The state of our groundwater in Wales – An assessment of groundwater quality

In our last blog, we looked at our assessment of groundwater levels in Wales and discussed a shift in how groundwater levels are becoming more variable in response to climate change and more extreme weather patterns

In this blog, we will focus on our assessment of groundwater quality - the overall physical, chemical and biological state of groundwater.

For nearly 20 years we have been measuring groundwater quality in Welsh aquifers.

We currently monitor approximately 170 sites, consisting of a mixture of boreholes, wells, springs and adits. We do this to monitor groundwater in its ‘natural’ state away from known sources of contamination such as industrial sites, landfills or airports.

We collect water samples once to four times a year and analyse this water to look for over 300 different substances. This includes substances such as metals, herbicides, pesticides and hydrocarbons.

Our water samples are assessed against several lists of standards, these are the United Kingdom Drinking Water Standards, Environmental Quality Standards, Groundwater Threshold Values and the groundwater hazardous substances standards.

As well as our own data, we assess groundwater data collected by local authorities, by water companies and by organisations such as the Drinking Water Inspectorate and British Geological Survey.

What does our latest assessment tell us about groundwater quality?

In our most recent (2021) classification of chemical status of groundwater, 17 out of our 39 groundwater bodies were considered as being at poor chemical status.

Fifteen substances were detected above the UK drinking water standards and the groundwater hazardous substances in samples collected and analysed between 2009 and 2021.

These included:

  • Metals – including manganese, iron, cadmium, arsenic, aluminium, lead and nickel. Most metals in groundwater are naturally occurring but can also be because of contamination. Our long history of metal and coal mine extraction means metal rich groundwater from over 1300 abandoned mines causes pollution in many of our rivers. Seven of the 15 exceedances were metals.

  • Chloroform - Chloroform or trichloromethane is organic compound and a by-product of chlorinated water, which degrades slowly in the soil. It can also form naturally in soils. Chloroform exceeds the standard in 13% of our groundwater samples and is found in 43 varied sample locations across Wales.

  • Nitrate – Nitrate is a common component of chemical and organic fertilisers (manure, digestate) and sewerage. Three percent of groundwater samples collected exceeded the drinking water standard. Nitrate exceedances occurred at 14 sampling sites, found in more intensively farmed areas.
  • Sodium – Sodium is a naturally occurring element and mostly known common salt. Sodium exceeds the drinking water standard in 2% of groundwater samples. Sodium exceedances occurred at just six sites, which appear to relate to seawater intrusion, mine discharge, and possibly food and industrial processing.

  • Pesticides – The pesticides Atrazine, Diazinon and Mecoprop exceed their respective standards in 3%, 2% and 1% of groundwater samples. Atrazine and Mecoprop are currently not approved for use in the UK but may be present in groundwater from historic use, continued illegal use of old supplies, or from landfills. Diazinon is still in use and is commonly found in products used to dip sheep.

  • Trichlorethane – Trichlorethane was banned in 1996 due to its Ozone damaging properties. Prior to this it was used for cleaning metal parts, as a solvent, a thinner and an aerosol propellant. On the surface it quickly evaporates but in soil and groundwater it is broken down to Dichloroethene which is also a hazardous substance. Trichlorethane only exceeded the standard in 1% of samples and at four sampling sites, which included an industrial estate and a farm.

  • Trichloroethylene - Trichloroethylene is used as an industrial degreasing solvent in metal cleaning and an extraction solvent in the textile manufacturing industry. It was also used in the dry-cleaning industry until the mid-1950s. Trichloroethylene was only detected above the standards in 0.5% of samples, and at just five of our sampling locations, including two industrial estates, a hotel, an agricultural market and a farm.

Chart showing substances found in groundwater sampling

Where we find substances above the relevant standard, we inform the owners of the water supply and other partners like local authority environmental health departments so they can make sure measures are put in place to make the water safe to drink. 

Where pollution is suspected we investigate and where necessary use our enforcement powers to remove the source of pollution and make sure groundwater is cleaned up.

Planning for the future

Groundwater is under threat from ‘emerging contaminants.’  This is how we describe substances which are not yet regulated but could potentially be harmful to environmental or human health.

In 2022 we analysed our water samples to look for 2,269 emerging pollutants and ‘lifestyle’ chemicals, such as medicines, cleaning and personal care products.

The most frequently identified included pharmaceuticals like bethanidine which is used to treat hypertension and paracetemol, nicotine, caffeine, and several pesticides and weedkillers.

Emerging pollution threats found in groundwater samples

In 2023, at 70 of our groundwater monitoring sites we’ve sampled for two Per- and Polyfluorinated Substances (PFAS) substances known as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS).  These are often referred to as ‘forever chemicals’ because of their persistence in the environment.

At two locations the concentrations of these substances were above the Drinking Water Inspectorate Tier one Guideline value. Our laboratory is currently developing its analysis technique which will allow a wider range of PFAS substances to be sampled for in future.

Further investigation into the threat of emerging contaminants is one the priorities we have identified following our assessment of groundwater quality. 

We need to ensure we have the capability and capacity to monitor for emerging contaminants to allow us to determine the current scale of the problem, where in Wales they appear most and assess if our regulatory approach remains fit for purpose.

Our assessment also identifies a need to reduce nutrient inputs into groundwater.

The spreading of slurries to land, digestate, sewage sludge and discharges of sewage effluent to ground are sources of nutrients. These nutrients, such as nitrogen compounds and phosphates risk polluting groundwater but can also travel through the subsurface and reach our streams, rivers and lakes.  They can also be a source of emerging contaminants to the wider environment.

We need to improve our understanding of how these substances behave in the ground. This will inform our policies for controlling the activity in high-risk locations and highlight where ground conditions in Wales are unsuitable for spreading and discharging effluent.



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