ODE Exam 3- Water Cycle and Ground Water

hydrologic cycle

the circulation of water among earth’s reservoirs

what percent of freshwater is on earth

3%

what percent of groundwater is on earth

30.1%

residence times

average time a H20 molecule spends in the reservoir

ice sheet residence time

500,000 plus years

atmosphere residence time

15 days

groundwater residence time

weeks to to 30,000 plus years

groundwater

water in the saturated zone, beneath the water table

water table

top of saturated zone

unsaturated zone

region beneath surface in which pores/cracks are filled with air and water

soil moisture

water in the unsaturated zone

saturated zone

region beneath surface in which all pores/cracks are filled with water

porosity

percent of void space in rock or sediment

what allows groundwater to flow?

porosity and permeability

permeability

how connected are the pore spaces

unconfined aquifer

not covered with a confining layer and has high porosity and permeability

aquitard (confining layer)

layer in between unconfined layer and confined layer. low porosity and permeability

confined aquifer

lowest layer below confining layer. low porosity and permeability

ogallala aquifer

high plains aquifer that follows from west to east on a regional scale

what rock is the best confining layer

shale

how is the water table related to surface topography

water table mimics surface topography in humid regions, total elevation difference is less extreme, water table topography is maintained because ground water flow is slow and water table cant return to being flat

why does ground water flow?

unsaturated zone- gravity pulls moisture downward

saturated zone- gravity and water pressure from the weight of water both drive the flow

if the water table is flat then

no flow of ground water

ground water flow is usually from a high water elevation to a low water table elevation along

curved and concave up pathways

effect of the water table position

elevation of water table at height one is higher than height two

pathways of groundwater flow

from recharge area to discharge area. flow is along curved, concave up paths

how fast does ground water flow

inches to feet per day. rate depends on hydraulic gradient

hydraulic gradient formula

triangle h/ L

triangle h

difference in total elevation between two points (in feet)

L

distance between two points (in feet)

Henry P. G. Darcy

1850s french engineer. field studies near dijon, france. conducted labs on rate of water flow thru various sediments. developed basic equation to describe rate of flow of ground water

darcy’s law

Q = K (triangle h/L) A

Q (darcy’s law)

volume/time. discharge across an area

triangle h/L (darcy’s law)

difference in total elevation between two points in feet

A (darcy’s law)

area across which water flows

K (darcy’s law)

hydraulic conductivity of aquifer. depends on properties of water and solid aquifer, esp permeability of rock unit

wells- ground water withdrawal

drawdown of the water table, brings water up to surface

drawdown of the water table and cones of depression

it is a unconfined aquifer. small vs large well. water table gets lowered on one side where the large well is used more (farming, ect).

natural artesian wells

natural confined aquifers. only small area is unconfined. rain recharges. Water flows under pressure to the surface, creating a well that does not require pumping

potentiometric surface

potential height where water would rise to if you drilled into a confined aquifer. only for confined aquifers

municipal wells

water towers. water is pumped from well into the tower. main water pipeline goes into individual pipes

problems with ground water supply and usage

lowering of water table, reversal of flow direction, saltwater intrusion

aquifer collapse

when too much water is pumped out of aquifers comprised of loose sediment they can permanently compact. water holds grains apart and keeps pores open, ground cracks and scarps develop, heavy irrigation, aquifer thins

subsidence example

San Joaquin, CA. 9 meters

reversal of flow direction

contaminant plume leaks and gets into well water and contaminates it