GEOL 101 1nd Edition Lecture 21 Outline of Last Lecture I. Stream ValleysII. Other Stream FeaturesIII. FloodsOutline of Current Lecture I. GroundwaterII. CavesIII. KarstCurrent LectureGroundwater-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 saturationThese 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 variableo Varies seasonably and from year to yearo 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 aboveo i.e. where there are topographic highs (hills or ridges), there will be a high point in the water tableo Where there are topographic lows (valleys), there will be a low point in the water tableo Groundwater flows from high points to low points- Variations in permeability from place to placeo 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 streambedo Losing Streams– streams lose water to the groundwater system by outflow through the stream-bed Loosing streams may be disconnected from the GWTo 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 movemento 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 tableo These are separated from the main groundwater table-Groundwater dissolves carbonate rocks- Groundwater is often mildly acidico 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 knownas 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 evaporateso 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