Contaminated Land Inspection Strategy
2.8 Broad Geological/Hydrogeological Characteristics
The District has a varied geology that has a major influence on the soils of the area, as well as the landscape, natural resources and cultural heritage.
2.8.1 Solid and Drift Geology
Three main geological series underlie the District. The western area is characterised by the red soils and generally low relief of the Triassic formation, while the eastern part of the District consists largely of the plateaus and valleys of the Jurassic series. The Carboniferous series outcrops in the centre of the District.
The hard Carboniferous limestone which characterises the Mendip Hills lies mainly to the south of the district, but the Upper Coal Measures underlie much of the District as well as outcropping in an area stretching from Hallatrow northwards to Clutton, Pensford and Compton Dando. The series is made up largely of sandstones, shales and mudstones but banded with coal seams.
The Triassic forms areas of low relief stretching from the slopes of the Mendips northwards across the Chew Valley to Dundry Hill. To the east the Triassic floors the valleys of the Somer, Wellow Brook and Cam before disappearing beneath the newer Jurassic rocks.
Most of the series is characterised by Keuper Marl, siltsones and mudstones. Occasional beds of Butcombe sandstone form local topographical features such as Pagans Hill and Chilly Hill near Chew Stoke. Fringing the northern slope of the Mendips are areas of Dolomitic Conglomerate which consist of rock debris.
The Jurassic series outcrop over about half the District and from the western flank of the Cotswolds Hills. The series comprises Lias limestones, clays and sands overlain by Oolitic limestones and clays. The Lias limestones give rise to the characteristic ‘tablelands’ or plateaus above the low-lying Keuper marl valleys and vales.
Lower Lias clay lies on top of the plateaux in places and is exposed in the narrow valley floors of the Cam, Wellow and Newton Brooks.
Midford Sands are areas of sandstone which are locally important in the valley of the Conygre Brook north of Priston as well as in the Midford area itself.
Rising above the Inferior Oolite is the Fullers earth formation, a mixture of limestone, mudstone and the famous Fullers earth clay. The Fullers earth outcrops in the steep valley sides in the east of the District and has a history of land slippage.
Above the Fullers earth lies the Great Oolite and Forest Marble limestone series forming the bold scarps and wide plateaus which typify the Cotswolds. They characterise the high ground between the valleys of the Wellow and Cam Brooks, the extensive plateau at Hinton Charterhouse and the Downs around Bath .
Gravels and alluvium feature in the valley bottoms of the Avon and its tributaries, and form extensive areas in the upper parts of the Chew and Cam Valleys, and at the foot of the Mendip slopes around Chew Valley Lake, and at Hollow Marsh.
The majority of the eastern region of the District is recognised as being a major aquifer, and therefore highly permeable. The area is also overlain with soils of a high leaching potential.
The only substantial area of this region where a minor aquifer exists is in the southeast, around Hinton Charterhouse and extending north to Midford, and the overlying soils have an intermediate to high leaching potential. The solid geology in this area is Jurassic Great Oolite Limestone, and the drift geology is alluvium, mainly silt.
There are major aquifers in the northwest of the District close to North Wick and Whitchurch, within the limestone geology and covered with soils of a high leaching potential.
Major aquifers and soils of a high leaching potential are also found in the far west, north of Nempnett Thrubwell, and the geology here is Keuper Marl, Sandstone, and Dolomitic Conglomerate.
The Chew Valley South area contains a major aquifer with soils of an intermediate leachate potential overlying, and Dolomitic Conglomerate solid geology. Compton Wood, in the south of the Chew Valley South area, lies on the District border and is adjacent to a major aquifer occurring in the neighbouring local authority and overlain by soil with a high leaching potential. The geology of this area consists of Hotwells Limestone.
A major aquifer lies beneath the Mendip area of the District; this is overlain by soils with an intermediate leachate potential, and is characterised by Dolomitic Conglomerate geology.
A small number of localised major aquifers, with overlying soils of mainly high leaching potentials, exist in the centre and southern area of the District. These are generally within areas of Oolitic limestone.
The remainder of the District, comprising large areas of the western region and sections of the central area, is classified as a non-aquifer and therefore negligibly permeable.
The map in Appendix I: Aquifer locations in SW England, details the locations and vulnerability of major and minor aquifers in the southwest region of England.
Source Protection Zones
Source Protection Zones (SPZs) have been defined by the Environment Agency for nearly 2000 groundwater sources (wells, boreholes and springs) used for public drinking water supply across England. SPZs are a tool to aid the decision making process when assessing risks to groundwater supplies posed by potentially polluting activities and release of contaminants. Generally the closer the activity or release is to a groundwater source, the greater the risk. Three zones are usually defined, and their size, shape and orientation is dependant on the hydrogeological characteristics of the aquifer and groundwater flow direction.
Zone 1 (Inner Source Protection) is defined by 50 day travel time from any point below the water table. Zone 2 (Outer Source Protection) is defined by 400 day travel time from any point below the water table. Zone 3 (Source Catchment) is the complete catchment area of a groundwater source. In effect, all groundwater supplies have an SPZ, not just those that have been defined and published by the Environment Agency.
Maps illustrating the areas where SPZs occur both within the District and close to the District boundary can be found on the Environment Agency’s website in the “What's in Your Backyard?” section. SPZs are also shown on the Council’s GIS and on the Proposals Maps in the Bath and Wansdyke Local Plans. SPZs are concentrated in the Chew Valley and the northern edge of the Mendip Hills and in areas north and south of Bath. The Proposals Map in the Bath Local Plan shows the Water Source Protection Area of the Midford Springs Water Supply Source
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