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Lecture 17: Ground Water

Chap. 11

STATISTICS AND DEFINITIONS

The hydrologic cycle describes theconstant exchange of water between

oceans, atmosphere and continents

- Has been operating on Earth forover 4 Ga

Hydrosphere: the global waters of the

Earth- Liquid and solid

- Fresh and salty

Total volume of water on Earth = 1.36

billion km397.20 % seawater

2.15 % glacier ice

0.62 % ground water

0.03 % inland seas, lakes, rivers, moisture insoil, atmosphere

Why do people rely on ground water?

- Ground water is the largest reservoir of freshwater readily available to humans

94% of Earths available fresh water is ground water (excluding glaciers)

- Surface water is often limited in quantity-Surface water needs to be extensively treated before drinking

26% of Canadians rely of ground water for crop, livestock and domestic use

- 2/3 of users in rural areas

Atlantic Provinces are heavily dependent on ground water

100% in Prince Edward Island90% of Ontario farms

1/3 of users in small communities-Many small communities in the Prairies use ground water wells for municipal supply

ZONES OF SUBSURFACE WATER

Unsaturated zone: near-surface zone of soil moisture, voids contain air and water

Capillary fringe: zone where ground water is held by surface tension in tiny conduits between grains,- fluid pressure < atmospheric

-Extending upward from the water table

Saturated zone: zone where all the open spaces in rock (pores, fractures) are completely filled with water

Rain water not held as soil moisture percolates down to form saturated zone

Ground water: water filling the open spaces in rocks in the saturatedzone

Water table: upper limit of the saturated zoneWetland: area where the water table is at the surface

RECHARGE AND DISCHARGE

Water flows from areas where topography is highest (recharge areas)to topography where head is lowest (discharge areas)

Variations of the water table depth

- Shape is usually a subdued replica of the surface topography- Seasonal and annual depth variations

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BASE FLOW

Ground water serves as an equalizer of stream flow- Much of the water in a stream is not direct runoff of rain and

snow melt, but comes from ground-water discharge (base flow)

- Example: 65% of Lake Ontario comes from base flow

Ground water acts as storage to sustain streams when rain is notfalling

SPRINGS

Spring: localized discharge point occurring where the water tableintersect the Earth`s surface

Recall: porosity and void ratio

Void SpaceTwo quantitative measures of the

relative amount of void space in a

material: porosity & void ratio

Both difficult to estimate inpractice

DARCYS LAW

Henri Darcy: 19th century

French engineer- Foundation of modern

groundwater

- Experiments of water flowthrough column of sand

Established volumetric flow rate of water through saturated sand

is proportional to the energy gradient- Loss of energy per unit length of flow path

Darcys Law: equation describing flow through a porous medium

v = Q / Av [m/s] : flow velocity

Q [m3/s] : volumetric flow rateA [m

2] : cross-sectional area of flow tube

Hydraulic head h [m]: level to which water rises above a datum- Measured by a manometer

Hydraulic gradient I [ ]: difference in hydraulic head between two points separated by a distance L[m]

I = (h2h1) / LABILITY TO TRANSMIT FLUIDS

One qualitative and two quantitative measures of the ability of a material to transmit fluids

Qualitative : PermeabilityQuantitative : Intrinsic permeability & Hydraulic conductivityPERMEABILITY

Permeability: general qualitative term describing the ability of a material to transmit fluidsPermeability is a compositeproperty of:

- Material properties: size, shape and interconnectivity of the voids

- Fluid properties: T, density, viscosity

Intrinsic permeability k[m2]: portion of permeability" which is representative of the properties of thematerial alone

Intrinsic permeability is a function of the characteristics of the voids through which the fluid moves

- Interconnectivity of voids - Cementation- Size, shape and packing of grains - Fracturing

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HYDRAULIC CONDUCTIVITY

Hydraulic conductivity K [m/s]: composite property describing the ability of a material to transmit water

K = k g / k : instrinsic permeability [m

2] g : gravity acceleration [m/s

2]

: density of water [kg/m3] : viscosity of water [Ns/m

2]

Important property in hydrogeology

Higher the hydraulic conductivity K, better the reservoir conducts the water

- Because density and viscosity of water does not vary much, K reflects mostly the intrinsic permeability

of the reservoirDARCYS LAW: in practice

v = Q / A = - K (h2 - h1) / L* Negative sign indicates a loss of energy along flow path

- Friction between water and rock/soil

- Friction between water moleculesDatum for measuring hydraulic head is sea level

- Difference between elevation and depth to water in well

Direction of ground-water flow is from high to low hydraulic

headFlow velocity

- Typically cm/dayAQUIFERS

Aquifer: saturated body of rock or soil that transmits economically significant quantities of ground waterAquitard: body of rock or soil that does not transmit economically significant quantities of ground water

Geological materials that form good aquifers:

- Sand and gravel: moraine, alluvial fans, etc.- Fractured rocks: jointed rock masses, fault zones, deeply weathered zones, etc.

- Soluble rocks: caves and channels in karst geology

- Reef carbonates

Two types of aquifers depending on the presence or not of an overlying aquitardUnconfined aquifer:

- Aquifer that lacks an overlying aquitard* The water table is the upper boundary

- Hydraulic head elevation of the water table

* Contour lines of the well water levels drawn on a map indicate the direction of flow

Confined aquifer (syn. artesian aquifer)- Aquifer bounded between overlying

and underlying aquitards

- No relation between the hydraulic

head and the water table In some cases, the fluid

pressure in the aquifer is so

high that the head is above theland surface

Flowing artesianwell: well flowing without a

pump**To ensure a continuous supply of water, well must penetrate into aquifer

PRODUCTION OF WATER

***Overpumping lowers the water table, making it necessary to dig a deeper well

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PRODUCTION OF WATER

Cone of depression in confined aquifers- Cone of depression develops in the

potentiometric surface without dewatering of

aquifer

- Decrease in fluid pressure- Reduction in porosity expels water

within pores

- Remains saturated- Decline in water table in vicinity of

well

- Cone-shaped area extending radiallyfrom the well

- Material within the cone changes from

saturated to unsaturated state

- Reaches a state of equilibriumWATER QUALITY

Ontario The Clean Water Act

Following the Walkerton Inquiry, Ontario

issued The Clean Water Act- Came into effect in July 2007

Multi-barrier approach to protecting drinking

water- Surface and ground water

Implementation

1.Source water protection zones established- In most cases, coincide with Conservation Authorities

2.Assessment report

- Describe in- and out-flow

- List threats and issues

- Rank risks3.Source protection plan