Nant y Mwyn Lead Mine

Nant y Bai Temporary Scour Protection

This year, with the cooperation of landowners, we plan to stabilise the eroding mine waste tips along a section of the Nant y Bai channel. This will reduce metal-rich sediment entering the stream, thereby improving water quality and safeguarding heritage features and nearby habitats. 

Details of planned works

Proposed Works: Installation of scour protection measures in eight separate locations along the Nant y Bai watercourse, which will include rock filled bags, geotextile matting, timber stakes and grassland translocation. Also, four brash sediment traps and a 28m pipe to carry a surface water discharge across legacy mining spoil, to reduce leaching of metals.

Start: We would like the works to commence in Spring 2026, however this timescale could easily be impacted, as we are working in complex environments with much sensitive flora and fauna and other constraints including working in a watercourse.

Duration: The works are scheduled to take approximately eight weeks. While you may occasionally see work vehicles in the area, we anticipate minimal disruption to local traffic.

Protected Site Considerations: Because our operations take place within a Site of Special Scientific Interest (SSSI), we are working with NRW protected sites team and taking precautions to protect the sensitive habitats during the works. This may include:

Delivery of materials like rock filled bags by helicopter, as this method will greatly reduce disturbance of the ground-based habitats. We are conscious of potential noise impacts, so helicopter use will be restricted and carefully scheduled.

The use of heavy machinery will be kept to a minimum. Most of the equipment deployed will be small and light, helping to limit disturbance to the area.

If you wish to learn more, receive updates, or give your views, please contact us on the details below, as your feedback is welcomed. Thank you for your continued support as we work together to protect this important site and enhance the health of our rivers for generations to come.

Contact us

General Enquiries: 0300 065 3000

Engagement Manager: Adam Bateman – 01623 637015 (Mining Remediation Authority)

Email: nantymwyn@metalmineswales.co.uk

There is evidence of ancient mining (pre-Roman) at the site however most of the workings date from the 16th through to the 20th centuries.

Mining activity at Nant y Mwyn mine during the early 20th century was intermittent and the site was abandoned during the early 1930s.

The workings can be divided into two main areas; the Upper Mine and the Lower Mine workings.

The Upper Mine workings are the more extensive and are found along the valley of the Nant y Bai to the north of Rhandirmwyn. They constitute the collapsed Upper Boat Level portal (or ‘mine entrance’), settlement ponds, dressing floors, engine house & chimney and substantial spoil tips.

The Lower Mine workings are situated adjacent to the southwest of Rhandirmwyn in the valley of the Nant y Mwyn watercourse, although workings extend up the length of the valley to the northeast. These workings comprise spoil deposits and a former mine process area, Pannau Adit portal and Deep Boat Level portal. The Deep Boat Level is the main discharge point for the Nant y Mwyn Mine and is located in the grounds of a residential property. The portal has collapsed causing the discharge water to issue from a shaft approximately 50m back from the original portal before entering the Nant y Mwyn via Church Terrace tributary.

Both Nant y Mwyn and Nant y Bai streams are heavily contaminated and contribute approximately 13 tonnes of zinc, 2 tonnes of lead and 40 kg of cadmium to the environment each year.

They are a significant source of metals to the Afon Tywi, causing the river to fail standards for zinc and cadmium, with zinc remaining elevated for approximately 25km downstream of the mine.

Over the past few years we have been gathering information to better understand the pollution sources from Nant y Mwyn mine, and to identify potential remediation options. In 2020 we completed a review of mining and mine water hydrology, including the discharges of mine water from the Lower Boat Level, Upper Boat Level and Pannau Adit. This review recommended additional work to improve our understanding of the mine system. We subsequently installed flow monitoring structures on the three discharges and continue to monitor the water quality monthly.

We have also completed archaeological, ecological and landscape assessments, topographical surveys, and commissioned an updated and expanded Scoping Study to identify a long list of possible mitigation options to improve the water quality in the Afon Tywi. It is likely that a combination of the high level potential mitigation options presented below, will be needed to successfully manage the range of pollution sources at Nant y Mwyn mine. We welcome your input as we continue to develop our options, as well any wider environmental, social and economic opportunities available.

Possible Migitation Options

• Mine water treatment
• Surface water management e.g. Mine site drainage
• Watercourse management e.g. Separation
• Sediment control
• Mine waste reprofiling
• Spoil heap erosion controls
• Mine waste capping

We will now undertake a Feasibility Study to assess the technical, environmental and economic suitability of the various possible mitigation options. This will lead to a short list of feasible options from which a preferred remediation strategy for the site will be selected. It is likely that this will be a phased strategy, targeting specific pollution sources over a number of years, allowing design, construction and then assessment of each intervention.

We would like the local community and other stakeholders to play a key part in this process. We will be holding public consultation events later in the project, and issuing newsletters to communicate progress on the project. An indicative timeline is presented below, which is subject to securing the required funding.

To date: Assessment

Baseline surveys and water quality monitoring to inform a Scoping Study and long list of remediation options.

2023: Defining the problem

Baseline surveys and water quality monitoring to inform a Scoping Study and long list of remediation options.

2024: Short List Options

Feasibility Study to assess long list options and identify a suitable short list.

2024/25: Preferred Remediation Strategy

Development of the preferred Remediation Strategy for the site with ranked intervention options for further consultation.

2026 Onwards: Construction and assessment

Design and construction of the highest ranked intervention option, followed by assessment of its impact on water quality.

Community Update January 2026

Nant y Mwyn Newsletter 2025

Nant y Mwyn Newsletter 2023