Hydro Lecture 7

Flow Lines continued…

Water flows from higher to lower hydrologic head
Geologic material is never actually homogenous, so flow lines refract when flows change subsurface structures or layers
If K1 is smaller than K2:

  • water tends to flow vertically in K1 more than K2
      • water flows nearly vertical in clay layers
      • aquifers have horizontal flow more than vertical

Ground Water Investigation

Determining Groundwater Levels and Pressures

  • Use existing wells
  • Use open borings
  • Use observation wells
  • Use piezometers

Determining Groundwater Flows: Well Studies

  • Useful for:
    • extracting water for study
    • relieving pressure under dams
    • lowering water table for construction
    • controling saltwater intrusion
  • The Basic Purpose:
    1. compute drawdown (change in water level, hydraulic properties must be known)
    2. determine hydrologic and geologic properties for computation
Drawdown

insert slide image #1 (pg 3 of notes)
Steady-state flow- time change is not considered, hydraulic head is constant
Transient flow- hydraulic head changes with time
Types of pumping wells:

  • Fully penetrating well (runs full thickness of aquifer)
  • Partial penetrating well (really difficult to study due to altered vertical flows)
  • Homogenous & isotropic (used for basic computations)

Flow in a completely confined aquifer

Basic Info:

  • No recharge zone, no inputs
  • Subsurface is always saturated
    • elastic storage due to pressure changes after pumping
  • Storativity is generally very small
  • Pumping will affect large area of aquifer

Theis

Theis%20equation.png
Q. insert
A. insert

Flow in a leaky, confined aquifer

Basic Info & Assumptions:

  • Aquitard located above, below or on both sides
    • if 2 aquitards, solution will be different
  • Hantush-Jacob formula deals with one leaky confining layer
  • Water table in the source bed is originally horizontal
  • Water table in the source bed does not fall during pumping
  • Source bed is the geologic layer above the leaky, confining layer

Hantush-Jacob Formula (1956)
insert math here

The Hantush-Jacob curve is very generalized and very theoreticl, but it serves as a basis for computation of simple systems.

Flow in an unconfined aquifer

Basic Info & Assumptions:

  • most difficult
  • Neuman's solution (1980s) (still a really important guy)
  • Vadose zone is ignored
  • @beginning, water pumped is from elastic storage
  • after beginning, water recharge due to gravity drainage
  • specific yeild is at least 10x the elastic storativity
Three phases of an Unconfined Aquifer
  • Early stage
    • pressure drops, mostly due to specific storage
    • behaves like an artesian aquifer
    • flow is horizontal
    • time:drawdown relates as Theis curve
  • Second Stage
    • water table declines, mostly due to specific yeild
    • flow is horizontal and vertical
    • time:drawdown is inversely related?
  • Later Stage
    • rate of drawdown decreases
    • flow returns to horizontal
    • time:drawdown reverts to Theis curve

Shlomo Neuman's Solution

  • involves three parameters and an even more complicated charts
  • vertical and horizontal hydraulic conductivity greatly affect the phases of the well
  • insert math here
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