In Situ Vapour Extraction of Soil Gas in a Clay/Shale Profile

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Environmental & Earth Sciences

Environmental & Earth Sciences Pty Ltd have designed and installed an insitu vapour extraction system which has been designed to effectively achieve the following:

  1. Volatilise and vent petroleum hydrocarbon vapour contaminant present on the site (can apply to groundwater rather than soils if the situation requires);
  2. To target contaminants present in the interface drainage layer between the shale and clay at between 0.8 and 1.2 metres depth (adjustable to depth required to suit particular site);
  3. Designed to treat the vapours extracted so that vapour emissions can be vented safely to the atmosphere, (or in the case of groundwater, separate contaminants and release an improved groundwater);
  4. Main feature that sets Environmental & Earth Sciences’ insitu vapour extraction system apart from other pump and treat remediation systems is that it can be installed at an operating site without altering the visual appearance of the site. All pipes, pumps, knockout tanks, activated carbon filters and gauges are located beneath the ground surface. The vent pipe for clean air emissions (or the water discharge pipe, depending on the system design) is the only external feature. This is easily tucked away on the site or disguised so as not to affect the visual appeal of an operating business. The electrical switch board for controlling the system can be installed in the main operations room of the business or in a locked box/shed in a small nook out of the way.
  5. Paper discusses the theory behind Environmental & Earth Sciences’ vapour extraction system and its effectiveness in a clay/shale environment. The construction and operation problems and considerations that need to be addressed during design and construction are also discussed.

Introduction

Many operating service station sites or sites where underground fuel tanks are located have the hidden problem; soil or groundwater contamination resulting from leaking tanks, pipes or from spills and overflows. Where there is no significant threat to human health, contaminated soil on an operating site is generally left undisturbed until site operations cease, unless contaminants are found to be moving off site or affecting the groundwater.

When offsite contaminant migration is present a number of problems are posed for the site owner, as they are required to prevent off site migration. Addressing these problems while trying to keep a business on the site successfully operating is often impossible. This paper discusses the use of Environmental & Earth Sciences’ vapour extraction system on a site with a clay/shale strata to aid in preventing offsite petroleum hydrocarbon vapour movement.

The vapour extraction system is a simple design that can be adapted for a variety of purposes, one in particular being the entrapment of contaminants migrating offsite. The installation of a permeable interception trench downgradient of the migration boundary points of the site will collect contaminants moving off site. The contaminants can then be drawn along the interception trench to a common location where the contaminants are treated and emitted.

The system is also economical and can readily complete with other insitu solutions, creating comparatively little disturbance during installation. This provides the site owner with the comfort of knowing that their site is still able to operate as normal and that any petroleum hydrocarbon related contaminants moving off site in the groundwater or as soil gas vapour are being intercepted and treated. Therefore, this system eliminates offsite movement and the consequence of fines.

Theory

There are several objectives that the vapour extraction system was designed to meet as outlined in the summary. Following is a discussion on how these objectives were achieved.

The vapour extraction system involves the application of a vacuum to the clay/shale interface layer, whereby the vacuum creates a pressure gradient in the subsurface materials. The pressure gradient will cause an advective flow of vapours towards the lowest pressure zone in the system. The lowest pressure zone is designed to be that of the interception trench filled with gravel and sand in which the extraction pipe is located.

A high vacuum is applied to the vapour extraction system as the contaminant vapours need to be drawn through clays and shales which are more impermeable than sands and gravels. The vacuum applied also enhances volatilisation of petroleum hydrocarbon contaminants from the dissolved, separate and residual phases into the vapour phase, thus increasing contaminant extraction.

Brief design details

The vapour extraction system is designed so that it will not require a large number of extraction wells, not interfere with the operation and development of the new service station (or business prevailing), and interfere as little as possible with the aesthetic look of the new service station (or business prevailing). Thus the service pit containing pumps, filters and other equipment is located underground, along with the trenches. The vent pipe will be the only section of the system to be constructed above ground.

A trench 2 metres deep and 0.5 metres wide was extended from one corner of the site along the intersecting boundaries of the site. This allowed the clean vapours to be vented from the filters in the service pit to the atmosphere.

The trench was backfilled with an evenly sorted coarse sand/gravel. The slotted extraction pipe was covered in filter sock and laid at 1.5 metres depth and the emissions pipe laid above this at 1.0 metres depth. The slotted extraction pipe was laid in such a manner as to allow gravitational flow towards the collection sump for the collection of any contaminants or liquids. A well compacted clay was backfilled between the depths of 0.8 metres and the surface. Plastic was inserted in the clay at approximately 0.4 metres depth. The clay and the plastic help prevent short circuiting of the extraction system, ie vapours will be drawn from the clay/shale interface zone rather than the surface.

It is important that the seal on the trench is strong. If there is a breach in the seal, a vacuum strong enough to draw vapours out of the clay/shale structure will not be created. Vapours or contaminants will migrate along the more permeable pathways such as service lines, sand/gravel lenses and interface between strata. The idea of the system is to make the collection trench more permeable than the surrounding strata. In this particular case with a clay shale strata it is not difficult to make the trench a more permeable structure, however, with the strong vacuum applied to draw vapours from the shale and clay interface, it becomes more important that the system is well sealed so air from the surface is not pulled into the system.

A service pit was designed to contain the sump, the activated carbon filter system and the vacuum and water pumps. The service pit contains a knockout tank in which water collecting from condensation in the pipes and any other sources was collected. Vapours were drawn out of the knockout tank through an activated carbon filter system and then emitted to the atmosphere via the emissions pipe to the vent point. The liquid ring vacuum pump used to extract vapours can be located before or after the activated carbon filters. In this case the activated carbon filters were located after the pump which was set to produce a vacuum of around 0.8 atmospheres pulling a flow rate of between 80 and 100 m³/hour.

A water pump controlled by a floatless level control was used to remove water from the knockout tank when the water level reached a set height and switched off when the water reduced to a lower level. On the particular site that this system is in operation, the water removed from the knock out tank was pumped to the oil water separator. On a site where no oil/water separator is in use, a water treatment system will need to be installed before the water can be removed to sewage or stormwater.

All pump motors and level controls need to be intrinsically safe and wired by a licensed electrician and passed by the relevant authority. It is very important that the electrics are well planned before the system is installed. The underground pump chamber is generally classified by Sydney Electricity as a Class 1, Zone 0 on a service station site, thus all pumps need to be assisted by intrinsically safe motors (Class EXD/EXE, depending on location and contaminant) and all wiring needs to be intrinsically safe and separate. Safety switch off circuits and boards need to be installed at the main control box.

The only feature required to stand above the surface is the vent pipe for the emission of the treated gases. In this case, the vent pipe will be located adjacent to the underground tank vapour points, venting approximately six metres above the ground level.

The structural design of the service pit is not shown here. The water pump intake from the knockout tank should not be located near either of the two vapour extraction pipes entering the knockout tank. The knockout tank is designed to be air tight, with needle valves and gate valves between pipe connections and pumps to allow different systems to operate separately.

The underground pump chamber needs to be strong enough to resist erosion and to support overhead traffic. The vapour extraction system can be located under garden beds if the trench is well sealed and the garden is not overwatered, otherwise infiltration can create problems.

Before entering the underground pump chamber it is always important to check the LEL levels. Remember the chamber is classified as a confined space and personnel entering this chamber should be certified to enter confined spaces.

Valves can be installed before the pumps and after the activated carbon filter to draw water or vapour samples from for analysis. Readings can also be collected via a Gas Chromatograph or Flame Ionisation Detector. Gauges on the vacuum pump will measure vacuum being pulled etc.

Decommissioning

Upon successful completion of vapour extraction, there are two options available for decommissioning the system.

Option 1 Leave the system in place in case of future spillage or leakage. Simply remove the pumps and filters, leaving all connections in place and capped, to allow the system to be reconnected at any time. If water is found to collect in the system regularly when the system was in operation, the water pump will need to be left in place when the system is not operating.

Option 2 If no future use for the system, all equipment and fittings would be removed from the service pit. The false bottom and the true bottom of the pit would be jack hammered out and the pit filled with gravel. The vent pipe can be removed and blocked at ground level.

In both options the piping in the trenches is left underground. These should be removed at the time that the service station (or business) is decommissioned.

Advantages

The main advantages of this system design are:

  • All systems underground, thus aesthetically pleasing;
  • The system is automatic, not manually operated, hence less running cost;
  • The system located on property boundaries and not over the entire site, thus allowing business to be relatively undisturbed during both installation and monitoring, making it more cost effective.

 

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