Section 4 Groundwater: Freshwater Unit - Honor Science

Groundwater

Water stored beneath Earth’s surface in the pore spaces and fractures within soil, sediment and rock (typically occupies small spaces between sediment particles or fractures within rock)

Groundwater Recharge

Water can move from surface into groundwater

Groundwater Discharge

Supplying water to streams, lakes, wetlands, and springs

How does water in the ground move?

It is pulled down by gravity through spaces between particles

What are the two major zones of subsurface based on water in pore space?

Unsaturated zone and Saturated zone (which is below the unsaturated)

Unsaturated Zone

Pore spaces contain air and water, soil can be wet but pore spaces not completely filled

Saturated Zone

Available pore spaces filled with water, groundwater is within this zone

Water table

Upper surface of saturated zone, the depth is not fixed and can rise and fall

Water table rises

Substantial recharge

Water table falls

Dry periods, groundwater withdrawn

Water Table Shape

Elevation varies across landscape, often follows surface topography (but there is a less pronounced change in elevation)

Aquifer *

A body of permeable rock or sediment that can store and transmit significant quantities of groundwater → well removes Ground water

Aquifer Material Must

1) Contain spaces in which water can be stored, spaces must be sufficiently connected for water to move through 2) Productive aquifers can be formed by sand, gravel, sandstone, fractured rock under appropriate conditions

Aquitard

A layer with relatively low permeability that restricts and slows groundwater movement

Aquiclude

A layer where groundwater movement is really limited or essentially negligible, can contain water but does not readily transmit it

The A categories that represent differences along a continuum

Aquifer - readily transmits groundwater, Aquitard - restricts groundwater movement, Aquiclude - essentially prevents groundwater movement

What are the aquifers classified according to relationship with surrounding geologic matter

Unconfined Aquifer and Confined Aquifer

Unconfined Aquifer

No confining layer separating the upper surface from land above. Upper boundary is water table. water can be recharged when precipitation infiltrates downward through permeable material.

Confined Aquifer (figure 4.2)

Seperated from surface by overlying material with low permeability (ex. aquitard) water can enter where permeable layer reaches surface, can be far from where groundwater is eventually withdrawn.

What does the arrangement of aquifers and confining layers influence?

Where groundwater can be recharged and how groundwater moves underground

Groundwater Storage and movement influenced by

Particle size, particle arrangement, connections between pore spaces

Porosity

Percentage of the total volume of a material that consists of pore space

Finding porosity percentage

(volume of pore space / total volume (sediment and pores)) x 100

What does high porosity mean

Large proportion of open space available

Fine and coarse sediments have substantial pore space when

Particles similarly shaped, particles well sorted (aprox. same size)

When sediment is well sorted VS poorly sorted

Well - greater porosity can happen because open spaces remain between similarly sized particles VS Poorly - particles occupy space between larger particles, reducing porosity

Packing

Describes how closely sediment particles are arranged - influences permeability

Loosely packed VS Tightly packed

Loosely packed v- more pore space - greater porosity VS Tightly packed - less pore spaces - lower porosity (particles size and porosity NOT the same)

What must pore spaces be

Connected so water can move through a material with pore space easily

Permeability

Describes the ability of a material to add fluids to pass through its connected pore spaces or fractures

Larger better connected pathways VS substantial porosity but small pore openings restricted water movement) (materials)

Coarse materials (sand, gravel) VS fine materials (clay, which can have high porosity but low permeability)

Hydraulic Conductivity

Describes how readily water can move through a particular material under hydraulic gradient

Hydraulic Gradient

Slope or rate of change in water pressure and height over a specific distance (tells us which way and how fast underground water will flow)

High hydraulic conductivity =

Groundwater move through material more readily

Whats important to know about groundwater in pore spaces

Not all groundwater in pore spaces drains freely from sediment

When saturated sediment drains from gravity…

Some water drains from pore spaces, some remains attached to particle surfaces or trapped in small spaces

Specific Yield

Proportion of water that saturated sediment will release through drainage under influence of gravity

Specific retention

Proportion of water that remains held within the material after gravity drainage

Porosity = specific yield + specific retention when

Values are expressed as % if the total

How much water can be drained out of saturated material by gravity (Particle size influences this equation)

Volume yield % = porosity % - specific retention %

Water amount that drains or remains in sediment depends on

Depends on size and arrangement of pore spaces

Size and arrangement of pore spaces are influenced by

influenced by particle size, sorting, and packing

What happens to precipitation when reaching the surface

Influenced by characteristics of earths soil at the surface

Soil texture

Describes relative proportions of silt, sand, and clay in soil (influences infiltration)

What to know about sand, silt, and clay

Sand - relatively large particles, silt - intermediate in size, clay - consists of very small particles

What do properties that control water movement also influence

Pollutant movement

Behavior of pollutant depends on

Chemical properties, amount of water present, biological + chemical reactions, other environmental conditions

Relationship between soil and sediment properties that allow predictions of pathways and rates water + dissolved materials move

Particle size + sorting - pore structure - permeability/ hydraulic conductivity - movement of water and dissolved pollutant

Groundwater depletion

Groundwater withdrawn faster than it is replenished, amount of stored groundwater within aquifer decreases (continued withdrawal causes the water table or groundwater pressure to decline)

Cone of depression (figure 4.6)

Water pumped from well and ground water level surrounding it falls. Groundwater from surrounding aquifer moves toward area of lower hydraulic pressure created from pumping

For a cone of depression, when the pumping lowers the water table enough, and shallow local well can dry, what does it depend on?

Pumping rate, distance between wells, aquifer properties, recharge

What can happen when groundwater levels are lowered in other areas

Effects can happen far from where water is being taken

Large quantities of groundwater removed =

Pore-water pressure decrease sediment may compact

Land subsidence

Lots of compaction = land surface sinks

What can compaction do to pore space

Permanently reduce pore space, and restored groundwater does not mean aquifer reaches original storage capacity again

Compaction

The act or process of pressing, crushing, or squeezing material to make it denser and reduce its volume by removing air or pore space

Saltwater Intrusion

When the pressure separating salt and freshwater in coastal aquifers becomes reduced because of excessive groundwater withdrawal and saltwater enters inland and toward the aquifer that initially contained fresh water

When water table intersects land surface

Ponds, streams, and wetlands can receive groundwater discharge (groundwater maintains water levels) (discharge can decrease when groundwater withdrawal lowers water table)

Ability of sediment and soil to store and transmit groundwater depends on and influences infiltration, groundwater recharge, and dissolved pollutants

Particle size, sorting, porosity, permeability, hydraulic conductivity, specific retention, and specific yield

Groundwater withdrawal exceeds recharge consequences

Cones of depression, dry shallow wells, land subsidence, saltwater intrusion, and reduced groundwater discharge