Nant y Mwyn Lead Mine

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.

Nant y Mwyn Newsletter 2023