ESS FINAL PREP: GROUNDWATER

what % of the Earth’s freshwater is ground water?

what % of the liquid freshwater?

30% of E’s freshwater, 96% of liquid freshwater

what is groundwater used for?

IRRIGATION (nearly half is groundwater).

Used for public supply/ individual supply.

GW has declined?

US water use declined due to conservation + efficiency improvement.

what is the water table?

rainfall partitions into runoff, evaporation+infiltration (primary source of GW.

what are the specific zones of ground water?

zone of soil moisture: near surface, held by molecular attraction.

zone of saturation: all pore spaces filled = GW

water table: upper boundary of saturation zone.

capillary fringe: transition zone above water table

unsaturated zone (vadose zone): pore spaces contain air + water

when does high runoff occur?

High runoff occurs: surface material is impermeable or saturated, urban areas have roads, buildings + parking lots.

GW & streams dynamic partnership

gaining stream: water table is higher than stream → GW flows in

losing streams: water table is lower, stream water infiltrates.

combination streams: gain in some sections, lose in others.

52% of stream flow in US comes from GW

what is porosity?

def: % of rock/sediment volume made of pore space (sands > clays > igneous rock)

what is permeability?

def: ability of rock/sediment to transmit water (connected pore spaces)

what are Aquifers & Aquitards?

low-permeability layers that hinder flow (eg: clay)

what is Darcy’s law?

how GW flows through porous materials, driven by difference in energy (hydraulic head).

Developed in 1856 based on experiments with water flowing through sand columns. Q= -KA (change in) H / (change in) L

what are Hot Springs?

def: spring w/ water typically 6-9 (degrees C) warmer than the avg annual air temperature of the area.

over 95% of hot springs+geysers in the US are found in the West, due to recent igneous activity

what are geysers?

def: intermittent hot spring that periodically erupts jets of hot water + stream. Require: underground of chambers in hot igneous rock. Heat from surrounding rock heats infiltrating GW

what are Geyser Deposits?

def: chemical sedimentary rocks are deposited when mineral-laden hot water reaches the surface

what is subsidence?

def: sinking of the ground when water is withdrawn faster than it can be replaced

why do sinkholes form?

form underground voids (often in limestone) collapse. can be natural or triggered by GW withdrawal, common in karst terrain where acidic dissolves carbonate rocks. o

how much freshwater does the US use everyday?

~306 B gallons of freshwater per day. Abt 25% of this is from groundwater.

what are some factors of the water table?

slope steepness, soil/rock permeability, rainfall intensity + vegetation cover)

what does the amount of runoff depend on?

rainfall intensity+duration, soil saturation, surface material, land slope, vegetation cover.

where does subsidence occur?

in areas with unconsolidated sediments (grains compact)

when is Travertine (calcareous tufa) formed?

when calcium carbonate precipitates due to a drop in pressure and loss of CO2 at the surface

what are the heat sources of hot springs?

geothermal gradient (~25 C per km depth) + Magmatic activity (esp in western US)