases with pumping rate and duration, decreases with distance from the well (inverse square law)
• Cone of depression is the 3D zone of lowered water levels around a pumping well
  • Shape and extent depend on aquifer properties (permeability, porosity), pumping rate, and duration
• Aquifer properties influence the magnitude and extent of drawdown
  • Transmissivity ($T$) is the rate water is transmitted through an aquifer per unit width under a unit hydraulic gradient (m²/day, ft²/day)
  • Storativity ($S$) is the volume of water released from storage per unit surface area of the aquifer per unit decline in hydraulic head (dimensionless)

Purpose of aquifer pumping tests

• Determine aquifer properties (transmissivity, storativity, hydraulic conductivity) and well performance (efficiency, specific capacity, radius of influence)
• Methodology:
  1. Install observation wells at various distances from the pumping well to monitor drawdown
  2. Measure pre-pumping static water levels in the pumping well and observation wells
  3. Pump the well at a constant rate for sufficient duration to achieve near-steady-state conditions
  4. Monitor and record water levels in the pumping well and observation wells during pumping
  5. Measure water levels during the recovery period after pumping has stopped
• Analyze data using various methods (Theis method, Cooper-Jacob method) to estimate aquifer properties

Pumping Test Analysis

Interpretation of pumping test data

• Theis method: Graphical method using a type curve to match observed drawdown data
  • Type curve plots Theis well function ($W(u)$) vs. $1/u$, where $u$ is a dimensionless time parameter
  • Matching observed drawdown data to the type curve yields estimates of $T$ and $S$
• Cooper-Jacob method: Semi-logarithmic graphical method using a straight-line approximation of the Theis solution
  • Plot drawdown data against the logarithm of time since pumping started
  • Slope of the straight line calculates $T$, intercept estimates $S$
• Determine well performance parameters (efficiency, specific capacity) from pumping test data
  • Well efficiency: ratio of theoretical drawdown to actual drawdown in the well
  • Specific capacity: pumping rate divided by drawdown in the well at a given time

Concepts in well performance

• Well efficiency: measure of how effectively a well converts available drawdown into usable discharge
  • Expressed as a percentage, affected by well design, construction, and development
  • 100% efficiency would have no additional drawdown due to well losses (friction, turbulence)
• Specific capacity: pumping rate divided by drawdown in the well at a given time (usually after 24 hours)
  • Expressed in volume per time per length (gpm/ft, m³/day/m)
  • Useful indicator of well performance, can estimate long-term yield
• Radius of influence: distance from the pumping well to the point where drawdown is negligible
  • Defines the extent of the cone of depression and the area affected by pumping
  • Depends on aquifer properties, pumping rate, and duration, estimated using pumping test data