Chapter 13: Groundwater Aquifers Groundwater Movement Erosion Deposition Resources

Groundwater Groundwater is an open, dynamic system Gravity is primary driving force Pulls water down Water enters the system at the ground surface through recharge Leaves the system through discharge

Porosity Percent of the total volume that is open space Affected by the size and shape of particles Increased by fracturing or dissolution Decreased by compaction and cementation Well sorted sediments have higher porosity than poorly sorted sediments

Permeability Permeability is a property of the rock Other liquids such as oil flow through rock Density and viscosity influence flow rate Size and connectedness of pores How easily fluid may pass through a rock A measure of how well the pores are connected, and how straight a path a fluid follows

The Water Table Water is found at some depth everywhere beneath the Earth’s surface The water table defines the boundary below which all pore spaces are filled with water The volume of pore spaces decreases as you go deeper into the crust Usable water is restricted to a few hundred meters below the surface

The Water Table Forms the boundary between the Saturated Zone and the Unsaturated Zone The unsaturated zone is also referred zone of aeration

Water Table The water table may be shallow or deep as a function of permeability of bedrock and climate. The water table intersects the land surface in lakes and streams. In the location shown at right, the water table lies at a depth of about two feet. In deserts, the water table may reside hundreds of feet below the surface.

Aquifers Aquifer Aquitard Aquiclude Confining Unit A layer or zone in the subsurface capable of producing water to a well (in usable quantity) Aquitard Aquiclude Confining Unit A layer or zone in the subsurface that does not easily allow water to pass through it

Aquifers Unconfined Aquifer An aquifer that has highly permeable material extending from ground surface downward to an aquitard at its base Recharge is from seepage, lateral flow of groundwater, or upward leakage through the aquitard Also called a water table aquifer

Aquifers Perched Aquifer A layer of saturated soil found in the unsaturated zone Underlain by an aquitard of limited aerial extent Recharge is only from seepage Water flows to the edge and continues down

Aquifers Confined Aquifer Aquitards form confining units top and bottom Recharge occurs in an area some distance away or by leakage through the aquitard The water level (potentiometric surface) may be above the top of the aquifer Artesian aquifers have a potentiometric surface above ground surface

Groundwater Flow Groundwater flows due to the force of gravity Hydraulic gradient defines the water table slope Hydraulic head is the elevation of the water table at a given point The water table surface usually mimics the ground surface contours

Groundwater Flow Flow is from high to low hydraulic head Gravity creates water pressure Pressure forces water to flow downward to the water table Water continues to flow downward and outward Flow is along curved path

Recharge Groundwater and surface water are intimately linked Recharge as meteoric water flows into the ground Usually at higher elevation Where permeable rocks are exposed at surface In arid regions, streams and lakes can act as recharge zones min maximum Steepness of slope: steep flat Rate at which rain falls: rapid slow Vegetation: sparse dense Permeability: non-permeable perm unpaved paved

Discharge Groundwater and surface water are intimately linked Discharge occurs to lakes, streams and wetlands Maintains flow in streams during dry periods Artificial discharge is the result of wells

Natural Discharge Springs include any natural flow of water from the ground surface Intersection of water table-ground surface Springs are caused by: Aquitards intersecting the ground surface Faults cutting through an artesian aquifer Fractures in volcanic rock Road cuts

Natural Discharge: Springs

Homework What controls how fast groundwater moves? Hydraulic conductivity Permeability Viscosity Gradient Is groundwater movement important?

Wells Before Pumping During Pumping Water table is nearly horizontal Water table is depressed as water is withdrawn from the aquifer Forms cone of depression

Thermal Springs & Geysers Geysers and hot springs require: Hot rock bodies to heat groundwater Fracture systems to transmit water Large supply of groundwater Geysers such as Old Faithful are the results of this process

Thermal Springs & Geysers Hot water contains thermal energy and dissolved ions Geothermal energy taps heat source Many mineral deposits are produced by hot water interacting with cooling igneous intrusions

Thermal Springs & Geysers Hot water contains thermal energy and dissolved ions Geothermal energy taps heat source Many mineral deposits are produced by hot water interacting with cooling igneous intrusions The One Ring to Rule Them All

Groundwater Erosion Groundwater chemically weathers & erodes bedrock through dissolution Subsurface dissolution forms: Caves Sinkholes Karst topography

Ha Long Bay, Viet Nam

Karst Topography Karst topography is found in areas that have carbonate, sulfate, or salt beds at or near the ground surface And the climate is…? Characterized by: Solution valleys with remnant walls and towers Disappearing streams Sinkholes

Karst in Chinese Landscapes

Groundwater Deposits Minerals may precipitate from dissolved ions in groundwater CO2 in water dissipates into air pH of water changes CaCO3 is precipitated Other soluble minerals may occur Gypsum

Groundwater Deposition Richard Cindric

Groundwater Deposits Minerals may precipitate from dissolved ions in groundwater SiO2 also moves Cement sediment Fossil replacement

Groundwater Resources Groundwater is the largest reserve of fresh water Much larger than surface water resources Used for drinking water, irrigation & industrial uses Excessive use causes drawdown, sudsidence, sinkhole collapse, pollution, and disruption of ecosystems. GROUND WATER ATLAS of the UNITED STATES Arizona, Colorado, New Mexico, Utah, USGS HA 730-C

Groundwater Resources Groundwater is the largest reserve of fresh water Much larger than surface water resources Used for drinking water, irrigation & industrial uses Excessive use causes drawdown, sudsidence, sinkhole collapse, pollution, and disruption of ecosystems. GROUND WATER ATLAS of the UNITED STATES Arizona, Colorado, New Mexico, Utah, USGS HA 730-C

How do we use groundwater? 97% of unfrozen fresh water supply 50% of our drinking water 25% of water used for industrial purposes Richard Cindric

Major Concepts The movement of groundwater is controlled largely by the porosity and permeability of the rocks through which it flows. The water table is the upper surface of the zone of saturation. Groundwater moves slowly through the pore spaces in rocks by the pull of gravity moving from high pressure to low pressure The natural discharge of groundwater is generally into springs, streams, marshes, and lakes. Groundwater occurs in unconfined and confined aquifers. Erosion by groundwater produces karst topography, with caves, sinkholes, solution valleys, and disappearing streams. Groundwater also precipitates minerals creating deposits in caves and along fractures and cements many kinds of clastic sedimentary rocks. Alteration of the groundwater system can produce many problems, such as pollution, subsidence, sinkhole collapse, and disruption of ecosystems.