Site Investigation

Question 1

Ground Penetration Radar (GPR)

Answer 1

Used to measure two way travel time and amplitude of the signal to identify the depth of the layer or objects underground.
Does not work well in saturated medium as waves are attenuated; lots of energy is required to move through water so signal gets lost before reaching the monitor i.e clay-rich sediment.
Higher dielectric = Slower travel time

Optimal conditions for this technique are sandy or rocky soils in unsaturated zones or bedrock with low hydraulic conductivity where water is not permanently present.

Question 2

Electromagnetic conductivity (EM)

Answer 2

The system consists of a transmitter coil energised with an alternating current at an audio frequency. This creates a primary magnetic field, Hp, that induces currents in the subsurface. These currents generate a secondary magnetic field Hs. Both magnetic fields are sensed by the receiver coil located a short distance away from the transmitter.
Results can be used to determine water quality, quantify soil salinity and determine the concentration of cations and heavy metals

Question 3

Drilling Methods

Answer 3

Percussion Drilling

• Commonest technique
• Cheap
• Can reach large depths
• Suitable in almost all soils

Auger Drilling

Hand Augers

• Very cheap
• Used in self-supporting strata without hard obstructions or gravel-sized particles
• Must be withdrawn at frequent intervals for the removal of soil

Mechanical Augers:

Flight Augers

• Suitable for cohesive soils
• Require considerable mechanical power and weight
• Give only a very rough indication of the levels and character of the strata
• Continuous-flight augers more efficient than short-flight augers

Bucket Auger
- Allows downhole logging

Hollow Stem Auger

• Can be used for both soft and stiff soil
• No drilling fluids used
• Minimal geologic material disturbance
• Able to collect continuous in-situ geologic samples without removal of the auger sections

Rotary Drilling

• Fastest drilling method
• Primarily intended for rock but used is soils also

Question 4

Steps for a Site Investigation

Answer 4

1) The geologic setting at and in the vicinity of the site: The conceptual model will distinguish between various geologic layers in terms of their hydraulic characteristics, and will attempt to indicate the significance of the various layers in influencing the groundwater flow system, and their potential control of the migration of pollutants in the subsurface.

(2) The regional and local surface and ground water flow systems: The conceptual model should identify the interaction between the groundwater and surface water systems in the vicinity of the site, and also indicate the inter-relationships between the regional and local groundwater flow systems. It should also incorporate
topographic and stratigraphic information into the schematic diagrams of the groundwater flow system;

(3) Identify impact of human activities on water flow and pollutant migration at the site: For example, buried pipelines, utilities, and sewers and their associated coarse-grained backfill often provide conduits for the preferred flow of non-aqueous phase liquids and groundwater through the subsurface. Groundwater pumping wells in the vicinity of a site may also alter hydraulic gradients and modify groundwater flow system
(4) Identify the natural and preferential pathways for pollutant migration: These pathways might include high hydraulic conductivity layers and fractures in clays and rocks
(5) Identify pollutants characteristics: It is important to include the pollutant characteristics in the development of the conceptual model to ensure that potential areas of occurrence and migration can become the focus of site monitoring and investigation program
(6) Identify potential receptors for evaluating the degree of environmental impact: Receptors may include people, plant, animals, and aquatic organisms.