• air sparging for volatile contaminants in groundwater
• bioventing for in situ bioremediation
• permeable reactive barrier technologies for a range of contaminants
• bioclogging to temporarily contain mobile contaminants
• multiphase extraction of petroleum products (non-aqueous phase liquids or NAPL)
• phytoremediation (use of plants)
• natural attenuation of a range of contaminants in groundwater and soil environments

Air Sparging

A series of air sparging (air injection below the groundwater table) trials has been carried out for dissolved gasoline constituents in groundwater and for residual non-aqueous phase liquid (NAPL) gasoline, ie separate phase. Initial trials indicated that dissolved benzene and other gasoline contaminants could be removed from groundwater within hours to days, but that the zone of effectiveness was within a 2 to 4-m radius of the air injection (sparging) well. Most cleanup was found to be due to volatilisation or physical stripping, rather than due to enhanced bioremediation from stimulation of aerobic bacteria by oxygen in the injected air. For the NAPL sparging trial, soil vapour extraction alone was found to remove only a small percentage of the total mass removed, compared to air sparging combined with soil vapour extraction. Another outcome of the work was that no significant reduction in the aquifer hydraulic conductivity was observed, even though residual air was found to be still at significant levels in groundwater some nine months after air sparging had ceased.

Bioventing

Bioventing was evaluated as a cleanup technique for diesel fuel contamination. Air and nutrients (ammonia and phosphate) were periodically injected into a diesel-contaminated zone 3.7 to 4.5 m below ground, to stimulate aerobic biodegradation. Over a 14-month period, biodegradation rates increased by a factor of 5-10 from background rates, i.e. up to 90 mg-diesel/kg-soil/day. Phospholipid analysis of soil materials showed that microbial biomass increased dramatically where nutrients and air (oxygen) were present. Ammonia was transformed to nitrite and nitrate, and was utilised variably in the contaminated zone. Most locations showed reductions of diesel concentrations in soil cores, with preferential biodegradation of some of the diesel constituents. At some locations, diesel concentrations were reduced to below 1,000 mg/kg from starting concentrations 100-fold higher.

Biodegradation rate changes during bioventing of diesel fuel contamination

Permeable Reactive Barriers

Permeable reactive barriers (PRBs) offer the potential to treat groundwater in situ, with the potential for lower long-term maintenance costs.  The principle is reasonably simple and involves placing a subsurface permeable ‘barrier’ across the direction of contaminated groundwater flow. The intention is to treat the contaminants of concern within the barrier and allow cleaned groundwater to flow downgradient beyond the PRB.  The ‘reactive’ portion of the PRB may be a range of treatments from aeration or addition of reductants (such as carbon) to stimulate biodegradation, to zero-valent iron that abiotically degrades chlorinated solvent chemicals. 

Proposed dual PRB strategy for remediation of ammonium-contaminated groundwater

CSIRO has shown proof-of-concept for PRBs for atrazine (a herbicide), petroleum hydrocarbons, and ammonium plumes in groundwater. Field-scale evaluations of PRBs have commenced, although few are complete. A dual barrier approach has been devised for the ammonium PRB concept. In this dual approach, oxygen is delivery in the first PRB to convert ammonium to nitrite/nitrate and a reductant (such as hydrogen or ethanol) is delivered in the second PRB to convert the nitrite/nitrate to harmless nitrogen gas. This is successfully occurring in laboratory-scale soil columns, and a field site has been instrumented with a small PRB to test the concept. It is hoped such a technique will knock out ammonium in groundwater that would otherwise discharge to the marine environment along the west coast of Western Australia. It is planned to expand this work for other contaminants, such as metals.

Multiphase Extraction of Petroleum Product (Non-aqueous Phase Liquids or NAPL)
Petroleum hydrocarbon liquids (non-aqueous phase liquids or NAPL) are notoriously difficult to recover from subsurface soil and groundwater environments. Failing to remove all of a NAPL gasoline, for example, may provide an ongoing source of soluble benzene (and other compounds) to groundwater flowing past the NAPL, for possibly decades. CSIRO has carried out field trials for enhancing NAPL recovery. These trials involved separately and simultaneously pumping the groundwater to induce drainage of NAPL to the recovery borehole, skimming/pumping of the NAPL product itself, and applying a vacuum to the well to recover vapours and avoid excess lowering of the water table. In the initial field trial in the sandy aquifers in Perth applying a vacuum during groundwater pumping increased the recovery by greater than 5-fold compared to recovery whilst only pumping groundwater. Further field trials are planned, and especially to assess recovery efficiencies during low and high water tables, since NAPL thicknesses in recovery boreholes can vary significantly seasonally.

Phytoremediation to Hydraulically Isolate and Remediate Soil and Groundwater
Plants are being increasingly trailed to remediate metals and hydrocarbon contaminated soils and groundwater. The Perth CSIRO group are one year into a field trial using eucalyptus trees (River Red Gums, Tasmania Blue Gums, etc) to hydraulically isolate and remediate petroleum contaminated soil and groundwater. The study has a dual aim: (i) to determine if the trees can reduce leaching of contaminants to groundwater and perhaps reverse hydraulic gradients in groundwater, and (ii) to determine the enhanced biodegradation potential in treed plots compared to an unplanted control area. It is early days, so no conclusive data are yet available. Other CSIRO groups have successfully carried out field trials on the uptake of selected metals into plants.

Schematic of possible phytoremediation processes for contaminants in soil and groundwater

Natural Attenuation (NA)
Natural attenuation stands with other technologies as a valid remediation/management option, where attenuation processes (biodegradation, dispersion, etc) can reduce contaminant concentrations below criteria at compliance boundaries (ie property boundaries, rivers, oceans, human exposure, etc). For a range of contaminants, nature may be able to do a better job at cleanup, than can be achieved by human intervention – ie nature may be able to cope. For some contaminants, chemical loadings are too high, or biogeochemical conditions are not conducive to ‘high-enough’ attenuation to be effective as a management strategy.
 

The CSIRO group has quantified at field scale the natural attenuation of gasoline compounds, solvents, munition residues and some pesticides. Some of these contaminants do degrade naturally given the right geochemical and microbiological conditions – but some do not. They have found, for example, at a gasoline-spill site in Perth that benzene under sulphate-reducing conditions shows only limited signs of attenuation, but toluene is readily attenuated (biodegraded). They have modelled the biogeochemical processes to determine the eventual fate of the gasoline plume. A current project seeks to determine the attenuation potential for petroleum hydrocarbons as they move towards a river environment, and especially at the groundwater/river water interface.

Innovative Measurement and Monitoring
Often detailed site characterisation and monitoring is required to properly implement and evaluate remediation (whether enhanced or natural). The CSIRO group has developed in-field techniques and on-line monitoring to assist in this task. They have trialed partitioning tracer techniques for quantifying the gasoline NAPL distribution and saturation in the zone of water table fluctuation - this technique promises to provide repeatable measurement of NAPL distributions in the subsurface.

They have developed on-line probes for measuring volatile organic compounds and oxygen – these probes can be buried at multiple locations and depths in soil or groundwater environments, and data can be reported remotely to the web. They are currently in use to monitor the effectiveness of air sparging remediation, to determine the fate of gasoline vapours in soil profiles and beneath houses, and to quantify the effectiveness of a geosynthetic capping strategy.

More Information
For more information about ‘The Remediators’ see the group’s web page at: http://www.clw.csiro.au/research/remediation/organic/ or e-mail: Greg.Davis@per.clw.csiro.au. The group also organised the 1999 and 2000 Contaminated Site Remediation Conferences held in Fremantle and Melbourne.

Principal supporters of the group’s research are BP, Western Australian Industry Groups, Shell, BHP, Rio Tinto, the Water and Rivers Commission of Western Australia, and the Centre for Groundwater Studies.

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