Download Characterization of Groundwater Systems - AGW-Net

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Groundwater differs from surface water
through its physical and chemical environment
Among aquifers there are huge differences
with respect to geological environments
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Resulting in their capacities to store water and to
transmit water flow as well.
Hence the availability of groundwater will
depend on hydrogeological setting
characterized by hydraulic parameters
Groundwater management requires reliable
aquifer characterization.
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Underlying resource, stored in underground
reservoir (rocks) and transmitted through
interconnected spaces
Aquifers have huge differences with respect to
their hydrogeological setting.
GW management based on a good understanding
of aquifer system.
Definition of Groundwater/Hydrogeology
•The rainfall that soaks into the ground and moves
downwards into spaces and cracks in the rocks
below the ground surface becomes groundwater.
•The study of groundwater is called hydrogeology.
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Storage capacity (storage
coefficient or specific
yield)
Transfer capacity
(transmissivity)
Physical and chemical
interaction capacity
(reservoir-rock vs GW)
Unconsolidated rocks:
Consolidated rocks:
Consolidated rocks:
• Pore spaces
• Fractures
• Karsts (enlarged fractures)
• Large storage
• Small storage
• Large storage
Why Groundwater?
•A readily available source at point needed
•Its relative low costs compared to surface water
•Availability in most areas
•Potable without treatment
•Employs low cost technologies
•The frequent drought problems enforce the use of
groundwater source as many small intermittent rivers
and streams dry out during the dry season
Minor aquifers
Critical issues:
• availability
• reliability
After MacDonald et al, 2005
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Groundwater characteristics:
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Groundwater occurs in most geological formations
because nearly all rocks possess openings (pores or
voids, or fractured).
All aquifers have two fundamental characteristics:
 A capacity for groundwater storage (Porosity ) and;
 A capacity for groundwater flow (Permeability).
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An aquifer is a geological formation capable of
yielding useful groundwater supplies to wells and
springs.
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Aquifer types:
unconfined (free
surface), called
water-table
aquifer or
phreatic aquifer;
 confined (under
pressure)
aquifers are
bounded by
impervious or
semipervious
layers.
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(Schmoll, O.,et al. – 2006.)
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Hydrogeological environments:
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The availability of groundwater depends primarily
on the geological environment
Aquifer unit storage capacity (storativity) varies
extremely between:
 Unconsolidated granular sediments, and;
 Highly-consolidated fractured rocks.
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Groundwater flow potential (transmissivity)
depends on aquifer saturated thickness and types
of aquifer layers (permeability)
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Hydrogeological environments:
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Ancient crystalline and metamorphic: highly
fractured or weathered:
 permeability is low, the storage capacity as well.
 GW potential moderate (0.1 – 1 l/s), since aquifer is
usually of minor extension.
 Targets: fractures at the base of the deep weathered
zone, and the vertical fracture zones.
Groundwater occurrence in weathered basement rocks (MacDonald et al., 2005)
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Hydrogeological environments:
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Consolidated sedimentary aquifers: large groundwater
storage and major regional extension, → different
aquifers types:
 Sandstones: moderate to high potential (1 – 20 l/s);
 Targeting for abstraction: coarse porous or fractured sandstones
 Mudstone and shale: fractured or interbedded with
sandstone layers;
 its potential is low (0 – 0.5 l/s) with minor regional flow;
 Targets: hard fractured mudstone or thin sandstone layers
 Limestones: slightly soluble in rainwater → fractures can
be enlarged to form karsts (developed conduits and
fracture systems)
 Its potential is moderate to high (1 – 100 l/s).
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Hydrogeological environments:
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Unconsolidated sedimentary aquifers: loose materials,
GW stored and transmitted through pore spaces, not
fractures.
Large to very large storage capacity and usually huge
regional extension. Distinguished major hydrogeological
units are:
 Major alluvial and coastal basins: sands, gravels and clays :
 High potential (1- 40 l/s), Sands and gravels are targeted layers.
 Small riverside deposits: mixed alluvium (cobbles, gravels,
sands, silts clays), and others deposits like coastal dunes
(sandy):
 potential moderate (1 – 20 l/s), sandy/gravel deposits targeted .
 Valley deposit in mountain: poorly sorted rock fragments,
sands, gravels, volcanic materials:
 Moderate to high potential: 1 – 10 l/s, Stables areas of sands and
gravels are interesting targets.
Groundwater occurrence in riverside deposits (MacDonald et al., 2005)
Summary of key properties of the most widely-occurring aquifer types (GW Mate2, 2006)
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Groundwater moves slowly through interconnected
pore/fracture spaces of aquifers materials.
GW in continuous slow movement from recharge areas
(usually upland areas) discharge areas (springs,
baseflow, wetlands and coastal zones).
The flow of groundwater through an aquifer is
governed by Darcy’s Law.
Aquifer storage transforms highly variable natural
recharge regimes into more stable natural discharge
regimes.
It also results in groundwater residence times, usually
counted in decades, centuries, even millennia:
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Large volumes of so-called ‘fossil groundwater’ still being held
in storage.
Residence time will depend also on aquifer geologic formations
that bear groundwater
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Where aquifers dip beneath much less permeable
strata, their groundwater becomes confined (to
varying degrees) by overlying layers.
This results in a corresponding degree of isolation
from the immediately overlying land surface, but
not from the groundwater system as a whole.
Drawdown induced by pumping from the
confined section of an aquifer is often rapidly
propagated to the unconfined section.
In various hydrogeological settings, shallow
unconfined and deep confined aquifer layers can
be superimposed → leakage downwards and
upwards between layers according to local
conditions.
Typical groundwater flow regime and residence times of major aquifers under semiarid climatic regimes (after GW-Mate2, 2006)
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GW in continuous slow movement from
recharge areas (usually upland areas)
discharge areas (springs, baseflow,
wetlands and coastal zones)
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Diagnosing the relationship surface water /
underlying aquifer is an important component
of groundwater system characterization.
It is important to distinguish between:
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Streams and rivers on which an aquifer is dependent
as a significant source of its overall recharge
Rivers that in turn depend significantly on aquifer
discharge to sustain their dry-weather flow.
It should be noted that in some cases rivers may
fluctuate seasonally between two of the conditions
depicted.
Spectrum of possible relationships between surface watercourses and underlying groundwater
systems (after GW-Mate2, 2006)
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Changes to surface water bodies in response to
groundwater development can cause significant
consequences:
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Long term reductions in stream flow can affect vegetation
along streams
Changes in the flow direction to/from streams may affect
temperature, oxygen levels, and nutrient concentrations in the
stream (affect aquatic life)
In gaining and in losing streams, affect hyporheic zones (active
sites for aquatic life)
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Lowering of lake diminish lakefront aesthetics, and shoreline
structures such as docks, may alter the natural fluxes to lakes of
key constituents (nutrients and dissolved oxygen)
Amplitude and frequency of water-level fluctuations affect
wetland characteristics (type of vegetation, nutrient cycling,
type of invertebrates, fish, and bird species)
Lead to reductions in spring flow, changes of springs from
perennial to ephemeral, or elimination of springs altogether
Plant and wildlife communities adapted to coastal areas can be
affected by changes in the flow and quality of groundwater
discharges
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Recharge may result from:
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Precipitation that percolates through unsaturated zone to water
table
Losses of surface water bodies (steams, lakes, wetlands)
Contemporary aquifer recharge rates, a fundamental
consideration in the sustainability of groundwater
resource development.
Understanding aquifer recharge mechanisms and their
linkages with land-use is essential for integrated water
resources management.
The quantification of natural recharge rates, subject to
significant methodological difficulties, data deficiencies
and resultant uncertainties because of:
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Wide spatial and temporal variability of rainfall and runoff
events, and;
Widespread lateral variation in soil profiles and
hydrogeological conditions.
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Recharge rates vary with:
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river flow diversion or control,
modifications to surface water irrigation,
changes in natural vegetation or crop type in recharge areas,
reduction in leakage from urban water-supply networks and
in-situ wastewater percolation,
lowering of water-table, etc.
A number of generic observations can be made
on aquifer recharge processes:
Areas of increasing aridity will have a much lower
rate and frequency of downward flux to the water
table,
 Indirect recharge from surface runoff and incidental
artificial recharge arising from human activity is
generally becoming progressively significant than
rainfall direct recharge.
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To understand groundwater systems and how
they respond to external and internal changes,
→need to be able to assess groundwater
movement:
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local scale (seepage phenomena, flow near
groundwater wells),
regional scale with recharge/discharge of aquifer
systems, interaction of different aquifers, interaction
with surface water bodies
Many difficulties encountered in groundwater
assessment due to various reasons:
groundwater cannot be readily observed, it is hidden
resource
 usually groundwater occurs in large, and complex
aquifer systems
 aquifers have high spatial variability of its
characteristics
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Safe yield: the attainment and maintenance of a
long-term balance between the amount of annual
withdrawal and the amount of annual recharge.
This allows water users to abstract groundwater no
more than it is replenished naturally.
So-called ‘safe yield’ is clearly bounded by the
current long-term average rate of aquifer recharge,
although should also consider:
value judgements about the importance of maintaining
some of the natural discharges from the aquifer system
 consideration of consumptive use and catchment export,
as opposed to local non-consumptive uses which result in
the local generation of an effluent.
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Recharge rate
quantification
Recharge area vs land-use
(GW protection)
Interactions (quantity/
quality) with surface
water bodies
Impacts of GW pumping
Interbasin/interboundary
aquifer
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Groundwater cannot be
readily observed;
GW may occur in large,
and complex aquifer
systems;
Aquifers have high
spatial variability of its
characteristics.
Policy level
Other stakeholders
Policies
Strategies
Regulatory framework
Demand side
Water provision and
management
Sustainable development
GW protection
Groundwater
Understanding of GW system
Information on unit system
Knowledge on aquifer properties, and
Technical solutions
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Different ways to assess groundwater:
Observation of groundwater levels
 Pumping tests, to test the response of groundwater
abstraction
 Hydrogeological investigations to built a first
concept on groundwater resource;
 Geophysical surveys to find the groundwater
resource, to sit borehole
 GW budgeting, modelling for better resource
development planning
 Groundwater Quality need to be assessed to
protect GW resources:
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 Salinity monitoring, Other field measurements of water
quality parameters
 Analysis of groundwater samples (in field or in laboratory)
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GW location, a big issue in
complex hydrogeological setting
Available GWR for current and
future use, for different
uses…sustainably!
Critical elements: safe yield ,
overexploitation (negative
impacts costs)
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Recharge ensures
renewability of GW storage,
input to the system!
Recharge rate estimate, a
critical issue for sustainability
of GW development
Recharge area map, for landuse planning and access to
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SW and GW (in many cases) hydraulically
connected…all too often ignored in water management
considerations and policies
Useful management information? maps of high risk
areas of extensive GW exploitation
Trend of characteristics as impacted by land-use/change,
climate change/variability, water use
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Areas and levels (vertical) of naturally-occurring hazards
Geological settings potentially bearing affected groundwater
Maps of occurrence of groundwater concentration for harmful
elements
Locate alternative (ground) water resources to ensure reliable
water supply
Thank You!
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Characterisation of groundwater
systems
Purpose: To appreciate the link between
understanding groundwater systems and
strategies for management
Duration: 30 Minutes
Activity: In 4 groups, participants discuss how the
knowledge of specific characteristics of aquifers
can improve the management of groundwater
(Local aquifers and international boundaries
aquifers.
Report Back: Each group presents a table with
their identified aquifer characteristics and how
each one improves groundwater management.