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– SFRA Groundwater Information

West Area of Thames region covers the northeastern tip of North Wiltshire County Council and the north half of Kennet District Council.

The boundary with EA South West region runs from north of Malmesbury to in North Wiltshire, and from north of Devices to Ham in Kennet DC in a line parallel with the A4 south of Marlborough and north of the Vale of Pewsey.

The geology covering the parts of Wiltshire discussed here may be obtained from BGS 1:50,000 geology sheets no. 252; Swindon, and 266; Marlborough.

Solid geology Clays underlie much of the central area of the northern tip of North Wiltshire area covered here (West Walton & Oxford Clay). The Environment Agency classifies these deposits as non-aquifer because they do not readily store or transmit groundwater. These deposits would not normally be prone to groundwater flooding.

Limestone members of the Great Oolite Group (Forest Marble and Cornbrash) are present in the northwest of the North Wiltshire area. Forest Marble is an inter-bedded limestone and mudstone and is mostly impermeable. Cornbrash Limestone is classified as a minor aquifer. Springs could occur from perched groundwater within these strata.

Limestones and some Mudstones are present to the southwest of the North Wiltshire area. Corallian deposits are present to the east of Swindon around Lydiard Millicent, south of Purton. The Corallian is a mix of inter-bedded sandstones, limestones and clays and is classified as a minor aquifer. This type of geology can also give rise to perched groundwater and spring flow.

The part of Wiltshire within the Kennet Council area consists mostly of chalk deposits. The Chalk is classified as a major aquifer and gave rise to groundwater flooding in the winter of 2000/2001 and 2003. The dry valley of Aldbourne was badly affected by groundwater flooding and sewerage flooding caused by high groundwater levels. Villages in the Og valley were also affected, with emergent groundwater in the dry valley near Ogbourne Maizey. Properties were flooded in Ogbourne Maizey and Ogbourne St. Andrew. Sewerage surcharging was reported in Avebury, as was property flooding in Beckhampton, near Avebury. As a result of this event, ‘ ’ we set trigger levels at a few key indicator boreholes. The levels were designed to provide enough time to prepare for similar events in the future. One of these indicator boreholes (SU 17/110) is located in Draycot Foliat to the north of the Og catchment. Another, Water Acre (SU 27/119) is the indicator for Aldbourne.

Deposits of Reading and Woolwich beds are present in the south east of the area. Consisting of clay, silt and sand, these deposits are not normally prone to groundwater flooding, but could give rise to perched groundwater.

Drift deposits Sand and gravel drift deposits are associated with the and the , and are present close to these rivers (refer to BGS Geology maps). These deposits tend to have a shallow water table. The drift deposits are drained by the surface watercourses running through them. When levels in these watercourses are high, less groundwater will be able to drain away. The drift deposits, which lie on top of the West Walton and Oxford Clays in the North Wiltshire area, may be poorly drained leading to water-logging and surface flooding. The clays tend to act as a barrier to the downward movement of groundwater through the sand and gravels.

In the Kennet Council area, there is a wide band of Clay deposits running from Aldbourne area through to Marlborough.

– Reports of suspected groundwater flooding (see map provided figure 1) We keep a record of telephone calls or enquiries from residents who suspect that they are suffering from groundwater flooding. There are only two enquiries of this nature within the Wiltshire study area covered by Thames region of the Environment Agency. One of the enquiries was not confirmed to be groundwater flooding, but rather poor drainage following heavy rainfall. It was not possible to confirm any details, as the house owner was not contactable after the initial enquiry.

As noted in the section on Geology, groundwater flooding occurred in Aldbourne and Ogbourne in the Kennet area. It is likely that these incidents do not appear on the database because we were not contacted directly by the residents.

Call_NoDate_ Town/VillagePostcodeNGR Geology Problem Action 99 22/01/2003 Purton SN5 4ELSU093538809C3orallian Cellar flooding. Man interested in buying Indicated that problems are likely to house. No varation across year be due to water table 124 17/08/2004 Purton StokeSN5 4JG409100 19060O0xford Clay. Floor is flooding in house. Cannot Does not appear to be a BGS sheet 252 contact owner to get more details as the groundwater flooding problem. (Swindon). telephone number provided by Possible a burst pipe or very wet Floodwatch is incorrect. ground due to recent intense rainfall. Table detailing suspected groundwater flooding incidents in study area.

– Groundwater Observation Boreholes (see map provided figure 2) There are several observation boreholes (OBH) within the study area; most of those in the Kennet Area are for the Chalk and Upper Greensand aquifers. The boreholes in the North Wiltshire area are mainly Oolite and gravel boreholes. All of these OBHs are shown on the attached map and are all currently monitored. The OBHs have different monitoring periods and there are many more boreholes, which are now closed, but we still hold the historical data. The monitoring data for all of these boreholes can be made available on request, and a table of the currently monitored boreholes is also attached.

– Groundwater contour maps (see map provided figure 3) Groundwater contour maps are available for the Chalk aquifer only. The contours are shown on the map enclosed and represent groundwater levels measured as metres above ordnance datum (mAOD). It may be possible to provide this map electronically, as an ArcGIS file if required.

Groundwater Emergence Zones Groundwater emergence zones have been asked for, but the Environment Agency does not “ ” currently map these. The DEFRA consultation Making Space for Water is currently assessing the feasibility of producing such maps. It is believed that BGS have produced Groundwater Emergence Maps.

– Groundwater vulnerability maps (see map provided figure 4) We do have access to such maps; however, they are designed to assess the vulnerability of an aquifer to pollution from ground sources, not for flooding vulnerability. It combines aquifer characteristics and soil leaching potential to provide groundwater vulnerability classes. These should be used with caution if being used to predict groundwater emergence because, even though it is a good indicator of permeability of the aquifer, not all areas on a high permeability aquifer would be prone to flooding. Other factors, such as topography and groundwater flow direction, would have to be considered.

However, a map is provided to show the groundwater vulnerability in the study area. It may be possible to provide this map electronically if required. The information is derived from National Rivers Authority Policy and Practice for the Protection of Groundwater: Groundwater Vulnerability 1:100,000 Map Series, Sheets 37 (Southern Cotswolds) and 38 (Upper Thames and Berkshire Downs).