gw lecture1

7/28/2019 GW Lecture1

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The Hydrologic Cycle

1 0 0P r e c i p i t a t i o n o n l a nd

Infiltration

Watertable

Groundwater flow

1 Groundwaterdischarge

38 Surface discharge

61Evaporation from land

39Moisture over land

385Precipitation

on ocean

424Evaporationfrom ocean

Surfacerunoff

Imperviousstrata

GroundwaterRecharge

Precipitation

Snowmelt

P - I - E - T - RO = dS/dt


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PrecipitationWater on Surface Overland Flow

ChannelFlow

The Hydrologic Cycle

Ground Water GroundWater

FlowOcean

Reservoir

AtmosphereEvaporation

Evapotranspiration

Evaporation


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Estimates of groundwater storage, annual recharge and

abstraction from aquifers in Sudan

Annual abstraction

M m3

Annual recharge

M m3

Groundwater

storage M m3

Major aquifer

9002000503,000Nubian

sandstone

250110060,000Um Rawaba/

Gezira

6608751,000Alluvial

1,8103,975564,000Total


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Terminology

In groundwater hydrology, several terms are used

to indicate groundwater situations

Terminology is hereafter explained in the context of

groundwater management


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What is groundwater?

Groundwater is water that is stored in a

porous media (soil/sand/gravel) under thesoil surface


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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


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UNDERGROUND WATER


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Misconception

Saturated zone is not the same as anunderground lake

A groundwater reservoir contains

more than only water !

Solid sandor

gravelparticle

Water


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Misconception (2)

Groundwater is completely different from

surface water

NOT TRUE

Groundwater is part of the water cycle,

the same water as surface water !


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Aquifer

Aquiferis the reservoir of porous media

(usually sand, gravel, limestone)

Aquitard is the name for a confining layer. A

confining layer restricts water flow (usually

clay or bedrock)


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Confined and unconfined aquifers


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HYDROGEOLOGICAL UNITS


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Unconfined Aquifers

GW occurring in aquifers: water fills partly anaquifer: upper surface free to rise and decline:UNCONFINED or water-table aquifer:

unsaturated or vadose zone

Near surface material not saturated

Water table: at zero gage pressure: separatessaturated and unsaturated zones: free surface rise

of water in a well


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Confined Aquifer

An aquifer can be confined, semi-confined or

unconfined

Confined: 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


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Wells

Observation 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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The causes of groundwater pollution are numerousand are as diverse as the activities of man


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Porosity

Ratio of openings (voids) to the total volume of a soil

n = (Vt-Vs)/Vt = Vv/Vt

Vt= total volume of the soil or rock

Vs=volume of solids in the sample

Vv= volume of openings (voids)

Expressed as percentage


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SPECIFIC YIELD AND RETENTION

n=Sy+SrSy=Vd/VtSr=Vr/Vt

n=porosity

Sy=specific yield

Sr=specific retention

Vd=volume of water that drains

from a total volume of Vt.,

Vr is the volume of water retained

in a total volume of VtVt=total volume of a soil or rock sample


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Specific Yield and Retention

Porosity: maximum amount of water that a rock cancontain when saturated.

Portion of the GW: draining under influence of gravity:SPECIFIC YIELD

Portion of the GW: retained as a film on rock surfaces andin very small openings: SPECIFIC RETENTION


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Functions of Groundwater Systems

Hydraulically: 1) stores water to extent of porosity.,

2)transmits water from recharge areas to discharge areas:

Reservoir and conduit

Water from recharge areasguided by hydraulicgradients and conductivity to discharge areas


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GROUNDWATER VELOCITY

Time reqd by groundwater to move from recharge to discharge areasFew days (zones adjacent to discharge) to millennia (central part of some recharge

through deeper GWsystems.


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Sewage network covers only about 5% of the state.

In residential areas and some industrial areas on site

sanitation system is used

Septic tank and disposal well tapping the unconfined

aquifer

Contamination Transport Modelling in Khartoum

State


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