CUADRO RESUMEN

Following on from section 8.2, where it has been shown that probability contours can be created for the 2-D case, two case study sites have been chosen to assess the viability and practicality o f a 2-D methodology. The selection o f these particular two case study sites came from a group o f six sites recommended by the Environment Agency following the completion o f a questionnaire sent out to all Regions. The questionnaire, developed as part o f this work, requested details o f sites known to exhibit fracture flow characteristics that were considered to be o f particular interest, and also the quantity o f data that were associated with each o f the sites. The six sites chosen are detailed in Table 8.2.

Further details o f the six case study sites are given in R&D Project Record W6-020-1 (Robinson and Barker, 2000a). Only the two case studies for which probability contours were ultimately produced are presented here, Coombe Farm and Alton Court.

Table 8.2: Summary of case study sites.

Case Study Site Location Geology

Alton Court Ross-on-Wye Devonian Old Red Sandstone

Coombe Farm Dover Cretaceous Middle Chalk

Forstal nr. Maidstone Cretaceous Hythe Beds

Meysey Hampton Cotswolds Jurassic Inferior Oolite

Scales north Cumbria Permo-Triassic Sandstone

Tadcaster nr. York Jurassic Limestone

The following discussion o f the case studies initially examines the relevant fracture data available at the site and data from literature sources, conceptual models for the sites are then developed, and finally numerical modelling and time-of-travel probability contours are simulated.

8.3.1 Coombe Farm

Coombe Farm public supply borehole is located on the Western outskirts of Dover in the base o f a Chalk valley that trends approximately WSW-ENE, as illustrated in Figure 8.9. This dry valley (Coombe Valley) forms one o f several valleys trending in this direction, and all are approximately parallel. The floors o f these valleys are narrow and flat due to the infill from the valley sides, with the infill up to 4 m thick. These valleys all connect with the Dour valley which trends NW-SE, passing through the centre of Dover and containing the River Dour. The Dour valley is known to follow a fault zone. The Coombe Farm borehole penetrates the Middle and the Lower Chalk and was drilled and acidised in 1974.

The Coombe Farm site lies on the northern limb o f the Wealden anticline structure which dips at 1 to 2 degrees to the NNE to NE, with the dip altered locally by folding and faulting. The folds are commonly monoclinal with an axis striking NW, with the northerly limbs being steeper. Faults are generally not o f large throws and not usually traceable away from outcrop. There are many dry valleys within the area which trend in two general directions: NW-SE and NNE-SSW. These directions mirror the conjugate joint directions o f N60°E and N25°E and it seems likely that they may be fault

controlled such as has been established in the River Dour valley (EA source proforma for Coombe Farm).

In previous porous media modelling, groundwater protection zones around Coombe were simulated using the East Kent Chalk model, which encompassed a large area with the following model boundaries:

• The Great River Stour to the west - approximately 28 km from Coombe Farm; • The English Channel to the east - approximately 8 km from Coombe Farm; • The Chalk escarpment forming the highest part o f the aquifer extending from

near Ashford to Folkestone formed the southern limit o f the area - Folkestone being approximately 8 km WSW o f Coombe Farm; and

• The Chalk dips gently to the NNE forming the southern limb o f the Richborough syncline, which forms the valley in which the River Stour flows eastwards towards the sea at Pegwell Bay. This formed the northern limit o f the model approximately 26 km north o f Coombe Farm.

Available Data i) Pumping Test Data

A pumping test was performed on the Coombe Farm borehole in November and December 1979 for 16 days. A representative transmissivity for the Middle Chalk was found to be 500 m^/d and a storage coefficient o f 0.4%. Only one monitoring well, CVIO, was found to have any drawdown over the pumping test period (2.49 m at the end o f the test period). This monitoring well was 180 m west o f the abstraction well; all the other monitoring wells were over 1 km from the abstraction well. Prior to the punq)ing test groundwater flow directions in Coombe valley were towards the WNW at a gradient o f approximately 0.008.

ii) Geophysical Logging

Geophysical logs were run in CVIO, 180 m west o f the abstraction well, in June 1980. The geophysical logs (tenq)erature, fluid conductivity, lateral resistivity, caliper and long normal resistivity) indicate fracturing in the Middle Chalk, but very few fractures in the Lower Chalk. The Lower Chalk boundary occurs about 55 m bgl. There is also evidence o f large fissures at the Lower Chalk/Middle Chalk boundary in the Melboum Rock. Data for fracture density and flowing fracture spacing has been obtained from

in the saturated part of the Middle Chalk. The temperature and conductivity logs indicate that of those 24 fractures approximately nine o f those are hydraulically active, o f which the majority are in the Melboum Rock. Scanline data from the Channel Tunnel and its associated works has also been used for obtaining fracture data.

iii) Channel Tunnel data

Much data are available for the Channel Tunnel and the associated links, with most of the data relevant to the Coombe Farm site coming from the “Engineering Geology of the Channel Tunnel” (Harris, 1996). The larger structural features of the Dover area can be seen in Figure 8.9. The engineering geologists working on the Channel Tunnel divided the area up into different structural sectors - Coombe Farm lies in sector D.

Mem I

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LI ^ ' River Dour

dustria

Ry STA

Coombe Farm ABH Jetties

Leisure Centre Eastern Docks

DOVER