Soil-Gas Vapour Monitoring

The Draft National Environment Protection (Assessment of Site Contamination) Measure April 2011 has a strong emphasis on soil gas monitoring and vapour risk assessment. In particular, Schedule B (2) - Guideline on Site Characterisation provides a description on a wide variety of soil gas monitoring technologies, some are off the shelf technologies and some require customisation.

The challenge for assessors is where to access the required technologies with associated technical support. HydroTerra has been steadily building our capacity to assist assessors in soil gas vapour monitoring with a range of specialised instrumentation and associated technical support.

What are soil-gas vapours?

Soil Gas Vapours are any form of gas that can emanate from a source in the ground. The vapours can be generated from a number of sources which have the potential to migrate through the subsurface by the processes of advection and diffusion, and be released into the atmosphere. Sources are varied and may include residual soil contamination, leaking underground storage tanks, contaminated groundwater and landfill gas to name a few.

Soil gas sampling methods

Soil gas samples may be taken from probes installed in open ground or recovered via access holes drilled through sealed surfaces (such as a driveway or parking area ’near-slab‘) or beneath building foundations (‘sub-slab‘). Sampling installations may be permanent, semipermanent or temporary depending on access and the need to re-sample.

Field Screening Devices

A field screening assessment of the contaminants present can be carried out by drawing the gas through an online PID, FID or other handheld detector for obtaining readings of parameters such as O2, CO2 and methane.(Draft NEPM, 2011, Schedule B2).

HydroTerra have a range of hand held gas detectors for sale and rent which are ideal for ground gas sampling.

Permanent multi-level probes/samplers

In some instances it is necessary to install permanent sub-surface soil-gas multi-level probes for depth profiling of vapour concentrations at fixed locations.

Single depth permanent probes can either be installed at depth (close to the source) or in the shallow sub-surface (particularly where the source is shallow).

For example:

Multiple depth (or multilevel) gas sampling installations may be undertaken by installing multiple sample ports at different depths (separated by a bentonite seal) within the one sampling well.

For Example:

Online VOC and specific gas monitoring.

Near-continuous measurement of total vapour (or VOCs) oxygen concentrations and other parameters is possible using data logging sensors. It is also possible to connect such sensors to telemetry. These probes allow seasonal trends in vapour fluxes and other parameters such as degradation rates to be assessed.

For Example:

Ground Gas Testing (Probe Integrity Seal)

When installing ground gas monitoring equipment it is important to have confidence in the Probe integrity/seal — soil gas probes (temporary or permanent) should be installed in a manner that ensures that ambient air is not drawn into the sampling system and that a representative soil gas sample can be collected. A number of tracer methods are available to test the seal integrity. NEPM, 2011, Schedule B2

HydroTerra have purchased a Helium gas detector to allow seal integrity testing of soil vapour wells available for hire.

For Example:

Passive implant sampling

Passive implant sampling refers to the burial or placement of an adsorbent or other material

in the ground, which is recovered for analysis after an appropriate period of time. HydroTerra have access to a wide range of absorbents/

For example:

Customisation of monitoring equipment

HydroTerra are happy to assist with the custom build of monitoring equipment to assist site assessors with their monitoring needs.

Reference: National Environment Protection (Assessment of Site Contamination) Measure April 2011 Schedule B2 GUIDELINE ON Site Characterisation

http://www.ephc.gov.au/sites/default/files/Schedule_B2__Guideline_on_site_characterisation__SEP10.pdf