Groundwater Resource Evaluation
Prepared by J. Mgaiwa
Why Groundwater
• Water security remains a key issue for much of
the World’s 2.2 billion people lacking access to
safely managed drinking water
(UN stats 2019 Sustainable Development
Goals)
• Groundwater is the solution for many
communities, especially in rural areas with
extended dry seasons, often made worse and
less predictable by climate change
Why Groundwater
• Water is essential for improved health,
economic development and food security and
as populations continue to grow and develop
there is a growing demand for greater
numbers of boreholes to be drilled
• Importantly, poorly sited and constructed
boreholes, which are common across the
globe, lead to high rates of borehole failure.
Poorly managed groundwater resources can
lead to over abstraction and degradation of
aquifer systems.
Why Groundwater
• Training needs to be given to professionals
working in the groundwater sector to ensure
boreholes are properly constructed and
aquifer are properly managed and monitored
to ensure that our precious groundwater
resources are available for current and future
generations
Development of Groundwater
Resources
The development of groundwater resources
can be viewed as a sequential process with
three major phases
exploration stage- in which surface and
subsurface geological and geophysical
techniques are brought to bear on the search
for suitable aquifers
Development of…cont
Evaluation stage-that encompasses the
measurement of hydrogeologic parameters,
the design and analysis of wells, and the
calculation of aquifer yields.
exploitation, or management phase- which
must include consideration of optimal
development strategies and an assessment
of the interactions between groundwater
exploitation and the regional hydrologic
system
Exploratory Geophysics
A wide range of geophysical surveying
methods exists, for each of which there is an
operative physical property to which the
method is sensitive. They may be surface or
down the hole methods.
They can be conducted as airborne, land,
down the hole or marine
What is groundwater exploration
• Groundwater exploration is the investigation
of underground formations to understand the
hydrologic cycle, know the groundwater
quantity and quality, and identify the nature,
number and type of aquifers
• Need to know whether the conditions of the
available water underground permits its
economic withdrawal through wells
What is groundwater….
The purposes of groundwater exploration are:
• To delineate the water bearing formation
• To estimate their hydrological characteristics
• To determine the quantity and quality of
water present in the formation
Evaluation and management
Might best be indicated by the following
series of questions:
• Where should the wells be located ?
• How many wells are needed ?
• What pumping rates can they sustain?
• What will be the effect of the proposed pumping
scheme on regional water levels?
• What will be the negative impacts of different
surface and subsurface activities to the identified
groundwater body?
Evaluation and management …cont
• What are the long-term yield capabilities of
the aquifer ?
• Will the proposed development have any
detrimental
influence
on other
components of the hydrologic cycle?
• Are there likely to be any undesirable side
effects of development, such as land
subsidence or seawater intrusion, that could
serve to limit yields?
Key words:
Investigation
Prospecting
Key words:
Survey
Key words:
Exploration
Exploitation
Key words:
Evaluation
Key words:
Monitoring
Key words:
Management
Concepts of basin yields
The maximum quantity of water that is
actually available from groundwater basin on
a perennial basis is limited by the possible
deleterious side effects that can be caused by
pumping and by operation of the basin.
Any withdrawal in excess of safe yield is an
overdraft
Concepts of basin yields…cont
The exploitation of a groundwater basin leads
to water-level declines that serve to limit
yields. One of the primary goals of
groundwater resource evaluation must
therefore be the prediction of hydraulic-head
drawdowns in aquifers
under proposed
pumping schemes
Mining yield
If groundwater is withdrawn at a rate
exceeding the recharge, a mining yield exists.
Many groundwater basins today are being
mined. Various arguments, economic and
other, have been advanced to justify mining of
groundwater
Mining yield…cont
One is that water in storage is of no value
unless it is used. In arid areas almost any
development of groundwater constitutes a
mining yield, but the needs are there and the
benefits are great, and so such exploitation
will continue
Perennial yield
Is the rate at which water can be withdraw
perennially under specified operating conditions
without producing an undesired result.
Undesired result is an adverse situation such
as:
Progressive reduction of water resource
Occurrence of land subsidence
Development of uneconomic pumping
conditions
Degradation of groundwater quality
Interference with prior water rights
Saline water intrusion
Occurrence of saline water intrusion in an
aquifer may be due to:
• Encroachment of seawater in coastal aquifers
• Seawater that entered aquifers during past
geologic time
• Salt in salt domes, thin beds or disseminated
in geologic formations
Saline water intrusion
• Water concentrated by evaporation in tidal
lagoons, playas or other enclosed areas
• Return flows to streams from irrigated land
• Human saline wastes
Saline water intrusion …cont
•
•
•
•
The mechanisms responsible saline water
intrusion include:
Reduction or reversal of groundwater
gradients which permits dense saline water to
displace fresh water (coastal aquifers)
Destruction of natural barriers that separate
fresh and saline waters )construction of
coastal drainage canals)
Subsurface disposal of waste saline water
Up conning effect
Land subsidence
Is a gradual settling or sudden sinking of the
Earth’s surface caused by the subsurface
movement of the Earth materials. This is
associated with decline in groundwater levels.
They develop in regions where water table
have been lowered due to excessive pumping
or regions underlain by soluble rocks like
dolomite, leading to sinkholes or karst
topography
Groundwater legislation
Among other issues, groundwater legislation
is required to regulate quantity and quality;
groundwater development and to constrain
activities that might compromise groundwater
availability; and to address increasing
competition
and
conflict
between
groundwater users, and increasing threat of
pollution.
Groundwater legislation…cont
It aims to regulate the relationship between
persons (physical and legal) and between the
people and the state administration on water
resources; it includes all legal provisions on
development,
use,
protection
and
management of groundwater resources,
which may be either scattered in various
enactments
or
integrated
into
a
comprehensive water law.
THE END