Hydro Lecture 5

Aquifers
insert water content and classification graphic

Unconfined Aquifer is made up of:

  • unsaturated zones aka vadose zones (pores with air and water)
  • saturated zones
  • capillary fringe
  • groundwater table

Generally speaking:
Groundwater tends to flow horizontally, but vadose zone water flows vertically: IF there are not any pumping wells.

  • see non-Darcy flow

In thoery, a confind aquifer is impermeable, but in reality, nothing is completely impermeable.
Groundwater tables generally follow the topography, more so with small scale examples
Water is always present within rocks

Confined Aquifer is made up of:

  • confining layers: less permeable layers of geologic units, the MOST important part
    • if you would pump from a confined aquifer under a lake or river, the confining layer permeability can hinder or help you
  • Recharge Area
    • In horizontal layers, recharge is very low as it moves through the aquiclude or aquitard; VERY slow flow rate
    • In more vertical layers (bent layers), the permeable water table zone reaches the surface; this zone is the recharge zone
  • Pressure Surface: height to which water would rise is aquifer was not confined
    • this creates flowing artesian wells
    • if pressure is decreased by pumping or releasing water, the pressure surface can decrease.
    • if pressure surface decreases below surface elevation, artesian wells cease to exist

Properties of Aquifers
Key Info for Geologists

  • Porosity: the percent of rock or soil composed of voids (air/water filled spaces)
    • voids can be microscopic or visible
    • % of voids in solid material (eg 10% porosity = 10% of the material is void, pumice has very high porosity)
    • Effective Porosity: the porosity available for fluid flow
      • sometimes pore spaces are so tiny that they do not connect, and no fluid can flow through, not even molecule of water
      • measurements use the notation: ne
    • Porosity is affected by:
      • sphericity of particles
      • grain size distribution
      • wide grain size distribution (cobbles to silt in one rock)
      • cementation: precipitation of sediment grains in diogenisis
        • noncemented sand vs cemented sandstone (= beach sand vs sandstone)
        • beach sand= high porosity; sandstone= relatively lower porosity
      • sorting: grain size variation
        • well sorted sands = high porosity; poorly sorted = lower porosity
        • insert chart on typical values of porosity
    • igneous and metamorphic rocks usually have 2% porosity or lower, not good for aquifers
      • however, if highly weathered or something like pumice, porosity can be up to 50%
    • fracture porosity: when this exists, it doesn't really matter how impermeable the rock is; what matters are the dimensions and orientations of fractures
      • multiple parallel fractures would not be ideal for aquifer material
      • 2-4% fracture porosity is common, can reach up to 10%
      • shale and fracking are great examples for this study
  • Permeability: the ease of a fluid to move through a geologic units (eg. transmitting water)
    • porosity and permeability are very similar, but not the same (clay is high in porosity (~60%) and low in permeability)
    • Permeability factors are very similar to porosity factors
      • difference- pore connectivity= the more connected the pores, the higher the permeability
    • permeability vs porosity
      • insert chart
    • extremely important factor in pumping aquicludes or aquitards (some aquitards are 800m thick)
      • once water is removed, subsidence cannot be recovered bc we can't restore the pressure required for the layers to hold water
  • Specific Yield: % of water in rock
    • more specifically, the volume of water that drains from a saturated rock
    • water won't completely drain out
    • clay avg 2%, medium sand avg 26%
    • very important in unconfined aquifers
  • Specific Retention: % water rock can hold
    • more specifically, the ratio of volume of water a rock can retain against gravity drainage
    • n = Sy + Sr; n = porosity, Sr = specific retention, Sy= specific yeild
  • Hydraulic Conductivity
    • how fast fuilds move through rock material
    • in order to know this, we have to know the rock material and type of fluid
    • insert better definition here
    • Darcy's Experiment & Law
      • most important aspect of hydrogeology, WILL be repeated, WILL be tested!!!
      • non Darcy flows are related to flows close to pumping wells, where normal physics can't apply
      • insert Darcy's equation here
        • Q= flow rate, K= hydraulic conductivity, A= surface area of cross section, L= distance between two points, ha= total hydraulic head, hb= something else about hydraulic head
        • pic darcyexp.gif
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