Naturally Occurring Radioactive Materials (NORMs) in the Mercia Mudstone Group and Beyond

October 22, 2025

Understanding the presence of Naturally Occurring Radioactive Materials (NORMs) in the ground is an important aspect of safe and responsible ground investigation. While most occurrences present no health or environmental hazard, awareness of their potential presence and distribution supports better planning, sampling, and interpretation, particularly when working within geological formations known to exhibit localised radioactive anomalies.

One such example is the Mercia Mudstone Group (MMG), where radioactive nodules, reduction spots or bleached haloes occasionally occur. These features form through geochemical precipitation of uranium and thorium during redox and evaporitic processes. Their occurrence has long been recognised in geoscientific and environmental assessments, particularly within research relating to nuclear waste disposal, where localised concentrations of radionuclides can complicate geochemical modelling and predictions of migration pathways.

In practical terms, such nodules may appear as hotspots during gamma logging or radiometric survey work. In most cases these represent very low-level anomalies and have no economic or health significance, yet their identification and correct interpretation are still important to the overall understanding of the ground.

Beyond the Mercia Mudstone Group, NORMs are found across a wide range of UK geological formations. Examples include the Branscombe Mudstone Formation near Newark, the Seaton Carew Formation extending from north Nottinghamshire to Middlesbrough, and the Waterstones Formation, which occurs offshore and within the onshore Cheshire–Fylde area through to the Irish Sea Basin. These and other radioactive marker beds can be clearly seen on gamma logs and are valuable for correlation between boreholes and for understanding wider basin geology.

The British Geological Survey’s publication OR/17/001, The Distribution of Natural Radioactivity in Rocks, provides a useful summary of the variability of uranium and thorium content in common rock types. Uranium concentrations can range from as little as 0.001 parts per million (ppm) in ultramafic rocks to over 1,000 ppm in organic-rich black shales, with typical values for granites and rhyolites between 2 and 50 ppm. Even groundwater and seawater contain measurable trace levels, typically expressed in parts per billion (ppb). These values demonstrate how variable and ubiquitous naturally occurring radioactivity is across different geological environments.

For those working in drilling and ground investigation, the key message is one of awareness rather than concern. Encountering NORMs, whether in the MMG or elsewhere, should form part of the site’s geological and environmental context. Such materials should be recorded and managed appropriately, particularly when radiometric equipment is used or when results are being interpreted for environmental or waste classification purposes.

The BDA Technical & Standards Sub-Committee continues to encourage contractors, consultants, and clients to be aware of the potential for encountering NORMs and to familiarise themselves with relevant guidance and research, including that produced by the British Geological Survey.

Author: Lee Healey, Chair – BDA Technical & Standards Sub-committee