Aquifer Tests in Unconfined Aquifers

1. Lauren Cameron Spring 2014 Aquifer Tests in Unconfined Aquifers

2. Topics • Unconfined vs. Confined • Parameters to Measure • Delayed Gravity Drainage Effects • Steady and Transient Solutions • Example Analysis with AQTESOLV

3. What does “Unconfined” Mean? • Upper boundary of aquifer is a water table, lower boundary is no-flow • Delayed gravity drainage occurs within the drawdown cone near well • Transmissivity is not constant near the pumping well • Vertical components to flow near well

4. Basic Conceptual Sketch Vertical components to flow – vadose & saturated zones Delayed Gravity Drainage in the drawdown cone Saturated thickness decreases near the well

5. Analytical Solution Accommodations • Variable transmissivity • Drawdown assumed to be small relative to the saturated thickness – so it can be neglected • Transmissivity is therefore assumed to be constant • Otherwise, one must use a numerical solution • Components of vertical flow • Vertical conductivity, Kv, is a parameter • Controls the duration of delayed yield • And the specific yield = aquifer storativity

6. Specific Yield • Volume of water that will drain by gravity per unit area per unit decline in head. • Inversely related to grain size – lab ranges: • Sand/gravel: 20 to 35% (0.2 to 0.35) • Silt/clays: < 10 % (< 0.1) • Strongly time-dependent parameter

7. Drainage Near Falling Water Table Source: Bear (1972)

8. Aquifer Storativity Ranges Inferred from Aquifer Tests • Confined: 10-7 to 10-4 • Semi-confined: 10-4 to 10-2 • Unconfined: 10-2 to 10-1

9. Consider the Following… • Given two aquifers that have the same transmissivity. • One is confined the other unconfined. • You pump both at the same rate for the same amount of time. • Which direction would the type curve shift when matching the drawdown-time data … • Up or down … along the vertical drawdown axis? • Right or left … along the horizontal time axis?

10. Answer: Shift Horizontally to the Right

11. Shift Right – Why? • We’re shifting along the time scale is in the direction of increasing Storativity. • The larger the storativity, the slower the drawdown response. • Recall hydraulic diffusivity, T/S … • The smaller the diffusivity, the slower the drawdown cone spreads from the pumping well.

12. Drawdown-Time at Observation Wells • Three drawdown segments observed • Early time: Behaves as confined aquifer response • Middle time: Flattens due to delayed yield • Late time: Behaves as delayed confined aquifer

13. Steady-State Solution • Based on Dupuit assumptions: • Flow is essentially horizontal • Drawdowns are small relative to total sat’d thickness • Well was pumped long enough that further drawdown is not measureable • Must be used with caution as these conditions are generally not met

14. Dupuit Solution with 2 Observation Wells

15. Transient Solutions • Jacob (1950) – Theis type curve solution when the first two Dupuit assumptions are met in late-time. • Neuman (1972, 1975) – Generates the three segments of drawdown curve. Accounts for delayed gravity drainage. Includes Kh and Kv, position of screen, and the change in storativity with time.

16. Neuman Equations & Type Curves