Unit 6 Groundwater Study Guide (Chapter 13; also pages 161-163) (Revised 7/12) UNIT 6 HOMEWORK WEB HIT HOMEWORK - part 1: ONE WRITTEN PARAGRAPH from any selected unit web hit site VIDEO WEB HIT HOMEWORK part 2: ONE WRITTEN PARAGRAPH from any selected unit video site For any Unit Web Hits and Unit Web Videos, go to the DMC HOME website; in Search box type Geology, select Vernon Kramer, scroll down to GEOL 1303, select Syllabus, select Web Hit Links, later then the select Video Web Hits, click on icon of interest for web sites OR: go to DMC Home website, select Degrees, Certificates, Courses, scroll down to Natural Sciences and select Geology, select Faculty Listings, select Walter Vernon Kramer, find Geol 1303, select Syllabus, and there you can find the web hit links click on icon of interest for web sites and then to Web video hits, click on icon of interest for video site [IF NONE OF THE WEB SITES COME UP, YOUR COMPUTER PROBABLY NEEDS TO BE REBOOTED (RESTARTED) Groundwater Generalities - Agriculture uses about 79% and municipals use 13% of our ground water supply - The oceans contain 97% of Earth s water; with only 2%-3% available for fresh water. Water, water everywhere, and all the boards did shrink, water, water everywhere, nor a drop to drink Rime of the Ancient Mariner - Of the freshwater on Earth, 69% is tied up with glaciers and most of the remaining 30% as groundwater. - Groundwater supplies almost 50% ( ½ ) US drinking water (and to virtually all people living in rural areas) - Most rivers and lakes maintain their water levels from groundwater. Water Cycle Groundwater system: open, dynamic system of underground water that uses heat from the sun and gravity as the primary driving energy forces Water cycle: water circulates from land to the sky and back to land again FYI: The water cycle includes evaporation (liquid to vapor), transpiration (by animals and vegetation), condensation (forms clouds), precipitation (rainfall), runoff (into rivers and seas) and infiltration (into the ground). -There is no new water on Earth. Almost all water has been here since Earth s creation. Groundwater Infiltration (percolation): the process of surface water entering into the ground or soil or rocks Groundwater System Groundwater: water below the Earth s surface that has infiltrated from surface sources 1
- Most shallow groundwater is available and suitable for human use - Most deep groundwater (+1,000 feet) contains high concentrations of minerals and salts and is generally less desirable for drinking water. Three General Classifications of Groundwater 1. Potable groundwater: water that infiltrated from surface sources. It is suitable for human use and generally found at relatively shallow depths (generally above 1,000 feet depth). [Potable water: water of quality, (relatively) suitable for drinking] 2. Connate groundwater: Water trapped in the pores of a rock during formation of the rock, can be described as fossil water, generally not potable (example water found in oil and gas wells) 3. Juvenile groundwater: Water brought to surface or added to underground supplies from magma and is never potable The difference between the right word and the almost right word is the difference between lightning and a lightning bug Mark Twain Pore space (porosity = % pore space) For Water to Enter the Earth There Must Be an Opening. Pore space: the spaces within a rock body that are unoccupied by solid material. This includes spaces between grains, fractures, vesicles, and voids formed by dissolution. Porosity: The percentage of the total volume of a rock or sediment that contains pore space Porosity and pore space determine the quantity of water that can be stored in rocks and sediments - Variety of porosity depends upon the rock type. Variety of Porosity: Intergranular Porosity Intergranular porosity: the open space between grains of sediments; the more space between the grains of sediments, the greater the porosity - This porosity is associated with sand, gravel, and soil (which are clastics or pieces of older rocks) and their rock types of sandstone and conglomerate: (clastic rocks) Factors Affecting Intergranular Porosity: A. Sorting: The separation of particles according to size or shape. This sorting occurs during transportation of this sediment by either wind or water. Better sorting can provide better porosity. Thus sorting can increase or decrease porosity. (Done during the sedimentary process) B. Compaction: grains pushed together will reduce porosity C. Cementation: the reduction of porosity by minerals precipitated from groundwater in the pore spaces of sediment or rocks. - Cementation by the process of precipitation can totally close pore spaces, or bond the sediments together as a rock (no longer a sediment) - Precipitation: formation of a solid substance by the mixture certain liquid solutions FYI: Two most common minerals that cement sediments: silica (quartz) and calcite 2
dissolution of limestone Variety of Porosity: Dissolution Porosity Dissolution: The dissolving of limestone by acid water which creates holes (solution cavities - which results in pore space) - Acid rains dissolve calcite (limestone and marble) which is carried away as a mineral solution by the groundwater. Solution cavities: (pores) openings remaining after the acid dissolution of limestone. Basically this is the removal of materials (calcite) by acid water. - The presence of a very large solution cavity (such as a cave) can create excellent porosity. Variety of Porosity: Fracture Zone Porosity fracture porosity Fracture zones: a zone where the bedrock is cracked or fractured and creates porosity. Fracturing intensity tends to decrease with depth. Variety of Porosity: Vesicles and Vugs Porosity Vesicles and vugs: small holes formed in volcanic or lava rocks by gas bubbles that became trapped as lava solidifies (unimportant for most aquifers) Moving Water within the Earth Permeability: the ability and capacity of rocks and sediments to transmit water - High permeability means that water moves easily through the rock or sediment. Examples could include: 1) good intergranular porosity found within coarse sand and gravel deposits, 2) a myriad of solution cavities and 3) intense, large-scale fracturing - (high permeability may yield hundreds of gallons of water per minute) 3
Moving Water Within the Earth (continued) - Low permeability means that water moves very slowly through certain rocks and sediments. Examples could include: 1) very fine-grain sand packed tightly together, 2) sediments that are poorly sorted with lots of clay, 3) sediments that are partially cemented filling pore spaces, 4) areas of widely-spaced fractures that don t easily interconnect - (low permeability may yield only a few gallons per day of water) - No permeability means that water does not flow through certain rocks and sediments. Examples would include: 1) sediments with abundant clay that has filled the pore spaces, 2) intense cementation and filling of the pore space by minerals or 3) areas with no fracturing or minor dissolution - (no permeability may yield a gallon of water every century) Groundwater Misconceptions: - Below the surface are underground rivers. (How do you keep open the space down there? Caves are not as wide and extensive as rivers.) - Below the surface are underground lakes (How do you keep open the space down there? Caves are not as big as lakes) - The deeper the well, the purer the water. (Not very often.) - Some people call water dowsers claim that they can use a forked stick or bent wires to find water. You can fool some people some of the time, you can fool all people some of the time, but you cannot fool all the people all the time Abe Lincoln Groundwater Generalities: 1. Water is found at some depth everywhere beneath the Earth s surface. 2. The volume of pore spaces generally decreases as you go deeper into the Earth. 3. Most usable water is limited to about 1,000 feet below the surface. 4. The groundwater surface usually mimics the ground surface contours. Groundwater Terms Aquifer: a permeable layer that allows the flow of groundwater Zone of aeration (vadose zone): the unsaturated zone in which water is being pulled down by gravity, located above the saturated zone. The pore space in this area is filled with air and minor amounts of water, so it is not truly saturated. Water that is trapped within the zone of aeration will either be used by plants, evaporate, or continue downward. Water table: the upper surface of the zone of saturation (an actual plane or surface) Zone of saturation: a zone in the subsurface in which all pore spaces are filled with water. This is the groundwater reservoir. (This zone lies below the water table.) Aquiclude (Aquitard or a Confining layer): a layer or zone in the subsurface that does not easily allow water to pass through it; such as clay layers and unfractured rocks (it can act as a bottom dam ) 4
Sketch of an aquifer Three Major Types of Aquifers (sketch below) 1. Unconfined aquifer or water table aquifer 2. Perched aquifer or temporary aquifer 3. Confined (artesian) aquifer Types of aquifers & types of water wells Unconfined Aquifer Properties (see sketch above) 1) Also called a water table aquifer 2) This aquifer has permeable material vertically extending from the ground surface downward to an aquitard 3) Adding water to the aquifer (the recharge zone) occurs by rainfall or flooded rivers and lakes 4) Water can leave the aquifer (discharge) by seepage into pumping wells, wetlands, lakes, springs, rivers, etc. This is why rivers and lakes exist even in times of drought. 5) Top of water table somewhat follows the surface topography or the slope of the land 5
Perched Aquifer Properties (see previous sketch) Perched Aquifer (water table): The upper surface of a local zone of saturation that lies above the regional water table on a small impermeable layer. Overtime, the groundwater will flow over the edge of the impermeable layer and no water will remain here. This should be considered as a temporary aquifer. Confined Aquifer (Artesian) Properties (see previous sketch) 1) This is basically a pressurized aquifer 2) The aquifer must be sloping to build up pressure. 3) The aquifer is between aquitards and the aquitards form both confining top and bottom layers and confines the water to the aquifer. Thus the water becomes pressurized like within a pipe. 4) Rainwater can only enter (recharge) this aquifer in a relatively small zone, some distance away from your water wells or spring (a discharge zone). 5) The aquifer must be filled with water to be pressurized as an artesian aquifer Water Well Levels (Carefully Note the Two Types of Artesian Wells!) (see sketch below) Artesian Water (Well): groundwater confined in an aquifer and under pressure great enough to cause the water to rise up within the well pipe (less pumping see two wells in the sketch below) Artesian Water (Flowing Well): groundwater confined in an aquifer and under pressure great enough to cause the water to rise up high enough in the well pipe to flow out on the surface (no pumping see one well in the sketch below) Unconfined Water (Well): Groundwater level within a pipe, within an unconfined aquifer, is at the same level as the water table. (see one small well in the sketch below) Artesian wells: water rises in well above the aquifer Earth Materials (That Make the Best Aquifers) - Soil and rocks that have good porosity and permeability 1. Sand within sand dunes and river sands 2. Sandstone, gravel & conglomerates 3. Cavernous limestone (lots of openings here) 4. Highly fractured rocks Earth Materials (That Make the Best Aquitards or Worst Aquifers): - Soils and rocks with poor permeability; restricts water flow 1. Clay and silt/mudstone and shale (clay can seal up openings and pores) 2. Strongly cemented sediments and rocks (pores filled with cementing minerals) 3. Most igneous and metamorphic rocks have few or no pores 6
water surface tension Water Extraction Water Surface Tension: intermolecular force that binds a film of water to a grain surface (thus we can never have 100% water extraction from an aquifer) The wars in the future will be about water Anonymous Some Texas Aquifers - Not all aquifers are created equal, IN REGARD TO WATER QUALITY - Corpus Christi has access to a large belt of Gulf Coast aquifers - The Edwards Aquifer is important to San Antonio; host rock is limestone - The High Plains (Ogallala) Aquifer (Texas Panhandle) of sand and gravel is rapidly running out of water because most of its recharge zone is covered by clays - Central Texas has a large area of fractured rock that serves as host for groundwater Local Aquifers: Present and ancient barrier islands that extend from Flour Bluff to Rockport have good permeability but poor quality water. Most of Pleistocene Fluvial (River) System to the West of Corpus Christi has good permeability but poor quality water. Goliad Formation (near Sinton) has good permeability, good quality water, is mostly but privately owned. - Corpus Christi uses river water because it is cheaper to acquire and process (especially when it rains). Local Groundwater Quality Problems - Clays on the land s surface and on farm lands usually provide excellent filters and many aquifers with some clay will form excellent filters to its contained water. - Surface clay and clay in aquifers cannot trap dissolved iron and salt molecules. Most of our local aquifers have an abundance of dissolved salts and dissolved iron. - Although dissolved iron does not affect water taste, it does stain everything orange as the water film dries. 7
Scale in a hot water pipe - Almost all local aquifers contain hard water or water with dissolved calcite (lime, scale, etc.). This scale can fill up (iron) hot water pipes and can fill hot water heaters with coatings of calcite. - Other local aquifers have areas that are severely contaminated by man. - Some of these local aquifers contain such metals as dissolved uranium and selenium. - Arsenic is a major contaminant in South Texas in many local aquifers. - Other isolated aquifers may contain sulfuric acid from leaking gas wells. The Romans realized, as have every civilized people since, that living in cities is impossible if the water supply is not reliably clean and fresh. - Frank & Francis Chapelle Desalination - This is an alternative tool that can produce fresh water from areas that have large reserves of brackish water. - In Texas there are more than 100 desalination units, including those at Brownsville, El Paso, Ft. Stockton, Odessa, etc. - The problem is that the cost of this water is double that of other water supplies (but better than no water!). Terms Associated with Groundwater Contamination (Pollution) Leaching: dissolving and removing the soluble constituents of a rock or soil or trash or anything (process) Leachate: A water solution or pollution produced by leaching that can enter an aquifer (product) Methods for Extracting Groundwater 1. By digging a well and use buckets or pumps 2. By driving a pipe into the ground and wait for water to enter, then pump it out 3. By drilling a well and pumping the water (this includes windmills) 8
Drilled Water Wells - Drilling is the most common method of getting to groundwater. After drilling a hole, a pipe is inserted into the well. At the base of this pipe will be a screen that keeps out the sand but permits water to pass. The water well pipe is cemented on the outside to prevent surface pollution from entering the water well. The installation of a pump type will depend upon the depth to the water level in the pipe. Cone of depression: a conical depression of the water table surrounding a well after heavy pumping. - Water moves faster straight down by gravity than sideways into the bottom of a water pump. Texas Water Laws: - The State owns the water that flows on the surface anywhere in the state. - However, groundwater is owned by the surface land owner - The rule of capture applies (different rules if you are in an election-mandated water district). The land owner can capture or pump all the water possible from a water well on his property, without regards to the effect of the water table on neighboring property. - If your well goes dry because of his pumping too bad (law of biggest pump and deepest well) - Law exceptions (never won in court) include malicious use, wasted water or sinking land. We used to think that energy and water would be the critical issues for the next century. Now we think water will be the critical issue. - Mostafa Tolba of Egypt Groundwater: Oil and Gas (Page 161 162) Oil and gas can develop from buried plant and animal remains. - Deep burial and heat combine to create oil and gas that are less dense (lighter) than the enclosing connate water. - Oil and gas will rises through the water in the sediments to a trap. - From top to bottom in the trap, expect to find the following layers : gas on top, then oil then water at the bottom. - Gas always rises higher in a trap than oil. - Oil always rises higher than water in a trap. 9
They say you cannot drill your way out of the oil crises, why is it then, that you can plant crops and eventually get out of a starvation crises- Kramer Salt Water Encroachment (Common with water wells adjacent to the sea) - This is usually associated with pumping freshwater aquifer too fast in wells. - Fast pumping allows sea water to enter the aquifer and thus the water well. Ground Subsidence -Withdrawing groundwater too rapidly can cause major ground subsidence such as at the Houston- Galveston area. You should pump out water only as fast as the water is entering the aquifer. Acid Groundwater - Rain water combines with carbon dioxide in the air to form carbonic acid: H 2 0 + CO 2 = H 2 CO 3 Acid ground water also comes from decaying plants, pollution and leaking gas wells. Caves - Acid water entering limestone pores can dissolve millions of tons of calcite (solution activity) in small and large areas to create caves. Caves or caverns: a naturally formed subterranean open area. The open chambers or series of chambers are produced in limestone by solution activity. Large caves take millions of years to form. Accessible Texas caves Local Texas Caves to Visit (Most other caves in TX are small and are on private land) - The beautiful Caverns of Sonora are located near the central-west TX town of Sonora. - Near San Antonio and Austin are six large, public-accessible caves including the following that should be visited by all: Natural Bridges Cavern, Cascade Cavern, Cave-Without-A-Name, Wonder Cave, Longhorn Caverns and Inner Space Caverns. 10
- Deposition (calcite) by groundwater in caves includes stalactites (on roofs of caves), stalagmites (on floor of caves) and columns that go from ceiling to floor. Travertine: form of massive calcium carbonate (calcite), resulting from deposition in caves or springs. Sinkholes Sinkhole: a depression formed by the collapse of a cavern roof. This can cause major problems in urban areas where buildings are constructed. Karst Topography Karst topography: a landscape developing in limestone areas that are characterized by numerous sinkholes. Abundant rainfall is required for strong karst development. - The Arecibo Radio Telescope of Puerto Rico is located on top of a large sinkhole. 11
Groundwater that Is Hot Geyser: a thermal spring that intermittently erupts steam and boiling water. Locally there must be hot fractured rocks, and a nearby heat source such as a recent volcano or intrusion. Geothermal energy: energy for human needs that can be extracted from steam and hot ground water Advantages: a) Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect b) Once you've built a geothermal power station, the energy can be almost free. Disadvantages: a) There are not many places where you can build a geothermal power station. You need hot rocks of a suitable type, at a depth where we can drill down to them. b) Hazardous gases and minerals may come up with the hot water or steam. These gases and minerals can be costly to safely dispose. The universe is full of magical things patiently waiting for our wits to grow sharper. E. Phillpotts 12