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Monitored Natural Attenuation (MNA) is defined as "the monitoring of groundwater to confirm whether Natural Attenuation (NA) processes are acting at a sufficient rate to ensure that the wider environment is unaffected and that remedial objectives will be achieved within reasonable timescale."

Natural attenuation relies on natural physical, chemical and biological processes that act, without human intervention, to decrease contaminant concentrations, flux and toxicity in soil and groundwater, and thereby reduce the risks posed by contamination. Attenuation occurs at most polluted sites, however the right conditions must exist underground to clean sites properly. These conditions are monitored to make sure natural attenuation is working.

MNA works most successfully where source removal of the pollutant has taken place. That is buried waste or a source such as a fuel storage tank must be treated/removed and disposed of properly. After the source is removed, the natural processes degrade residual contaminants in the soil and groundwater. During this time the soil and groundwater are monitored regularly to make sure they are improving.

MNA is suitable for almost any hydrocarbon spill in soils or groundwater. Application usually restricted by duration of monitoring required to attain acceptance and potential "blight" of any properties constructed on site undergoing monitoring. This is most suited to commercial industrial sites where processes are being continued. Duration of monitoring is dependant upon the type of hydrocarbon impact and availability of nutrients, oxygen and soil type. A network of upgradient and downgradient boreholes are required together with advanced chemical testing.

MNA is not a "do nothing" way to clean up sites. Regular monitoring is needed to make sure pollution doesn't migrate off site or pose a significant risk to other receptors. Depending on the site, MNA may work just as well and almost as fast as other methods. Because MNA takes place underground, physical remediation techniques are not needed. As a result there is no waste generated through dig and dump operations. MNA requires less equipment and labour than most methods as such it can be cheaper, however monitoring for many years can be costly, and regulators may place restrictions on development whilst it is ongoing.

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