Metal mining in the UK has occurred for thousands of years and was widespread during the 18th and 19th centuries. This activity has resulted in a legacy of abandoned metal mine sites across the UK, many of which are either contaminated or result in the release of metals to the aquatic environment.
Four hundred and seventy waterbodies have been identified as “impacted” or “probably impacted” by abandoned non-coal mines across England and Wales. The first Water Framework Directive (WFD) surface water classification in England and Wales identified 133 waterbodies failing to meet environmental quality standards (EQS) for metals (Cd, Pb, Ni, Zn, Cu) in metal mining areas.
We were recently commissioned by the Environment Agency to undertake a project to investigate whether conventional EQS are suitably protective of aquatic life in waterbodies impacted by abandoned metal mines, or if higher metal concentrations than the EQS can be acceptable in these waterbodies without risk to aquatic life.
Ecological status of waterbodies impacted by abandoned mines (i.e. the biodiversity of the aquatic community) is often found to be good, even where EQS are exceeded. Importantly, we were asked to explore how these instances could be accommodated within the WFD to enable the status of the water body to be correctly classified.
Additional objectives of the project were to provide guidance on:
- The course of action when biological and chemical measurements differ, that is, when EQS for metals are breached but the biological community is in a good condition.
- How to implement water quality targets for metals that represent good chemical and ecological status in mining-impacted rivers.
- The evidence required to enable the appropriate refinement of water quality targets in mining-impacted rivers. For example, whether site biological data are required.
The project required a combination of desk and field work. The “tiered risk-based framework” for incorporating biotic ligand models (metal bioavailability models) and ambient background concentrations (ABCs) for metals risk assessment under the WFD was used as the basis for the project.
The initial phase of the project was focussed on the identification of sub-sets of waterbodies that share similar WFD compliance characteristics for further investigation in subsequent phases of the project. For example, scenario three has poor chemical status but good ecological status.
This “screening assessment” applied both simple bioavailability screening tools and default Ambient Background Concentrations for zinc and was used to identify candidate waterbodies that offered the greatest scope for developing robust guidance for the implementation of alternative approaches to assess the status of mining impacted waters.
Tiered risk-based framework – click to enlarge
The conclusions of the project were as follows:
- Accounting for the bioavailability of copper in waterbodies affected by abandoned metalmines using simple screening tools reduces the face-value EQS failure rate by approximately 66 per cent, and the “significant” EQS failure rate by more than 70 per cent.
- Accounting for the bioavailability of Zn and Ni as well as Cu using simple screening tools does not affect the overall burden of EQS failure across waterbodies impacted by abandoned metal mines, but reduces the proportion of mining impacted waterbodies affected by one or more “significant” EQS failures from 54 to 36 per cent.
- An assessment of the risk posed by mixtures of metals can be readily conducted on compliance data expressed as “confidence of failure”. However, the lack of metals data in the majority of waterbodies included in the WFD surface water classification limited the useful application of this approach.
- Collection of site-specific physicochemical data can be used to refine the predictions of bioavailability screening tools made using conservative defaults, and are likely to result in improved compliance (particularly for the copper EQS).
- The full BLM for copper provides less precautionary estimates of site-specific PNECs than the simple Cu bioavailability screening tool and could be readily applied by the Environment Agency to refine EQS compliance where risks remain after application of the bioavailability screening tool.
- Waterbody-specific Ambient Background Concentrations (ABC) for zinc can be estimated by sampling in the headwaters of waterbodies, but care must be taken to ensure that these estimates are reliable.
- Site-specific quality targets for zinc, based on the macroinvertebrate ecology predicted or observed at a site, can be derived and can result in improved compliance compared to the use of both conventional and bioavailability-based EQS. In addition to zinc, the approach is likely to be applicable to other metals and possibly other types of chemical stressors (for example, pesticides). However, the methodology for deriving site-specific quality targets requires additional development and validation before they can be robustly applied during surface water classification.
- Guidance for Environment Agency staff on the appropriate application of bioavailability-based tools in waterbodies affected by abandoned non-coal mines has been produced.