Presentation on theme: "Chapter Fifteen Groundwater. Groundwater Earth’s hydrosphere extends from top of atmosphere to ~ 10 km (6 mi) below the Earth’s surface. Groundwater,"— Presentation transcript:
Chapter Fifteen Groundwater
Groundwater Earth’s hydrosphere extends from top of atmosphere to ~ 10 km (6 mi) below the Earth’s surface. Groundwater, 0.6% of world’s water, accounts for 97% of Earth’s supply of unfrozen water. Surface waters from rivers, lakes or precipitation enter the ground under the influence of gravity The amount of water that enters the ground depends on the topography, geologic composition and vegetation The surface water will percolate downwards until it reaches a zone where all the pore spaces are saturated with water
Factors affecting infiltration
Moisture through tree
Surface distribution of water
Movement and Distribution of GW Water bound to clay minerals Zone of aeration: unsaturated zone Zone of Saturation Water Table: Upper surface of the zone of saturation Capillary Fringe: Lower part of the aeration zone – range from few tens of cm to several meters
Groundwater Movement For groundwater to flow- need porous and permeable materials Porosity is the percent of pore spaces in relation to the total soil/rock/sediment volume (Primary Porosity: Porosity that develops as a rock forms; Secondary Porosity: Develops after a rock has formed) Permeability is a measure of the ability of rock/sediment to transmit fluid Groundwater flow occurs when there is a hydraulic gradient Hydraulic conductivity is the ability to transmit groundwater. Typical groundwater flow is slow ~0.5 – 1.5 cm/day (0.2 - 0.6 in/day). The slow movement stores the water making it available to humans.
The subsurface distribution of water
Primary porosity of sedimentary rocks
Pore connection and Permeability
Pore connection and Permeability – contd.
Variations in water table depth
Water Table configuration
Perched water pools
Tapping Groundwater aquifer Groundwater depth is a reflection of local topography and prevailing climate Aquifers are permeable water-bearing bodies of geologic materials Unconfined and Confined aquifers Aquiclude and Aquitard Artesian flow- water flows to the surface from wells without pumping Potentiometric surface Natural Springs Geysers- intermittent surface emission of hot water/steam
How groundwater flows Groundwater flow potential – Gravity – Pressure from overlying water and rocks – Hydraulic Gradient: Difference in potential / distance – Hydraulic conductivity: Measure of conductivity – size, shape & degree of sorting of its grains – coarse, well-rounded, well-sorted and gravel – high conductivity – Darcy’s principle (19 th Century): Rate at which water flows is difference in potential hydraulic conductivity
Groundwater flow – contd. Mesuring the Rate of GW Flow: – Dye injection method – Carbon –14 dating method Groundwater Flow Rate: Average: 0.5 to 1.5 cm/day Fastest: 100 m / day
Municipal water tower
Natural springs – contd.
Locating groundwater and features associated with pumping Locating Groundwater Mapping Water table from well records, lake, river levels, drilling test holes Features Associated with Pumping Cone of depression- local depression in water around pumping well Subsidence- compressed land due to aquifer depletion Salt water intrusion- due to over pumping along coast line Greater demand for water- leads to groundwater depletion and introduction of pollutant Solutions- enhance recharge, water transfer, conservation, desalinization, and use of iceberg
Search for Groundwater Characteristics of the Water Table: – Depends on Typical depth – Relationship to topography – Response to Climatic Fluctuations AQUIFERS: – Aquifer: Permeable, water-bearing bodies of geological material – Unconfined aquifer: Not overlain by impermeable cap tock – Confined Aquifer: Found in greater depths, between aquiclude (impermeable rock layers) and Aquitard
Search for water
Overuse of groundwater
Water table around a well
The effect of development on a water table
Subsidence in a coastal area
Subsidence in a coastal area – contd.
Groundwater contamination / purification Natural occurring solutes- Chlorine, Arsenic, Mercury, and Selenium Manufacturing contaminants- from household products: cans of paint, solvent, cleanser etc. Natural Groundwater Purification Filtration- adhere to clay particles Decomposition- some decompose completely by oxidation Bacteria action- organic solids consumed by microorganis
Salt water intrusion – contd.
Stratigraphy of Yucca Mountain
Landfill sites – contd.
Products of Groundwater Karsts Water reacts with carbon dioxide to form weak carbonic acid which then attacks limestone H 2 O + CO 2 => H 2 CO 3 + CaCO 3 => Ca ++ HCO 3 – Caves - natural underground cavities and most common geological product of limestone dissolution Cave deposits - spelotherms are deposits on cave surfaces in a variety of forms: travertine, Stalactite, Stalagmite, banded draperies or drip curtains Growth of spelotherm- depends on the solution and porosity of surface material, climate, topography, and vegetation
State 1 of cave formation
State 2 of Cave formation
Stalactite, stalagmite and travertine
Speleothem growth and climate
Rise and fall of sea level
Karsts Topography Surface expression of the geology of dissolved limestone and work of near surface water Cave and Karsts landscapes are extremely sensitive- so need to be protected Landform Sinkholes-circular surface depression Disappearing Streams- flow through sinkholes may emerge as spring several kilometers away Natural Bridge- series of neighboring sinkholes expand and join together
Worldwide distribution of karst landform
Typical landforms associated with karst topography