Soil vapor extraction well and manifold design determine how effectively an SVE system directs blower capacity through contaminated soil. Even a correctly selected roots blower cannot compensate for poorly positioned screens, leaking wellheads, undersized branches or condensate-filled pipelines.
The system should collect contaminated soil gas from the intended subsurface zones while limiting uncontrolled atmospheric-air entry, groundwater extraction and excessive pressure loss. Base wellfield and manifold design on site investigation, pilot testing and measured soil response.
Extraction Well and Screen Design
An extraction well provides a controlled connection between the vacuum system and the target unsaturated soil. When vacuum is applied, soil gas moves through the formation toward the screened interval. Volatile contaminants enter the gas stream and travel to above-ground treatment.
The well should intersect the target zone and transmit vacuum with limited resistance. Screen and filter-pack design should limit particle entry, while seals and surface completion should limit atmospheric short-circuiting. Screen placement should also reduce unintended groundwater extraction. The well construction should allow required airflow and vacuum to be measured, provide access for vapor sampling, and remain serviceable through the remediation project. High airflow alone does not prove that gas is being drawn from the intended treatment zone; verify the source against site geology and measured response.
Vertical wells are commonly used where contamination extends through several depths and surface access is available above the treatment area.
A typical well has a solid upper casing, a perforated or slotted screen across the target interval, a filter pack around the screen, and a low-permeability seal above the pack. The surface completion provides the vacuum connection and access for sampling and measurement.
Choose the screened interval from soil stratigraphy, contaminant distribution, and groundwater elevation. A long screen may appear to increase coverage, but most air can enter through the most permeable layer, leaving lower-permeability zones with little airflow. Where depth-specific control matters, several shorter screened intervals may be more effective than one long screen.
Horizontal wells can suit shallow, laterally extensive contamination where vertical access is restricted, including beneath buildings or active facilities. A horizontal screen may also follow a linear source along a pipeline or trench. Installation accuracy matters: changes in elevation can create low points that collect groundwater or condensate.
Design the drilling route, screen position, pipe strength, settlement allowance, condensate drainage, and cleaning access together. Air can enter unevenly along a long screen, so multiple vacuum connections or separate control zones may be needed.
The screen should intersect the contaminated unsaturated zone and, where practical, remain above the expected seasonal groundwater level. Select its depth with the top and bottom of contamination, seasonal groundwater range, capillary fringe, soil layering, preferential pathways, foundations, utilities, and required vertical control in view.
A screen installed too high may draw mostly clean surface air. A screen set too low may extract water or become submerged as groundwater rises. The capillary fringe can contain substantial moisture and create high resistance to soil-gas flow.
Screen length affects airflow capacity and control over the depth being treated. A longer screen increases open area, may reduce gas-entry velocity at a given flow, and can cover uncertain contaminant depths. It can also connect layers with different permeability, allowing the most permeable layer to dominate airflow and making depth-specific performance harder to assess. Longer screens may also increase the risk of groundwater entry.
Shorter screens provide better vertical control but may require more wells. Choose between broader extraction capacity and targeted treatment based on the site geology and remediation objective.
The well diameter should provide the required gas capacity without adding unnecessary installation cost. Screen openings should admit gas with limited resistance while retaining the filter-pack material, withstanding the applied vacuum, and remaining compatible with site contaminants.
Openings that are too small can clog with fines, biological growth, or mineral deposits. Openings that are too large can allow filter material or soil particles into the well. Select the screen opening and filter-pack gradation together.
The filter pack supports gas entry around the screen and limits fine soil particles entering the well. Select clean, appropriately graded material that is chemically compatible, mechanically stable, and low in fines.
Install the pack continuously around the screened interval. Gaps or bridging can create uneven airflow or vertical pathways along the outside of the casing.
Install an annular seal of low-permeability material above the filter pack to prevent atmospheric air or gas from another soil layer travelling down the annular space. Hydrated bentonite, grout, or another site-compatible material may be suitable.
A defective seal can cause surface-air short-circuiting, misleading pressure measurements, low contaminant concentration, poor depth control, or gas migration between layers. Document seal placement and hydration during well construction.
Wellhead Controls and Measurements
The wellhead should remain leak-tight, mechanically protected, and accessible for monitoring. A typical completion includes a sealed cap, vacuum connection, isolation and flow-control valves, vacuum-gauge and vapor-sampling ports, and an identification label. Use a lockable cover or vehicle-rated chamber where site exposure requires it. Flush-mounted completions should prevent surface water from entering the chamber.
Protect above-ground wells from impact, weather, and unauthorized adjustment.
The wellhead connection should allow operators to isolate the well, adjust flow, measure vacuum and temperature, collect vapor samples, and perform maintenance.
Flexible hose may be used for short connections if it is rated for the full operating vacuum and compatible with the gas. It should resist chemical exposure, ultraviolet radiation, weather, collapse, kinking, and mechanical impact. A hose can collapse internally under vacuum while appearing undamaged from the outside.
Provide an isolation valve at each well so operators can test it individually, remove it from service, change well combinations, investigate water carryover, perform maintenance, or conduct rebound testing. The operating plan should allow vacuum to be redirected when a well is isolated without exceeding the equipment’s operating limits.
Use valves rated for vacuum service and compatible with the extracted vapor. Make the valve position visible or record it in the operating log.
A balancing valve controls the share of total airflow drawn from each well. It should allow gradual adjustment; a valve that moves from nearly closed to nearly open with a small turn makes commissioning difficult. Base balancing on measured data, not valve position alone.
Relevant measurements include branch airflow, wellhead vacuum, contaminant concentration, moisture carryover, subsurface pressure response, and contaminant mass-removal rate. Identical valve positions can produce different flows because soil permeability and branch resistance vary.
Place sampling ports where the sample represents the intended gas stream. Avoid dead legs and condensate-filled tubing, which can distort results.
A sampling connection should be leak-tight, isolatable, chemically compatible, protected from blockage, and safe to access. Locate it away from uncontrolled dilution air. Purge the sample tubing sufficiently before collecting a representative sample.
Individual well airflow measurements support balancing and performance assessment. Select instruments for the expected vacuum, flow range, gas temperature, moisture, contaminant composition, required accuracy, and available straight-pipe length.
Portable instruments may be sufficient for periodic balancing; continuous measurement may be justified for critical wells or automated systems. Record the flow reference condition. Actual volume at sub-atmospheric pressure differs from standard or normal volume and must be converted before values are compared.
Manifold, Piping, and Condensate Management
The main manifold carries combined gas flow from active wells to the separator and blower. Size it for simultaneous airflow, permitted pressure loss, pipe length, fittings, gas temperature, and expected condensate. Account for planned future connections, and do not assume the main header should match the diameter of an individual branch. An undersized header can consume a significant part of the available blower vacuum before it reaches the wells.
Possible arrangements include a common header, zoned or ring manifolds, separate headers for different site areas or screened depths, and independent groups for wells requiring different vacuum levels. A common header is simple but can be difficult to balance when well resistance varies widely.
Separate zones may provide better control where soil conditions, contaminant concentrations, treatment methods, well distances, moisture carryover, or required vacuum differ. Depending on scale and design, a zone may have its own separator, control valve, or blower.
Above-ground piping is easier to inspect, modify, and clean, but needs protection from weather, vehicles, vandalism, and ultraviolet exposure. Underground piping reduces surface obstruction but makes leaks and condensate blockages harder to locate.
Buried vacuum lines should withstand settlement and external loads, use leak-tight joints and chemically compatible materials, and include a controlled slope and accessible inspection points. Account for freezing, future excavation, and pipe identification. Flexible buried piping must resist both vacuum collapse and external soil loading.
Condensate can form anywhere in the extraction system. Slope pipes toward controlled collection points so liquid does not form an uncontrolled seal that partially or completely blocks gas flow.
A liquid-filled branch may cause pulsating airflow, unstable vacuum, gurgling, sudden loss of well flow, or high blower vacuum. The drainage arrangement should account for settlement after installation.
Use drain pots, knockout vessels, low-point sumps, automatic or manual drains, or pumped collection tanks as required by the system design. Every drain connected to a vacuum line must remain leak-tight during operation; an open or defective drain can admit atmospheric air. Treat collected liquid as potentially contaminated until analysis shows otherwise.
Install the main gas-liquid separator upstream of the blower, normally close to the blower inlet while allowing the manifold to drain toward it.
The separator should have sufficient gas capacity and liquid-holding volume for routine condensate and credible surges. Depending on the gas and liquid load, it may include a demister, level indication, high-level alarm, and high-high-level blower shutdown. Provide controlled liquid removal, inspection access, and vacuum-rated construction.
Select pipe materials for contaminant composition, vacuum, temperature, outdoor exposure, mechanical loading, and expected service life. Steel, stainless steel, and selected vacuum-compatible plastics may be suitable, depending on the site.
For plastic pipe, verify resistance to vacuum collapse, temperature limits, chemical compatibility, ultraviolet exposure, support spacing, and thermal expansion. A pipe rated for positive internal pressure is not automatically suitable for external atmospheric loading under vacuum.
All joints must remain leak-tight under vacuum. Depending on the pipe material and service conditions, connections may be welded, flanged, solvent-welded, mechanically coupled, or made with vacuum-rated flexible connectors.
Use threaded connections cautiously because they can leak or introduce unsuitable sealant into the gas stream. Provide serviceable flanges or unions where components need regular cleaning or removal.
Long above-ground pipelines expand and contract as temperature changes. Supports should carry pipe weight, maintain the designed slope, and allow controlled thermal movement without loading the blower flanges. They should also resist wind and accidental impact.
Do not use the blower or separator as a pipe support. Pipe strain can distort flanges, damage flexible connectors, and create vacuum leaks.
Leakage, Commissioning, and Maintenance
Vacuum leaks reduce useful soil extraction and dilute contaminant vapor. Common leak points include wellhead caps, flexible hoses, valve stems, drain valves, sampling ports, separator covers, instrument connections, damaged buried joints, and unused branches. Leak-test the manifold during commissioning and investigate again if performance changes unexpectedly. A low contaminant concentration caused by atmospheric leakage can be mistaken for successful remediation.
Short-circuiting can also occur when surface air travels directly to a well screen through the annular space, cracked pavement, floor drains, utility openings, poorly sealed monitoring wells, trenches, or building foundations. Use surface sealing or controlled air-inlet points where the site design requires them. The objective is to route replacement air through the treatment zone, not to eliminate all replacement air.
Measuring vacuum only at the blower inlet is insufficient to evaluate the wellfield. Depending on the design, useful points include individual wellheads, the main manifold, separator and filter inlets and outlets, blower inlet and discharge, and vapor-treatment inlet and outlet.
These readings help distinguish subsurface resistance from losses in the piping, separator, filter, or treatment train. High blower vacuum combined with weak wellhead vacuum can indicate substantial pressure loss between the well and blower.
Commissioning establishes a baseline for later operation and troubleshooting. A typical sequence is:
- Inspect wellheads, valves, pipe supports, drains, and separators; confirm low points are clear.
- Leak-test the manifold and confirm instruments are functioning.
- Start the vacuum system at controlled capacity and test each well individually. Record wellhead vacuum, airflow, vapor concentration, and moisture carryover.
- Test selected combinations of wells and measure subsurface pressure response.
- Balance the branches, then record valve positions and operating values as the commissioning baseline.
Wellfield maintenance should include inspecting wellhead seals, exercising isolation valves, checking flexible connectors, draining condensate, cleaning flow instruments, inspecting access chambers, and checking pipe slope and settlement. Remove accumulated sediment, leak-test the manifold, and compare current airflow with commissioning data. A gradual decline in one well’s airflow may indicate a blocked screen, water accumulation, valve damage, or changing soil conditions.
Design Inputs for Blower Selection
Provide the following information before finalizing the wellfield, manifold, and blower duty:
- Site plan, soil profile, contaminant distribution, and groundwater elevations.
- Pilot-test results and required extraction airflow and wellhead vacuum.
- Well count, screened intervals, active-well combinations, and expected condensate.
- Vapor composition, proposed treatment system, and expected pressure losses.
- Manifold arrangement, pipe lengths and elevations, and planned future expansion.
Glossary
Annular Seal: A low-permeability seal placed around a well casing to prevent uncontrolled vertical or surface-air migration.
Branch Balancing: Adjustment of individual branch restrictions so connected wells receive their approved flow or pressure.
Extraction Well: A screened subsurface well used to collect soil gas, groundwater or both under applied vacuum.
Filter Pack: Graded granular material placed around a well screen to support the formation and maintain flow access.
Gas-Liquid Separator: A vessel that removes entrained or collected liquid from a gas stream before downstream equipment.
Short-Circuiting: Unwanted airflow through an easy pathway that bypasses contaminated material or the intended treatment zone.
Vacuum Manifold: The piping header that connects multiple extraction wells to separators, treatment equipment and the vacuum source.
Well Screen: The perforated or slotted interval of a well that admits air, vapor or liquid from the target formation.