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Non-Ideal Aquifers
| Question | Answer |
|---|---|
| Why is the bounded aquifer non-ideal? | The aquifer is discontinuous due to the presence of an impermeable boundary or “pinching out” of the aquifer. • Greater drawdown must occur to maintain the pumping rate, Q |
| Why is the recharge boundary non-ideal? | The aquifer is connected to a surface water body (or other recharge source). • Less drawdown occurs because water is being pumped from a source other than just elastic storage |
| What are leaky aquitards? | not perfectly confining, H2O will leak across them and recharge the aquifer drawdown is less since water is being added |
| How can we assess aquifers w/ boundary conditions> | Using image well method (application of superposition) |
| How does the slope of the Jacob slope change? | double for an impermeable boundary halved for a recharge boundary 1 at water table |
| What can we do for both the impermeable and recharge boundary conditions? | • estimate T & S using the early-mid time data • estimate the location and distance to the boundary by using drawdown data from multiple observation wells |
| What are assumptions for the analysis of leaky aquitards? | all Theis assumptions apply vertical leakage across the aquitard(s) into the pumped aquifer • adjacent unpumped aquifer has a constant hydraulic head •aquitard is incompressible (transmits H2O but does not release H2O from storage) |
| Aquitard Compressibility | most aquitards are more compressible than aquifers |
| How does one differentiate leakage from a recharge boundary? | install two obs wells at equal distances from pump well |
| Transient unconfined aquifer response assumptions | same as Theis • the water table forms the upper surface of the aquifer • water is released from both elastic storage and gravity drainage • drawdown is small in comparison to the saturated thickness of the aquifer |
| what is phase 1 of Neumans's method of analysis? | • Drawdown is relatively rapid • Water is released from elastic storage (compression of matrix and expansion of water) • Response follows a Theis curve • obtain estimates of T and S (=Ss∙b) |
| what is phase 2 of Neumans's method of analysis? | • Rate of drawdown decreases due to delayed drainage • h2o released from the new vadose zone result of lowering of WT • Vertical hydraulic gradients increase • Produces “recharge effect” and response a leaky aquifer (deviates below the Theis curve) |
| what is phase 3 of Neumans's method of analysis? | • Drainage reaches an “eqm” • can take a very long time to reach this stage • Water released by drainage of pores balances drawdown • Response again follows a Theis curve with storage associated with specific yield • obtain estimates of T and S (=SY) |
| How can we determine what kind of non-ideal aquifer? | all are v similar Use of other hydrogeologic evidence (borehole logs, maps, geophysics, other wells in area) will help to arrive at a reliable interpretation |
| briefly describe pumping test procedures. drawdown data accuracy is dependent on these | constant Q measure s in several obs well measure s at approp. t intervals measure external influences record pump and recovery data continuing tests for no less than 24hrs for confined and 72 hrs for unconfined |
| what are the effects of partial penetration? | A partially penetrating well is one where the screen does not cover the full thickness of the aquifer. Results in vertical gradients & alters the observed drawdown measurements. The effect is diminishedfarther from the pumping well |
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