|Buried valleys and
In areas where
buried valleys are eroded into low permeable sediments e.g. clay,
the aquifers in the buried valley typically constitute the only
possibility for large-scale ground water abstraction.
Surrounding clayey sediments define the lateral extent of the
aquifers. There will be only limited water exchange between the
aquifers in the buried valley and the surrounding layers.
Groundwater flow and groundwater recharge in the aquifers then
occurs inside the valley and in areas outside the valley via
surficial sand layers.
Internal erosional surfaces may form sharp contacts between
individual aquifers inside the valley. The individual aquifers
tend to be elongate and follow the overall valley trend.
Distinct hydraulic barriers in buried valleys indicated by large
potentiometric surface discontinuities are found both parallel
and perpendicular to buried valleys. The longitudinal barriers
are supposed to be a result of erosional features inside the
valley, whereas transverse barriers may be the result of
cross-cutting and clay-filled valleys or glaciotectonic
deformation within the valley itself.
Buried valleys filled with coarse-grained sediments eroded into
a succession of predominantly sandy sediments will constitute
one large aquifer system. Sand-filled valleys eroded into chalk
or limestone will also constitute one aquifer system.
Clay-filled valleys, on the other hand, may restrain the
groundwater flow in the aquifer system. Accordingly, differences
in the hydraulic properties of the infill and the surrounding
layers either due to structural or textural differences are
likely to affect the groundwater flow in the aquifer system.
The vulnerability of the groundwater resources to potentially
harmful chemicals introduced at the ground surface is a
complicated matter. On the positive side the chemical compounds
may be degraded or retained before they reach the aquifer, or
protective clay layers or an upward hydraulic gradient may
prevent the contamination finding its way to the aquifer.
However, in areas with buried valleys the physical conditions
may be very complex because of the generally complicated
architecture of the valleys and the surroundings. In this way
preferred flow paths for transport of contaminated water from
shallow aquifers to deeper aquifers may exist.
Due to the generally complex internal structure of the valleys
potentially protective clay layers above the aquifers are likely
to be discontinuous. The aquifers inside the valley will thus
have a varying degree of natural protection. Even if laterally
extensive clay layers are present, the protective effect will
only have local importance if the surrounding sediments are sand
dominated. In cases where small sand-filled valleys eroded into
alternating clay and sand layers the buried valley in-fill may
constitute highly vulnerable zones as the clay cover is
penetrated. The valleys may therefore create short-circuits
between the aquifers in the valley and the aquifers in the