Beneath the Water Table: Profiling Groundwater at SVE Sites

A soil vapor extraction system looks deceptively simple from the surface: an array of extraction wells, a blower skid, a stack of pipework, and a process skid stripping contaminants from the off-gas. The real engineering, however, sits below ground, in the relationship between the unsaturated zone where vapors travel and the saturated zone that sets the lower bound for treatment. Interpreting that relationship begins with a groundwater profile, the vertical record of a site’s hydrogeology, chemistry, and physical stratigraphy that determines whether an SVE campaign will hold its design vacuum or stall within weeks of being commissioned. Learn more about Turbidity Monitoring In Flood Events Rapid Response And Data Collection.

Across Australia, the redevelopment of former industrial land in places like Sydney’s Inner West, Melbourne’s Fishermans Bend, and Perth’s older industrial corridors has put hundreds of hectares under formal investigation. Most of those projects fall under the National Environment Protection (Assessment of Site Contamination) Measure, commonly referred to as the NEPM, together with its 2013 amendment. Site auditors working under that framework expect a defensible hydrogeological picture before they will sign off on the conceptual site model that underpins any SVE design, and that picture is built one boring at a time.

Building the Conceptual Site Model Before the Probe Goes In

A profile is only as honest as the thinking that frames it. Before a single monitoring well is installed, the investigator should already be carrying a working conceptual site model built from titles searches, historical aerial photography, trade directories, sand allotment records, and council archives. In New South Wales, the NSW EPA Practice Note on contaminated sites and the underpinning state environment planning policy provide the regulatory scaffolding; in Victoria, the Victorian EPA’s audit framework achieves a similar purpose. The model should already predict the depth of fill, the likely depth to water, and the contaminants most probably present, even if those predictions turn out to be only partly correct.

The practical benefit of investing in this desktop stage is that the field program can be targeted rather than generic. A site suspected of holding dense non-aqueous phase liquid beneath a clay layer needs nested piezometers screened across that interface, not a single well finished at the water table. A site over a shallow limestone aquifer in the southern Adelaide Plains may need a different approach altogether, with higher-yield airlift tests and conservative assumptions about lateral migration. Building the CSM first keeps the drilling contractor, the analytical laboratory, and the site auditor working from the same blueprint rather than chasing surprises.

Stratigraphic Logging and the Language of Aquitards

Few field tasks are more pedestrian and more consequential than describing soil in a core tray. The colour change from a mottled orange clay to a grey silty sand is often the line between a treatable interval and a dead zone for vapor flow. Australian logging practice generally follows AS 1726, with logs recording moisture content, plasticity, structure, and obvious indicators of contamination such as hydrocarbon odours, sheen, staining, or discolouration. Anything resembling a fill horizon should be flagged separately, because fill across an old manufactured gas plant site, for instance, may include demolition rubble, ash, lime residues, and decayed timber that distort both airflow and recovery calculations.

Aquitards deserve close attention during logging. A continuous clay layer two metres thick will behave as an effective barrier to vapor migration, forcing extraction wells to be screened above it and requiring careful assessment of whether contamination has been driven deeper by past drilling or trenching. A discontinuous clay lens, on the other hand, can act as a perched reservoir that bleeds contamination downward during recharge events, especially after heavy summer storms that pulse through the Sydney Basin or the wet-season rises that lift water tables across the Top End. The log needs to record thickness, lateral extent where observable in adjacent holes, and any secondary porosity such as root channels, bioturbation, or desiccation cracks.

Hydraulic Gradients, Flow Direction, and Tidal Influences

Once wells are installed and properly developed, the next task is to read the water itself. Three rounds of depth-to-water measurements, ideally captured across different tidal stages for any coastal site, are the minimum for a defensible potentiometric surface. In Botany Bay around the historic Orica precinct and along the Yarra estuary near Fishermans Bend, tidal oscillations of half a metre or more can flip local gradients twice a day. Ignoring that signal leads to misinterpretation of contaminant migration direction and to a poorly placed extraction wellfield that draws clean groundwater past the source rather than vapor from it.

Further inland, gradients tend to be driven by topographic relief, irrigation pumping, and seasonal recharge. On the Gnangara Mound north of Perth, summer drawdown from public open-space irrigation can steepen gradients enough to pull plumes kilometres from source within a single irrigation season. Calculating a meaningful gradient requires correcting water levels for temperature, density, and barometric pressure, then plotting a contour map that the auditor can overlay against the inferred plume footprint. Where the gradient is essentially flat, the profile interpreter should flag potential diffusion-dominated transport and recommend longer equilibration times before sampling commences.

Dissolved-Phase Contaminant Plumes and Their Vertical Expression

A vapor extraction system treats the gas phase, but the mass it removes originates in liquids held in pore spaces. The vertical distribution of dissolved contaminants, light non-aqueous phase liquid, and dense non-aqueous phase liquid within the saturated zone dictates how much of that mass is accessible. LNAPL, when present, tends to sit above the water table as a separate phase, smearing across the capillary fringe and creating a long-term secondary source that can sustain vapor recovery for years. DNAPL, being heavier than water, sinks to the base of the aquifer and pools on low-permeability lenses where it is effectively untouchable by SVE alone and must be addressed by other means.

Sampling protocols matter at this stage. In the Australian context, low-flow purge-and-sample techniques using dedicated bladder pumps have largely replaced bailers for routine work, and most accredited laboratories now run low-level volatile analysis to method detection limits around one microgram per litre. The profile is only credible when field duplicates, trip blanks, and equipment rinsates are reported alongside the primary results, and when the chain of custody satisfies the NEPM’s data quality objectives. Any unusual vertical profile, such as a dissolved chlorinated solvent peak well below the water table, deserves a second look at the geology before being accepted at face value.

Coupling Vapor Readings with the Saturated Zone Profile

Once vapor probes have been driven across the unsaturated zone and groundwater has been characterised beneath it, the two pictures must be stitched together. A high vacuum reading at a probe set two metres above the water table paired with a clean groundwater sample fifty metres downgradient tells the engineer that the immediate source has been depleted, not that the site is clean. The reverse is equally informative: falling vacuum combined with rising dissolved-phase concentrations during extraction suggests that vapor recovery is drawing mass upward from the saturated zone and that the SVE run will need to extend longer than initially scheduled.

Major recharge events complicate the picture further. After the kind of rainfall that closes roads across Brisbane’s bayside suburbs or the cloudbursts that sweep through McLaren Vale, groundwater mounding can saturate the smear zone and temporarily halt vapor flow across the wellfield. Practitioners planning long extraction campaigns increasingly look to https://d-a-instruments.com/blog/2026/08/turbidity-monitoring-in-flood-events-rapid-response-and-data-collection to understand how rapid-response water-quality monitoring captures the suspended sediment and particulate loads that accompany these events. Knowing how the aquifer responds to a flood pulse allows the extraction schedule to be throttled rather than abandoned and helps explain why vapor concentrations sometimes dip for weeks after a major storm.

Reporting Findings That Regulators and Auditors Will Accept

A profile that lives only on a field log sheet is a profile that does not exist for regulatory purposes. The final interpretive report should present lithology, hydraulic data, groundwater chemistry, and vapor readings on a single composite figure for each boring, supported by tabulated analytical results with full quality assurance and quality control reporting. Cross-sections linking the profiles clarify the three-dimensional geometry of the plume, while concentration-versus-depth plots help the reader see whether mass is concentrated near the water table, dispersed through the smear zone, or migrating toward a deeper receptor such as a confined production aquifer.

Auditors across Australia have grown increasingly sceptical of any site assessment that treats the vapor and groundwater regimes as independent systems. The contemporary expectation, set out in guidance issued by CRC CARE and referenced by state environment agencies, is that the two are integrated within a single conceptual model. Consultants who can deliver that integration efficiently tend to rely on consistent instrumentation and a shared data platform rather than a patchwork of incompatible loggers and sensors. Site teams interested in that approach can review the optical sensing platform used across the broader D & A Instruments product family to see how turbidity, suspended-solids, and water-level measurements can be brought under one telemetry umbrella. When the next investigation begins, that kind of workflow is what turns a stack of paper profiles into a defensible case for the regulator and a reliable basis for designing an extraction system that actually works in the field.