GROUNDWATER SYSTEMS

Delivered by: Prodeo Yao Agbotui KAAF University, DoCE. 24th 09 and 01/10 2011

Groundwater systems

What is GW ?

Ground water is that part of the subsurface water which occurs within the saturated zone of the earth’s crust where all pores are filled with water.

It can be reached artificially via wells or by natural flow mechanisms (artesian conditions, springs etc)

Groundwater systems

GW characteristics

Ground water is a huge water resource, but is exhaustible and is unevenly available.

Ground water and surface water resources are interrelated should be integrated in water studies.

Excessive and continued exploitation of ground water must be avoided because of slow replenishment.

Ground water is generally better than surface water in respect of biological characteristics. However surface water is generally better than ground water in terms of chemical characteristics.

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GW X’tics contd

Considering coverage area and evaporative losses, underground reservoirs storing ground water are more advantageous than surface reservoirs.

There is no construction cost involved in underground reservoirs. However, well construction, pumps and energy for pumping water, and maintenance of pumps and wells require money.

Siltation of recharge areas may considerably affect groundwater reservoirs.

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GW Xtics contd

Ground water is generally of uniform temperature and mineral quality and is free of suspended impurities.

Ground water source has indefinite life, if properly managed

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GW occurence GW occurs in either porous or

fractured media GW is found in 2 zones within

the subsurfce. These are Vadose(unsaturated) and

saturated zones. Vadose zone is under negative or

suction pressure.The zone is mad up of has the soil zone, intermediate zone and capillary zones.

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GW occurence

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The water table separates the 2 zones within the gw zones. The gw table is the upper limit of the saturated zone, and it is at 0 piezometric pressure.

The saturated zone travels to several 1000m depths, but reachable fresh water occurs within the first 100 – 200m.

GW storage terminology Aquifer: A saturated underground layer of water-

bearing permeable rock or unconsolidated materials (gravel, sand, or silt) which stores and transmits water ineconomic quantities. (eg Ogalla(USA), Great Artesian Aquifer (Australia), mesozoic and cenozoic formations and limestone formations in Ghana).

Aquitard: A formation of low permeability layer eg clay.

Aquiclude (or aquifuge: A solid, impermeable area underlying or overlying an aquifer eg unweathered granite

Groundwater systems

Aquifer types

Groundwater systems

Unconfined aquifer Confined aquifer Artesian aquifer Perched aquifer

Transmission Xtics of aquifers

Porosity: This is the ratio of the volume of pores to the total volume of the porous medium. There are 2 types of porosity (primary and secondary).

The amount of interconnected porosity for the flow of water is called effective porosity.

Porosity for saturated materials range between 0 – 50%.

<5% is considered small

5 – 20% medium

>20% is largeGroundwater systems

Transmission xtics of aquifers

Rock type Total porosity, % Effective porosity, %

Anhydrite 0.5-5 0.05-0.5

Chalk 5-40 0.05-2

Limestone, dolomite 0-40 0.1-5

Sandstone 5-15 0.5-10

Shale 1-10 0.5-5

Salt 0.5 0.1

Granite 0.1 0.0005

Fractured crystalline rock - 0.00005-0.01

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Aquifer transmission xtics

Specific yield (Sy): Is the percentage volume of water that a rock sample yields by gravity when saturated.

Specific retention (Sr): Is the percentage volume of water a rock sample retains after gravity flow has ceased.

What is Sr + Sy?

Groundwater systems

Transmission xtics of aquifersMaterial Porosity Specific yield

Clay 45 - 55 1 – 10

Sand 25 - 40 10 – 30

Gravel 25 - 40 15 – 30

Sand and gravel 10 – 35 15 – 25

Sandstone 5 - 15 5 – 15

Shale 1 - 10 0.5 – 5

Limestone 0 - 40 0.5 – 5

Groundwater systems

Transmission xtics of aquifers

Specific yield is applicable to unconfined aquifers only. For confined however, we use storativity.

Storativity: Is the volume of water an aquifer would release from or take into storage per unit surface area of the aquifer for a unit change in head. Its around 5 x 10-2 – 1 x 10-5

Permeability: This is the ease with which water flows through a porous media.

Groundwater systems

Transmission xtics of aquifers

Permeability depends on porosity, packing, size distribution, and shape of grains.

Transmissivity: This is the product of hydraulic conductivity and aquifer depth. T = KB

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Worked examples

1. A ground water basin consists of 20 km2 of plains. The maximum fluctuation of ground water table is 3 m. Assuming a specific yield of 15 %, determine the available ground water storage.

2. In an aquifer whose area is 100 ha, the water table has dropped by 3.0 m. Assuming porosity and specific retention of the aquifer material to be 30 per cent and 10 %, respectively, determine the specific yield of the aquifer and the change in ground water storage.

Groundwater systems

Flow of water through porous media

• Ground water flows whenever there exists a difference in head between two points. This flow can either be laminar or turbulent.

• Henry Darcy discovered through empirical experimentation that the rate of flow, Q through a column of saturated sand is proportional to the difference in hydraulic head, Δh, between the ends of the column and to the area of flow cross-section A, and inversely proportional to the length of the column, L.

• Please refer to eqn 1• Please note that Darcy’s law is applicable for Re of 1 -10.

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Flow through porous media

• When Darcy’s law is substituted in the continuity equation of motion, we obtain eqns 4 and 5, called the Boussinesq’s and Dupuit’s equations respectively for the confined and unconfined cases.

• The equations assume that that the medium is homogeneous, isotropic, and water is incompressible. Equation 4 also assumes that large pressure variations do not occur. Equation 5 further assumes that the curvature of the free surface is sufficiently small for the vertical components of the flow velocity to be negligible in comparison to the horizontal component.

• For steady flow, Eqs. 4 and 5 become 6 and 7.

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Well Hydraulics• A well is a hydraulic structure which, if properly designed and

constructed, permits economic withdrawal of water from an aquifer.

• Cone of depression: The surface of a lowered water table resembles in an aquifer during pumping.

• Radius of influence of a well: The horizontal distance from the centre of a well to the practical limit of the cone of depression.

• Drawdown at a location: The difference, measured in the vertical direction, between the initial water table (or the piezometric surface in the confined aquifer) and its lowered level due to pumping at any location within the radius of influence.

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Well Hydraulics

• Well yield : The volume of water discharge, either by pumping or by free flow, per unit time.

• Specific capacity of a well: Well yield per unit drawdown.

• Steady or equilibrium condition: When pumping and recharging rates balance each other, such that there is no appreciable drawdown.

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Equillibrium Eqns• Please refer to diagram on Asawa G.L.(2007) for the

derivation of the equillibrium equations.

But the notable ones are from eqns 8 – 19.• Example 3

A well with a radius of 0.3 m, including gravel envelope and developed zone, completely penetrates an unconfined aquifer with K = 25 m/day and initial water table at 30 m above the bottom of the aquifer. The well is pumped so that the water level in the well remains at 22 m above the bottom of the aquifer. Assuming that pumping has essentially no effect on water table height at 300 m from the well, determine the steady-state well discharge. Neglect well losses.

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Non-equillibrium eqns

• Is not required for this level. So ignore, but lets talk a little of it.

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Groundwater exploration• Ground water exploration is the delineation of the

water-bearing formations, estimation of their hydrogeologic characteristics and the determination of the quality of water present in these formations.

• Methods employed are:• Remote sensing• Near surface geophysics (electrical, seismic

reflection and refraction, ground penetrating radar).• Limitations of geophysics• Well logging (gamma logging, neutron probing, self

spontaneous potential, temperature logging, CCTV)Groundwater systems

Groundwater exploration

Well logging set up An integrated well log

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Groundwater exploration

CCTV view of a well

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Pumping tests

• The most effective method for determining aquifer parameters like T, S and K on the basin scale is most effectively done using pumping tests.

• Measurements during an aquifer test include water levels at observation wells (before the start of pumping, at intervals during pumping, and for some time after pumping), the discharge rate, and the time of any variation in the discharge rate.

• The following equations can be used from pumping tests for the evaluation of aquifer properties (Pls refer to eqns 20 -22)

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Groundwater exploration

• Test drilling: Test drilling can predict the true hydrogeologic character of subsurface formations by drilling through them, obtaining samples, recording geologic logs, geochemical characterisation of aquifer material and conducting aquifer tests.

• The methods of drilling include cable tool. forward rotary, reverse drilling, auger drilling, horizontal drilling.

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End of slide

Thank you for your attention

Questions & comments?

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Reading / references1. Asawa G.L. (2005). Irrigation and water resources engineering.

New Age International Publishers, New Delhi.

2. Schwab G.O. et al (1992). Soil and water conservation engineering. John Wiley and Sons, Inc, Ohio.

3. Images from www.en.wikipedia.org

4. Hiscock, K. 2005. Hydrogeology, Principles and Practice. Ch2

5. Domenico, P.A. and Schwartz, FW, 1990. Physical and chemical hydrogeology. Chapters 2 and 3.

Groundwater systems