Intro To Hydrogeology Owen
Intro To Hydrogeology Owen
Intro To Hydrogeology Owen
Water
on the
ground?
? ? ? ? ?
What is groundwater ?
What is Groundwater?
• Groundwater is water stored in the pore spaces of the soil and rock
• The porosity is the volume of voids / volume of the rock
What is Groundwater?
• Rocks that supply ‘useful’ quantities of groundwater are called aquifers
• Permeability (hydraulic conductivity) is a measure of the flow rate of
groundwater through the rock mass and depends on the size and degree of
interconnection of the pore spaces
Groundwater fluxes and the water balance
What is Groundwater?
• Rainwater that infiltrates into the soil / rock becomes either soil moisture
in the vadose zone and or groundwater in the phreatic zone.
Aquifers
• Aquifers may be unconfined or confined. Aquifers may have primary or
secondary porosity. Rocks / soil that impede the flow of water are called
aquitards.
Confined / Unconfined Aquifers
If the drawdown is small when compared to the thickness of the aquifer, the
situation may be considered as similar to the previous section, namely steady
confined flow
Groundwater
•Positive points
•Vast stored groundwater resources
•Usually available at point needed
•Relative low cost compared to surface water
•Potable without treatment
•Employs low cost technologies
•Drought resistant – no evaporation
•Requires little land
•Negative points
•Storage & conveyance only
•Must be pumped
•Limited flows
•Mineralized
•Difficult & expensive to assess
Groundwater Storage
The groundwater resource is vast:-
many times the size of surface water resources
Regional distribution of global
fresh water resources
Reliable
Regional
supply and
Groundwater development
good quality
resources
management: Reduce
Full cost conflicts
recovery
Enough Aquifer
water at recharge
low cost & safe yield
Equal
access for Protect
the poor Base flows
Industry National Water
Policy Makers
Water Regional/Local
Utilities Water Managers
Groundwater
Farmers Experts
Environmental
Local NGO
NGOS
Groundwater Flow.
• A confined aquifer has a specific storage (Ss) of 2.3 x 10-6 m-1 and a thickness of
78m. How much water would it yield if the piezometric surface declined by an
avg. 1.75m over an circular area with radius 1350m? The Storativity (S –
dimensionless) of the aquifer is given by
S = Ss x b where b is the thickness of the aquifer.
Vw (volume of water discharged) = S x A (surface area) x ∆h (avg. decline in
head)
Ss = ρwg(α + ηβ) where ρw is the density of water, g is the acceleration of
gravity, α is the compressibility of the aquifer 1/(N/m2) , η is the porosity and β is
the compressibility of water.
Exercise: In the figure below, for locations A, B, C, D, E and F give values for the
Elevation Head; Pressure Head and the Total Head.
Identify a recharge point, R, in the figure. And a discharge point, D.
Draw in three flow lines from points X, Y and Z.
Draw flow lines showing the major groundwater movement.
Draw flowlines showing the major groundwater movement.
Comment on the difference with the last image
Exercise: Fig 4.31 is a map showing groundwater elevations in wells in an
unconfined aquifer at Milwaukee, Wisconsin. The aquifer is in good hydraulic
connection with Lake Michigan, which has a surface elevation of 580 masl. Lakes
and streams are shown on the map.
1. Make a water table contour map with a 50m contour interval, starting with
550m.
2. Draw 5 flow lines at right angles to the groundwater contours, showing the
flow pattern.
3. Can you suggest a reason why the water levels are below lake level at some
points?
Kh avg = Sum Khmbm / b Kv avg = b / Sum bm / Kvm
33 30
Groundwater 3500 SEA
400m
3
Groundwater 4500 3
Natural
Surface water (rivers & lakes) 225,000 8,000 2,000
Artificial
(Large dams) 6.000 3,800
Natural
Ground water (< 400 meters) 3,500.000 300,0000 3.000