HYDROLOGIC PROPERTIES HYDROLOGIC PROPERTIES OF ROCKSOF ROCKS

Advanced Center for Water Resources Advanced Center for Water Resources Development and Management (ACWADAM)Development and Management (ACWADAM)Development and Management (ACWADAM)Development and Management (ACWADAM)Plot 4, Lenyadri society, Sus road, Pashan, Plot 4, Lenyadri society, Sus road, Pashan, PunePune--411021. 411021. ��020020--2587153925871539��020020--2587153925871539Email: Email: acwadam@vsnl.netacwadam@vsnl.netWebsite: Website: www.acwadam.orgwww.acwadam.org

Accumulation and movement of water in rocksAccumulation and movement of water in rocks

�� Rocks must possess Rocks must possess openings in order to allow openings in order to allow a zone of saturation to a zone of saturation to form…form…form…form…

�� The properties of rocks The properties of rocks that allow the storage and that allow the storage and that allow the storage and that allow the storage and transmission of water are transmission of water are known as the hydrologic known as the hydrologic properties of rocks.properties of rocks.properties of rocks.properties of rocks.

�� POROSITY and HYDRAULIC POROSITY and HYDRAULIC CONDUCTIVITY are the CONDUCTIVITY are the hydrologic properties of hydrologic properties of CONDUCTIVITY are the CONDUCTIVITY are the hydrologic properties of hydrologic properties of the the rock framework rock framework in in which groundwater occurs which groundwater occurs andand through which it through which it andand through which it through which it moves.moves.

PorosityPorosity

�� Material that contains Material that contains voids or openings is voids or openings is said to be said to be porousporous. The . The said to be said to be porousporous. The . The property is called property is called POROSITY.POROSITY.

�� Rocks have openings of Rocks have openings of �� Rocks have openings of Rocks have openings of different types.different types.

�� The presence and type The presence and type of openings depend of openings depend largely upon the largely upon the of openings depend of openings depend largely upon the largely upon the characteristics of rocks characteristics of rocks (this is the reason why (this is the reason why GEOLOGY is so GEOLOGY is so GEOLOGY is so GEOLOGY is so important in important in understanding understanding groundwater).groundwater).groundwater).groundwater).

Types of rock openings that attribute Types of rock openings that attribute porosity to rocksporosity to rocksporosity to rocksporosity to rocks

�� Between grains Between grains ––intergranularintergranularintergranularintergranular

�� Within grains Within grains ––intragranularintragranularVesicles in basaltsVesicles in basalts�� Vesicles in basaltsVesicles in basalts

�� Joints and fractures Joints and fractures ––divisional planes that divisional planes that have some openinghave some openinghave some openinghave some opening

�� Solution channels Solution channels from where rock from where rock material has been material has been material has been material has been removed in solutionremoved in solution

�� Lava tubes or tunnelsLava tubes or tunnels

Porosity in alluvium (gravel, sand & clay) is Porosity in alluvium (gravel, sand & clay) is different from porosity due to fractures in hard rockdifferent from porosity due to fractures in hard rockdifferent from porosity due to fractures in hard rockdifferent from porosity due to fractures in hard rock

Alluvium

Hard rock

What is porosityWhat is porosity

�� Represented as Represented as percentage or fraction percentage or fraction related to the finite related to the finite

�� e.g. if the total volume e.g. if the total volume of rock is 100 mof rock is 100 m33 and and related to the finite related to the finite

volume of the rock.volume of the rock.

�� Porosity in percentage is Porosity in percentage is a ratio given bya ratio given by

of rock is 100 mof rock is 100 m33 and and the volume of the volume of openings within the openings within the rock is 40 mrock is 40 m33 then the then the a ratio given bya ratio given by rock is 40 mrock is 40 m then the then the porosity in % porosity in %

= (40/100)x100= (40/100)x100

= 0.4x100= 0.4x100

ήή or or φφ==(Vol of openings/Total vol of rock (Vol of openings/Total vol of rock

sample)x100sample)x100

= 0.4x100= 0.4x100

= 40%= 40%

�� 100% porosity is 100% porosity is considered as 1.00 considered as 1.00 units in fraction. units in fraction. sample)x100sample)x100considered as 1.00 considered as 1.00 units in fraction. units in fraction. Hence, 40% porosity Hence, 40% porosity in fraction means a in fraction means a value of 0.4.value of 0.4.10% porosity means 0.1 value of 0.4.value of 0.4.10% porosity means 0.1

50% porosity means 0.575% porosity means 0.75 and so on…

Types of porosity in different rocksTypes of porosity in different rocks

�� Primary porosity Primary porosity –– rocks rocks acquire porosity during acquire porosity during their formation their formation (intergranular porosity in (intergranular porosity in their formation their formation (intergranular porosity in (intergranular porosity in sedimentary rocks or sedimentary rocks or vesicles in volcanic rocks).vesicles in volcanic rocks).

�� Secondary porosity Secondary porosity –– rocks rocks acquire porosity due to acquire porosity due to processes like weathering processes like weathering and fracturing acting on and fracturing acting on and fracturing acting on and fracturing acting on the rock after its formation the rock after its formation (weathering and (weathering and fracturing) fracturing) –– joints, joints, fractures, solution fractures, solution Porosity is also an index of how fractures, solution fractures, solution channels.channels.

Porosity is also an index of how much water can be stored in the openings within a given finite volume.

Porosity is that part of the volume of rock (open phase) not occupied by rock material (solid rock phase).

Practical manifestation of porosity Practical manifestation of porosity –– the the floor mop case 1floor mop case 1floor mop case 1floor mop case 1

�� Water spilled on the floor Water spilled on the floor can be wiped with a mop can be wiped with a mop can be wiped with a mop can be wiped with a mop or sponge.or sponge.

�� The mop absorbs the water The mop absorbs the water because it is because it is porousporous in in because it is because it is porousporous in in nature nature -- POROSITY.POROSITY.

�� Squeezing the mop drives Squeezing the mop drives out some water.out some water.out some water.out some water.

�� Does the mop go Does the mop go dry…completely???dry…completely???

Can you relate this concept to Can you relate this concept to water in rocks?water in rocks?

Practical manifestation of porosity Practical manifestation of porosity –– the the clothesline case 2clothesline case 2clothesline case 2clothesline case 2�� A wet kurta hung out to A wet kurta hung out to dry...dry...dry...dry...

�� After a while, water starts After a while, water starts draining out of the kurta draining out of the kurta draining out of the kurta draining out of the kurta in the form of droplets in the form of droplets under the influence of under the influence of gravity gravity –– is this porosity is this porosity gravity gravity –– is this porosity is this porosity or is the kurta “yielding” or is the kurta “yielding” water?water?

However, the kurta still However, the kurta still �� However, the kurta still However, the kurta still remains wet after all the remains wet after all the water “drains” out…what water “drains” out…what is this “retained” water is this “retained” water is this “retained” water is this “retained” water called?called?

Porosity….from a more practical anglePorosity….from a more practical anglePorosity….from a more practical anglePorosity….from a more practical angle

�� PorosityPorosity is an indicator is an indicator of how much of how much of how much of how much groundwater the rock groundwater the rock will store.will store.

�� In reality, however, we In reality, however, we �� In reality, however, we In reality, however, we are interested in the are interested in the quantity quantity of water a of water a saturated volume of rock saturated volume of rock saturated volume of rock saturated volume of rock will will releaserelease or or yieldyield from from this storage.this storage.

Porosity= Specific yield + Porosity= Specific yield + specific retentionspecific retention

Specific yield and specific retentionSpecific yield and specific retention

�� Specific yield (SSpecific yield (Syy)) is a part of is a part of the total porosity of the rock…the total porosity of the rock…

�� Specific retention (SSpecific retention (Srr)) is is that part of the total porosity that part of the total porosity indicating the quantity of indicating the quantity of water retained by the rock water retained by the rock the total porosity of the rock…the total porosity of the rock…

�� In other words, it indicates the In other words, it indicates the specific quantity of water specific quantity of water released by the saturated rock released by the saturated rock

water retained by the rock water retained by the rock after gravity drainage is after gravity drainage is complete.complete.

�� Molecular forces of cohesion, Molecular forces of cohesion, adhesion and capillarity are adhesion and capillarity are released by the saturated rock released by the saturated rock

under the influence of gravity under the influence of gravity (that is under gravity (that is under gravity drainage).drainage).

adhesion and capillarity are adhesion and capillarity are responsible for holding the responsible for holding the water back against the force water back against the force of gravity.of gravity.

�� Gravity is a major force that Gravity is a major force that drives the water out of the drives the water out of the pore space in a rock.pore space in a rock.

�� Not all water in the pore space Not all water in the pore space is removed by gravity…some is is removed by gravity…some is retained as specific retention.retained as specific retention.

Both specific yield and specific retention are represented in percentage or fraction.

An exampleAn example�� Total vol. of saturated Total vol. of saturated rock= 100 mrock= 100 m33

Porosity is 40%, i.e. 40 Porosity is 40%, i.e. 40 �� Porosity is 40%, i.e. 40 Porosity is 40%, i.e. 40 mm3 3 of water can be of water can be stored in the rockstored in the rockstored in the rockstored in the rock

�� The rock released 25 mThe rock released 25 m33

of water under gravity of water under gravity (SPECIFIC YIELD), (SPECIFIC YIELD), i.e. i.e. 25% of the total volume 25% of the total volume 25% of the total volume 25% of the total volume of 100 mof 100 m33

�� The rock retained 15 mThe rock retained 15 m3 3

of water (SPECIFIC of water (SPECIFIC of water (SPECIFIC of water (SPECIFIC RETENTION), RETENTION), i.e. 15% of i.e. 15% of the total volume of 100 the total volume of 100 mm33

…example (continued)…example (continued)

PorosityPorosity == Sp. YieldSp. Yield ++ Sp. RetentionSp. Retention

40%40% == 25%25% ++ 15%15%40%40% == 25%25% ++ 15%15%

In fraction, we can write this asIn fraction, we can write this asIn fraction, we can write this asIn fraction, we can write this as

0.40.4 == 0.250.25 ++ 0.150.150.40.4 == 0.250.25 ++ 0.150.15

1. In hydrogeology, the specific yield is more significant than the porosity.

2. The specific yield of a rock depends upon the size, shape and the arrangement of grains.

Comparative table of values for typical rocksComparative table of values for typical rocks

Rock Rock typetype

Size of Size of grains grains in mmin mm

Porosity Porosity as %as %

Specific Specific yield as yield as %%

ClayClay <0.004 <0.004 to 0.1to 0.1

45 to 5045 to 50 ≤ 3%≤ 3%

SandSand 0.1 to 20.1 to 2 35 to 4535 to 45 ≤ 25%≤ 25%SandSand 0.1 to 20.1 to 2 35 to 4535 to 45 ≤ 25%≤ 25%

GravelGravel 2 to 202 to 20 30 to 3530 to 35 ≥ 25%≥ 25%

Increase in grain size means the solid rock phase increases and porosity decreases.decreases.

But as the grain size increases, the pore size (openings) becomes bigger, with an increase in the specific yield.

Fine grained material has a higher porosity with smaller pore size, increasing the dominance of molecular forces and therefore resultingin higher specific retention of stored water.

Practical manifestation of porosity Practical manifestation of porosity –– the the floor mop case 1floor mop case 1floor mop case 1floor mop case 1

�� Water spilled on the floor Water spilled on the floor can be wiped with a mop can be wiped with a mop can be wiped with a mop can be wiped with a mop or sponge.or sponge.

�� The mop absorbs the water The mop absorbs the water because it is because it is porousporous in in because it is because it is porousporous in in nature nature -- POROSITY.POROSITY.

�� Squeezing the mop drives Squeezing the mop drives out some water out some water ––out some water out some water ––SPECIFIC YIELD.SPECIFIC YIELD.

�� However, some water is However, some water is retained by the mop retained by the mop –– it it remains wet remains wet –– SPECIFIC SPECIFIC remains wet remains wet –– SPECIFIC SPECIFIC RETENTION.RETENTION.

Practical manifestation of porosity Practical manifestation of porosity –– the the clothesline case 2clothesline case 2clothesline case 2clothesline case 2�� A wet kurta hung out to A wet kurta hung out to dry dry –– POROSITY.POROSITY.dry dry –– POROSITY.POROSITY.

�� After a while, water starts After a while, water starts draining out of the kurta draining out of the kurta in the form of droplets in the form of droplets in the form of droplets in the form of droplets under the influence of under the influence of gravitygravity–– SPECIFIC YIELD.SPECIFIC YIELD.

�� However, the kurta still However, the kurta still �� However, the kurta still However, the kurta still remains wet because remains wet because water is held back in its water is held back in its pores against the force of pores against the force of gravity gravity –– SPECIFIC SPECIFIC gravity gravity –– SPECIFIC SPECIFIC RETENTION.RETENTION.

Different rock types show different openings…Different rock types show different openings…the specific yield depends upon these openingsthe specific yield depends upon these openingsthe specific yield depends upon these openingsthe specific yield depends upon these openings

Unconsolidated Unconsolidated sedimentssediments

CLAY, SAND AND CLAY, SAND AND GRAVELGRAVEL

Intergranular openingsIntergranular openingssedimentssediments GRAVELGRAVEL

Consolidated Consolidated sedimentssediments

SANDSTONE, SANDSTONE, SILTSTONESILTSTONE

Intergranular openings Intergranular openings and jointsand joints--fracturesfractures

LimestoneLimestone Intergranular openings and solution channelsIntergranular openings and solution channels

BasaltBasalt Vesicles, fracturesVesicles, fractures--joints and weathered joints and weathered portions, contacts between basalt unitsportions, contacts between basalt units

Crystalline rocks Crystalline rocks (granite, gneisses, (granite, gneisses, quartzite, schists etc)quartzite, schists etc)

Weathering, jointWeathering, joint--fracturesfractures

Porosity deals with the accumulation of Porosity deals with the accumulation of groundwater…permeability / hydraulic conductivity groundwater…permeability / hydraulic conductivity groundwater…permeability / hydraulic conductivity groundwater…permeability / hydraulic conductivity deals with its movement or flow…deals with its movement or flow…next presentationnext presentation