Groundwater

groundwater

water found within void spaces of geological material beneath the surface of the earth

• exists in saturated layers of sand, gravel, certain type of clay material, and cracks within crystalline rock

water budget equation

P = R + ET + I + ∆S

• P = precipitation

• R = runoff

• ET = evapotranspiration

• I = interception

• ∆S = groundwater storage

water distribution

All Water

• 97.6% ocean and saline lakes

• 2.4% freshwater

Freshwater

• 87% ice and snow

• 13% liquid water

Liquid Freshwater

• 95% groundwater

• 3% lakes, rivers, streams

• 2% soil moisture

groundwater

• major source of all fresh drinking water supplies in some countries

• stored underground in aquifers

• highly vulnerable to pollution

• understand for management and protection of water resources

• comes from precipitation that filters through vadose zone (important environmental role in GW systems) to reach the zone of saturation where GW flows

groundwater rate of infiltration

a function of soil type, rock type, antecedent water, and time

vadose zone

area between land surface and the top of the aquifer where the material are not saturated with water, but are filled primarily with air

• includes all the material between the Earth’s surface and the zone of saturation

• unsaturated zone; means shallow

water table

the upper boundary of the zone of saturation

capillary fringe

a layer of variable thickness that directly overlies the water table

• drawn up into this layer by capillary action

World Water Day (1998)

international observance of this day is an initiative that grew out of the 1992 United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro

• United Nations General Assembly designated March 22

• invisible resource playing a major role in global economics, political conflicts, and personal health

groundwater hydrology

the study of the characteristics, movements, and occurrence of water found beneath the land surface

ancient groundwater use

• Qanats development in the Middle East, Africa, China, and South America

• hydrologists and engineers of Roman Empire used GW from springs as water resources for aqueducts to their cities

• early Greek philosophers - Homer (800 BC), Thales (636-546 BC), Plata (427-347 BC) → GW originated from sea water → purified as move through land

Roman Architect and water management

Marcus Vitruvius Pollio (70-25 BC) hypothesized that precipitation (P) and infiltration (I) source groundwater

• precipitation falling in mountains infiltrated the earth’s surface and led to streams and springs in the lowlands

17th century

Pierre Perrault (1611-1680) measured P and its relationship to runoff in the seine river watershed

• discovered that P increased discharge of the seine river by 600%

• confirmed and supported Vitruvius theory 1700 years earlier

Leonardo da Vinci & Bernard Palissy (15th century)

independently reached an accurate representation of the hydrologic cycle

• hydrologic variables quantified in 17th century

Pierre Perrault, Edme Mariotte, and Edmund Halley (16th century)

• by measuring rainfall, runoff, and drainage area, Perrault showed rainfall was sufficient to account for the flow of the Seine

• Mariotte combined velocity and river cross-section measurements to obtain discharge in the Seine

• Halley showed evaporation from Mediterranean Sea was sufficient to account for the outflow of rivers flowing into the sea

18th century advancements

• Daniel Bernoulli’s piezometer

• Bernoulli’s equation (classical formulation of energy loss during fluid flow)

• Pitot tube

• Chezy formula

19th century

• development in groundwater hydrology

• Darcy’s Law

• Dupuit-Thiem well formula

• Hagen-Poiseuille’s capillary flow equation

lithology

the physical characteristic of a mineral composition: grain size and density of geologic material

stratigraphy

describes composition and age of sediment, lenses, and other formations

structure

cracks, folds, and other formation of geologic systems

sedimentary rocks

weathered and eroded igneous and metamorphic rock

conglomerate

sedimentary rock composed of rounded and gravel-sized particles

sandstone

sedimentary rock composed of sand-sized particles

siltstone

sedimentary rock composed of particles smaller than gains of sand

shale

sedimentary rock whose original constituents were clays or muds

fines

very small particles form weathered or eroded sedimentary rocks similar in size to talcum powder

geologic work of groundwater

• dissolves rocks

• mildly acidic (weak carbonic acid; forms when rainwater dissolves carbon dioxide form the air and from decaying plants)

• carbonic acid reacts with calcite in limestone to form calcium carbonate (CaCO3) a soluble material

karst

geological terrain where significant solution of carbonate rocks has occurred due to flowing groundwater

• sedimentary

sinkhole

dissolved limestone, carbonate rock, or salt beds, and the creation of large holes at the land surface

• sedimentary

caverns

landscapes that have been shaped mainly by the dissolving power of GW (karst hydrology)

• composed of dripstone (calcite deposited das dripping water evaporations, includes stalactites from the ceiling and stalagmites from the floor)

• irregular terrain, sinkhole or sinks (accompanied by collapse), lack of surface drainage (streams)

glacial till

rock debris varying from clays to mixtures of clay, sand, gravel, and boulders

glacial outwash

deposit of boulders, pebbles, gravel, sand, and fines which were carried by the floodwaters after glaciers melted (valleys)

alluvial valley

permeable formation that can hold considerable amount of water

alluvium

sediment deposited by flowing rivers

• fluvial is the process

tectonic formations

tectonic activities can create fissures and fractures that hold GW

arid locations

can have low infiltration rates due to hard, sunbaked land surfaces

• impermeable surfaces

saturated zone

all open spaces are filled with water

GW table

• top of the saturated zone of GW

• water level in a well

• water level in an unconfined aquifer

pressure (potentiometric) surface

the level to which water will rise in wells in a confined aquifer

confined aquifer

aquifer that is overlain by a confining bed of geologic material

• recharge area

unconfined aquifer

aquifer with no confining bed of material between the saturated zone and the land surface

confining layer

impermeable

percolation

downward movement of water from the land surface to the aquifer

• factors: climate, terrain, geology, and vegetation groundcover

aquifer

underground water-bearing geologic formation that can store and yield usable amounts of water

• materials: sand, gravel, sandstone, limestone, and fractured rock

• means to bear or carry water

consolidated rock aquifer

tightly bound geologic formation composed of material almost impervious, and doesn’t allow GW to move easily

• ex: sandstone, limestone, granite

unconsolidated rock aquifer

loosely bound geologic formation consisted of granular material (sand, gravel), and generally yields larger amount of GW

fractured aquifer

found in consolidated rock that contains usable amount of water in cracks, fissures, joints

• ex: granite, basalt

saturated thickness

total water bearing thickness of an aquifer

aquitard

a geologic formation that restricts the flow of GW from one aquifer to another

aquiclude

formation containing water, do not transmit significant quantities of water

aquifuge

formation does not contain nor transmit water

perched aquifer

localized zone of saturation above he main water table created by an underlying layer of impermeable material

artesian conditions in confined aquifers

higher precipitation causing water to emerge at the land surface as a spring, from a well, or fault

• similar to city water systems

Ogallala aquifer

(high plains aquifer)

• largest aquifer in North America

• 175,000 square miles (groundwater = Lake Huron)

• 1 ft < aquifer thickness < 1300 ft

• most GW from it used for irrigation

• GW mining in some areas (pumping greater than recharge)

pore spaces in aquifers

openings within the geologic material, providing GW to reside or move through under the force of gravity

porosity of aquifers

ratio of void space in a geologic formation to the total volume of the formation

• p = Vvoid/Vtotal

porosity ranges for sediments

• well-sorted sand or gravel 25-50%

• sand and gravel, mixed 20-35%

• glacial till 10-20%

• silt 35-50%

• clay 33-60%

cubical packing

(porosity)

each sphere sits directly on the crest of the underlying sphere

• porosity = 47.65%

rhombohedral packing

(porosity)

the spheres lie in the hollows formed by 4 adjacent spheres of the underlying layer

• lower overall porosity as void spaces occupied by grains of smaller diameter

• porosity = 25.95%

total porosity

includes all pore space in the sample

effective porosity

accounts only for interconnected pore space of the sample

primary porosity

intergranular porosity if it was formed during the formation of the sediment

secondary porosity

the result of fracturing, faulting, and dissolution of rocks (karsts) created after a rock is formed

representative elementary volume (REV)

(porosity)

a sample that is sufficiently large to contain a great number of pores allowing to define a mean property and sufficiently small so that the parameter variation from one domain to the next may be approximated by continuous functions

• size of this for a fractured rock is much larger than that for porous medium

transmissivity

rate at which GW moves laterally through the saturated thickness of an aquifer with a hydraulic gradient = i = 1

• T = Kb

• volume of water flowing through a cross-sectional area of an aquifer that is 1ft x aquifer thickness (b) under a hydraulic gradient (K) of 1ft/1ft in a given amount of time (usually a day)

specific yield (Sy)

the ratio of the volume of water that drains from a saturated rock owing to the attraction of gravity to the total volume of the rock

• Sy = V water drain due to gravity / V total sample

surface tension

some of the water molecules cling to the surface because of surface tension of the water

specific retention (Sr)

(aquifer)

the ratio of the volume of water an aquifer can retain against gravity drainage to the total volume of the aquifer

• porosity (n) = Sy + Sr

permeability (k)

the ability of a material to transmit a fluid; measure of how fast the fluid can travel through the rock or sediment

• usually correlates with porosity, but not always

• function of the size of the pore opening

• smaller grains = larger surface contacts = greater frictional resistance to flow = lower intrinsic permeability

grain-size classification

• well sorted sand - grain of sand of uniform size

• poorly sorted sand - small particles fill in pore space between larger grains

• porosity of 30% in sand/gravel formations = significant storage space for GW = a good aquifer

• porosity of 15% in small grain sand with finer-grained clay = fairly tight & restrictive

• igneous rock with a porosity <1% = water-tight barrier

• clay & shale with numerous porous openings, yet voids too small for water movement (impervious) = GW barrier

hydraulic conductivity of aquifers (K)

the proportionally constant in Darcy’s law, which relates the amount of water which will flow through a unit cross-sectional area of an aquifer under a unit gradient of hydraulic head

• K = ease with which water can move through pore spaces or fractures

• components: porosity, permeability, specific yield, specific retention

• units: length/time = velocity

hydraulic head

fluid potential to flow through porous media = mechanical energy per unit mass of fluid

• elevation = Z; pressure = P; velocity = v; density = ρ; volume of unit mass = V=1/ρ

manometer

(hydraulic head)

P = ρgψ +po

• Ψ = height of liquid above P

• po = atmospheric pressure (standard state)

ψ = h−z p = ρg (h−z) +po

• fluid potential at any point P in a porous medium is the hydraulic head at that point multiplied by the acceleration due to gravity

Darcy’s Law

an empirical law equation by which the discharge (flowrate) of GW can be calculated; valid for flow in any direction; linear law (y=ax); works for laminar flow; macroscopic law

• Q = A x K x (∆H/L)

• Q = discharge; A = cross-sectional area of flow; K = hydraulic conductivity; i = ∆H/L (hydraulic gradient); L = distance between the 2 head drops

constant head permeameter

water in the supply reservoir is kept at a constant head

• used for non-cohesive sediments (sand)

• head should never be more than about ½ the sample length

• water moves through sample at a steady rate

• hydraulic conductivity determined from variation of Darcy’s Law

falling-head permeameter

homogeneous

a unit that has the same properties at all locations

• ex: an aquifer where K doesn’t vary over space

• equivalent hydraulic conductivity (Ke) for a __________ formation hydraulically equivalent to a system of heterogeneous formations

heterogeneous

formation where properties change spatially

• ex: aquifer where K varies over space

• most aquifers

• equivalent hydraulic conductivity (Ke) treats this as a homogeneous one

anisotropic

materials where the permeability or conductivity at a point has a directional dependency

groundwater flow gradient graphical method

• draw a line to connect each well

• note the water elevation in each well

• measure the map distance between a well pair

• find the difference in elevation between a well pair

• find map distance for each unit change in head for a well pair by dividing the head difference by the map distance between the well pairs

• mark even increments along the line between the well pair, select the increment length so that each increment is a convenient length