Residence time

• flow velocity/effective flow velocity (ve)

  • larger than flux density
• flow velocity equation: ve = (k*ne)*[delta(H)/delta(x)]

  • ve (m/d) = flow velocity
  • k (m/d) = conductivity
  • ne = porosity
  • [delta(H)/delta(x)] = hydraulic head gradient
• equation to determine how far a water droplet has travelled in certain time period or how long it takes to travel a certain distance

  • ve = [delta(x)/delta(t)]
  • x = distance
  • t = time
• residence time (Tr): time water remains in certain volume
• Tr = (D*ne)/R

  • D (m) = thickness of aquifer
  • ne = porosity
  • R (mm/year) = precipitation surplus
• longer residence time in soil results in better water quality
• buffer strips are beneficial for spreading animals and plants, landscape and recreation
• residence time in extraction wells

  • water flows faster closer to well
  • radial flow pattern
  • use area of 25 year residence time to protect groundwater to make sure its water quality meets standards
  • measured with the use of radius of 25 year residence time
• volume for extraction well = π*r2*ne*D

  • r = radius
  • ne = porosity
  • D = thickness of aquifer
• calculating radius = sqrt[(Q*Tr)/(π*D*ne)
• calculating residence time = [(π*D*ne)/Q]*r2