CHAPTER 5 UNCONFINED AQUIFERS Outline Basics Assumptions Case

CHAPTER 5: UNCONFINED AQUIFERS

Outline ■ Basics ■ Assumptions ■ Case study set-up ■ Dynamic – state flow – Neuman’s curve-fitting method ■ Steady – state flow’ – Thiem - Dupuit’s method ■ AQTESOLV

Basics ■ Top of aquifer is the watertable ■ Pumping unconfined aquifer: – causes dewatering – cone of depression – vertical flow components ■ Differences from confined aquifers: – Confined are not dewatered – Water in confined comes from reduction in pressure, ■ Time drawdown curves usually follow S-shape expansion of water and – Steep early-time segment compaction of aquifer – Flat intermediate-time segment – Flow is horizonal – Relatively steep late-time segment

Outline ■ Basics ■ Assumptions ■ Case study set-up ■ Dynamic – state flow – Neuman’s curve-fitting method ■ Steady – state flow’ – Thiem - Dupuit’s method ■ AQTESOLV

Assumptions ■ Unconfined aquifer ■ Infinite areal extent ■ Homogeneous and uniform thickness – If drawdown is large compared to aquifer’s original thickness apply correction to late-time data ■ Watertable is horizontal ■ Constant discharge rate ■ Full penetration well

Outline ■ Basics ■ Assumptions ■ Case study set-up ■ Dynamic – state flow – Neuman’s curve-fitting method ■ Steady – state flow’ – Thiem - Dupuit’s method ■ AQTESOLV

Case study set-up ■ ‘Vennebulten’ in the Netherlands ■ ~10 m thick ■ Shallow (3 m) and deep piezometers (12 -19 m) ■ Pumped for 25 hours – Q = 873 m 3/d – D = 21 m

Outline ■ Basics ■ Assumptions ■ Case study set-up ■ Dynamic – state flow – Neuman’s curve-fitting method ■ Steady – state flow’ – Thiem - Dupuit’s method ■ AQTESOLV

Dynamic – State Flow: Neuman’s curve-fitting method Additional assumptions: 1. Isotropic or anisotropic 2. Flow is in unsteady state 3. Influence of unsaturated zone on drawdown in negligible 4. Sy / Sa > 10 5. Observation well is fully penetrating 6. Diameters of pumped and observation wells are small

Divides data into early-time and late-time with different curves

Early-time Late-time

Similar to Theis method: fit a curve to early and late time segments, find a match point for early and late time segments, then solve for relevant information Early time: Late time: Horizonal and vertical hydraulic conductivity: Verify Sy / Sa > 10

Outline ■ Basics ■ Assumptions ■ Case study set-up ■ Dynamic – state flow – Neuman’s curve-fitting method ■ Steady – state flow’ – Thiem - Dupuit’s method ■ AQTESOLV

Steady – State Flow: Thiem – Dupuit’s method Additional assumptions: 1. Isotropic aquifer 2. Flow is in steady state the 3. Velocity of flow is proportional to tangent of hydraulic gradient instead of since as it is in reality the 4. Flow is horizontal and uniform everywhere in a vertical sections through axis of the well

Steady horizontal flow can be described by Using h = D –s and the corrected drawdown equation it can be transformed into Which is identical to the Thiem equation

Outline ■ Basics ■ Assumptions ■ Case study set-up ■ Dynamic – state flow – Neuman’s curve-fitting method ■ Steady – state flow’ – Thiem - Dupuit’s method ■ AQTESOLV