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Groundwater investigation techniques 189

Dye-tracer test in the Chepstow Block Carboniferous limestone aquifer, BO X
south-west England 5.3

As part of an investigation into the risk of landfill leachate con- Great Spring in November 1982 and its recovery monitored at the
tamination of the Chepstow Block Carboniferous limestone aquifer Great Spring (Fig. 1b). The recovery curve shows a minimum travel
in south-west England, a dye-tracer test was conducted to study time of 42 days and a peak travel time of 130 days over 7 km giving
−1
the groundwater flow system (Clark 1984). The study area is shown a groundwater velocity of 54 m day . The curve can be simulated
in Fig. 1a with groundwater discharge from the Chepstow Block using an advection-dispersion model with a longitudinal dispersiv-
almost entirely focused on the Great Spring located in the Severn ity (Section 6.3.1) of 720 m. This high dispersivity, even allowing for
rail tunnel. The karstic nature of the limestone suggests that the scale of the test, suggests that groundwater flow is through a
groundwater flow is through solutionally widened fissures or joints, complex fissure system rather than as pure conduit flow (Clark
with the joints parallel to the main north-westerly oriented faults 1984).
that trend towards the Great Spring. The spring is an important The total recovery of the fluorescein tracer was about 23% sug-
water resource for the area, having a mean flow of about 650 L s −1 gesting losses by adsorption and degradation of some 77%. The
3
3
−1
(56 × 10 m day ) (Fig. 1c). maximum concentration of dye-tracer recovered was 0.92 mgL −1
7
A dye-tracer test using fluorescein was designed to determine giving a dilution of the dye input of 6 × 10 and leading to the con-
groundwater flowpaths and residence times and confirm the theory clusion that the impacts of waste disposal activities on the ground-
of conduit flow. A mass of 30 kg of fluorescein was injected into a water quality of the Great Spring will be considerably lessened by
sinkhole in the Cas Troggy Brook located to the north-west of the dilution in the limestone aquifer.

Fig. 1 (a) Location map of the
Chepstow Block and Great Spring, (b)
recovery of fluorescein dye-tracer and
(c) variation in discharge at the Great
Spring during the tracer test. After Clark
(1984).