Geology Reference
In-Depth Information
the risk of leachate (liquid rich in waste products) migration from
the quarry was low, but it turned out that this was based on an
incorrect interpretation of local geology. The land
ll was then
operated using internal clay bunds and a series of drains and
lagoons but, crucially, these were constructed overlying a thin
layer of limestone that had been left in place in the
floor of the
quarry as an operating surface for vehicles, both during quarrying
and land
ll operations. When the quarry was about half full of
refuse, leachate emerged at a spring in an adjacent valley about a
kilometreaway.Thispollutedastream,whichthenimpacteda
fish hatchery downstream as well as water supply. Reappraisal of
the geological model using available published maps indicated that
the leachate was probably passing laterally through the lower
stratum of jointed limestone that had been left in the base of the
quarry and then channelled down a fault to the spring. Tracer
tests were commissioned and these con
rmed the link between the
quarry source and the polluted spring (Smart, 1985). Various
options were considered to improve the situation, including grout-
ing, but the preferred remedial measures involved the excavation
of land
ll along the downstream margin, cutting a trench through
the lower limestone and into the underlying Fullers Earth clay and
then using geotextile membrane on the inside and an impermeable
sheet on the outside wrapped around a drain falling to a sump
where leachate could be collected regularly for separate treatment.
Upstream of the quarry, a similar membrane and drain system was
keyed into the Fullers Earth, collected groundwater through
ow
and channelled it around and away from the quarry. As the
land
ll was completed, it was to be capped to prevent direct
rainfall ingress. The works led to signi
cant reductions in migra-
tion of leachate and improvement in the quality of water emerging
from the spring.
It is to be noted that at the time the quarry was operated a dilute and
disperse approach was generally adopted to land
lling in the UK and
worldwide, whereby it was assumed that the overall migration of
leachate would be insigni
cant as it mixed with a large volume of
groundwater. This case illustrates that where the leachate is trans-
ported by local channel
flow, rather than volumetric dispersion
through a porous mass, dilution cannot be relied upon. Nowadays,
in most countries the base and sides of land
lls will be sealed using
a 0.6 to 1.0m layer of low-permeability soil followed by
flexible
geomembranes, which again will be covered by a layer of soil to
prevent puncturing by traf
c or other means. Levels of leachate will
also be controlled by internal drainage. While these systems can
seldom be completely leak proof, pollution levels should generally
be low.
