REM/General/TBT-REM-016

Monitored Natural Attenuation

Remediation & Contaminated Land › General › Monitored Natural Attenuation

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Monitored Natural Attenuation (MNA) relies on natural processes to reduce contaminant concentrations in soil and groundwater.
MNA is not a 'do nothing' approach — it requires systematic long-term monitoring to confirm natural degradation is occurring.
Natural processes include biodegradation, dilution, adsorption, and chemical transformation of contaminants over time.
Monitoring involves regular sampling of groundwater from boreholes to track contaminant concentration trends.
MNA is typically used alongside other remediation methods as part of a combined remediation strategy.
The Environment Agency requires evidence that MNA is achieving the agreed remediation targets before accepting it.
Monitoring boreholes must be installed, maintained, and sampled safely — they are often in contaminated ground.
Groundwater sampling exposes workers to contaminated water and volatile organic compounds during collection.
MNA monitoring programmes can last years or decades depending on the nature and extent of contamination.
COSHH assessments are required for all contaminants that workers may be exposed to during monitoring visits.

Verify effectiveness	Without monitoring, there is no evidence that natural processes are actually reducing contamination to safe levels.
Regulatory compliance	The Environment Agency requires monitored proof that MNA is working before it will sign off remediation completion.
Worker protection	Sampling contaminated groundwater exposes workers to harmful chemicals requiring PPE and safe handling procedures.

Don't

Complete a COSHH assessment for the contaminants expected in groundwater before sampling begins
Wear chemical-resistant gloves and eye protection when collecting groundwater samples on site
Follow the sampling protocol precisely to ensure results are representative and legally valid
Decontaminate all sampling equipment between boreholes to prevent cross-contamination of results
Monitor air quality for volatile organic compounds when opening boreholes for sampling
Secure all borehole headworks to prevent unauthorised access and contamination of the monitoring point
Record sample identification, date, time, depth, and field measurements accurately for every visit
Review monitoring data trends quarterly to confirm natural attenuation is progressing as expected
Report results to the Environment Agency at the frequency specified in the remediation plan
Maintain safe access to monitoring boreholes throughout the long-term monitoring programme

DON'T treat MNA as a passive process — it requires active monitoring and data analysis throughout
DON'T sample groundwater without chemical-resistant PPE as specified in the COSHH assessment
DON'T open borehole headworks without checking for toxic or flammable gas emissions first
DON'T cross-contaminate samples by using undecontaminated equipment between boreholes
DON'T delay reporting monitoring results to the Environment Agency beyond agreed timescales
DON'T assume contaminant levels are decreasing without analysing the monitoring data trends
DON'T leave borehole headworks unsecured where they can be tampered with or contaminated
DON'T dispose of purged groundwater from boreholes into surface drains or watercourses
DON'T skip scheduled monitoring visits — data gaps undermine the credibility of the MNA evidence
DON'T end the monitoring programme without Environment Agency approval that targets are met

See also: Contaminated Land Safety Awareness | Contaminated Groundwater Handling

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