Research and analysis

Mapping of brackish aquifers and deep sourced springs

Published 29 October 2020

1. Chief Scientist’s Group research summary

Many regions of the UK rely heavily on near-surface freshwater aquifers for drinking water and other purposes including irrigation. However, deeper aquifers that contain brackish groundwater may also provide a valuable resource in water-stressed areas. Very little is known currently about the location of these waters and their potential to supply useable water. There is also limited understanding of the sources of ‘deep’ spring waters, some of which have reputed health benefits. This project reviewed the occurrence of these resources. It presents methods for identifying and mapping these resources and makes recommendations for their protection.

1.1 Why was this project needed?

Activities carried out deep underground, including exploitation of onshore oil and gas resources, could have an impact on groundwater at depth. Although such groundwater may have relatively high mineral content and may not be potable, it may still be a useful resource that could be used in cases where lower quality water is acceptable. Its use could ease pressure on fresh water sources. It is also important to identify where deep groundwater rises and emerges as springs and, if possible, identify their flow paths, so that they can be mapped and protected.

1.2 What did the project involve?

We reviewed how other countries define and use brackish groundwater. Criteria were considered for defining ‘deep springs’ and a catalogue was prepared of known deep springs. Methods for identifying and mapping geological formations with brackish groundwater and the sources of deep springs were identified, with the aim that maps could be prepared in future. We considered how the Environment Agency’s approach to groundwater protection in England can also be used to protect these waters.

1.3 What did we find?

We confirmed the importance of protecting deep brackish groundwater and deep-sourced springs. Brackish groundwater found at depth, despite its relatively poor quality, may still be directly useful for irrigation or livestock. Groundwater that contributes to deep-sourced springs can help sustain ecosystems.

Aquifer permeability and water quality together determine how useful brackish water might be. The permeability of a rock formation was estimated by reference to near-surface aquifers in similar rocks. Trends of increasing salinity with depth for each rock type were identified, although data were limited at increasing depths. A case study was prepared for a site in the Cheshire Basin.

A method was proposed for constructing an England-wide map, based on the British Geological Survey (BGS) 3D geology cross-sections of bedrock geology, to indicate the possible presence of brackish groundwater. Mapping was not considered to be practical for brackish groundwater in formations at the surface, for example in estuarine sediments or river terrace gravels. These should instead be assessed on a site-specific basis.

Deep-sourced springs may be identified on the basis of characteristics indicative of significant depth, such as mineralisation typical of deeper rock formations, rather than setting a specific depth. A literature review was undertaken to catalogue known deep-sourced springs, although the origin of some of the most widely studied springs in the UK remains uncertain. Often, there are few data to determine the underground catchment of deep-sourced springs, so a methodology for national mapping was not proposed. Instead, lines of evidence to confirm a deep-sourced origin are presented, including water quality, age, uniformity of spring discharge and favourable geological structures.

1.4 What is the next step?

Springs and brackish groundwater may be valuable water resources. To aid identification and protection of brackish groundwater a phased programme of mapping could be considered, using the methods proposed in this project

1.5 Publishing details

This summary relates to information from project SC18009 reported in detail in the following output:

Report:

SC180009

Title:

Mapping of brackish aquifers and deep sourced springs

October 2020

Project manager:

Ian Davey, Research, Analysis and Evaluation

Research contractor:

Mott MacDonald Limited
Mott MacDonald House
8-10 Sydenham Road
Croydon
CR0 2EE

This project was funded by the Environment Agency’s Research, Analysis and Evaluation 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

© Environment Agency