Frequently asked questions

Groundwater is the water that lies beneath the Earth’s surface and fills the pores and fractures in soil, sand and rock. It forms when precipitation (rain; hail; snow, etc.) filters down through the ground until it becomes trapped in a layer of rock called an aquifer.

The level of groundwater is often called the water table.

The study of groundwater is called hydrogeology.

Groundwater can be a crucial source of fresh water for drinking, agriculture and industry, and it provides vital support for ecosystems like rivers and wetlands, especially during dry periods. It is a stable resource, often requiring less treatment than surface water and acting as a natural reservoir. Arid places that have little surface water such as rivers or lakes often rely on groundwater for fresh water supplies.

An aquifer is an underground unit of porous rock or sediment that can hold and transmit groundwater. Aquifers are the source of fresh water in wells and springs. They are replenished through a process called groundwater recharge, where precipitation (rain; hail; snow, etc.) seeps into the ground and accumulates in the porous material.

Groundwater flooding occurs when the water table in permeable rocks rises so that water enters cellars or comes up above the ground surface. Groundwater flooding is generally of longer duration than other causes of flooding, possibly lasting for weeks or even months.

There are several different types of flooding, such as pluvial or fluvial flooding, and coastal inundation, besides groundwater flooding.

Pluvial flooding occurs when heavy rainfall overwhelms drainage systems, sewers and the ground’s ability to absorb it, causing water to flow over the land.

Fluvial flooding happens when a river overflows its banks because the water volume exceeds its capacity. It is typically caused by heavy or prolonged rainfall or rapid snowmelt.

Costal inundation is the flooding of dry, low-lying coastal areas by seawater, caused by factors like storm surges, high tides and long-term sea level rise.

A borehole is simply a long, narrow hole that has been drilled into the ground for various purposes such as extracting resources, geotechnical investigations or environmental assessments. They can be used to create a private water supply from an underground aquifer. 

Groundwater is found almost everywhere beneath the Earth’s surface, but the amount and accessibility vary significantly. Water is present in the tiny gaps between soil and rock particles, and areas where these spaces are filled with water and can be extracted in useful quantities are called aquifers. However, some geological formations are impermeable or their pores are not saturated with water, which means there might not be a usable amount of groundwater.

Groundwater is found exclusively underground. There are underground lakes and rivers, but the term groundwater covers everything from the large scale, such a underground voids (caves) filled with water, to the microscopic scale of tiny gaps between the individual particles that make up a rock such as sandstone.

The groundwater level (also known as the water table) isaffected by the balance between recharge (water entering the ground) and discharge (water leaving the ground).

The primary source of recharge is precipitation such as rainfall or melting ice, snow and hail. When recharge from precipitation exceeds discharge, levels rise; when it is less than discharge, levels fall.

Human activities such as abstraction can increase discharge, as can natural factors such as evaporation. Other factors, such as geology type, topography, climate change and land use, can also affect groundwater.

Flooding can occur from many sources including groundwater, surface water, runoff from intense rainfall and cracked utility pipes.

Groundwater flooding occurs when the water table in permeable rocks rises to enter cellars or comes up above the ground surface. Groundwater flooding is not necessarily linked directly to a specific rainfall event and is generally of longer duration than other causes of flooding (possibly lasting for weeks or even months).

Groundwater levels are generally highest in early spring and lowest in early autumn. Flooding that occurs from June to September is therefore not normally caused by groundwater. Flooding that responds rapidly to rainfall events is also unlikely to be caused by high groundwater levels; exceptions do occur if the water table is normally very shallow and it has been very wet.

Surface water flooding

Information on the risk of surface water flooding is held by:

• Environment Agency (for England)
• Natural Resources Wales
• Scottish Environment Protection Agency
• Rivers Agency of Northern Ireland

Pluvial flooding

Flooding relating to rainfall — pluvial flooding — is caused by a combination of rainfall intensity and local drainage conditions. BGS does not hold any information on pluvial flooding.

Leaks

Flooding can also be caused by leaking pipes. Your water supplier and/or water and sewerage company or your local council can provide information on the distribution of pipes that could have become cracked or leak.

If the water contains trichloromethane (TCM) it is likely to be caused by mains leakage, as this is a residual of the chlorination process. Your water supply company may offer to analyse the flood waters to see if it contains TCM.

If the flooding is related to rainfall events, it could be caused by a fractured storm drainage pipe.

Please note: BGS cannot advise on specific flooding events or queries on individual home buyer reports.

Under the Flood and Water Management Act, the Lead Local Flood Authority (LLFA) has a duty to investigate to the extent it feels necessary or appropriate. An internet search should enable you to find out the LLFA for your area. The LLFA may have delegated the duty to investigate flooding to local district councils.

The duty to investigate does not require the LLFA to tackle groundwater flooding but merely keep a record of the investigation. However, they may be able to advise on a course of action.

The measures that you can implement to stop groundwater flooding are limited and often not totally effective. These include the construction of additional drainage on your property or the installation of dewatering pumps in your basement.

Measures to reduce the impact of the flooding that occurs include tanking of basements and adaptations within your house similar to those suggested by  environmental regulators, such as the Environment Agency, for flooding from overland flows.

Please note: BGS is not able to advise on specific flooding events.

New springs discharging at locations where there is no local knowledge of previous occurrences are most likely due to a leaking pipe or some other artificial cause. However, natural springs can flow temporarily due to periods of sustained rainfall in combination with abnormally high groundwater levels.

Springs occur where the groundwater level rises above the ground surface or where there is a barrier to the flow of groundwater (either natural or artificial) in the underlying rock, causing water to discharge to the ground surface. If the spring does not normally flow then there is unlikely to be a drainage channel to direct the flow to an established water course. The water will find its own course, potentially flooding downgradient property.

With regards to hazards occurring on their land, the land’s occupiers have a general duty under common law to take reasonable steps to remove or reduce the impact on their neighbours. This would apply to the emergence of a natural spring or a spring caused by leaking pipes that then floods a neighbouring property. A breach of this duty could constitute a nuisance, which is actionable within the civil courts. In determining what amounts to ‘reasonable steps’ to reduce or remove such hazards, the courts will consider the costs of the steps required to alleviate the hazard and the financial means of the occupier.

BGS has produced the first dataset for Great Britain that provides an assessment of the susceptibility to groundwater flooding. Based on geological and hydrogeological information, the digital data can be used to identify areas where geological conditions could enable groundwater flooding to occur and where groundwater may come close to or above the ground surface.

The method to derive the dataset involved initially identifying those areas with rocks that are sufficiently permeable to allow groundwater flooding to occur. Where the rock layer is made up of a range of rock material that may have different permeabilities, we use the worst case scenario and the highest likely permeability.

Where the rocks are permeable, we assess the potential for groundwater flooding based on whether groundwater may be present at shallow depths and could rise to enter cellars or, under certain circumstances, come above the ground surface.

The rock permeability is combined with a groundwater surface for periods when groundwater levels are high. This is an approximated surface based on:

• regional groundwater level contours
• point measurements of groundwater level
• river levels (making the assumption that the river is an expression of the water table at that location)

The groundwater-level surface that results from modelling this data is then used to calculate a depth to groundwater in areas where the rocks are sufficiently permeable. This depth is then used to classify the susceptibility to groundwater flooding.

BGS has produced maps of the susceptibility to groundwater flooding for England, Wales and Scotland, which we provide to companies that produce home buyer reports. However, BGS is not the sole provider of groundwater flooding data and you should check the details of the report to find out who supplied the data.

These maps are based on a model that takes into consideration the height of the land around a property but not the height of the property itself. The model is not at the individual property scale, so is not definitive and should be used as guidance only.

Susceptibility is not the same as risk. These maps show where there is the potential for groundwater flooding but do not give any indication of the probability that it will occur.

We classify our groundwater flooding susceptibility/potential into three categories, which may have been included in your home buyer’s report (Table 1).

Please note: BGS is not able to answer queries relating to individual home buyer reports.

If the advice, based on our groundwater flood susceptibility classification, is that you check there has not been a problem in the past at your property, we suggest you ask the current owners and neighbours or undertake an internet search to try to identify evidence of past flooding events that may indicate that groundwater flooding is an issue.

The age of the property may be a help in assessing the likely potential for or impact of flooding, as older houses are more likely to have been designed with flood risk in mind.

Even if an area is prone to groundwater flooding, a property’s construction (such as the presence of a basement, the level of the ground floor, the construction of local drainage, etc.) can determine whether or not this poses any problems to a householder.

Not necessarily.

It is important to remember that if an area is susceptible to groundwater flooding it does not mean that groundwater flooding has ever occurred in the past or will do so in the future, as the maps do not contain information on how often flooding is likely to occur (the probability or ‘return period’) or to what depth but just that the physical setting is such that it could occur.

The susceptibility maps are designed for planners, identifying areas where groundwater flooding might be an issue. The computer model we use to classify areas produces maps with a pixel size of 50 m. Because of this coarse resolution, it is possible that the actual susceptibility at your property may be different due to small-scale variations in rock and soil type and local landscape.

So, although information from the maps is often included in home buyer reports, it really should not be used in isolation as an indicator of groundwater flooding susceptibility and risk at individual properties. You may be able to find more specific information about the site you are interested in by viewing our geological maps or by searching our borehole records for information on water strikes and groundwater levels.

You should also be aware that home buyer reports often look within a relatively large radius from the property, sometimes over 100 m, and report back the highest susceptibility that they find. Therefore, it may be that our maps give a lower susceptibility at your property than the report indicates.

Yes. You can obtain further details on the BGS Susceptibility to Groundwater Flooding dataset and BGS Digital Products.

This includes information on the resolution, cost and format of the data licensed.

If you are a developer and only need to purchase a small area of the dataset, it may be more cost effective to obtain the data from one of the value added resellers that the BGS supply.

Groundsure and Landmark both resell our data in their reports, while FIND resells the data in GIS format and as a viewer service.