GEOL 101 1nd Edition Lecture 21 Outline of Last Lecture I Stream Valleys II Other Stream Features III Floods Outline of Current Lecture I Groundwater II Caves III Karst Current Lecture Groundwater Groundwater is the water found in the pores of soil and sediment plus narrow fractures in bedrock Unlike surface water groundwater generally moves very slowly seeping its way through the pore spaces In certain areas that are dominated by cave networks the water can flow more quickly through the large voids in the rocks Groundwater is the largest reservoir not counting glaciers of fresh water that is readily available to humans Runoff water that flows along the surface and does not seep in to the ground As water seeps down into the ground it forms the following layers Zone of Aeration Undersaturated zone above the water table Includes Belt of soil moisturethe water that holds on to soil particles in the near surface zone Water here cannot be pumped by wells Water Table the upper limit of the zone of saturation These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Zone of Saturation Water not held as soil moisture percolates downward Water reaches a zone where all of the open spaces in sediment and rock are completely filled with water Water within the pores is called groundwater The water table is the contact between the zone of aeration and the zone of saturation Variations in the water table Variations in rainfall Depth is highly variable o Varies seasonably and from year to year o The GWT will be deeper during the dry season or years and closer to the surface during the wet season or years Variations in the water table Water table tends to mimic the topography shown above o i e where there are topographic highs hills or ridges there will be a high point in the water table o Where there are topographic lows valleys there will be a low point in the water table o Groundwater flows from high points to low points Variations in permeability from place to place o Permeability ability of a substance to transmit fluid Links between the groundwater and the rest of the hydrologic cycle Surface water percolates downward to the surface Groundwater discharges to the surface through springs and via root uptake Interaction between groundwater and streams Two types of interactions o Gaining Stream streams gain water from the inflow of groundwater through the streambed o Losing Streams streams lose water to the groundwater system by outflow through the stream bed Loosing streams may be disconnected from the GWT o Stream may be gaining in one area and losing in another depending on varying conditions upstream and downstream Permeability aquitards and aquifers Permeability the ability of a material to transmit a fluid Aquifers permeable rock strata or sediment that transmits groundwater freely o Rock types that commonly act as aquifiers include medium coarse grained sandstones and gravels Aquitard an impermeable layer that hinders or prevents water movement o Shale and crystalline igneous and metamorphic are good aquitards because water cannot flow easily Ogallala Aquifier Eroded material that was shed off of the Rock Mountains formed very porous and permeable rock layers that stretch from South Dakota to Texas Underlies a 174 000 mi2 area In 2005 it contained an estimated 950 trillion gallons Is the main source of water for agricultures in the high plains Springs Occur when water table intersects Earth s surface Natural outflow of groundwater Can be caused by an aquitard creating a localized zone of saturation which is called a perched water table o These are separated from the main groundwater table Groundwater dissolves carbonate rocks Groundwater is often mildly acidic o Contains weak carbonic acid which is formed as water combined with carbon dioxide from the air The groundwater comes into contact with limestone Carbonic acid reacts with calcite in limestone dissolving the rock Caverns Caves Groundwater that dissolves soluble rock below the surface will form caverns also known as caves Caves have a delicate balance of solution and dissolution Caves which have this balance are called living caves and are continuously producing new cave formations such as stalagmites flowstone etc Caves that have stopped interacting with groundwater are called dead caves Features found within caves Composed of dripstone travertine o Calcite deposited as dripping water evaporates o Collectively they are called speleothems Flow Stones massive deposits of travertine Stalactites hang from ceiling Stalagmites form on the ground Soda straws hollow tubes earliest growth of stalactites Columns form when stalagmites and stalactites meet Curtains or bacon thin sheets of travertine that hang from the ceiling or wall Shelf Stone limestone precipitates on the top of a pool of water creating a shelf with a column underneath Shields Box work Popcorn Calcite aragonite and gypsum crystals Dissolved limestone that is very close to the surface results in landscape features known as Karst Topography Some common features include Sinkholes Lack of streams and or disappearing streams Springs Natural Bridges Towers Sinkholes When caverns from close to surface the roof may becomes unstable and collapse under its own weight making a sinkhole Areas that are dominated by sinkholes have very irregular terrain Sinkholes are major sources of water quality issues because of people dumping garbage into them They may cause severe damage to homes and property Disappearing streams Streams running along the surface may be swallowed up into a sinkhole or a cave These streams may reemerge downstream through the mouth of a cave or by spring Natural bridges remnants of a cave passage formed when sinkholes collapse on two sides Note this is not the only way that natural bridges may form Karst towers Last remnants of massive limestone layers that form enormous towers These form in tropical regions where the limestone is eroded more rapidly by plentiful rainfall as well as high humidity