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How Groundwater moves
All freshwater found underground must have had a source of recharge. This is normally precipitation
(rainfall/snow-melt), but can also sometimes be seepage from rivers, lakes or canals. The aquifer fills up until
water reaches the land surface, where it flows from the ground as springs or seepages, providing the dryweather flow (or baseflow) of lowland rivers. Thus the aquifer becomes saturated to a level where the
outflow matches recharge.
Shallow aquifers in recharge areas are generally unconfined, but elsewhere, and at greater depths,
groundwater is often confined by overlying impermeable strata (an aquiclude), or partially confined by low
permeability strata (an aquitard). In confined conditions water may be encountered under pressure, and
when wells are drilled, rises above the top of the aquifer, even as far as ground elevation, to a level called
the potentiometric surface (A).
IMPERMEABLE DRIFT OVER
AQUIFER A
± LOCAL PERCHED AQUIFER
A UNCONFINED
AT OUTCROP
OVERLAIN BY
PERMEABLE
QUATERNARY
A CONFINED
OVERLAIN BY LOW
PERMEABILITY QUATERNARY
Water table
RIVER/AQUIFER
INTERACTION
CO
Potentiometric
surface
?
NF
INI
NG
BE
AQ
D /
AQ
UIT
AR
UIF
ER
CON
F IN IN
A
G BE
D /A
QUIT
ARD
D
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A.
Typical aquifer situations in the UK
Groundwater systems are dynamic and water is continuously in slow motion down gradient from areas of
recharge to areas of discharge. In large aquifer systems, tens or even hundreds of years may elapse in the
passage of water through this subterranean part of the hydrological cycle (B). Such flow rates do not
normally exceed a few metres per day and compare to rates of up to 1 metre per second for riverflow. There
are however some fracture-flow aquifers such as karstic limestones where flow along major fissure systems
can be as rapid as 1 km per day. Thus supplies located in different aquifers, or in different parts of the same
aquifer can tap water of widely different residence time. This is a important factor for contaminants which
degrade over time and for the control of disease-causing micro-organisms (bacteria, viruses, protozoa).
intermittent
discharge area
aquifer
recharge area
unsaturated zone
minor perennial
discharge area
major perennial
discharge area
artesian
discharge area
KEY
RS
MONTH
S
A
YE
groundwater piezometric
level (with maximum and
minimum levels in the nonconfined aquifer)
YEA
RS
DECADE
S
CENTUR
IES
A
NI
LEN
MIL
aquitard
(low-permeability strata)
aquiclude (virtually
impermeable strata)
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B. Typical groundwater flow system in large aquifer (The residence periods indicated are typical order-of-magnitude
values from time of recharge to point of discharge ) (After Foster and Hirata 1988)
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The presence of layering is a characteristic geological feature, and is present in both confined and
unconfined aquifer systems. As well as controlling the yield, design and depth of the wells that tap such
systems, layering is hydraulically important because the presence of beds with different permeabilities
affects the rate at which contaminants entering below the ground surface can move into the aquifer. Even in
crystalline and very old sedimentary rocks, where structural and other controls dominate the effects of any
original bedding, layering occurs as weathering processes enlarge fractures and introduce interstices near to
the ground surface in rocks of otherwise very low permeability.
Such rocks may also be overlain by a thin superficial layer of much more recent river or glacial deposits
that, if permeable, can provide a temporary storage medium for rainfall recharge, thereby increasing the
productivity and apparent storage of the underlying hard rock formation. This is a not-uncommon situation
for private supplies tapping older formations in the UK. It results in much more localised flow systems
because the aquifer is limited in vertical or lateral extent, as in the case of relatively recent glacial (C) or
alluvial (D) sediments, or because the bedrock is highly consolidated and usable water is found only either
in certain fracture situations or in a thin weathered zone near the ground surface (E).
Glacial sands and gravels
Glacial
clays (till)
Bedrock
C. Glacial outwash aquifer
Narrow alluvial aquifer
Bedrock
D. Narrow alluvial aquifer
zone
red
athe
We
Typical productive well locations
Joint
Fault zone
Dykes
Bedrock
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E. Hard-rock aquifer
C,D,E. Localised groundwater flow systems in minor aquifers (adapted from Freeze and Cherry 1979,
Davis and De Wiest 1966)
Residence times in such aquifers are much less predictable either because the degree of interconnection
with nearby rivers or lakes is uncertain or there is more scope for rapid by-pass flow along fracture networks.
Typically the shortest residence times (hours➙days➙weeks) occur in limestones where solutionally enlarged
fissures or conduits (karstic features) are well developed.
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