Topic 4: Groundwater Flashcards
what is an aquifer?
a geological unit saturated with water able to store and transmit water.
Modern groundwater vs young groundwater
modern:
- -less than 50 years since recharge
- -most vulnerable to global environmental change
young:
- -less than 100 years since recharge
groundwater use in BC
25% of BC population
45% of public water systems
derived mainly from infiltration and snow melt, recharge is seasonally variable
groundwater use in USA
over 62% of groundwater extraction goes towards agriculture.
$20 Billion a year
GRACE satellite purpose
helped make shallow groundwater drought indicator
4 types of streams in relation to groundwater
- -gaining stream: water table above stream height
- -flow through stream: water table higher and water flow through from one side to the other
- -perched losing stream: water table below stream
- -losing stream: water table connected to stream and water flowing out
unconfined aquifer
open to atmosphere, top of the saturated zone and is free to fluctuate. fluvial sediments involved.
confined aquifer
saturated geologic zone overlain by confining layer
pressures are greater than atmospheric pressure
groundwater does not cycle as fast
groundwater ages can be greater than 10,000 years
receives water from smaller area
Aquitard vs aquiclude
Aquitard: does not transmit significant amounts of water, very low hydraulic conductivity, could have significant storage capacity/porosity
Aquiclude: does not transmit any water, NO hydraulic conductivity and little to no porosity.
what 2 things do aquifers serve as?
- -reservoirs for groundwater storage
- -pipelines for groundwater movement
what does porosity depend on
shape, arrangement and degree or sorting
specific yield
volume of water that can freely drain from a saturated rock or soil under the influence of gravity
specific retention
volume of water retained by surface tension forces as films around individual grains and capillary openings
list the 3 ways recharge inputs are transferred
- flow to adjacent areas via throughflow
- re-surface as return flow (springs) or baseflow (rivers)
- long term storage in deep aquifers
what is hydraulic head
h: measure of mechanical energy at a location- measured from base to water height
what is elevation head
z: elevation above some chosen reference level. from middle of well screen to bottom of ground
what is pressure head
P: to define water pressure at free interface between air and water as zero, measured from top of water table to middle of well screen.
general rule for groundwater flow
- water table reflects topography, deepest below uplands and shallow in valley
- equipotential contours lines perpendicular to divides, parallel to boundaries.
- streamlines perpendicular to equipotential contour lines
- both horizontal and vertical flow components (generally down in uplands and up in lowlands)
- flow nets complicated by changes in geology an topography
what are equipotential contours
lines of equal hydraulic head
what is darcys law
equation that describes the flow of a liquid through a porous medium
what is hydraulic gradient
gradient slope of the top of the groundwater table that indicates the direction of movement
T or F: groundwater flows in all directions
True, groundwater flows in all direction along potentiometric gradient from H to L
vertical hydraulic gradient
determined from h levels between wells or piezometers
horizontal groundwater flow
piezometric surface map of h contours. i=delta h/delta x
what are groundwater flow nets and why are they useful?
they are lines of equal hydraulic head. useful for determining flow direction, distance and travel times
conditions needed for effective recharge
soils should be permeable to conduct infiltration
vadose (unsaturated) zone must be permeable and free from clay layers
aquifer must be unconfined, permeable and thick to avoid groundwater mounds
groundwater table must be deep, beyond 10m below ground surface
what is artificial recharge
practice of artificially increasing the amount of water that enters a groundwater reservoir.
purposes for artificial recharge
conserve and dispose of runoff waters
supplement natural recharge of groundwater
reduce of balance salt water intrusion
suppress ground subsistence
suitable infiltration sites
flood plains, alluvial fans, sand dunes, weathered zones, glacial outwash plains, permeable vadose zones
importance of groundwater
- water supply
- supports aquatic ecosystems, moderates water temperature
- geologic processes
what year did hydrogeology emerge as a distinct branch of science from hydrology and geology?
1800
what are the key hydrological question you want to ask when there is a risk for site contamination?
- what is the geology underlying the area?
- what are the contaminants?
- which way is groundwater flowing?
- how long will it take for contaminants to reach the stream?
Darcy’s law is applicable to which type of flow? turbulent or laminar?
laminar.
