The state of our groundwater in Wales – why we monitor it and what it is telling us about groundwater levels

In recent years there has been much interest in the state of our rivers and seas, but what about the waters which flow in the rocks under our feet?

Groundwater is a valuable resource, soaking up large volumes of water into the land during wet weather and providing a much needed supply of water during prolonged dry spells.

Natural Resources Wales is responsible for managing and protecting groundwater in Wales from overuse and from pollution. To do this we monitor and measure groundwater and use this data to assess the state of groundwater, identifying where action is needed. 

In this blog, we will discuss our recent assessment of groundwater resource in Wales. In a follow-up blog, we will focus on groundwater quality.

What is groundwater resource?

Groundwater resource refers to the overall quantity of groundwater found beneath the land.

Groundwater levels, also known as the water table, refers to the level below which the soil or rock is fully saturated with water. 

Just like river levels change, groundwater levels change too. When rain or snow falls, some flows over the ground into rivers and streams, but some soaks into the ground and becomes groundwater. We call this process groundwater recharge.

When water is stored in small pore spaces in sand, gravel and cracks in the rock, we call this an aquifer. Water can leave the aquifer either through natural discharge into rivers, streams and springs, or through human abstraction by pumping boreholes.

Groundwater levels are monitored using groundwater monitoring boreholes.  We have almost 130 monitoring boreholes measuring groundwater level in Wales, some of which have been monitored for 40 years.  The average depth of our monitoring boreholes is around 50 meters, with the deepest being 212 meters.

Why do we monitor groundwater levels?

We analyse our groundwater level data to look for trends and monitor the amount of groundwater held in storage.

While a resilient resource, groundwater levels can become strained if recharge is reduced over a long period, or if abstraction is increased. This can lead to a groundwater drought.

Groundwater drought can be identified by consistently low groundwater levels, reduced flow in groundwater fed rivers and springs, and groundwater abstractions becoming unreliable. 

Comparisons of measured groundwater levels with long term averages provide an indication of the status of groundwater resources. Observation over long periods allows us to assess groundwater responses to weather conditions, abstraction, and climate change.

What is our monitoring telling us?

In our most recent (2021) classification of the status of groundwater quantity, all 39 groundwater bodies were considered as being at good status.

But an exercise with the British Geological Survey is showing that groundwater trends are changing and becoming more variable.

The British Geological Survey have worked with us to develop a Standard Groundwater Index (SGI) in the Clwyd, Pembrokeshire and Usk and Wye catchments.

This is a tool to identify times when groundwater levels are higher and lower when compared to long term historical averages or norms and assigns a number accordingly. For example, a positive SGI number indicates groundwater levels are above average for that time of year, while a negative value indicates levels are below normal.

The associated data show the number of months that groundwater levels have spent in each these categories across two different 20-year periods; 1980 to 2000 and 2001 to 2021.  The following observations have been made:

• The number of average months has reduced by 20% from 228 (1980 to 2000) to 180 (2001 to 2021).
• In all but one location, the number of months spent in moderate drought have increased by 30 months between 2001 to 2021 compared to 1980 to 2000.
• There were no months classified as severe and extreme drought between 1980 and 2000. However, between 2001 and 2021, one extreme and six severe drought months were identified.
• In every location analysed, there are more wet months recorded in the latter 20-year period.

Across Wales, the overall trend is that groundwater levels are becoming more variable. There are fewer months recording average levels for the time of year with more evidence of extreme events (drought or wet). This suggests there is a changing resilience in resource, and effects upon contributions to river flow and water supplies may be more severe in future.

Planning for the future

We will continue to track groundwater level trends and use these to better identify and understand groundwater drought. This will help us sustainably manage groundwater resources to meet the needs of the environment and people.

We have used the information collected through this work to help develop a series of priorities for both groundwater resource and groundwater quality. We hope these will help us to better understand groundwater levels and observe on-going changes.

For groundwater resource, we will prioritise reviewing our monitoring networks. Our current monitoring focusses on deeper groundwater, but we recognise that shallow groundwater is equally as important. Our review may include having better coverage of shallow groundwater monitors to better understand these shallow groundwater levels and how they will be change in future.

We will also focus on gaining a better understanding of the threats posed by climate change.

As the climate changes we are likely to see further changes in groundwater.  We expect to see an increase in seawater intrusion to groundwater near the coast. Flooding from groundwater may become more common in place. Elsewhere changes in rainfall patterns means springs and other shallow sources of groundwater are likely to dry up more often. We need to understand where this might happen and advise what mitigation, if any, is possible.

Demand for renewable sources of heating and cooling such as ground source heat pumps is likely to increase the pressures on groundwater in urban environments. We need to assess where increased demand for groundwater might occur and ensure we have appropriate ways of regulating these uses.