Hydro Lecture 6

Continued from previous lecture
Aquifers are the major topics in Hydrogeology

  • you MUST understand Darcy's law (in theory and math)
  • be able to calculate hydraulic head

Hydraulic Conductivity

  • ease of flow of water through subsurface
  • similar to permeability but related to geologic fluid AND structure (eg. water and sandstone; oil and shale)
  • NAPL- non-aqueous phase liquids

Intrinsic Permeability vs. Hydraulic Conductivity

  • hydraulic conductivity equation: K=k(pg/mu)
    • K- hydraulic conductivity, k- permeability
    • p-

Measuring these in the lab: at least four diff ways

Transmissivity (another aquifer property)

  • the total amount of water that can be transmitted horizontally through a geologic unit's single unit width (1 unit length, not 2 or 3)
  • T=KB, T- transmissivity, K- hydraulic conductivity, B-
  • definition is based on Darcy's Law

Specific Storage (another aquifer property)

  • super duper important
  • aka elastic storage coefficient
  • the vol of water an aquifer releases/produces from storage (fully saturated): per unit vol per unit decline in hydraulic head
  • insert specific storage equation here
  • hydraulic head measured in length, Ss measured in vol like ac ft or ft cubed
  • it's caused by pressure changes, water moves from higher hydraulic head to lower
    • water and rocks have elastic properties
      • as pressure increases, more space becomes avilable for water due to expanding pore space and contracting geologic unit
      • as pressure decreases, less space becomes available for water due to expansion of water molecules
      • material and water compete for space

Storativity (ditto)

  • amount of water expelled per unit surface area per unit change in hydraulic head
  • S=bSs
  • storativity is different for confined and unconfined aquifers, due to specific yeild's relation to storativity
    • Confined
      • aquifer is always saturated
      • Ss less than .005
    • Unconfined
      • saturation varies, therefore thickness of aquifer varies:: always fluctuating
      • groundwater drainage is much more important here
      • S=Sy + h(Ss)
      • Ss between .02 to .3
      • highly dependent on grain size

Lecture Notes 5
all about flow

isotropic similar to homogeneous, isotropic is about permeability in 3 directions
anisotropic similar to heterogeneous
generally, vertical and horizontal permeability are very idfferent
generally, vertical hydrulic conducivity is 10% of horizontal
you can have a homogenous structure that is anisotropic
aquifers are naturally heterogeneous
for aquifers with layers, we use the average hydraulic conductivity

  • insert avg horizontal hydr conductivity formula, can be used with any number of layers

Potentiometer Surface
From physical geology labs: contours and elevations (topographic maps)
Potentiometer = total hydraulic head (instead of elevation)
Map that shows flow direction of groundwater or other fluids

  • made of equipotential lines (instead of contour lines) that are generally parallel to eachother
  • flow lines are perpindicular to equipotential lines
  • ada_att2.gif
  • when three wells form a right triangle, we can calc hydraulic gradient exactly
    • insert super complicated formula here for Grad h=

Specific Discharge or Darcy's flux
Q/A = k (hydraulic gradient) or Q/A = k (dh/dl)

Flow Lines aka Flow Nets
shows the paths that gw takes through an aquifer

  • solid lines show flow direction
  • dashed lines show hydraulic gradient markers
  • assumptions:
    • aquifer is homogenous, isotropic and fully saturated (never happens in real life)
    • Darcy's law is valid, flow is laminar
  • Flow Net Theory
    • ?
  • total flow can be calc from these maps
    • insert formulas here
    • and here
    • there might be more
    • but we don't have to know them
    • just the theories they support
  • Seepage Under Dams
    • many diff types
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