introduction to ground water hydrology
TRANSCRIPT
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Introduction to Ground Water Hydrology
Dr. Mohsin Siddique
Assistant Professor
Dept. of Civil & Env. Engg
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Hydrological Cycle
2Earth’s Water Cycle or Hydrologic Cycle
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Ground Water Hydrology
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� Groundwater is water that is stored in a porous media (soil/sand/gravel) under the soil surface.
� Ground water hydrology may be defined as the science of occurrence, distribution and movement of water below the surface of earth.
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Ground Water Hydrology
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Shallow groundwater:
Quick recharge (weeks, months, years)
More prone to outside contamination (organic pollution, effluents)
Deep groundwater:
Slow recharge (decades, centuries, fossil)
Sometimes natural contamination (salts, fluor for instance)
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Ground Water Hydrology
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� Saturated / Unsaturated zone
� The porous media can be fully filled with water, called the saturated zone
� The porous media can be partially filled with water, and partially with air,
called the unsaturated zone
� Groundwater management usually only considers the saturated zone
(unfortunately)
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Aquifer
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� Water holding media: Aquifer (Unconfined, Confined or Semi Confined)
� Confining layer: Aquitard and aquiclude
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Aquifer
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� Aquifer is a geologic unit that stores and transmits water.
Aquifer is a underground “reservoir”.
An aquifer can be confined, semi-confined or unconfined
� Confined: (also called artesian) Groundwater is between two restricting layers
� Unconfined: Top of the groundwater is not confined by a restrictive layer
� Semi-confined: Groundwater has semi-restrictive layer
� Aquitard and aquiclude is the name for a confining layer. A confining layer
restricts water flow (usually clay or bedrock)
� Aquitard (partly permeable, sandy clays)
� Aquiclude (contains water but does not transmit it, clays)
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Water Table
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� Top of an unconfined aquifer is the water table
� Top of the pressure level in a confined aquifer is the phreatic level
or piezometric level or potentiometric surface
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Water Table and Wells
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� Top of an unconfined
aquifer is the water
table
� Top of the pressure
level in a confined
aquifer is the phreatic
level or piezometric
level
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Water Table
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� Piezometric level is HIGHER than the actual water level in the confined
aquifer !
� The pressure at the top of the water table equals the atmospheric pressure
� In groundwater hydrology, the pressure at the top of the water table is
defined as 0
� Thus: To lift water ABOVE the water table, energy is needed !!
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Wells
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Groundwater well
� To measure the level of the water table
Piezometer
� To measure the level of the piezometric or phreatic level
Pumping well
� To extract water from the groundwater to the surface (a pump needs
energy to lift the water)
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Physical Properties That Affect Groundwater
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Physical Properties That Affect
Groundwater
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� Porosity:
� The ratio of voids in a rock or sediment to the total volume of material is referred to as porosity and is a measure of the amount of groundwater that may be stored in the material.
� Permeability:
� It measures the transmission property of the media and the interconnection of the pores.
� It is related to hydraulic conductivity and transmissivity.
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Darcy’s Law and Hydraulic Conductivity
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� Henry Darcy, a French hydraulic engineer, observed that the rate of laminar flow of a fluid (of constant density and temperature) between two points in a porous medium is proportional to the hydraulic gradient,i, (=dh/dl) between the two points (Darcy 1856).
� The equation describing the rate of flow through a porous medium is known as Darcy’s Law and is given as:
Q = K i A
i=dh/dl� Q = volumetric flow rate [L3T-1]
� K = hydraulic conductivity [LT-1]
� A = cross-sectional area of flow [L2]
� h = hydraulic head [L]
� l = distance between two points [L]
� Darcy law is valid only for laminar flow (Re= <= 1)ν
10VD
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Hydraulic Conductivity & Transmissivity
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� Hydraulic Conductivity is the flow rate through a porous medium for unit cross sectional area under a unit hydraulic gradient.
k=Q/iA� Hydraulic conductivity is usually expressed in units of length/time: feet/day,
meters/day, or gallons/day/square-foot.
� Transmissivity, T ,is a measure of the amount of water that can be transmitted horizontally through a unit width by the fully saturated thickness of an aquifer under a hydraulic gradient equal to 1.
� It is equal to the hydraulic conductivity multiplied by the saturated thickness of the aquifer and is given by:
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Ground Water Management
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� For sustainability, we must have a balance between Ground Water Withdrawal and Ground Water Recharge.
� If groundwater is pumped faster
than the recharge, water levels
drop
� Porous media lose water, pores are
filled with air
� Porous media could consolidate,
resulting in land subsidence, sink
holes, loss of water storage
capacity
land level in 1925
land level in 1955
land level in 1977
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Artificial Aquifer Recharge
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� Artificial aquifer recharge (AR) is the enhancement of natural ground water supplies using man-made conveyances such as infiltration basins or injection wells.
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Aquifer Storage and Recovery
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� Aquifer storage and recovery (ASR) is a specific type of AR practiced with the purpose of both augmenting ground water resources and recovering the water in the future for various uses.
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Aquifer Storage and Recovery-UAE
19 LIWA ASR Project
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Aquifer Storage and Recovery-UAE
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LIWA ASR Project
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Aquifer Storage and Recovery-UAE
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LIWA ASR Project Inset Map
� Source:http://www.environmentalatlas.ae/cartography/resourceOfLife/waterInfrastructureDesalinationSupplyDistributionAndASRProjects
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Thank you
� Questions….
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