Research and analysis

Determining background concentrations of metals in waterbodies impacted by abandoned metal mines: summary

Published 5 February 2026

Applies to England

1. Chief Scientist’s Group report summary

This project explored methods to determine the naturally occurring ‘background’ concentration of metals in rivers polluted by abandoned metal mines. The findings will inform the assessment of the statutory Environment Act target to reduce pollution from abandoned metal mines.

1.1 Background

Metals discharged from abandoned metal mines pollute about 3% of rivers in England and can harm river wildlife and ecosystems. The Environmental Targets (Water) (England) Regulations 2023 set a statutory target to halve the length of rivers and estuaries polluted by abandoned metal mines in England by 2038.

Metals occur naturally in the environment, and their concentrations are typically higher in waters which are influenced by mineral deposits. In some rivers, this can mean metal concentrations are greater than the Standards used for water quality compliance assessments.

A background concentration can be applied in water quality compliance assessments to account for the naturally occurring concentrations of metals in the environment. However, the current approach for determining the background concentrations of a metal is not suitable for rivers influenced by metal deposits, such as those impacted by abandoned metal mines. An approach that accounts for natural mineralisation would enable more accurate assessment of progress towards the target.

1.2 Approach

A systematic review of academic and grey literature was conducted to identify methods that had previously been used to determine background concentrations of metals in river, lake, estuarine and groundwater. These methods were evaluated against four suitability criteria. To be considered suitable, the method had to:

• determine a background concentration for the target in surface waters
• be scientifically justified or validated
• account for local mineralisation
• be compatible with existing Environment Agency data and/or other open data sources

A pilot study then tested the applicability of suitable methods in four English catchments using existing Environment Agency monitoring data.

1.3 Results

The review identified 14 methods for determining background concentrations of metals in surface and groundwaters. Two methods met all four of the suitability criteria. Both methods require metal concentration data from monitoring points located in or proximal to the catchment of interest because background concentrations are dependent on local geology and hydrology. Monitoring points must also be located downstream of metal deposits but upstream of all abandoned mining activity. The background concentrations derived using these methods are specific to the catchments where data was collected. Therefore, suitable monitoring data is needed for all catchments impacted by abandoned metal mines.

The location of Environment Agency water quality monitoring points relative to the location of mineral deposits, abandoned metal mines and mining spoils was investigated in four pilot catchments. In all four catchments, the locations of current and historical monitoring sites were impacted by pollution from abandoned metal mines. Therefore, neither method can be reliably applied.

1.4 Conclusions

This work identified two scientifically robust methods for determining background concentrations of metals in mineralised catchments. Both require monitoring data obtained from monitoring points downstream of mineralisation and upstream of abandoned metal mines and mining spoils. A pilot study could not identify current or historic Environment Agency monitoring points which met these requirements in four test catchments.

Background metal concentrations depend on local mineralisation and hydrology and so cannot be extrapolated between catchments. The lack of suitable monitoring points in the four pilot catchments means that neither of the two suitable methods identified will be able to be applied consistently across catchments in England impacted by abandoned metal mines, using existing Environment Agency monitoring data.

There are several options which could progress the development of a protocol for determining background concentrations of metals in mineralised catchments:

• monitoring metal concentrations at new sites in catchment headwaters that are not impacted by metal mining
• testing other methods identified in this project for determining background concentrations
• modifying existing methods to account for local mineralisation

Data from new sites located in catchment headwaters, which are unimpacted by mining but downstream of mineral deposits, would enable the two suitable methods identified by this work to be used. This would be the most pragmatic and scientifically justified approach for determining background concentrations of metals in mineralised catchments.

1.5 Publication details

This summary relates to information from project SC240021, reported in detail in the following output:

• Report: SC240021/R
• Title: Determining background concentrations of metals in waterbodies impacted by abandoned metal mines.
• Project manager: Laura Hunt, Chief Scientist’s Group

This project was delivered by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

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