A SHORT OVERVIEW ON HYDROLOGY

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PRECIPITATION

  • Precipitation : Any atmospheric water coming on Earth’s surface

  • Infiltration : Vertical movement of water from earth’s surface to below earth’s surface

  • Run off : Horizontal or sub-horizontal movement of water on Erath’s surface

  • Precipitation = Infiltration + Run-off

TYPES OF GROUNDWATER

  • Connate water : Sedimentary origin (Fossil water) - during compaction of sediments

  • Meteoric water : Atmospheric water formed due to precipitation

  • Juvenile water : Magmatic water formed from magma

  • Metamorphic water : Metamorphic origin

  • Hydrogen-3 (Tritium) and Carbon-14 used to date groundwater

VERTICAL DISTRIBUTION OF GROUNDWATER

1) ZONE OF AERATION

  • Voids are partially filled with water and air

  • Movement of groundwater is vertical

  • Water lies in this zone is called vadose water

  • Atmospheric water > Water pressure

  • divided into 3 zone : Soil water zone, Intermediate zone and Capillary zone

2) ZONE OF SATURATION

  • Also known as phreatic zone

  • Voids are fully saturated with water

  • Movement of water is horizontal

  • Water pressure > Atmospheric water

TYPES OF ROCK FORMATION

  • Aquifer : Porous and permeable , eg: Sandstone, Conglomerate

  • Aquiclude : Porous and non-permeable , eg: Clay,shale,limestone

  • Aquitard : Porous and semi-permeable, eg: Sandy clay beds

  • Aquifuge : Non-porous and non-permeable, eg: Granite, Basalt

TYPES OF AQUIFER

  • UNCONFINED AQUIFER

    • Surrounded by impermeable layer only at bottom

    • Also known as water table aquifers

    • Water pressure = atmospheric pressure

    • Non-artesian well

  • CONFINED AQUIFER

    • Surrounded by impermeable layer at both top and bottom

    • Which restrict movement of groundwater

    • Water pressure is greater than atmospheric pressure

    • Therefore if we dug a well in confined aquifer the water will come automatically to piezometric surface

    • Piezometric surface above earth’s surface : Flowing artesian well

    • Piezometric surface below earth’s surface : Non-flowing artesian well

  • PIEZOMETRIC SURFACE :

    • Imaginary surface upto which groundwater comes automatically from confined aquifer

PROPERTIES OF FORMATION

  • Porosity

  • Permeability

  • Specific yield

  • Specific retention

  • Transmissivity

  • Specific storage or storativity

POROSITY

  • Percentage of void space in the rock

  • Types : Primary and secondary porosity

1) PRIMARY POROSITY

  • Formed during deposition of sediments

  • Inter-granular: Porosity resulting from interstitial spaces of grains

  • Intra-granular: Pores within the grain itself.

2) SECONDARY POROSITY

  • Form later to the time of formation of sedimentary rocks.

  • Its post-depositional i.e. during diagenesis

  • Form due to secondary processes like solution, recrystallisation, fracturing etc.

  • 5- types :

    (i) Inter-crystalline : Porosity between individual crystals of a crystalline rock

    (ii)Fenestral porosity : primary penecontemporaneous gap in rock

    framework larger than grain supported interstices.

    (iii) Moldic porosity : Form due to solution confined to grains

    (iv) Vuggy porosity: Cross cut primary depositional fabri, larger than moldic

    porosities

    (v) Fracture porosity : Develop due to fracturing

PERMEABILITY

  • Property of rock to transmit fluid

  • Permeability increases with increasing

    • Grain size

    • Grain shape

    • Sorting

  • Permeability decreases with increasing Tortuosity

  • Types: Intrinsic, Relative (Hydraulic conductivity), Effective permeability

    • Intrinsic : If it is saturated with single type of fluid (Unit = Darcy)

      1 Darcy = 9.87 × 10 -13 m2

    • Hydraulic conductivity : Measure of ease with which a medium transmit water ( Unit = m/day)

STORATIVITY

  • Unitless and dimensionless

  • Volume of water that can release from an aquifer from unit cross-sectional area declined by unit length normal to the water table

  • Used mainly in case of confined aquifer ( V=SAdh)

  • Specific storage : Unit = 1/m or m-1

TRANSMISSIVITY

  • Product of hydraulic conductivity and thickness (T=kb)

  • Unit : m2/day

TYPES OF STREAMS

  • Influent stream : river recharges the groundwater (losing stream)

  • effluent stream : groundwater recharges the river (gaining stream)

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