lecture hydrogeology

Lecture Hydrogeology

1. Water cycle

2. Groundwater Properties

3. Aquifers & Pumping tests

4. UK Aquifers

5. Hydrogeology and Construction

6. Groundwater Contamination

The Water Cycle

Reservoir Average

residence time

Antarctica 20,000 years

Oceans 3,200 years

Glaciers 20 to 100 years

Seasonal snow cover 2 to 6 months

Soil moisture 1 to 2 months

Groundwater: shallow 100 to 200 years

Groundwater: deep 10,000 years

Lakes 50 to 100 years

Rivers 2 to 6 months

Atmosphere 9 days

PhysicalGeography.net

Price; Introducing Groundwater

• Describes the continuous movement

of water on, above and below the

Earth’s surface

• Water moves from one reservoir to

another:

• River to ocean

• Ocean to atmosphere

• By physical processes :

• Evaporation

• Condensation

• Precipitation

• Run-off*

• Subsurface flow*

*Of most interest to hydrogeologists and

engineering geologists

Earth’s Water Resources

(C) Copyright, 1996 by Purdue Research Foundation, West Lafayette, Indiana 47909, All Right

Reserved. This material may be reproduced and distributed in its entirely for non-profit, educational use,

provided appropriate copyright notice is acknowledged.

Surface/fresh water excluding ice

• 1400 M km3: Total Volume of water

• 96.5 %: saline water of oceans

• 2 %: glaciers and polar ice-caps • 0.02 %: rivers and streams

• ~1-1.5 %: GROUNDWATER

• 7-60 M km3

• Estimates difficult due to permafrost,

seasonal ice & impermeable rocks

Groundwater in rocks and soils

(C) Copyright, 1996 by Purdue Research Foundation, West Lafayette, Indiana 47909, All Right Reserved. This material may be

reproduced and distributed in its entirely for non-profit, educational use, provided appropriate copyright notice is acknowledged.

• Water exists in the ground within the saturated and unsaturated

zone

• Unsaturated zone: pores/voids partially filled

• Saturated zone: pores/voids completely filled

Groundwater in rocks and soils

Un

sa

tura

ted

Zo

ne

S

atu

rate

d

Zo

ne

• Unsaturated zone: water held as a film around grains, water at less than

atmospheric pressure

• Capillary fringe: ~saturated water above the water table at pressure less than

atmospheric due to surface tension and capillary phenomena

• Groundwater recharged by downward flow of water due to gravity

• Saturated zone: below the water table ‘groundwater’ at a pressure above

atmospheric

‘Groundwater’

Groundwater in rocks and soils • How/why does water flow in the ground?

• Differences in water table heights (elevation and pressure)

• Water works to equalize the difference by flowing

• Therefore flow occurs from high ‘head’ to low ‘head’

• Flow cells Recharge

zone Discharge

zone Groundwater

circulatory systems

Discharge at

rivers

Recharge during

Winter

impermeable

permeable

Perched water tables

(C) Copyright, 1996 by Purdue Research Foundation, West Lafayette, Indiana 47909, All Right Reserved. This material

may be reproduced and distributed in its entirely for non-profit, educational use, provided appropriate copyright notice is

acknowledged.

• Water table within low permeability lenses

• Above regional water table

Groundwater Properties • At what depth is groundwater reached?

Northumberlandtoday.com

Wikipedia

Water table at 2-3 m BGL 20-200 m deep, Qanats

Alluvial fan, Iran

Groundwater Properties • Term ‘groundwater’ either refers to 1) water in rocks 2) the

exploitable commodity

• Is not evenly distributed throughout the Earth’s crust;

– some lithologies concentrate groundwater and permit flow easily through it

– others hold groundwater and only permit very slow flow rates and small volumes

• Some areas do not get sufficient recharge

– Water not available to enter subsurface, high rates of evapotranspiration (Evp)

• Large areas of the UK where groundwater exists but can not be exploited as a resource

– Mainly due to properties of the lithologies in which the water exists

Groundwater Properties Porosity:

• Measure of pore space

• How porous the rocks are; 𝑛 = 𝑉𝑝 𝑉𝑏 ∙ 100 (= Volume of void

space/total rock volume)

Permeability (=hydraulic conductivity):

• A measure of the ease with which water can flow through a rock

• Permeable materials permit water to flow through them (impermeable contrary)

• A function of connectivity and grain size of geological material

• High porosity does not mean high permeability, an example: Cretaceous Chalk

Groundwater Properties Permeability: Flow takes place by: • Intergranular flow – diffuse flow, between grains

in sands and gravels, poorly cemented sandstones and young porous limestones

• Fracture flow – through joints, bedding etc; erratic flow in faults; dense joint sets provide diffuse flow in chalks

• Secondary flow - groundwater flow increasing permeability by dissolution, notably in limestones, karst systems,

• Limestones at Castleton,

• Derbyshire

Inchnadamph

Groundwater as a resource

• Like many natural resources, if groundwater is to be exploited as a resource;

• It must exist in economically viable quantities

• This situation is met where:

1. Layers of rock are sufficiently porous to store water

2. Permeable enough to allow flow through

• These conditions exist: Aquifer

• Unconfined aquifer: upper surface water table

• Confined aquifer: low permeability confining layer overlying aquifer

Types of Aquifers

(C) Copyright, 1996 by Purdue Research Foundation, West Lafayette, Indiana

47909, All Right Reserved. This material may be reproduced and distributed in its

entirely for non-profit, educational use, provided appropriate copyright notice is

acknowledged.

Unconfined

Aquifer

Confined

Aquifer

Groundwater must be abstracted

by pumping Groundwater flows as

under pressure

Aquifer Properties

One of the most important and easiest properties:

• Hydraulic Conductivity, k, m/s:

– A measure of how much water can naturally flow

– Is dependent on the hydraulic gradient, 𝑖:

𝒊 =𝑯

𝒍

𝒊

𝑯

• Slope of the water table

• Typical 𝑖 for an aquifer =

1:100

• Hydraulic Conductivity, k

– Flow rate, Q

– Area of the aquifer:

– B = aquifer thickness, m

– w = aquifer width, m

• Darcy’s Law is used to describe the flow through an aquifer

• For a given material, K, remains constant; proportionality constant

Aquifer Properties

𝑲 =𝑸

𝑩𝒘𝒊

Darcy’s apparatus to experimentally

verify his concept in Dijon, 1803

Other Aquifer Properties

• Specific yield - % volume of water that can drain freely from a rock; indicates the groundwater resource value of an aquifer (some water not extractable)

• Storage Coefficient/Storativity – volume of water released from an aquifer for each unit change of water table height

• Transmissivity, T – hydraulic conductivity of a vertical section of aquifer, hence: 𝑇 = 𝐾𝐵

– How readily water can move through aquifer to wells

Typical Aquifer Values

Material Permeability K (m/day)

Porosity % Specific yield %

Granite 0.0001 1 0.5

Shale 0.0001 3 1

Clay 0.0002 50 3

Fractured sandstone 5 15 8

Sand 20 30 28

Gravel 300 25 22

Cavernous limestone erratic 5 4

Chalk 20 20 4

Fracture zone (e.g. fault) 50 10

K< 0.01 m/day – impermeable K>1 m/day – exploitable aquifer

Field Measurement of Aquifer Properties

Regional Aquifer Properties • Cone of depression forms in piezometric surface during abstraction

• Shape: pumping rate, transmissivity and storativity of aquifer

• Pump water out at a steady rate while monitoring the fall in water table in at

least 2 monitoring wells

• Draw down proportional to pump rate

Local Aquifer Properties • Packer test used to determine individual contributions to overall aquifer

transmissivity of particular layers or fissures

• Can be either constant head or falling head

Field Measurement of Aquifer Properties

UK Aquifers

UK Groundwater Forum

Cretaceous Chalks

• Shell fragments

• Porosity 40 %

• Specific yield = ~1 %

• T = 1000 m2/day

• Cracks and fissures

• Bedding flow

Permo-Triassic Sandstones

• Penrith Sandstone

• Dune sands

• Intergranular flow

• Porosity 20-35 %

• K = 1-10 m/day

Most important UK aquifers occur in ‘Younger Cover’

• Annual abstraction:

2400M m3/day

• 85% from two aquifers

UK Aquifers Example of a hydrogeological

map

(BGS)

Hydrochemistry &

abstraction licenses

Relief & Annual Rainfall

data

Hydrograph data

Geological cross sections

Hydrogeological Map: • Piezometric contours

• Surface water courses

• Rainfall catchments

Stratigraphic column with

hydrogeological properties

UK Aquifers

Example of a hydrogeological map: close up BGS

1 km

Permian

Breccias

Flow direction

Piezometric

contours

Contours of base

Otter Sandstone aquifer

Rivers,

streams &

Sea

Rainfall

catchments

Drift material

Hydrogeological model • More than one groundwater flow path exists

Groundwater Resources Abstraction Well Design

• Example abstraction well

connected to three aquifers

• Borehole supported by steel

casing

• Grout between casing and rock

for sanitary reasons

• Perforated screen used for loose

sands prevents up flow of

sediment

• Gravel filter pack for fine sands

• Unlined for rocks as support not

needed

Resource Considerations

• Aquifer abstraction stability is only assured if the rate of abstraction < recharge.

• If abstraction > recharge = groundwater mining (e.g. Great Man-made River)

• Groundwater quality is ensured by:

– aquifer filtration while flowing

– underground residence time in contact with absorptive clays and cleansing bacteria in soils

Tapping an aquifer…

“Eighth wonder of the world” Colonel Gaddafi

• Worlds largest water irrigation project

• Supply 70 % of overall water demand

• 6,500,000 m3 per day

• Abstract groundwater from Nubian Sandstone

Aquifer in Sahara

• Fossil aquifer; no recharge

• Potential water reserves: 150,000 km3

• Transport to cities of Tripoli, Benghazi & Sirte

Tapping an aquifer…

“Eighths wonder of the world” Colonel Gaddafi

• Worlds largest water irrigation project

• Supply 70 % of overall water demand

• 6,500,000 m3 per day

• Abstract groundwater from Nubian Sandstone

Aquifer in Sahara

• Fossil aquifer; no recharge

• Potential water reserves: 150,000 km3

• Transport to cities of Tripoli, Benghazi & Sirte

• 1300 abstraction wells down to 500 m

• 2820 km of pipes and aquaducts

• Several huge reservoirs

Groundwater & Engineering Works

• Any construction project working in the saturated zone will be affected by groundwater

• Groundwater exclusion techniques:

Sheet Piling

Diaphragm Walling

Prweb.com Menardbachy.au



• Groundwater exclusion techniques:

Grouting

Bachysoletanche

.com



• Dewatering techniques:

• Pumping to lower

water table

• Also used in deep

mining

Moorcroft Quarry,

Plymouth

Price 2002



• Drainage:

• Remove water that

enters works

• Remove water before

it can be an issue



• Drainage:

• Reduces strength

of foundations

• Remove and

prevent water from

flowing

Pump water out

via gallery and borehole

Grout curtain

Groundwater Contamination EU Water Framework Directive

• Europe’s Water protection policy

In terms of groundwater:

1. Prevent input of pollutants

2. Recharge-discharge

balance

3. Reverse current pollutant

concentration trends

4. Do all the above within 15

years

Groundwater Contamination Common contaminants:

1. Petroleum products

2. Fertilizers

3. Pesticides

4. Human waste (sewerage)

5. Nitrates

Contaminant sources:

1. Storage tanks (point source)

2. Septic systems

3. Fly-tipping of waste

4. Contaminated water courses

5. Landfills

6. Roads and railways (line

source)

7. Salt water intrusion

8. Farming (diffuse source)

Contaminant Transport

Advection; with

groundwater

Diffusion & dispersion



2. Pesticides


4. Nitrates



2. Septic systems



5. Landfills


source)


8. Atmospheric contaminants


Advection; with

groundwater


• Herbicides, pesticides and fungicides used to kill

weeds & insects

• Soluble and susceptible to leaching; easily reach GW

• Restrictions in use near public supply wells



2. Pesticides


4. Fertilisers: Nitrates



2. Septic systems



5. Landfills


source)


8. Atmospheric contaminants


Advection; with

groundwater


• High concentrations detrimental to health (infants)

• Risk of groundwater contamination managed

• Designated ‘Nitrate Vulnerable Zones’

• Farmers encouraged to adapt farming practices

Summary • Groundwater as a resource

• Types of aquifers

• Groundwater movement & field measurement

• Aquifer well design & aquifer exploitation example

• Hydrogeology and engineering projects

• Groundwater contamination

lecture hydrogeology

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