A guide to groundwater and potential leachate migration.

 

Summary

  • Investigations have indicated that the site for the proposed Auckland Regional Landfill offers good natural containment and attenuation characteristics.
  • The liner system design and construction controls provide good protection against leachate escaping into the environment.
  • Leachate is continually extracted from the landfill during the consented operating and aftercare periods, reducing the presence of leachate over time together with associated future risks.
  • Should any leachate escape through the primary liner, the migration of leachate through the Geosynthetic Clay Liner and compacted natural clay liner and subsequently through the natural soils will be extremely slow, during which time chemical contaminants will be attenuated resulting in the level of such contaminants, if any, reaching a river or stream being below detectable levels.

What is groundwater?
Groundwater is any water that is present below the surface of the ground, either in soil or rock. Groundwater is generally replenished by rainfall, although the slope of the ground surface and the type of soils present below the surface will determine how much of the rainwater becomes groundwater and how much becomes surface runoff or storm water.


Groundwater movement.
Frequently, the movement of the groundwater can be so slow that it may take months or even years to travel one metre.
In some places, such as dense rock, it could take up to 3000 years to travel one metre. For other places, such as gravels, it could only take two minutes to travel one metre. Typically, it would take 32 years for water to travel one metre through a landfill clay liner.
There can be a significant difference between the time it takes for groundwater to move horizontally compared to vertically. This is because of the layering effect that occurs when rock is formed. Where there is layering, the vertical movement of groundwater can take 40-240 times longer than horizontal movement.


Groundwater at the proposed Auckland Regional Landfill site?
Fourteen investigation bores were drilled across the site to determine the nature of the underlying soils and groundwater systems and compare shallow and much deeper groundwater movement. These bores varied in depth between 33 to 50 metres, with a single bore drilled to approximately 250 metres deep to provide information on the regional groundwater system and for a potential potable water supply.
Shallow groundwater information was gathered from hand auger and excavator test pits.
These investigations showed that groundwater would move significantly slower through the rock at the level of the base of the proposed landfill liner compared to higher ground.


The bores showed three groundwater systems present in the area.
The first is a shallow groundwater system in the higher ground. Groundwater in this system is replenished by rainwater and, because it will move horizontally much easier than vertically, is the likely source of the springs for which Springhill Farm is named.
The second groundwater system is lower down in siltstones and sandstones at the valley floor level. Within this system, the groundwater will move horizontally at a rate of up to 30 years per metre. The vertical movement was found to be at least 40 times lower.
A very small amount of groundwater will move down to the third groundwater system, but very slowly. Downward migration of groundwater from the second to the third system is between 12 to 80 years per metre.


The third groundwater system, which is the lowest, is also called the regional aquifer. It is below the level of the Hoteo River.


How will the landfill lining system protect the groundwater?
Components of the lining system
A lining system will be installed at the base of the landfill as a barrier system to prevent seepage of any leachate (contaminants) into the surrounding environment and groundwater. Overlaying the liner system is a gravel layer and pipe network to remove leachate from the landfill.
The lining system proposed for ARL has three main components (from top to bottom);

  • A fully welded continuous 1.5 mm thick High Density Polyethylene (HDPE/ “plastic”) liner with an anticipated life of 300-1000 years. This is protected from damage by a geotextile cushion blanket and the leachate drainage layer.
  • A Geosynthetic Clay Liner (GCL). This comprises a layer of bentonite clay held in place between two geotextile layers. Leachate passing through a defect (if any) in the HDPE liner would take approximately 20 years to travel through this layer.
  • A compacted clay or mudstone liner of 600 mm minimum thickness. Leachate passing through defects (if any) in the above two layers would take up to 32 years per metre to travel through this layer.

Best practice design to address any risk of leachate leakage through a hole in the HDPE liner is to place the HDPE immediately above GCL and clay liners as is proposed at the Auckland Regional Landfill.
A strict quality assurance and approval programme will be in place for construction of the lining system to minimise the risk of any defects that could result in leakage paths (for example through poor quality welds between the HDPE sheets or any damage). The
underlying GCL/clay effectively seals any HDPE defect that may be present. Measured leakage through liners (Rowe, 2012) show that a HDPE layer over GCL reduces potential leakage by a factor of 50,000 compared with HDPE alone and by a factor of 32,500 compared with a clay liner alone.

What happens to leachate if it does leak through the liner defect?
Chemicals in leachate and groundwater will attach to clay and soil particles. This process, in combination with other processes, is called attenuation.
Any contaminants in leachate that might leak through the HDPE liner will be subject to this same attenuation process as they pass through the clay components of the GCL and clay liners and again through the underlying natural soils, reducing the concentration of any contaminants in the leachate.
Given the slow rate of groundwater movement (as explained above), any leachate that might escape through the liner system would move through the natural soils extremely slowly, allowing for this attenuation of contaminants and resulting in the level of contaminants (if any) reaching a river or stream being below detectable levels.


Testing and monitoring
Regular testing of groundwater from subsoil drains and monitoring boreholes positioned around the perimeter of the landfill will detect any changes in chemical concentrations in the groundwater. In the unlikely event of detecting changes associated with leachate, remedial actions would be taken to control the source of contamination or prevent further migration.

After the landfill closes:
Leachate continues to be drained and removed from the landfill through the operating life and during the consented aftercare period that follows (potentially 30 – 35 years).
At the start of the aftercare period the landfill will be fully capped and the leachate will continue to be drained without the addition of further rainfall, thereby reducing the presence of leachate over time together with associated future risks.
Perimeter monitoring bores will continue to be monitored for changes in chemical concentrations of groundwater that might indicate the presence of leachate. Remedial actions will be undertaken if necessary.
During this aftercare period when the landfill is capped, the waste will continue to decompose. In tandem, the volume of leachate and level of chemical contaminants within the leachate will reduce to a point where they will not pose an environmental concern to future generations.

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