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Where the 5 layer rock
sequence is still in tact and thus the Upper Coralline Limestone lies above the
Blue clay formation the perched
aquifers can be found. These groundwater bodies are allowed to form due to the
impermeable nature of the Blue Clay lying beneath
a permeable layer of Upper Coralline Limestone. The Blue Clay layer acts as an
aquiclude and thus water is stored within the fissures and fractures
found within this zone. Since such aquifers outcrop beneath a relatively thin
soil cover infiltration of rainwater runoff is direct. A number of fresh water
springs can also be found in cases where this water table outcrops. In the case
of Malta this is most common in the Northern part of the Island as indicated by
the geological map located above. On the southern
side of Malta and eastern part of Gozo only the Globigerina and Lower Coralline
limestone formation are mainly exposed. Here the Lower Coralline Limestone
aquifer or the mean sea level aquifer has
formed. The Lower Coralline Limestone is in lateral and vertical contact with
sea water. Due to the differences in density between the fresh rainwater and the
saline sea water, fresh water being much less dense than saline sea water
floats above it in the shape of a lens that thins out at the coast (Ghyben-Herzberg
System). Porosity and permeability of this rock formation depends to a large
extent on the fissure and micro fracture recharge. Since the rock structures of
the Maltese Islands are carbonate and have high secondary porosity,
rainwater easily seeps through.
Today the mean sea level Aquifers are being
abstracted at high rates both for public and private use. Problems
associated with high abstraction rates include localized upconing of sea
water and thus an increase in salinity. Other human induced pressures
include various activities that can be categorized as point and diffuse
pollutants of groundwater.
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