July 4, 2013
With fracking increasingly in the news it is worth taking a few minutes to look at the environmental concerns with hydraulic fracturing of shale particularly in relation to the potential contamination of groundwater. Cases of suspected groundwater contamination have been documented but it is not for us to judge whether fracking does or will cause disruptions to groundwater supplies however we should look at the remediation techniques available to treat contaminated groundwater and in particular ‘pump and treat’ as a remediation technique.
Pump and treat involves pumping out contaminated groundwater with the use of a submersible (SS Geosub pump) or a vacuum (Advanced Peristaltic) pump for smaller site contaminations or more industrial pumping kit and allowing the extracted groundwater to be treated either by purification (slowly filtering the groundwater through a series of vessels that contain materials designed to adsorb the contaminants from the groundwater) or separation (collecting/containerising the contaminant and treating ex-situ).
For petroleum-contaminated sites the material used for filtration purification is usually activated carbon in granular form. Chemical reagents such as flocculants followed by sand filters may also be used to decrease the contamination of groundwater. Air stripping is a method that can be effective for volatile pollutants such as BTEX compounds found in gasoline.
For most biodegradable materials like BTEX, MTBE and most hydrocarbons, bioreactors can be used to clean the contaminated groundwater to non-detectable levels. With fluidised bed bioreactors it is possible to achieve very low discharge concentrations which will meet or exceed discharge standards for most pollutants.
Depending on the geology and soil type, pump and treat as a remediation technique may be a good method to quickly reduce high concentrations of pollutants but it is more difficult to reach sufficiently low concentrations to safely class a remediation as satisfactory, due to the equilibrium of absorption (chemistry)/desorption processes in the soil. Also, pump and treat as a remediation technique is an expensive way to treat contaminated groundwater and typically is a very slow process.
Pump and treat is best suited to controlling a hydraulic gradient and keeping a contamination release from spreading further. Better options of in-situ treatment often include air sparge/soil vapour extraction (AS/SVE) or dual phase extraction/multiphase extraction (DPE/MPE). Other methods include trying to increase the dissolved oxygen content of the groundwater to support microbial degradation of the compound (especially petroleum) by direct injection of oxygen into the subsurface, or the direct injection of a slurry that slowly releases oxygen over time (typically magnesium peroxide or calium oxy-hydroxide).
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