GROUNDWATERGROUNDWATER

ERT 246- HYDROLOGY AND WATER ERT 246- HYDROLOGY AND WATER RESOURCES ENGINEERINGRESOURCES ENGINEERING

Siti Kamariah Md Saat, Siti Kamariah Md Saat,

PPK Bioprocess 2010PPK Bioprocess 2010

GroundwaterGroundwater

• Groundwater is part of the water cycle. It is water that is located beneath the earth’s surface in pores and crevices of rocks and soil.

• The process of water entering the ground to become groundwater is called recharge.

• The volume of recharge and how fast it enters groundwater is dependent on climate, the depth to the water table, the types of plants that use water in the soil and the types of soil and rock it must pass through.

Groundwater flowGroundwater flow

• Because groundwater has to move between pores and crevices in soil and rock, it moves much more slowly than surface water.

• Water can move down a river in hours, days or perhaps weeks.

• Groundwater in an aquifer may take ten, one hundred or many thousand years to flow through an aquifer.

• Management of groundwater needs to consider the amount of water going into the aquifer, how much is stored in the aquifer, and how long it takes to move through it.

Groundwater FlowGroundwater Flow

• Groundwater velocity– Depends on permeability and hydraulic

gradient (slope of water table)– Ranges from 100 m/day to mm/day

Groundwater problem and issueGroundwater problem and issue

• Groundwater is a complex resource. The unseen nature of groundwater makes it difficult to quantify, however careful monitoring and management of groundwater resources helps to guard against over-extraction and ensure reserves do not become stressed or drop below sustainable levels.

• It may also be impacted by climate change. However, by recording water levels and assessing recharge rates, it is possible to better-understand the resource and the impacts that climate change may have upon it.

Groundwater problem and issueGroundwater problem and issue

• While groundwater can be a reliable source of water, its overuse can result in failure of supply.

• The risk of loss of supply or changes to groundwater quality is increased as surface water systems become fully allocated and more people access groundwater to meet their water needs.

Water ProfileWater Profile

Bound Water in Minerals

Capillary Water

Intermediate Vadose Water

Water in Unconnected Pores

Groundwater

Soil Water

Inte

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Zo

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Sa

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Un

sat

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Subsurface FlowSubsurface Flow

• Infiltration– flow entering at the ground surface

• Percolation– vertical downward unsaturated flow

• Interflow– sub-horizontal unsaturated and perched saturated flow

• Groundwater flow– sub-horizontal saturated flow

Groundwater zonesGroundwater zones

• Water in the ground is found in three general zones – Vadose, or unsaturated zone (saturation <1) – Saturated zone (saturation = 1)– Capillary zone, lies above the water table

AquifersAquifers

• Groundwater is the water found in an aquifer

• Aquifer: – The saturated underground formation that will

yield usable amounts of water to a well or spring. The formation can be sand, gravel, limestone or sandstone

– Any geologic unit through which water can move easily (i.e. it’s permeable)(= high permeability)

Permeability: The ease with which water flows through porosity. Most important variable is grain / pore size.

Confined aquiferConfined aquifer

– The saturated formation between lowpermeability layers that restrict movement of water vertically into or out of the saturated formation

– Water is confined under pressure similar to water in a pipeline

– In some areas confinedaquifers produce water without pumps(flowing artesian well)

Unconfined aquiferUnconfined aquifer

– The saturated formation in whichthe upper surface fluctuates with addition orsubtraction of water

– The upper surface of an unconfined aquifer is calledthe water table

– Water, contained in an unconfined aquifer, is freeto move laterally in response to differences in the water table elevations

Aquifer TypesAquifer Types

• Unconfined - storage LARGE depends on specific yield• Confined - storage SMALL depends on compressibilities

Confined and unconfined aquifer

Perched aquifer:Groundwater ponded above an impermeable layer within a larger unconfined system.

Ground Water Ground Water and Surface Waterand Surface Water

• These are almost always connected– If a stream contributes water to the aquifer

it’s called a “losing stream”– If a stream receives water from the aquifer

it’s called a “gaining stream”– Same stream can be both at different

places or at different times

Aquifer propertiesAquifer properties

1. Porosity

2. Specific retention

3. Specific yield

4. Permeability

5. Transmissibility

6. Storage coefficient

PorosityPorosity• Percentage of open spaces in rock and

sediment that can hold water.• This determines the amount of water that a rock

can contain.

• In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation.

Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments

Poorly sorted sediments usually have lower porosity because the

fine-grained fragments tend to fill in the open space.

Specific retention, SrSpecific retention, Sr

• Sr the ratio of the volume of water a geomaterial can retain against gravity drainage to the total volume of the geomaterial.

• Known as ability to retain water

• Sr= Wr/V, where – Wr is a water retention volume and – V is total volume

Specific yield, SySpecific yield, Sy

• Sy the ratio of the volume of water that drains from a saturated geomaterial flowing to the attraction of gravity to the total volume of the geomaterial

• Sy= Wy/V, where– Wy= volume of water discharge– V= total volume

n= Sy + Sr

Specific YieldSpecific Yield

PermeabilityPermeability

• A measure of the degree to which the pore spaces are interconnected, and the size of the interconnections.

• Low porosity usually results in low permeability, but high porosity does not necessarily imply high permeability. It is possible to have a highly porous rock with little or no interconnections between pores.

• A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability.

• Unit m/day or m/year

Storage coefficientStorage coefficient

• Storativity(S) or Storage coefficient

• •The volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head.

Specific StorageSpecific Storage

• Specific storage (Ss) or Elastic storage coefficient

• •The amount of water per unit volume of a saturated formation that is stored or expelled from storage owing to compressibility of the mineral skeleton and the pore water unit change in head.

Specific StorageSpecific Storage

• Ss = ρwg (α + nβ), where

• ρw= density of water

• g =the acceleration of gravity• α = compressibility of aquifer skeleton

(1/(M/LT2))

• β = compressibility of water (1/(M/LT2))

• n = porosity (L3/L3)

SpecificsSpecifics

Unconfined

Sy = n - Sr

n porosity

Sy specific yield (gravity drainage)

Sr specific retention (like field capacity)

Confined

S = b.Ss

b thickness

Ss specific storage

Ss = .( + n. )

specific weight

matrix compressibility

water compressibility

Transmissivity

• Amount of water that can be transmitted horizontally through a unit width by the full saturated thickness of the aquifer under a hydraulic gradient of 1.

T= K B• T: transmissivity(or m2/d)

K: hydraulic conductivity (L/T)

B: saturated thickness of the aquifer (L or m)

Groundwater Flow rate-Darcy LawGroundwater Flow rate-Darcy Law

• Simple relationship that states that flow velocity is directly proportional to: – hydraulic gradient: slope of the WT. – hydraulic conductivity (K): parameter

describing the permeability of the aquifer (also depends on the density and viscosity of the fluid).

• Typical rates are on the order of 1-10 cm/day for most aquifers.

Pollution of GroundwaterPollution of Groundwater

• Need a sense of ground water flow– Warm up responses to the velocity of

groundwater flow is dependent on:• porosity and permeability

28% • permeability and hydraulic gradient

61% • porosity and hydraulic gradient 7% • pressure gradient 4%

ApplicationsApplications

• Recharge surficial and deep aquifers

• Hazardous waste and landfill leachate

• Swale Design

• Percolation or Retention Pond Design

• Yield of an Aquifer

• Green Roof Design and Operation

• Seepage Through Reservoirs

• Exfiltration Design and Operation

• Yearly Volume Budgets

Darcy’s LawDarcy’s Law

• Darcy’s law provides an accurate description of the flow of ground water in almost all hydrogeologic environments.

• Flow rate determined by Head loss dh = h1 - h2

Darcy’s LawDarcy’s Law

• Henri Darcy established empirically that the flux of water through a permeable formation is proportional to the distance between top and bottom of the soil column.

• The constant of proportionality is called the hydraulic conductivity (K).

• V = Q/A, V – ∆h, and V 1/∆L

V = – K (∆h/∆L)

and since

Q = VA (A = total area)

Q = – KA (dh/dL)

Hydraulic ConductivityHydraulic Conductivity

• K represents a measure of the ability for

flow through porous media:

• Gravels - 0.1 to 1 cm/sec

• Sands - 10-2 to 10-3 cm/sec

• Silts - 10-4 to 10-5 cm/sec

• Clays - 10-7 to 10-9 cm/sec

ConditionsConditions

• Darcy’s Law holds for:

1. Saturated flow and unsaturated flow2. Steady-state and transient flow3. Flow in aquifers and aquitards4. Flow in homogeneous and heterogeneous systems5. Flow in isotropic or anisotropic media6. Flow in rocks and granular media

Example of Darcy’s LawExample of Darcy’s Law

• A confined aquifer has a source of recharge.

• K for the aquifer is 50 m/day, and n is 0.2.

• The piezometric head in two wells 1000 m

apart is 55 m and 50 m respectively, from a

common datum.

• The average thickness of the aquifer is 30

m, and the average width of aquifer is 5 km.

Compute:Compute:• a) the rate of flow through the aquifer

• (b) the average time of travel from the head of the aquifer to a point 4 km downstream

• *assume no dispersion or diffusion

The solutionThe solution

• Cross-Sectional area= 30(5)(1000) = 15 x 104 m2

• Hydraulic gradient = (55-50)/1000 = 5 x 10-3

• Rate of Flow for K = 50 m/day Q = (50 m/day) (75 x 101

m2) = 37,500 m3/day

• Darcy Velocity: V = Q/A = (37,500m3/day) / (15 x 104

m2) = 0.25m/day

• Seepage Velocity: Vs = V/n = (0.25) / (0.2) = 1.25 m/day (about 4.1 ft/day)

• Time to travel 4 km downstream: T = 4(1000m) / (1.25m/day) = 3200 days or 8.77 years

• This example shows that water moves very slowly underground.

Limitations of the Darcian Limitations of the Darcian ApproachApproach1. For Reynold’s Number, Re, > 10 or where the flow

is turbulent, as in the immediate vicinity of pumped wells.

2. Where water flows through extremely fine-grained materials (colloidal clay)

Darcy’s Law:Darcy’s Law:Example 2Example 2• A channel runs almost parallel to a river, and they are 2000 m

apart.

• The water level in the river is at an elevation of 120 ft and 110m in the channel.

• A pervious formation averaging 30 m thick and with K of 0.25 m/hr joins them.

• Determine the rate of seepage or flow from the river to the channel.

Confined AquiferConfined Aquifer

Confining Layer Aquifer

30 ft

m

Example 2Example 2

• Consider a 1-m length of river (and channel).Q = KA [(h1 – h2) / L]

• Where:A = (30 x 1) = 30 m2

K = (0.25 m/hr) (24 hr/day) = 6 m/day

• Therefore,Q = [6 (30) (120 – 110)] / 2000 = 0.9 m3/day/ft length = 0.9 m2/day

Thank YouThank You

End of ClassEnd of Class

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