Groundwater Contaminants and Salination

Description 

Fresh groundwater is a precious and limited resource. With a growing population and urbanization, the threats upon groundwater are growing, with both contamination due to human activities and salination due to over-use.

Both newly identified and existing aquifers may be at some risk of contamination and an understanding of the geological setting within which the water is contained is needed. This can provide information on natural barriers to pollution such as clay layers, or the presence of potential groundwater ‘pathways’. These pathways can be beneficial when acting as transfer routes from aquifers to wells, or as recharge paths to restore water levels in the aquifer. However, the same pathways could just as easily carry pollutants from the surface, or some other contamination source, into the aquifer.

Investigation of salination is made to determine if a mapped aquifer is fresh, brackish or saline. Beyond this, there is the potential for salination to occur during the life cycle of an aquifer through over-extraction. This over-use can draw up more mineralised waters from depth which results in an increase in salinity. In coastal regions, severe extraction can cause recharge paths to change, with the primary source of replacement fluid becoming the sea rather than surface water percolating in from land surrounding the reservoir.

 

 

 

Cross section of ground showing water aquifer pathways

Solutions 

Guideline Geo´s wide range of geophysical investigation techniques can solve most of the common questions regarding groundwater contamination and salination. They will provide a non-destructive and cost-efficient way of gaining a better understanding of the ground conditions, supplying better data coverage than is normally achieved with traditional, discrete, point-by-point geotechnical investigations and sampling, such as drilling or digging.

For groundwater applications in general it is necessary to use a physical property that can distinguish water from surrounding geologies, regardless of whether the water is found in unconsolidated materials (e.g. gravel, sand or silt), in rock fractures or in water-bearing permeable rock. Preferably it should also be possible to give information about the geological structures and layers. For contaminants and salination studies the method needs to be able to detect the difference between fresh and potentially hazardous water. As some aquifers can be found very deep, the geophysical method should preferably be able to ‘reach’ depths of 200 – 300 meters or maybe even deeper.

Guideline Geo provide efficient resistivity and TEM instrumentation, both very capable of mapping a physical property associated with the presence of contaminants and salination, a change of resistivity.

Typically, water has a low electrical resistivity, and thus most materials in which water is present will experience a change in resistivity from its original value. The more saturated with water a material is, the more its resistivity will change. Add to this the fact that salt has very low electrical resistivity, we can deduce that the higher the ion content of water, the lower the resistivity. This makes it possible to detect the difference between fresh water and saline water. If water is contaminated with a pollutant, in most cases its electrical properties will also change. Exactly how it will change depends on the pollutant but typically it will lower the resistivity due to an increased amount of dissolved ions.

 

 

ABEM WalkTEM 2 hero image

The ABEM WalkTEM 2 is perfectly suited to locating groundwater and minerals, but also useful for applications such as geological mapping and environmental surveys

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Example of investigations

Resistivity and TEM measurements are most often used as part of a single investigation, identifying anomalous regions at a particular moment in time, but they are also very suitable for monitoring or time-lapse measurements. To be able to track the distribution and pace of leachate spreading around a landfill area, is an incredibly powerful tool.

Note! All ABEM resistivity equipment is capable of also recording the chargeability of the ground (how quickly it charges up and subsequently discharges with the application and removal of current) using time domain induced polarisation methods. This additional dataset can be very useful in mapping different types of waste in a landfill, or interpretation of targets such as mineral deposits and clays.

What method and technique to use?

Whilst there are many factors involved in deciding upon the correct solution for a given project, these are some of the key considerations:

  • TEM is unsuitable in urban areas due to sources of electromagnetic disturbance.
  • TEM is suitable for deep investigations (>500 m) where resistivity methods might be limited by access, equipment power or general survey practicalities.
  • Resistivity requires galvanic contact via electrodes of some description which can be both hard and time consuming in environments with bare rock or asphalt etc.
  • TEM or VES may be sufficient for broad prospection but ERT methods are best for more detailed results.
  • TEM is not well suited to differentiating between highly resistive materials; the method responds better to conductive bodies.
  • 1D methods (VES, TEM, VSP) are not well suited to identifying confined features like fracture zones and intrusions compared to 2D methods which offer lateral detail.

Combining methods

It is often beneficial to combine different geophysical methods to get the best resulting picture of the groundwater contamination or salination. For instance:

  • 1D measurements with VES or TEM can be used to pin-point the most interesting area, where more thorough investigations with ERT can be made.
  • It may be beneficial to supplement ERT with TEM soundings through those areas where investigation depth needs to be greater than that obtained with ERT.
  • The combination of resistivity and IP can help refine an interpretation, for example resolving if waste is of organic or inorganic origin and thereby the type of groundwater contamination.

 

 

The ABEM Terrameter LS 2 uses a galvanic method and is well suited to differentiating between highly resistive materials

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Other geophysical methods

Guideline Geo also provide GPR and seismic solutions, which can be beneficial for ground investigations relating to the mapping of contaminants or salination.

GPR

GPR can be used to map the bedrock topography (in other words pathways for groundwater and contaminants), the extent of protective clay layers, the thickness of friction soils etc. In some cases GPR can also be used to map the groundwater table, but this is limited to coarse grained soils.

Seismic methods

The seismic method is most commonly used for measure depth to bedrock, bedrock quality, soil stability or mapping of geological structures. But depending on the aquifer type it can in some situations be easy to detect the groundwater table. The porosity and water saturation decides if seismics will resolve a groundwater surface. Seismics will not be able to differentiate fresh water from salt water or polluted water.

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To contact sales, please fill in below form. Since all our products are modular, include any information you have on the intended application, geology/terrain, structural/ground-conditions, depth requirements and previous experience using our equipment - and we will get back to you with a recommendation.
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