environmental issue on Mendip has been disruption of the underground
water flow and the destruction of cave systems.
The Carboniferous Limestone is an important public water resource
for much of the surrounding region, including around 500 000 people
in the Bristol area. As water consumption has increased rapidly
over the last 50 years, so the importance of the Mendip Hills as
a source of groundwater has grown.
A major planning study carried out by Somerset County Council
in the late 1960s (reported in 1971) highlighted the areas where,
in the Water Authorities' view, quarrying would be detrimental
to the water supply. In the main Mendip ridge, the only areas falling
outside those zones were between Moon's Hill and Whatley Quarries,
that between Nunney and Waterlip, a small area around Dulcote,
and between Axbridge and Crook Peak. West Mendip is presently covered
by Groundwater Protection Areas defined by the Environment Agency.
Groundwater flow in the Carboniferous Limestone is mostly through
a network of underground conduits or caves. When quarries intercept
these flow systems, firstly water can flood workings and alternatively
be diverted by pumping, and in the process cut off the original
flow, causing springs to diminish or dry up. As the springs themselves
are often used for public water supply, this too can be interrupted.
Thirdly, quarrying can cause fissures to become blocked e.g. by
blasting vibration dislodging the rocks beyond the quarry void
itself or by generating fine material which can be washed into
fissures. In addition, the limestone has a significant water storage
capacity, both above and below the water table. By removing the
rock, quarrying can reduce the storage available in the aquifer.
Claims have been made at various times that Mendip quarrying has
affected the flow in the famous thermal springs in Bath, but the
evidence appears to be inconclusive.
Stringent limits are placed on activities which could interfere
with or contaminate water, for example workings are often not permitted
to intercept the water table and if they are, strict regimes of
pumping and water disposal are imposed.
Quarries also destroy
any caves that they intersect. A classic example of a quarry intersecting
a cave system draining to a spring is Fairy Cave Quarry, near Stoke
St Michael. Here quarrying intersected 4.5 km of cave passage feeding
St Dunstan's Well. By concentrating quarrying in the less karstic
areas of eastern Mendip, the quarry companies can avoid intersecting
major cave systems.
Furthermore, the larger quarry operators have made efforts to
reduce the impact of sub-water-table working. The operators of
Whatley quarry pump any water flowing into the quarry into the
Mells River just upstream of the Mells River Sink, where water
sinks underground, thus recharging the aquifer. Around Torr Works
quarry a comprehensive groundwater monitoring and tracer testing
scheme has been in operation since the mid 1980s.