Vacuum and airflow distribution in contaminated soil determine how effectively SVE, bioventing and other air-based methods reach their target zones. The blower creates the pressure difference; when a roots blower is used, its operating point must match the required airflow and inlet vacuum.
Subsurface airflow is rarely uniform. It follows permeable layers, fractures, utility trenches and other low-resistance paths. As a result, a system may produce high total airflow while leaving some contaminated areas with little or no effective treatment.
The vacuum system must therefore be designed around soil permeability, geological layering, moisture conditions and measured pressure response, not simply around the size of the contaminated site.
Soil Conditions and Airflow Behavior
Air moves through soil when a pressure difference exists between two locations. In an SVE system, the extraction blower lowers pressure inside the well, drawing soil gas from surrounding higher-pressure areas toward the extraction point.
The resulting airflow depends on well vacuum, distance from the extraction well, soil permeability and moisture, thickness of the unsaturated zone, and well-screen length and position. Surface cover, nearby wells, underground structures and groundwater level also affect the pressure field. Vacuum generally declines with distance, but the rate depends on subsurface structure. High vacuum at the blower does not prove that a sufficient pressure difference reaches the contaminated soil.
Air permeability describes how easily gas can move through connected soil pores.
Coarse materials generally provide less resistance than fine-grained materials. A simplified qualitative comparison is:
Indicative soil conditions and airflow response
| Soil condition | Typical airflow behaviour |
|---|---|
| Gravel | Very high airflow with relatively low vacuum |
| Coarse sand | High airflow and good pressure transmission |
| Fine sand | Moderate airflow with greater resistance |
| Silt | Low airflow and limited pressure response |
| Clay | Very low airflow except through cracks or preferential pathways |
| Fractured soil or rock | High localized flow through fractures |
| Wet soil | Reduced air-filled porosity and greater resistance |
These descriptions are general. Compaction, grain distribution, cementation and water content can cause soils with similar names to behave differently. Site measurements are more reliable than selecting blower capacity from soil classification alone.
Porosity represents the total amount of void space within the soil. High porosity does not automatically mean high air permeability.
The pores must be interconnected for air to move effectively. Clay can have substantial total porosity while still resisting airflow because its pores are extremely small and poorly connected for gas transport.
The amount of connected, air-filled pore space therefore changes with moisture; the resulting operating effects are discussed below.
Natural soil profiles commonly contain layers with different permeabilities.
A site may include surface fill, sand, silt lenses, clay layers, gravel channels, and weathered rock, as well as fractured bedrock. Air tends to travel horizontally through permeable layers rather than crossing a low-permeability layer. This can cause the vacuum system to treat one depth effectively while bypassing contamination above or below it. A long well screen passing through several layers may collect most of its airflow from the most permeable zone. The total measured airflow may appear satisfactory even though resistant layers contribute almost nothing.
Possible solutions include separate extraction wells for different depths, shorter screened intervals, independently controlled well zones, nested wells, pulsed operation, and targeted air-inlet wells, as well as hydraulic or pneumatic fracturing where suitable and permitted. The appropriate arrangement depends on site geology and remediation objectives.
Soil permeability can differ according to direction. Layered sediments often allow air to move more easily horizontally than vertically. This condition is called anisotropy.
Anisotropic soil can create an elongated pressure-response area rather than a circular one. The actual influence of an extraction well may extend farther along a permeable layer and remain limited across it. Assuming a circular radius of influence can therefore result in incorrect well spacing. Pressure measurements should be taken in several directions and, where necessary, at several depths.
The radius of influence describes the distance from an extraction well over which a measurable pressure response occurs. It describes pressure propagation, not a fixed boundary of contaminant removal, and should not be used alone to set well spacing.
The measured radius depends on applied well vacuum, instrument sensitivity, soil permeability and moisture, observation-well construction, surface condition, test duration and operation of nearby wells. A measurable pressure response does not demonstrate that meaningful airflow passes through the contaminated soil. A response below an instrument’s detection threshold does not prove that no airflow occurs.
Evaluate treatment coverage using pressure and airflow data from target zones, contaminant-vapor concentration, oxygen and carbon-dioxide changes, contaminant mass-removal trends, and rebound after shutdown. Include measurements at multiple depths where site layering warrants them. Set well spacing from site geology, contaminant distribution and the airflow needed through target zones, using pilot-test data rather than a single radius value.
Air follows paths of lower resistance.
Common preferential pathways include gravel layers, utility trenches, abandoned pipelines, foundation backfill, soil cracks, and fractured rock, as well as animal burrows, previous excavation areas, poorly sealed boreholes, and gaps around underground structures. These pathways can cause short-circuiting. Instead of travelling through the contaminated soil, atmospheric air enters through an easy route and moves directly toward the extraction well.
Indicators of short-circuiting can include high airflow with low vacuum, low contaminant concentration, strong response near a trench, weak response across a low-permeability zone, rapid atmospheric oxygen entry, and unexpected changes after opening or closing one well. Surface sealing and branch balancing may reduce this problem, but subsurface pathways should first be identified.
Atmospheric air entering through the ground surface can be beneficial or harmful depending on the remediation design.
Controlled entry provides replacement air and can help sweep contaminants toward extraction wells. Uncontrolled entry near a well can bypass deeper contamination.
Surface-air entry is affected by pavement, concrete floors, building foundations, cracks, drainage systems, and unsealed monitoring wells. Performance also depends on vegetation, soil dryness, and rainfall. Where excessive short-circuiting occurs, the surface may be sealed using a suitable low-permeability cover. A cover should not be installed without considering stormwater drainage, vapor accumulation beneath structures and access to underground services.
Water occupies pore space that would otherwise be available for gas flow. Higher soil moisture may reduce air permeability, raise the vacuum needed to maintain a given airflow, reduce extraction flow, alter the pressure response, increase condensate production, and redistribute some soluble contaminants.
Heavy rainfall can cause a sudden reduction in airflow. As the soil dries, performance may partially recover. Increasing blower speed immediately can raise vacuum without producing a proportional improvement in useful subsurface airflow. Seasonal moisture changes should be included in the expected operating range.
A rising groundwater table reduces the thickness of the unsaturated zone and may submerge part of the extraction-well screen.
This can result in reduced vapor-extraction area, increased liquid carryover, lower soil-gas flow, higher blower vacuum, unstable separator operation, and movement of contaminants into different soil zones. A falling groundwater table can expose previously saturated contaminated soil, creating a new source of vapor. Groundwater elevation should therefore be monitored together with airflow, vacuum and contaminant concentration.
Wellfield Design and Flow Distribution
The screened section of an extraction well should intersect the intended treatment zone.
A screen that is too long may draw most of its air from the easiest layer. A screen that is too short may fail to cover the complete contaminated depth.
Important considerations include depth of contamination, seasonal groundwater range, soil layering, building foundations, required vertical control, and potential liquid entry. The design should also consider nearby underground services. Where several layers require different vacuum conditions, separate screened intervals can provide better control than one long screen.
Well spacing should reflect site geology, contaminant distribution and pilot-test results, not a standard distance applied to every site. Pressure-response measurements inform the design, but they should not be treated as a direct spacing rule; the design should promote airflow through contaminated zones.
Closely spaced wells can improve coverage but may interfere with one another. If several wells draw from the same permeable zone, their pressure fields overlap and the additional airflow from each new well may be limited. Widely spaced wells can leave untreated areas between them, especially in heterogeneous soil.
Evaluate individual well airflow, pressure response at monitoring points, interactions between wells, contaminant distribution, soil stratigraphy and required remediation time. Account for available blower capacity. Well spacing may need to vary across the site because soil and contamination conditions are rarely uniform.
When several wells operate simultaneously, each affects the pressure field around the others. Opening a high-flow well can reduce vacuum available to more distant or resistant wells; closing it may increase vacuum elsewhere. The whole system therefore cannot always be understood by testing each well independently.
During commissioning, test wells individually and in selected combinations. Record airflow and vacuum, measure pressure response at observation points, and document changes caused by well interaction. Record normal valve positions so the system can be returned to a known balance. Repeat measurements after seasonal or process changes.
Some SVE systems use controlled air-inlet wells to direct replacement air through a specific contaminated zone.
An inlet well may help extend the treatment pathway, deliver oxygen for biodegradation, reduce uncontrolled surface-air entry, sweep a low-flow region, improve vertical airflow, and influence vapor movement beneath a surface cover. An inlet well should not be positioned so close to an extraction well that it creates a direct short circuit. The effect should be confirmed through pressure, airflow and concentration measurements.
Pressure loss also occurs above ground between the extraction wells and blower.
A remote well can have weak vacuum because of an undersized branch pipe even when the soil itself is permeable.
The manifold should be evaluated for combined airflow, pipe diameter, branch length, number of bends, valve restriction, and condensate accumulation. The system should also account for filter loading, separator resistance, and elevation differences. Vacuum should be measured at both the blower inlet and individual wellheads. A high blower vacuum combined with low wellhead vacuum can indicate excessive manifold resistance.
Balancing distributes the available blower capacity across the extraction wells.
The objective is not to give every well the same airflow. Different wells may require different operating conditions because they serve different soil zones.
Balancing should consider contaminant concentration, contaminant mass-removal rate, soil permeability, wellhead vacuum, branch airflow, and treatment priority. The design should also consider vapor-treatment capacity, moisture carryover, and interaction with nearby wells. A well with high airflow but very low contaminant concentration may consume much of the blower capacity while contributing little to total mass removal. Temporarily reducing flow from that well may improve extraction from more contaminated areas.
Field Measurements and Pilot Testing
Measure airflow in each well with instruments suitable for vacuum service and the expected gas conditions. Accuracy can be affected by sub-atmospheric pressure, gas temperature, moisture, contaminant composition, pulsation, insufficient straight-pipe length, low flow and instrument fouling. Install instruments where they can be inspected and cleaned.
State both the measurement location and the flow reference conditions. Actual flow at the blower inlet or wellfield differs from standard or normal flow; compare values only after converting them to consistent pressure and temperature references.
Monitoring points are used to evaluate how extraction pressure is transmitted through the soil.
Each monitoring point should have a known screened depth, leak-tight construction, a protected connection, an appropriate pressure range, and an identifier tied to the site plan. Pressure should be recorded after the system has operated long enough to approach a stable response. Short tests may reflect the volume of connected pores near the well rather than the full subsurface pressure field. Measurements should also be taken when selected wells are turned off to understand interaction and rebound.
A pilot test provides site-specific information before full-scale design. Connect a temporary vacuum unit to an extraction well and operate it at several controlled vacuum levels. Measure airflow at each condition, pressure response at surrounding points, contaminant concentration, temperature and moisture carryover. Test selected well combinations and observe pressure recovery after shutdown.
Continue long enough for pressure response to stabilize and to identify changes in airflow, surface-air entry or condensate accumulation. Short-duration data can underestimate achievable airflow or miss liquid carryover.
Pilot-test data should be reviewed as a complete set.
Possible observations include:
Possible interpretations of pilot-test patterns
| Pilot-test result | Possible interpretation |
|---|---|
| High vacuum and low airflow | Low permeability, blocked screen, wet soil or restricted piping |
| Low vacuum and high airflow | Permeable soil, surface leakage or preferential pathway |
| High airflow and low contaminant concentration | Clean-air short circuit or previously depleted zone |
| Moderate airflow and high concentration | Potentially strong contaminant mass removal |
| Strong nearby pressure response only | Limited influence or highly resistant soil |
| Strong response in one direction | Layering, fracture or utility pathway |
| Rising moisture carryover | Well screen near groundwater or excessive vacuum |
| Rapid vacuum loss after shutdown | High permeability or strong atmospheric connection |
| Slow pressure recovery | Low permeability or trapped subsurface vacuum |
These interpretations should be verified using geological, chemical and groundwater information.
Operating Changes and Blower Control
Subsurface airflow conditions can change as volatile contaminant mass is removed, soil dries, rainfall and groundwater levels vary, biological activity develops, or the site settles. Well-screen fouling, new surface construction, cracks, utility work and increasing vapor-treatment resistance can also alter performance. Review the blower operating point and well balance periodically; commissioning conditions may no longer represent the current site.
A variable-frequency drive can adjust total airflow and vacuum as the active wellfield changes. Confirm that the blower can meet the required duty at the minimum and maximum combinations of active wells while remaining within its permitted speed range. Branch valves remain necessary for well-by-well balancing.
Blower Selection Considerations
Develop the blower duty from field measurements or a validated site model. Specify simultaneous airflow, required vacuum at the extraction wells, airflow reference conditions, and pressure losses through the manifold, separator, filter and vapor-treatment equipment. Include minimum and maximum active-well combinations, expected moisture and seasonal ranges, site altitude, inlet-gas temperature and composition, and the control range. Verify the selected blower against its performance data at the required duty points.
Do not add a large design margin without justification. An oversized positive-displacement blower can create excessive vacuum when only a few wells operate. Staged blowers or variable-speed control may provide better flexibility where site demand spans a broad range.
Glossary
Permeability: A measure of how readily a porous medium transmits air or liquid under a pressure gradient.
Pneumatic Pilot Test: A field test that applies controlled air injection or extraction and measures airflow and pressure response.
Porosity: The fraction of a material’s bulk volume occupied by void spaces, whether or not those voids are well connected.
Preferential Pathway: A high-conductance feature such as a fracture, utility trench or coarse layer that channels subsurface airflow.
Radius Of Influence: The observed distance over which a test well produces a measurable pressure or vacuum response.
Radius Of Treatment: The area actually receiving sufficient airflow or pressure change to achieve the remediation objective.
Short-Circuiting: Unwanted airflow through an easy pathway that bypasses contaminated material or the intended treatment zone.
Wellfield Balancing: Adjustment and verification of branch flows and wellhead pressures across a network of remediation wells.