EES215

Lecture 12

 

Examples of groundwater situations:

  • unsaturated zone (zone of aeration; vadose zone)
  • saturated zone
  • water table


Correlation between topography and water table

  • Hydraulic head
  • General flow patterns
  • Toth's scheme: local, intermediate, regional water flow

Artesian well
Confined aquifers; unconfined aquifers; perched water table; presence of fresh water under oceanic islands; encroachment of salt water

While most groundwater systems are local with distances between recharge and discharge areas of less than 50 km, there exist also continent-wide groundwater systems.  The most famous one is the Great Artesian Basin of Australia (Fig. 1) which occupies a large part of the north-east section of the continent.  The discharge area is in the tropical (wet) north, the discharge in the arid south, often on top of travertine hills (Fig. 2). 

Approaches to ground water systems

Measurement of hydraulic conductivity

Construction of flow nets; boundary conditions; flow lines and equipotential lines; refraction of flow at geologic boundaries - the tangent law;
Change of flow velocities with depth
Modeling of ground water flow: numerical approaches - finite difference
basic approach - replacement of differentials with differences
assumptions and simplifications:  two dimensional - three-dimensional; distribution of permeability; hydraulic head; assumptions for fluids; rock;

Regional and local flow systems; gravitational flow - heat driven convection
Examples of aquifers - numerical models
Investigation of flow systems - geologic investigation
Tracer studies - cosmogenic isotopes
Ogallala Aquifer; Great Artesian Basin
 

Geophysical methods:

Approach:
Determinations of specific physical signals and comparisons to expected values allow conclusions concerning the makeup of the crust or the presence of 'unusual' formations.

Scope and reasons for investigations:

  • exploration - oil and other hydrocarbons; ore deposits
  • siting: stability of formations; presence of faults etc.
  • environmental concerns: hydrology; stability
  • history of formation

Seismic methods: based on the propagation of seismic waves, typically released from specified source (explosive charges; thumper) - we have already talked about it - reflection, refraction of seismic waves; give structure and stratigraphy of crust; distribution of 'reflectors'

Electric methods: rocks in general have low conductivity; exceptions: salts; conductivity is also function of presence of water - ionic conductivity; performance of measurement: current is sent through formation; field measurement allows determination of electrical conductivity;
used for the assessment of aquifers; makeup of formations; determination of porosity; permeability

Gravitational field:  we discussed the expected gravitational field; specific investigations compare the expected value at given location to the measured value; for calculation of the expected value all 'obvious' characteristics of the location are taken into effect: latitude; altitude (Free Air Correction); presence of additional mass (Bouguer Correction). Anomalies are related to presence of bodies with higher/lower density: ore deposits; rarely used for other reasons

Other methods:

magnetic methods:  earth has dipole field with axis close axis of rotation. Three components: strength; inclination; declination.
Show distribution
Correlation between latitude and inclination.
Determination of expected field
Certain minerals have magnetic moment (magnetite; hematite) - get magnetized at time of solidification: igneous rocks at temp. below Curie T; sedimentary rocks at time of settlement
Determination of specific magnetic characteristics give indication concerning history of material; make-up of material (presence of magnetite etc)

Heat flow:  determination of potential for geothermal areas; history of formations; potential for U deposits