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 ( 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.

Hydraulic gradient

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

Composite Landscape

Artesian aquifer

Desert Oases

Municipal water tower

Natural springs

Natural springs – contd.

Geysers

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

Saltwater intrusion

Salt water intrusion – contd.

Stratigraphy of Yucca Mountain

Landfill sites

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

Cave formation

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